physicsworld--2020-12-08--Things we don’t know about Uranus (and Neptune) - Physics World

Uranus and Neptune are the least-explored planets in our solar system .
Apart from the Voyager 2 spacecraft , which flew past them in the 1980s , no human-built probe has ever ventured near them .
They are also small and far away , covering just 3.8 and 2.3 arc-seconds of the Earth ’ s sky , respectively .
That makes it hard for even the best telescopes to pick out their features .
Indeed , one of the few things we do know is that much of our knowledge about their appearance is no longer accurate , because they @ @ @ @ @ @ @ are incredibly active worlds , with convective clouds that rise up like thunderheads and send streamers flying through their slushy atmospheres and giant storms that last for years .
The other thing we know about Uranus and Neptune is that planets of this type are very common .
According to Heidi Hammel , vice president for science at the Association of Universities for Research in Astronomy ( AURA ) , most of the exoplanets discovered so far are in “ the Neptune to sub-Neptune range ” in terms of their size , yet @ @ @ @ @ @ @ never been thoroughly investigated .
“ We ’ d like to change that , ” Hammel told audience members at the annual Appleton Space Conference , which was held virtually this year on 3 December .
During her talk , Hammel laid out numerous reasons for sending spacecraft back to these icy outer worlds .
The ice giants , she explained , have interiors that differ fundamentally from those of rocky planets like Mercury , Venus , Earth and Mars .
But they aren ’ t composed of hydrogen like Jupiter and Saturn @ @ @ @ @ @ @ ices , which in astronomical terms are mixtures of various frozen stuff .
Theoretical models suggest that both planets have cores composed of iron , nickel and silicates , but “ the bottom line is , we don ’ t know what the interior is , ” Hammel said .
Hammel went on to explain that our knowledge of Uranus is especially limited because of the timing of the Voyager 2 flyby .
For reasons that are not fully understood , but likely stem from an impact early in its formation , Uranus @ @ @ @ @ @ @ with respect to its orbit .
This is very unusual for a planet , and unfortunately it meant that Uranus was in its southern solstice when Voyager 2 made its nearest approach .
In this configuration , the planet ’ s northern hemisphere is completely dark – meaning that Voyager 2 couldn ’ t image half its surface ( or half of its moons ) , and that half of it was not receiving any energy from the Sun .
This lack of sunlight helps to explain why images obtained during the 1986 @ @ @ @ @ @ @ words , “ fairly dull ” , with only 10 features visible .
By 2004 , when the 10 m diameter ground-based Keck telescope was trained on Uranus , it was a different story .
With the planet now side-on to the Sun , Hammel noted , “ funny things happened : the planet turned on ” .
Instead of a smooth , pale-blue dot , the Keck images showed popcorn-like clouds similar to the ones the Cassini spacecraft saw on Saturn .
Similarly , an image obtained by the Hubble Space Telescope @ @ @ @ @ @ @ absent when Voyager 2 made its historic visit .
Neptune , in contrast , was plenty active during its Voyager 2 flyby in 1989 .
At the time , it exhibited a huge dark patch on its flank that observers likened to the “ Great Red Spot ” on Jupiter .
Within five years , though , images obtained by Hubble showed that Neptune ’ s dark spot had vanished .
( Jupiter ’ s spot , in contrast , has been around for at least 400 years . )
The dark spot @ @ @ @ @ @ @ Neptune was imaged again , but by 2016 it – or , rather , something like it – had reappeared .
The star of the Uranus-Neptune system , though , is Neptune ’ s moon Triton .
This icy chunk of rock is a twin of the dwarf planet Pluto , and it was probably captured from the Kuiper belt by the pull of Neptune ’ s gravity .
Unlike Pluto , though , Triton has active cryovolcanoes , and Voyager 2 flew past during an eruption that sent a plume of material @ @ @ @ @ @ @ tenuous atmosphere before the winds captured it .
That makes Triton one of only five moons in the solar system known to be geologically active , and thus an attractive target for a space mission like the ones sent to explore the others : Saturn ’ s moons Enceladus and Titon ( Cassini ) and , soon , Jupiter ’ s moons Io and Europa ( JUICE ) .
At present , no mission is scheduled for either of the ice giants .
Though a Uranus orbiter was discussed during the last decadal @ @ @ @ @ @ @ exercise conducted every 10 years by the US National Academy of Science , Engineering and Medicine on behalf of NASA – it wound up third on the list , behind missions to Mars and Europa .
Several missions have , however , been proposed for the next decadal survey , including a Triton-only mission called Trident and a wider system explorer called Neptune Odyssey .
And with the results of that survey due out in 2023 , it ’ s certainly not too early for lobbying .
Speaking with stuffed toy versions of @ @ @ @ @ @ @ background , Hammel reminded her audience that Uranus ’ rings are a particular mystery .
It has grown new ones since the Voyager 2 flyby , and we know they are coloured red , blue and grey .
But what are they made of ?
Where do they come from ?
So far , the answer isn ’ t clear .
“ We must send a spacecraft there to really study the rings in detail , ” Hammel concluded .
“ We are all hoping that this is finally the era for going @ @ @ @ @ @ @

Paler blue dot An image of the planet Uranus taken by the spacecraft Voyager 2 on 14 January 1986 from a distance of approximately 12.7 million km. (Courtesy: NASA/JPL)
Uranus and Neptune are the least-explored planets in our solar system. Apart from the Voyager 2 spacecraft, which flew past them in the 1980s, no human-built probe has ever ventured near them. They are also small and far away, covering just 3.8 and 2.3 arc-seconds of the Earth’s sky, respectively. That makes it hard for even the best telescopes to pick out their features. Indeed, one of the few things we do know is that much of our knowledge about their appearance is no longer accurate, because they are not static. Instead, they are incredibly active worlds, with convective clouds that rise up like thunderheads and send streamers flying through their slushy atmospheres and giant storms that last for years.
The other thing we know about Uranus and Neptune is that planets of this type are very common. According to Heidi Hammel, vice-president for science at the Association of Universities for Research in Astronomy (AURA), most of the exoplanets discovered so far are in “the Neptune to sub-Neptune range” in terms of their size, yet these so-called “ice giants” have never been thoroughly investigated. “We’d like to change that,” Hammel told audience members at the annual Appleton Space Conference, which was held virtually this year on 3 December.
During her talk, Hammel laid out numerous reasons for sending spacecraft back to these icy outer worlds. The ice giants, she explained, have interiors that differ fundamentally from those of rocky planets like Mercury, Venus, Earth and Mars. But they aren’t composed of hydrogen like Jupiter and Saturn. Instead, they are made of ices, which in astronomical terms are mixtures of various frozen stuff. Theoretical models suggest that both planets have cores composed of iron, nickel and silicates, but “The bottom line is, we don’t know what the interior is,” Hammel said.
Limited knowledge
Hammel went on to explain that our knowledge of Uranus is especially limited because of the timing of the Voyager 2 flyby. For reasons that are not fully understood, but likely stem from an impact early in its formation, Uranus’ rotation is tilted at 90 degrees with respect to its orbit. This is very unusual for a planet, and unfortunately it meant that Uranus was in its southern solstice when Voyager 2 made its nearest approach. In this configuration, the planet’s northern hemisphere is completely dark – meaning that Voyager 2 couldn’t image half its surface (or half of its moons), and that half of it was not receiving any energy from the Sun.
Another view of Uranus A 2006 image taken by the Hubble Space Telescope shows bands and a new dark spot in Uranus’ atmosphere. (Courtesy: NASA/Space Telescope Science Institute)
This lack of sunlight helps to explain why images obtained during the 1986 flyby were, in Hammel’s words, “fairly dull”, with only 10 features visible. By 2004, when the 10 m diameter ground-based Keck telescope was trained on Uranus, it was a different story. With the planet now side-on to the Sun, Hammel noted, “funny things happened: the planet turned on”. Instead of a smooth, pale-blue dot, the Keck images showed popcorn-like clouds similar to the ones the Cassini spacecraft saw on Saturn. Similarly, an image obtained by the Hubble Space Telescope in 2006 showed dark bands that were absent when Voyager 2 made its historic visit.
Neptune, in contrast, was plenty active during its Voyager 2 flyby in 1989. At the time, it exhibited a huge dark patch on its flank that observers likened to the “Great Red Spot” on Jupiter. Within five years, though, images obtained by Hubble showed that Neptune’s dark spot had vanished. (Jupiter’s spot, in contrast, has been around for at least 400 years.) The dark spot was still absent in 2011, when Neptune was imaged again, but by 2016 it – or, rather, something like it – had reappeared.
The star of the Uranus–Neptune system, though, is Neptune’s moon Triton. This icy chunk of rock is a twin of the dwarf planet Pluto, and it was probably captured from the Kuiper belt by the pull of Neptune’s gravity. Unlike Pluto, though, Triton has active cryovolcanoes, and Voyager 2 flew past during an eruption that sent a plume of material 8 km into the moon’s tenuous atmosphere before the winds captured it. That makes Triton one of only five moons in the solar system known to be geologically active, and thus an attractive target for a space mission like the ones sent to explore the others: Saturn’s moons Enceladus and Titan (Cassini) and, soon, Jupiter’s moons Io and Europa (JUICE).
Visual aid Heidi Hammel answers a question about Uranus’ rings during her Appleton Space Conference talk with the help of a stuffed toy model. (Courtesy: Margaret Harris)
At present, no mission is scheduled for either of the ice giants. Though a Uranus orbiter was discussed during the last decadal survey for planetary science – a prioritization exercise conducted every 10 years by the US National Academy of Science, Engineering and Medicine on behalf of NASA – it wound up third on the list, behind missions to Mars and Europa. Several missions have, however, been proposed for the next decadal survey, including a Triton-only mission called Trident and a wider system explorer called Neptune Odyssey. And with the results of that survey due out in 2023, it’s certainly not too early for lobbying. Speaking with stuffed toy versions of Uranus and Neptune displayed prominently in the background, Hammel reminded her audience that Uranus’ rings are a particular mystery. It has grown new ones since the Voyager 2 flyby, and we know they are coloured red, blue and grey. But what are they made of? Where do they come from? So far, the answer isn’t clear. “We must send a spacecraft there to really study the rings in detail,” Hammel concluded. “We are all hoping that this is finally the era for going back to the ice giants.”


physicsworld--2020-12-08--Spin guided scattering of light is observed in a liquid crystal - Physics World

Just as topological insulators provide protection to electrons travelling along their edges and surfaces , photons can also be topologically protected .
This can occur when photon scattering modes are associated with just one spin state .
Now , researchers in India and the Netherlands have found that spin-selective or spin asymmetric scattering modes can be observed using a twisted nematic-liquid-crystal-based spatial light modulator .
Their work indicates that liquid crystals can host topological states by controlling the interaction between disorder of the material and the spin-orbit interaction of light .
Topology is the field of mathematics concerned with geometric @ @ @ @ @ @ @ deformed .
Properties that are invariant when deformed are said to be topological .
Recently , physicists have become more and more interested in applying topological principles in physical systems with a view to exploiting topological invariance whereby particles are not affected by local perturbations .
A spin-polarized current of electrons , for example , can flow without scattering in topological materials that could have significant technological applications .
Although much of the research focus has been on electronic , many research groups are now turning their attention to photonics .
In the @ @ @ @ @ @ @ arise when photons travelling through a medium are controlled so that their spin is “ locked ” to a particular propagation direction .
This can be achieved by controlling the interplay between the spin angular momentum of the beam ( its circular polarization ) with its orbital angular momentum .
When travelling through certain materials , it is thought that a Gaussian beam can be manipulated so that its scattering modes are spin selective .
In their recent study , published in Physical Review A , Ankit Kumar Singh and colleagues at the @ @ @ @ @ @ @ Kolkata and Delft University of Technology have demonstrated a method for producing spin-selective scattering modes in the Fourier spectrum of a Gaussian beam passing through a liquid crystal-based modulator .
Their experiment used a helium-neon laser to produce a Gaussian beam which was spatially filtered and then linearly polarized .
The beam then passed through the medium — a transmission spatial light modulator made of a liquid crystal .
This had properties that could be controlled using an applied voltage , resulting in a random phase distribution .
Following this , the beam @ @ @ @ @ @ @ consisting of a quarter wave plate and another polarizer .
This enabled researchers to measure the distribution of left circularly polarized and right circularly polarized light once the beam was focused onto a photodetector .
They found that randomly scattered modes were only observed for the right circularly polarized projection , whereas the left circularly polarized projection showed no scattering modes .
From this , they concluded that the randomly scattered modes only occur for one spin state of the light , while the other travels through the medium mostly unaffected by the @ @ @ @ @ @ @ in its infancy , the field of topological photonics provides exciting possibilities for the future of light-based technologies .
Potential applications of spin-controlled photon systems range from high resolution imaging techniques to microparticle trapping .
Understanding the topological properties of light as it interacts with different materials will enable researchers to develop more robust technologies in the future .

New spin on topology: spin guided light scattering has been observed in a liquid crystal (Courtesy: Shutterstock/Serg-DAV)
Just as topological insulators provide protection to electrons travelling along their edges and surfaces, photons can also be topologically protected. This can occur when photon scattering modes are associated with just one spin state. Now, researchers in India and the Netherlands have found that spin-selective or spin asymmetric scattering modes can be observed using a twisted nematic-liquid-crystal-based spatial light modulator. Their work indicates that liquid crystals can host topological states by controlling the interaction between disorder of the material and the spin-orbit interaction of light.
Topology goes photonic
Topology is the field of mathematics concerned with geometric objects that do not change when continuously deformed. Properties that are invariant when deformed are said to be topological. Recently, physicists have become more and more interested in applying topological principles in physical systems with a view to exploiting topological invariance whereby particles are not affected by local perturbations. A spin-polarized current of electrons, for example, can flow without scattering in topological materials that could have significant technological applications. Although much of the research focus has been on electronic, many research groups are now turning their attention to photonics.
In the case of photons, topological protection can arise when photons travelling through a medium are controlled so that their spin is “locked” to a particular propagation direction. This can be achieved by controlling the interplay between the spin angular momentum of the beam (its circular polarization) with its orbital angular momentum. When travelling through certain materials, it is thought that a Gaussian beam can be manipulated so that its scattering modes are spin selective.
Shining a light on anisotropic materials
In their recent study, published in Physical Review A, Ankit Kumar Singh and colleagues at the Indian Institute of Science Education and Research Kolkata and Delft University of Technology have demonstrated a method for producing spin-selective scattering modes in the Fourier spectrum of a Gaussian beam passing through a liquid crystal-based modulator.
Their experiment used a helium-neon laser to produce a Gaussian beam which was spatially filtered and then linearly polarized. The beam then passed through the medium — a transmission spatial light modulator made of a liquid crystal. This had properties that could be controlled using an applied voltage, resulting in a random phase distribution. Following this, the beam was passed through a polarization state analyser consisting of a quarter wave plate and another polarizer. This enabled researchers to measure the distribution of left circularly polarized and right circularly polarized light once the beam was focused onto a photodetector.
They found that randomly scattered modes were only observed for the right circularly polarized projection, whereas the left circularly polarized projection showed no scattering modes. From this, they concluded that the randomly scattered modes only occur for one spin state of the light, while the other travels through the medium mostly unaffected by the disorder of the system.
The future of photonics
Though still in its infancy, the field of topological photonics provides exciting possibilities for the future of light-based technologies. Potential applications of spin-controlled photon systems range from high-resolution imaging techniques to microparticle trapping. Understanding the topological properties of light as it interacts with different materials will enable researchers to develop more robust technologies in the future.


physicsworld--2020-12-09--Online MR imaging verifies proton beam range - Physics World

Proton therapy holds the promise of delivering highly conformal dose distributions to a target tumour volume , due to the finite range of the proton beam .
However , the accuracy of this delivery can be affected by factors such as target motion , which may lead to overdosing regions of healthy tissue and underdosage of the tumour volume .
“ Proton therapy is very sensitive to changes in the position and shape of the patient as a whole and the patient ’ s organs , both between fractions and within one fraction , ” explained Sonja Schellhammer , recipient @ @ @ @ @ @ @ best abstract presented at the recent ESTRO 2020 congress .
“ It would be ideal if we could monitor the proton range simultaneously with the patient anatomy in real time.
” Currently , verification of proton range is mostly based on detection of secondary radiation such as prompt gamma rays or acoustic waves .
Another indirect approach involves using MRI to visualize long-term radiation-induced biological effects .
MRI benefits from excellent soft-tissue contrast , offers real-time imaging , delivers no radiation dose and could prove ideal for integration with proton therapy .
With @ @ @ @ @ @ @ Dresden-Rossendorf ( HZDR ) are working to integrate proton therapy with MRI .
Previous studies by others have shown that MRI can determine proton beam range using an off-line approach , with contrast-enhanced images recorded weeks or months after irradiation .
“ The question we asked in this study , is whether it would be possible to visualize the proton beam with online MRI during irradiation , ” Schellhammer explained in her ESTRO presentation .
To test this idea , the researchers designed a phantom experiment using the Dresden proton therapy facility , @ @ @ @ @ @ @ and ASG Superconductors – Paramed MRI Unit , had integrated an open 0.22T MRI scanner with a fixed proton research beamline .
The proton beam was incident upon a plastic bottle filled with deionized water and placed in the scanner ’ s magnetic isocentre .
The team used a PMMA range shifter to tailor the proton energy such that the beam stopped within the bottle , and recorded MR image slices through the centre of the beam .
The researchers performed one scan during proton irradiation , and three immediately after irradiation , @ @ @ @ @ @ @ For two of the six MR sequences – the proton density-weighted gradient echo ( GE ) sequence and the inversion recovery gradient echo ( IRGE ) sequence – they saw signatures in the MR image that were likely induced by the proton beam .
They also observed that these signals didn ’ t disappear immediately after irradiation , but faded over tens of seconds .
“ With this experiment , we confirmed that there is a measurable proton beam-induced signal on the MR images , ” said Schellhammer .
“ We then asked @ @ @ @ @ @ @ verification.
” Next , the team irradiated the water phantom using proton beams with energies of between 190 and 225 MeV ( corresponding to different beam ranges ) .
The observed MR signal changed in depth with increasing range , and the measured residual ranges agreed with the calculated values to within 2 mm .
Repeating the experiment with four different beam currents ( 1 , 3 , 9 and 27 nA , corresponding to dose rates of 1.7 , 5 , 15 and 45 Gy/s ) demonstrated that the signal intensity increased @ @ @ @ @ @ @ ( recorded for 20 s ) were only visible using beam currents of 3 nA or more .
To investigate whether this approach may work in a patient , the team used the same experimental set-up to irradiate other liquid phantoms ( ethanol and petroleum ) , highly viscose materials ( sugar syrup , mayonnaise and gelatine ) and a tissue-mimicking material ( a pork chop ) .
While images of ethanol and petroleum showed similar beam-induced signals as seen with water , for the more viscose materials and the pork chop , @ @ @ @ @ @ @ appears that the signal is only present in liquids , so likely may not be transferable to patients , ” explained Schellhammer .
“ But this approach may well prove useful for quality assurance of proton range in future hybrid MR-proton therapy systems.
” The mechanisms underlying the observed effects are still to be unravelled .
The most probable hypothesis , Schellhammer suggested , is that irradiating a liquid increases its temperature locally , creating a density difference in which the lighter heated water rises out of the imaging volume and creates a @ @ @ @ @ @ @ to test this hypothesis , she noted .
“ I have demonstrated that proton beam range can be accurately verified with online MRI , ” Schellhammer concluded .
“ However , today only at high doses and in fluid-filled phantoms .
The method holds potential for dosimeter-free online quality assurance of MR-integrated proton therapy .
Further research towards MR-based range proton beam verification is clearly justified .
”

MR images acquired using GE (top row) and IRGE (bottom row) sequences during proton irradiation, with expected proton ranges of (left to right) 4.2, 6.4, 8.6 and 12.2 cm at 9 nA. (Courtesy: S Schellhammer, HZDR/OncoRay)
Proton therapy holds the promise of delivering highly conformal dose distributions to a target tumour volume, due to the finite range of the proton beam. However, the accuracy of this delivery can be affected by factors such as target motion, which may lead to overdosing regions of healthy tissue and underdosage of the tumour volume.
“Proton therapy is very sensitive to changes in the position and shape of the patient as a whole and the patient’s organs, both between fractions and within one fraction,” explained Sonja Schellhammer, recipient of the Donal Hollywood Award for the best abstract presented at the recent ESTRO 2020 congress. “It would be ideal if we could monitor the proton range simultaneously with the patient anatomy in real time.”
Currently, verification of proton range is mostly based on detection of secondary radiation such as prompt gamma rays or acoustic waves. Another indirect approach involves using MRI to visualize long-term radiation-induced biological effects. MRI benefits from excellent soft-tissue contrast, offers real-time imaging, delivers no radiation dose and could prove ideal for integration with proton therapy.
With this in mind, scientists at OncoRay/Helmholtz-Zentrum Dresden-Rossendorf (HZDR) are working to integrate proton therapy with MRI. Previous studies by others have shown that MRI can determine proton beam range using an off-line approach, with contrast-enhanced images recorded weeks or months after irradiation. “The question we asked in this study, is whether it would be possible to visualize the proton beam with online MRI during irradiation,” Schellhammer explained in her ESTRO presentation.
To test this idea, the researchers designed a phantom experiment using the Dresden proton therapy facility, where OncoRay, in collaboration with IBA and ASG Superconductors – Paramed MRI Unit, had integrated an open 0.22T MRI scanner with a fixed proton research beamline. The proton beam was incident upon a plastic bottle filled with deionized water and placed in the scanner’s magnetic isocentre. The team used a PMMA range shifter to tailor the proton energy such that the beam stopped within the bottle, and recorded MR image slices through the centre of the beam.
The researchers performed one scan during proton irradiation, and three immediately after irradiation, using a variety of MR sequences. For two of the six MR sequences – the proton density-weighted gradient echo (GE) sequence and the inversion recovery gradient echo (IRGE) sequence – they saw signatures in the MR image that were likely induced by the proton beam. They also observed that these signals didn’t disappear immediately after irradiation, but faded over tens of seconds.
“With this experiment, we confirmed that there is a measurable proton beam-induced signal on the MR images,” said Schellhammer. “We then asked whether this could be useful for range verification.”
Next, the team irradiated the water phantom using proton beams with energies of between 190 and 225 MeV (corresponding to different beam ranges). The observed MR signal changed in depth with increasing range, and the measured residual ranges agreed with the calculated values to within 2 mm.
Repeating the experiment with four different beam currents (1, 3, 9 and 27 nA, corresponding to dose rates of 1.7, 5, 15 and 45 Gy/s) demonstrated that the signal intensity increased with increasing beam current. MR images (recorded for 20 s) were only visible using beam currents of 3 nA or more.
To investigate whether this approach may work in a patient, the team used the same experimental set-up to irradiate other liquid phantoms (ethanol and petroleum), highly viscose materials (sugar syrup, mayonnaise and gelatine) and a tissue-mimicking material (a pork chop). While images of ethanol and petroleum showed similar beam-induced signals as seen with water, for the more viscose materials and the pork chop, no signal was seen.
“It appears that the signal is only present in liquids, so likely may not be transferable to patients,” explained Schellhammer. “But this approach may well prove useful for quality assurance of proton range in future hybrid MR-proton therapy systems.”
The mechanisms underlying the observed effects are still to be unravelled. The most probable hypothesis, Schellhammer suggested, is that irradiating a liquid increases its temperature locally, creating a density difference in which the lighter heated water rises out of the imaging volume and creates a signal void. Further investigations are needed to test this hypothesis, she noted.
“I have demonstrated that proton beam range can be accurately verified with online MRI,” Schellhammer concluded. “However, today only at high doses and in fluid-filled phantoms. The method holds potential for dosimeter-free online quality assurance of MR-integrated proton therapy. Further research towards MR-based range proton beam verification is clearly justified.”


physicsworld--2020-12-09--‘Smart speaker, tell me about your acoustic sensor’ - Physics World

“ Alexa , play some Christmas music.
” “ OK Google , turn on the fairy lights.
” “ Hey Siri , how long do you need to cook a turkey ?
” This festive season , we ’ ll undoubtedly be chatting to our smart speakers like they ’ re another member of the family , and every time , the disembodied response will be almost instantaneous .
These devices – which include Amazon ’ s Echo , Google ’ s Nest and Apple ’ s Homepod – have already become an extra presence in more than a fifth @ @ @ @ @ @ @ 2019 almost 147 million units were sold globally and sales for 2020 are expected to be 10 % higher still .
Quite simply , smart speakers have reached an astonishing level of capability for recognizing what we say .
Although speculation remains as to exactly how much they are listening to and what the collected data are used for , there is no doubt that the voice-recognition technology is amazing in its accuracy .
This comes down to ultrasensitive acoustic sensors and sophisticated machine-learning algorithms interpreting speech ( see box “ From speech @ @ @ @ @ @ @ enough to have made it into our homes , the development of sensors for voice recognition is by no means finished .
It ’ s not clear which technology has the most promise , and novel ideas are frequently gaining commercial attention .
It also seems likely that this field , like so many others , will be altered in light of the coronavirus pandemic , with wearable sensors that can detect speech through vibrations of the throat potentially providing important diagnostic tools for diseases , such as COVID-19 , that can that @ @ @ @ @ @ @ text from live speech , two things need to happen : an acoustic sensor has to convert the incoming sound waves into an electrical signal , and then software must be used to figure out what words have been said .
For the second stage , the electrical signal is traditionally first converted from analogue to digital , before being analysed using a fast Fourier transform technique to find the variation in amplitude of different frequencies over time .
Little time sections of the graphs , representing small sounds known as “ phones @ @ @ @ @ @ @ short sound , or “ phoneme ” , that they probably represent .
Then algorithms build the phonemes into full speech .
Because voices vary so much , these programmes can not just rely on a phonetic dictionary to piece together the phonemes , which is why machine learning is so useful to improve accuracy over time .
When using Alexa , for example , it will “ remember ” when we correct it on what we said , and so become more accurate at interpreting our individual voices .
Most algorithms also @ @ @ @ @ @ @ which phonemes are most likely to follow each other – this is called a hidden Markov model .
A single-step approach , called “ end-to-end deep learning ” , is now gaining popularity and is used in current voice-recognition technology .
This uses a single learning algorithm to go from the electrical signal representing audio to a text transcript without extracting the phones .
The story of how acoustic sensors reached such extraordinary sensitivities begins in the late 19th century when the first acoustic sensor , the carbon or “ contact ” microphone @ @ @ @ @ @ @ – Emile Berliner and Thomas Edison in the US , and David Hughes in the UK .
These devices consist of carbon granules pressed between two metal contact plates with a voltage applied across the plates .
Incoming sound waves cause one of the plates , the diaphragm , to vibrate .
During compression , the graphite granules deform , increasing the contact area between the plates so that the resistance of the set-up drops , and current increases .
These changes as the diaphragm moves mean that the sound is encoded in @ @ @ @ @ @ @ wasn ’ t until 1952 that voice-recognition technology was first developed .
A team at Bell Telephone Laboratories ( now Nokia Bell Labs ) in the US created a program called “ Audrey ” – the Automatic Digit Recognition machine – that could understand the digits 0–9 spoken into a standard telephone ( which most likely featured a carbon microphone ) .
Audrey could be used for hands-free dialling , but had to be trained to the user ’ s voice and required a room full of electronics to run .
While the @ @ @ @ @ @ @ come a long way since Audrey , acoustic sensors have also gone through rigorous development .
Various designs , such as ribbon , dynamic and carbon microphones , have come in and out of fashion , but the one that has prevailed for voice recognition is the capacitor , or “ condenser ” , sensor .
Originally developed in 1916 by E C Wente of Western Electric Engineering Department in the US ( later becoming Bell Telephone Laboratories ) , the design hinges on the fact that the voltage across a capacitor depends @ @ @ @ @ @ @ To this end , the sensor features a stationary backplate and a moving diaphragm , charged up by an external voltage .
As the diaphragm vibrates from the incoming sound waves , the capacitance – and hence the voltage across the capacitor – varies with diaphragm displacement , from which the amplitude variation of different frequencies in time can be calculated .
The inventors Gerhard Sessler and Jim West at Bell Telephone Laboratories further developed the capacitor sensor in 1962 by using an electret diaphragm , creating the electret condenser microphone ( ECM @ @ @ @ @ @ @ Teflon , have a pre-existing surface charge , which means they keep a permanent voltage across the capacitor , thereby reducing the power input needed .
Roughly 3–10 mm in diameter , ECMs dominated the general microphone market for nearly 50 years , but the move towards compact devices led to drops in signal-to-noise ratio and decreased stability , particularly in variable temperature environments .
When it comes to voice recognition , ECMs have therefore been mostly replaced by micro-electro-mechanical system ( MEMS ) capacitive microphones .
At only 20–1000 μm in diameter @ @ @ @ @ @ @ find in smart speakers .
MEMS sensors differ from ECMs in the internal circuitry , notably in that they convert the signal from analogue to digital while still inside the microphone .
Apart from being less susceptible to electrical noise than ECMs , the design is smaller and easier to make , since the silicon wafers required can be made on semiconductor-manufacturing lines .
The drawback with MEMS microphones is that they are not very durable , so do not cope well with harsh environments .
This is because dust particles tend to @ @ @ @ @ @ @ vibration , and rain can also damage it .
Even in ambient settings , the trapped layer of air between the diaphragm and backplate makes it harder for the diaphragm to vibrate at maximum amplitude , limiting the device ’ s sensitivity .
Although capacitor sensors have dominated the industry for decades , the technology might not hold all the answers for the future .
US firm Vesper Technologies is paving the way in the commercial development of piezoelectric acoustic sensors , with the endorsement of Amazon Alexa .
Founded in 2014 , @ @ @ @ @ @ @ based on the PhD research of Bobby Littrell , the company ’ s chief technology officer , and the work has since won many awards .
These devices work using a diaphragm made of a piezoelectric material , such as lead zirconate titanate , that directly converts the mechanical energy from sound waves into an electrical response .
When the piezoelectric diaphragm stretches as a sound wave hits , the distances between ions are increased , creating small electric dipoles in the new most energetically stable ionic arrangement .
The lack of a @ @ @ @ @ @ @ s unit cell means no equivalent dipoles are formed on the other side of a centre of symmetry ( which would cancel out , to leave no net dipole ) .
The cumulative effect of all of these tiny dipoles across the crystal is the generation of a voltage , which varies in time as the strain in the crystal varies .
Compared with capacitor acoustic sensors , piezoelectric devices have the distinct advantage of containing just a single layer , meaning they do not trap dirt , air or rain and so @ @ @ @ @ @ @ are also self-powered , meaning the range of applications , particularly when there is limited room for a battery , is much wider .
However , thin-film devices like these – and the capacitive designs – tend to be quite difficult to make .
“ You need a high or ultrahigh vacuum , ” explains Judy Wu , a physicist at the University of Kansas in the US , “ and you need to select a good substrate because [ otherwise ] you will not be able to get epitaxial growth.
” Epitaxial @ @ @ @ @ @ @ film grows as a single crystal with one orientation of the unit cells .
This is needed so that the dipoles formed under mechanical strain all point in the same direction .
“ You have to really raise the temperature , ” Wu continues , “ to give the thermal energy and the mobility when you put the atoms on the substrate [ for them ] to find the minimum energy position to form a perfect lattice.
” As Vesper has shown , these conditions can be produced , but they do limit @ @ @ @ @ @ @ growing a thin film of a single crystal on a flexible substrate is difficult , since single crystals have to grow on an ordered structure to be ordered themselves and most flexible materials are not crystalline .
“ You can not provide a perfect lattice there – it ’ s just amorphous material , ” explains Wu .
However , Wu and her team are working on a potential solution .
They have used graphene in solution to grow piezoelectric zinc oxide nanowire arrays for a strain sensor ( ACS Applied Nano Materials @ @ @ @ @ @ @ is flexible but crystalline , the nanowires grown are still piezoelectric crystals .
The difficulty is that it is very delicate .
The researchers overcame this by using a solution to grow the nanowires , so as not to destroy the graphene ’ s perfect structure with sputtering techniques , which would reduce its conductivity .
Not only this , but the ambient pressure and relatively low temperature used ( 90 °C ) mean the process is very cheap .
Their strain sensor works by detecting changes to graphene ’ s conductivity , @ @ @ @ @ @ @ extra surface charge that develops on the zinc oxide nanowire array when it is mechanically strained .
Wu says they are also working on a flexible version of the sensor , encased in PET plastic .
The research is still in fairly early stages , and the sensor does not have a definite application just yet , but in the hope of filling some niche function needing a sensitive , flexible sensor , they have patented the graphene process .
“ We want to do something the ceramic sensor can not do , @ @ @ @ @ @ @ sensor ] might recognize your skin , or recognize your voice , because it ’ s very sensitive to acoustics.
” Adding to the whirlpool of ideas in the voice-recognition field , Keon Jae Lee and his team at the Korea Advanced Institute of Science and Technology ( KAIST ) have been developing a new piezoelectric sensor design ( Advanced Materials 32 1904020 ) that mimics human hearing .
Speaking to Physics World , Lee light-heartedly explained his faith in the concept , “ If nature is doing it , it ’ s @ @ @ @ @ @ @ piezoelectric sensors have a similar shape to the basilar membrane in our ear ( see box “ How ears have inspired a new kind of voice-recognition device “ ) , thereby allowing about twice as much information to be harvested than conventional capacitor sensors .
This advantage comes from the fact that , rather than collecting a single signal containing all the frequencies and analysing that to figure out frequency amplitudes , many signals ( in Lee ’ s case , seven ) are analysed from various positions along the membrane .
This @ @ @ @ @ @ @ accurate .
Lee and colleagues found their 2018 design exhibited 97.5 % accuracy in speech recognition ; a 75 % improvement on a reference MEMS condenser microphone .
“ I think the two advantages are accuracy and sensitivity , ” Lee concludes .
“ We can pick up sound from a long way away and recognize individual voices.
” The tricky part of their research is analysing the data from the channels to give the relative amplitudes of different frequencies , since amplitudes are modulated by the resonance behaviour of the channel .
@ @ @ @ @ @ @ few research groups have taken up the idea .
But his group seems to have cracked it , and even founded a spin-off company manufacturing its unique sensors , Fronics , in 2016 .
Lee is optimistic for the future of the design commercially and believes the team has found the right number of channels for the sensor .
It ’ s a fine balance , he explains , between improving your accuracy by collecting more data , and needing a bulky machine to process it all .
How ears have inspired a @ @ @ @ @ @ @ Lee and his team at Korea Advanced Institute of Science and Technology are developing a new voice-recognition sensor that draws inspiration from hearing in nature .
It has a piezoelectric membrane that mimics the basilar membrane found in ears , which in humans is curled into a spiral inside our cochlea .
Lee ’ s design , however , is more similar to the straight membrane found in the ears of many birds and reptiles .
Shaped like a trapezium when viewed from above , a basilar membrane is tapered : it is @ @ @ @ @ @ @ located in the centre of the spiral in our ears ) and becomes wide and thin .
The membrane can be thought of as a series of oscillating strings lying perpendicular to the axis of symmetry of the trapezium , except that the strings really form a continuous spectrum that waves can pass between .
An incoming sound creates a travelling wave that passes down the membrane , beginning at the wide , thin end .
Essentially a superposition of individual waves of different frequencies , as each wave travels down the membrane @ @ @ @ @ @ @ oscillating strings that has a resonant frequency equal to the wave ’ s own frequency and will vibrate most strongly here .
The membrane is covered by more than 10,000 hair cells , which transmit information to the brain about the amplitude of the frequency at their position along the membrane .
Ears are more complex than this membrane alone : for example the hair cells also amplify the “ correct ” frequency for that position on the basilar membrane .
But the concept of a collection of resonating sections of a membrane @ @ @ @ @ @ @ design .
Voice-recognition technology is not limited to a device sitting in the corner of your room or in your pocket .
Sensors that work using neck vibrations , instead of sound waves travelling though air , would be very useful where sound propagation is almost entirely prohibited , such as noisy industrial environments or when people have to wear bulky equipment like gas masks .
A breakthrough occurred late last year when Yoonyoung Chung and his team from Pohang University of Science and Technology , South Korea , reported creating the first @ @ @ @ @ @ @ can solve this problem by perceiving human voices through neck-skin vibrations on the cricoid cartilage ( part of the larynx ) .
Demonstrating that skin acceleration on the neck is linearly correlated with voice pressure , they realized that they could measure the variation of voice pressure in time by designing a device that detects skin acceleration through changes in capacitance ( Nature Communications 10 2468 ) .
The sensor Chung ’ s team has created is less than 5 μm thick and the diaphragm is made from epoxy resin ( figure 1 @ @ @ @ @ @ @ a flexible microphone sensor , so it was natural for us to use a polymeric material , which is intrinsically flexible , ” explains Chung .
The individual sensor panels are small enough that any neck curvature is negligible in them and will not affect vibrations – a bit like how the Earth ’ s curvature seems small enough to ignore from our perspective , since we occupy such a small proportion of its surface .
What ’ s also vital for capacitive sensors is that they have a “ flat frequency ” @ @ @ @ @ @ @ appear falsely high in amplitude because of a resonance in the device , and it makes electrical signals much easier to analyse .
In order to achieve this , the diaphragm must have a narrow resonance peak that lies well above the frequency range of the human voice .
Chung achieved the narrow width by making the diaphragm from a fully cross-linked epoxy resin with a low damping ratio .
Cross-linking prevents the movement of molecules past each other and the oscillation and conformational flip of the phenyl rings , which would all @ @ @ @ @ @ @ second condition , a high resonant frequency , could be achieved by either a high stiffness or a low mass .
The snag is that a device that ’ s less stiff is actually more sensitive because the diaphragm ’ s vibrations are bigger .
Chung has found a solution to this stiffness dilemma : instead of reducing the stiffness of the bulk material , he added holes to the diaphragm .
The holes reduce the diaphragm ’ s mass to satisfy the high resonant frequency , while keeping stiffness low to increase @ @ @ @ @ @ @ been commonly used in the silicon MEMS area in order to reduce the air resistance , ” adds Chung .
The group has submitted patent applications for the design and is collaborating with several industry partners , though Chung is careful to note that commercialization is still in the early stages , since there is more research to be done in the lab .
“ We are now trying to make a sensor interface circuit on a flexible substrate so the entire sensory system can be attached on the neck skin without any @ @ @ @ @ @ @ current design has a rigid circuit board , which can cause occasional problems by suppressing neck vibrations .
Throat sensors like these could also be used to diagnose illnesses like COVID-19 , which have symptoms that manifest themselves in the vocal cords .
Researchers from the Massachusetts Institute of Technology in the US have found that COVID-19 changes our voice signals by affecting the complexity of movement of the muscles in the throat ( IEEE Open Journal of Engineering in Medicine and Biology 1 203 ) .
They hypothesized this to be due @ @ @ @ @ @ @ preventing them from moving independently .
Sensor devices paired with an app could provide early-stage screening to alert people to seek further testing for COVID-19 .
“ It can also be used as a cough detector to diagnose people , ” speculates Chung , while considering clinical uses for sensors like his .
Even Vesper , though its current designs are not flexible , has shown interest in this potential outlet for innovation .
“ Our team is already brainstorming new ideas such as acoustic respiratory health monitors , ” wrote its chief @ @ @ @ @ @ @ in March about the pandemic .
An advantage of sensors being repurposed from a voice-recognition context is that there is money available for developing the technology .
Well-resourced tech companies are always on the lookout for technology that has exciting prospects for smart devices – such as sensors like Lee ’ s and Chung ’ s , which are sensitive enough to recognize individual voices in lieu of a password or fingerprint .
And given the human need , as a result of the pandemic , to quickly detect respiratory illnesses on a @ @ @ @ @ @ @ be on a fast-track to becoming a viable diagnostic technology .
It will be interesting to see how both of these areas of innovation evolve over the next few years following the pandemic .
Likewise , the debate between capacitive and piezoelectric sensors will continue to change , possibly as a result of manufacturing improvements , or the changing demand for certain features , like flexibility and resistance to harsh environments .
Perhaps , one crisp December morning as you wait for the Brussels sprouts to boil , you might ask your smart @ @ @ @ @

Physics World. Members of the Institute of Physics can enjoy the full issue Taken from the December 2020 issue of. Members of the Institute of Physics can enjoy the full issue via the Physics World app
Smart speakers that can register our everyday commands have become commonplace in homes around the world. While the acoustics sensors inside these devices are far flung from the first microphone, the technology is still evolving, as Pip Knight explains
(Courtesy: Shutterstock/David Ferencik; IOP Publishing/Mat Ward)
“Alexa, play some Christmas music.” “OK Google, turn on the fairy lights.” “Hey Siri, how long do you need to cook a turkey?”
This festive season, we’ll undoubtedly be chatting to our smart speakers like they’re another member of the family, and every time, the disembodied response will be almost instantaneous.
These devices – which include Amazon’s Echo, Google’s Nest and Apple’s Homepod – have already become an extra presence in more than a fifth of UK households. Indeed, in 2019 almost 147 million units were sold globally and sales for 2020 are expected to be 10% higher still. Quite simply, smart speakers have reached an astonishing level of capability for recognizing what we say. Although speculation remains as to exactly how much they are listening to and what the collected data are used for, there is no doubt that the voice-recognition technology is amazing in its accuracy. This comes down to ultrasensitive acoustic sensors and sophisticated machine-learning algorithms interpreting speech (see box “From speech to text”).
While good enough to have made it into our homes, the development of sensors for voice recognition is by no means finished. It’s not clear which technology has the most promise, and novel ideas are frequently gaining commercial attention. It also seems likely that this field, like so many others, will be altered in light of the coronavirus pandemic, with wearable sensors that can detect speech through vibrations of the throat potentially providing important diagnostic tools for diseases, such as COVID-19, that can that affect the vocal cords.
From speech to text (Courtesy: Shutterstock/SergeyBitos) To generate text from live speech, two things need to happen: an acoustic sensor has to convert the incoming sound waves into an electrical signal, and then software must be used to figure out what words have been said. For the second stage, the electrical signal is traditionally first converted from analogue to digital, before being analysed using a fast Fourier transform technique to find the variation in amplitude of different frequencies over time. Little time sections of the graphs, representing small sounds known as “phones”, are matched to the ideal short sound, or “phoneme”, that they probably represent. Then algorithms build the phonemes into full speech. Because voices vary so much, these programmes cannot just rely on a phonetic dictionary to piece together the phonemes, which is why machine learning is so useful to improve accuracy over time. When using Alexa, for example, it will “remember” when we correct it on what we said, and so become more accurate at interpreting our individual voices. Most algorithms also use a probabilistic approach to work out which phonemes are most likely to follow each other – this is called a hidden Markov model. A single-step approach, called “end-to-end deep learning”, is now gaining popularity and is used in current voice-recognition technology. This uses a single learning algorithm to go from the electrical signal representing audio to a text transcript without extracting the phones.
Audrey and the capacitors
The story of how acoustic sensors reached such extraordinary sensitivities begins in the late 19th century when the first acoustic sensor, the carbon or “contact” microphone, was developed independently by three inventors – Emile Berliner and Thomas Edison in the US, and David Hughes in the UK. These devices consist of carbon granules pressed between two metal contact plates with a voltage applied across the plates. Incoming sound waves cause one of the plates, the diaphragm, to vibrate. During compression, the graphite granules deform, increasing the contact area between the plates so that the resistance of the set-up drops, and current increases. These changes as the diaphragm moves mean that the sound is encoded in an electrical current.
However, it wasn’t until 1952 that voice-recognition technology was first developed. A team at Bell Telephone Laboratories (now Nokia Bell Labs) in the US created a program called “Audrey” – the Automatic Digit Recognition machine – that could understand the digits 0–9 spoken into a standard telephone (which most likely featured a carbon microphone). Audrey could be used for hands-free dialling, but had to be trained to the user’s voice and required a room full of electronics to run.
While the computing side of voice recognition has obviously come a long way since Audrey, acoustic sensors have also gone through rigorous development. Various designs, such as ribbon, dynamic and carbon microphones, have come in and out of fashion, but the one that has prevailed for voice recognition is the capacitor, or “condenser”, sensor. Originally developed in 1916 by E C Wente of Western Electric Engineering Department in the US (later becoming Bell Telephone Laboratories), the design hinges on the fact that the voltage across a capacitor depends on the distance between the plates. To this end, the sensor features a stationary backplate and a moving diaphragm, charged up by an external voltage. As the diaphragm vibrates from the incoming sound waves, the capacitance – and hence the voltage across the capacitor – varies with diaphragm displacement, from which the amplitude variation of different frequencies in time can be calculated.
The inventors Gerhard Sessler and Jim West at Bell Telephone Laboratories further developed the capacitor sensor in 1962 by using an electret diaphragm, creating the electret condenser microphone (ECM). Electret materials, such as Teflon, have a pre-existing surface charge, which means they keep a permanent voltage across the capacitor, thereby reducing the power input needed. Roughly 3–10 mm in diameter, ECMs dominated the general microphone market for nearly 50 years, but the move towards compact devices led to drops in signal-to-noise ratio and decreased stability, particularly in variable temperature environments.
When it comes to voice recognition, ECMs have therefore been mostly replaced by micro-electro-mechanical system (MEMS) capacitive microphones. At only 20–1000 μm in diameter, these devices are typically what you find in smart speakers. MEMS sensors differ from ECMs in the internal circuitry, notably in that they convert the signal from analogue to digital while still inside the microphone. Apart from being less susceptible to electrical noise than ECMs, the design is smaller and easier to make, since the silicon wafers required can be made on semiconductor-manufacturing lines. The drawback with MEMS microphones is that they are not very durable, so do not cope well with harsh environments. This is because dust particles tend to gather under the diaphragm, limiting its vibration, and rain can also damage it. Even in ambient settings, the trapped layer of air between the diaphragm and backplate makes it harder for the diaphragm to vibrate at maximum amplitude, limiting the device’s sensitivity.
A new way?
Although capacitor sensors have dominated the industry for decades, the technology might not hold all the answers for the future. US firm Vesper Technologies is paving the way in the commercial development of piezoelectric acoustic sensors, with the endorsement of Amazon Alexa. Founded in 2014, the company’s initial designs were based on the PhD research of Bobby Littrell, the company’s chief technology officer, and the work has since won many awards.
These devices work using a diaphragm made of a piezoelectric material, such as lead zirconate titanate, that directly converts the mechanical energy from sound waves into an electrical response. When the piezoelectric diaphragm stretches as a sound wave hits, the distances between ions are increased, creating small electric dipoles in the new most energetically stable ionic arrangement. The lack of a centre of symmetry in the crystal’s unit cell means no equivalent dipoles are formed on the other side of a centre of symmetry (which would cancel out, to leave no net dipole). The cumulative effect of all of these tiny dipoles across the crystal is the generation of a voltage, which varies in time as the strain in the crystal varies.
Changing times There have been multiple acoustic-sensor designs since the carbon microphone was first developed by Emile Berliner (left, courtesy Granger Historical Picture Archive/Alamy Stock Photo), Thomas Edison and David Hughes in the late 19th century, including electret condenser microphones (middle, courtesy CUI Devices) and micro-electromechanical system capacitive microphones (right, courtesy CUI Devices).
Compared with capacitor acoustic sensors, piezoelectric devices have the distinct advantage of containing just a single layer, meaning they do not trap dirt, air or rain and so are much more durable. The devices are also self-powered, meaning the range of applications, particularly when there is limited room for a battery, is much wider.
However, thin-film devices like these – and the capacitive designs – tend to be quite difficult to make. “You need a high or ultrahigh vacuum,” explains Judy Wu, a physicist at the University of Kansas in the US, “and you need to select a good substrate because [otherwise] you will not be able to get epitaxial growth.” Epitaxial growth is what happens when a thin film grows as a single crystal with one orientation of the unit cells. This is needed so that the dipoles formed under mechanical strain all point in the same direction. “You have to really raise the temperature,” Wu continues, “to give the thermal energy and the mobility when you put the atoms on the substrate [for them] to find the minimum energy position to form a perfect lattice.”
As Vesper has shown, these conditions can be produced, but they do limit the device applications. For instance, growing a thin film of a single crystal on a flexible substrate is difficult, since single crystals have to grow on an ordered structure to be ordered themselves and most flexible materials are not crystalline. “You cannot provide a perfect lattice there – it’s just amorphous material,” explains Wu.
However, Wu and her team are working on a potential solution. They have used graphene in solution to grow piezoelectric zinc oxide nanowire arrays for a strain sensor (ACS Applied Nano Materials 3 6711). As the graphene is flexible but crystalline, the nanowires grown are still piezoelectric crystals. The difficulty is that it is very delicate. The researchers overcame this by using a solution to grow the nanowires, so as not to destroy the graphene’s perfect structure with sputtering techniques, which would reduce its conductivity. Not only this, but the ambient pressure and relatively low temperature used (90 °C) mean the process is very cheap.
Their strain sensor works by detecting changes to graphene’s conductivity, which occur as a result of the extra surface charge that develops on the zinc oxide nanowire array when it is mechanically strained. Wu says they are also working on a flexible version of the sensor, encased in PET plastic. The research is still in fairly early stages, and the sensor does not have a definite application just yet, but in the hope of filling some niche function needing a sensitive, flexible sensor, they have patented the graphene process. “We want to do something the ceramic sensor cannot do,” Wu explains, “[Our sensor] might recognize your skin, or recognize your voice, because it’s very sensitive to acoustics.”
Following nature
Adding to the whirlpool of ideas in the voice-recognition field, Keon Jae Lee and his team at the Korea Advanced Institute of Science and Technology (KAIST) have been developing a new piezoelectric sensor design (Advanced Materials 32 1904020) that mimics human hearing. Speaking to Physics World, Lee light-heartedly explained his faith in the concept, “If nature is doing it, it’s probably the most efficient way.”
Their piezoelectric sensors have a similar shape to the basilar membrane in our ear (see box “How ears have inspired a new kind of voice-recognition device“), thereby allowing about twice as much information to be harvested than conventional capacitor sensors. This advantage comes from the fact that, rather than collecting a single signal containing all the frequencies and analysing that to figure out frequency amplitudes, many signals (in Lee’s case, seven) are analysed from various positions along the membrane. This wealth of information makes voice predictions more accurate. Lee and colleagues found their 2018 design exhibited 97.5% accuracy in speech recognition; a 75% improvement on a reference MEMS condenser microphone. “I think the two advantages are accuracy and sensitivity,” Lee concludes. “We can pick up sound from a long way away and recognize individual voices.”
The tricky part of their research is analysing the data from the channels to give the relative amplitudes of different frequencies, since amplitudes are modulated by the resonance behaviour of the channel. It’s why Lee thinks so few research groups have taken up the idea. But his group seems to have cracked it, and even founded a spin-off company manufacturing its unique sensors, Fronics, in 2016. Lee is optimistic for the future of the design commercially and believes the team has found the right number of channels for the sensor. It’s a fine balance, he explains, between improving your accuracy by collecting more data, and needing a bulky machine to process it all.
How ears have inspired a new kind of voice-recognition device (Courtesy: Claus Lunau/Science Photo Library) Keon Jae Lee and his team at Korea Advanced Institute of Science and Technology are developing a new voice-recognition sensor that draws inspiration from hearing in nature. It has a piezoelectric membrane that mimics the basilar membrane found in ears, which in humans is curled into a spiral inside our cochlea. Lee’s design, however, is more similar to the straight membrane found in the ears of many birds and reptiles. Shaped like a trapezium when viewed from above, a basilar membrane is tapered: it is narrow and thick at the base (located in the centre of the spiral in our ears) and becomes wide and thin. The membrane can be thought of as a series of oscillating strings lying perpendicular to the axis of symmetry of the trapezium, except that the strings really form a continuous spectrum that waves can pass between. An incoming sound creates a travelling wave that passes down the membrane, beginning at the wide, thin end. Essentially a superposition of individual waves of different frequencies, as each wave travels down the membrane, it will reach one of the oscillating strings that has a resonant frequency equal to the wave’s own frequency and will vibrate most strongly here. The membrane is covered by more than 10,000 hair cells, which transmit information to the brain about the amplitude of the frequency at their position along the membrane. Ears are more complex than this membrane alone: for example the hair cells also amplify the “correct” frequency for that position on the basilar membrane. But the concept of a collection of resonating sections of a membrane is the key to Lee’s design.
Neck sensors and COVID-19
Voice-recognition technology is not limited to a device sitting in the corner of your room or in your pocket. Sensors that work using neck vibrations, instead of sound waves travelling though air, would be very useful where sound propagation is almost entirely prohibited, such as noisy industrial environments or when people have to wear bulky equipment like gas masks. A breakthrough occurred late last year when Yoonyoung Chung and his team from Pohang University of Science and Technology, South Korea, reported creating the first flexible and skin-attachable capacitor sensor, which can solve this problem by perceiving human voices through neck-skin vibrations on the cricoid cartilage (part of the larynx). Demonstrating that skin acceleration on the neck is linearly correlated with voice pressure, they realized that they could measure the variation of voice pressure in time by designing a device that detects skin acceleration through changes in capacitance (Nature Communications 10 2468).
The sensor Chung’s team has created is less than 5 μm thick and the diaphragm is made from epoxy resin (figure 1). “We wanted to develop a flexible microphone sensor, so it was natural for us to use a polymeric material, which is intrinsically flexible,” explains Chung. The individual sensor panels are small enough that any neck curvature is negligible in them and will not affect vibrations – a bit like how the Earth’s curvature seems small enough to ignore from our perspective, since we occupy such a small proportion of its surface.
1 Electronic skin The throat sensor developed by Yoonyoung Chung and colleagues at Pohang University of Science and Technology in South Korea, illustrated above, consists of a crosslinked ultrathin polymer with a hole-patterned diaphragm structure. (CC BY 4.0 Nature Commun. 10 2468/CC BY Christian Rambow)
What’s also vital for capacitive sensors is that they have a “flat frequency” response. This means that no frequencies appear falsely high in amplitude because of a resonance in the device, and it makes electrical signals much easier to analyse. In order to achieve this, the diaphragm must have a narrow resonance peak that lies well above the frequency range of the human voice. Chung achieved the narrow width by making the diaphragm from a fully cross-linked epoxy resin with a low damping ratio. Cross-linking prevents the movement of molecules past each other and the oscillation and conformational flip of the phenyl rings, which would all lead to friction between molecules. The second condition, a high resonant frequency, could be achieved by either a high stiffness or a low mass. The snag is that a device that’s less stiff is actually more sensitive because the diaphragm’s vibrations are bigger. Chung has found a solution to this stiffness dilemma: instead of reducing the stiffness of the bulk material, he added holes to the diaphragm. The holes reduce the diaphragm’s mass to satisfy the high resonant frequency, while keeping stiffness low to increase sensitivity. “This solution has also been commonly used in the silicon MEMS area in order to reduce the air resistance,” adds Chung.
The group has submitted patent applications for the design and is collaborating with several industry partners, though Chung is careful to note that commercialization is still in the early stages, since there is more research to be done in the lab. “We are now trying to make a sensor interface circuit on a flexible substrate so the entire sensory system can be attached on the neck skin without any difficulties,” he explains. The current design has a rigid circuit board, which can cause occasional problems by suppressing neck vibrations.
Throat sensors like these could also be used to diagnose illnesses like COVID-19, which have symptoms that manifest themselves in the vocal cords. Researchers from the Massachusetts Institute of Technology in the US have found that COVID-19 changes our voice signals by affecting the complexity of movement of the muscles in the throat (IEEE Open Journal of Engineering in Medicine and Biology 1 203). They hypothesized this to be due to an increased coupling between muscles, preventing them from moving independently. Sensor devices paired with an app could provide early-stage screening to alert people to seek further testing for COVID-19. “It can also be used as a cough detector to diagnose people,” speculates Chung, while considering clinical uses for sensors like his. Even Vesper, though its current designs are not flexible, has shown interest in this potential outlet for innovation. “Our team is already brainstorming new ideas such as acoustic respiratory health monitors,” wrote its chief executive Matt Crowley in a blog post in March about the pandemic.
An advantage of sensors being repurposed from a voice-recognition context is that there is money available for developing the technology. Well-resourced tech companies are always on the lookout for technology that has exciting prospects for smart devices – such as sensors like Lee’s and Chung’s, which are sensitive enough to recognize individual voices in lieu of a password or fingerprint. And given the human need, as a result of the pandemic, to quickly detect respiratory illnesses on a mass scale, throat-monitoring devices will likely be on a fast-track to becoming a viable diagnostic technology. It will be interesting to see how both of these areas of innovation evolve over the next few years following the pandemic. Likewise, the debate between capacitive and piezoelectric sensors will continue to change, possibly as a result of manufacturing improvements, or the changing demand for certain features, like flexibility and resistance to harsh environments. Perhaps, one crisp December morning as you wait for the Brussels sprouts to boil, you might ask your smart speaker for an update.


physicsworld--2020-12-08--Spin-guided scattering of light is observed in a liquid crystal - Physics World

Just as topological insulators provide protection to electrons travelling along their edges and surfaces , photons can also be topologically protected .
This can occur when photon scattering modes are associated with just one spin state .
Now , researchers in India and the Netherlands have found that spin-selective or spin asymmetric scattering modes can be observed using a twisted nematic-liquid-crystal-based spatial light modulator .
Their work indicates that liquid crystals can host topological states by controlling the interaction between disorder of the material and the spin-orbit interaction of light .
Topology is the field of mathematics concerned with geometric @ @ @ @ @ @ @ deformed .
Properties that are invariant when deformed are said to be topological .
Recently , physicists have become more and more interested in applying topological principles in physical systems with a view to exploiting topological invariance whereby particles are not affected by local perturbations .
A spin-polarized current of electrons , for example , can flow without scattering in topological materials that could have significant technological applications .
Although much of the research focus has been on electronic , many research groups are now turning their attention to photonics .
In the @ @ @ @ @ @ @ arise when photons travelling through a medium are controlled so that their spin is “ locked ” to a particular propagation direction .
This can be achieved by controlling the interplay between the spin angular momentum of the beam ( its circular polarization ) with its orbital angular momentum .
When travelling through certain materials , it is thought that a Gaussian beam can be manipulated so that its scattering modes are spin selective .
In their recent study , published in Physical Review A , Ankit Kumar Singh and colleagues at the @ @ @ @ @ @ @ Kolkata and Delft University of Technology have demonstrated a method for producing spin-selective scattering modes in the Fourier spectrum of a Gaussian beam passing through a liquid crystal-based modulator .
Their experiment used a helium-neon laser to produce a Gaussian beam which was spatially filtered and then linearly polarized .
The beam then passed through the medium — a transmission spatial light modulator made of a liquid crystal .
This had properties that could be controlled using an applied voltage , resulting in a random phase distribution .
Following this , the beam @ @ @ @ @ @ @ consisting of a quarter wave plate and another polarizer .
This enabled researchers to measure the distribution of left circularly polarized and right circularly polarized light once the beam was focused onto a photodetector .
They found that randomly scattered modes were only observed for the right circularly polarized projection , whereas the left circularly polarized projection showed no scattering modes .
From this , they concluded that the randomly scattered modes only occur for one spin state of the light , while the other travels through the medium mostly unaffected by the @ @ @ @ @ @ @ in its infancy , the field of topological photonics provides exciting possibilities for the future of light-based technologies .
Potential applications of spin-controlled photon systems range from high-resolution imaging techniques to microparticle trapping .
Understanding the topological properties of light as it interacts with different materials will enable researchers to develop more robust technologies in the future .


physicsworld--2020-12-09--Fast quantum random number generator could advance cryptography on the cheap - Physics World

While world events are often difficult to predict , true randomness is surprisingly hard to find .
In recent years , physicists have turned to quantum mechanics for a solution , using the inherently unpredictable behavior of photons to generate the truly random numbers that underpin many modern cryptographic protocols .
Now , a new study promises to make this process of quantum random number generation more accessible , by showing that it is possible to produce certifiably random numbers quickly using a system built with off-the-shelf components .
When numbers are used to securely encode information , the randomness @ @ @ @ @ @ @ string of truly random numbers is one that a hacker can never guess .
In classical physics , however , all processes – even chaotic ones – are deterministic , making true randomness impossible .
To illustrate this , study lead author David Drahi , a physicist at the University of Oxford , UK , notes that classically , a simple coin flip is about as random as it gets .
However , he continues , “ if you know the mass of the coin , if you can see the coin , @ @ @ @ @ @ @ , you can predict where it is going to land ” .
Classical randomness is therefore limited by the existence of information about the physical process meant to produce it .
In the quantum world , in contrast , “ there are these fundamentally non-deterministic processes , ” says Nathan Walk , a physicist at Freie Universitat Berlin , Germany and a co-author on the study .
The results of quantum measurements , he adds , are inherently unpredictable , because their outcome does not exist in any meaningful way until the measurement @ @ @ @ @ @ @ the system has , famously , collapsed .
In developing their random number generator ( RNG ) , the study ’ s authors focused not only on producing randomness , but also on confirming that this randomness originates from a non-deterministic quantum process rather than some incidental classical noise in the experiment .
“ There is not a test you can do on a string of numbers to tell if it ’ s random , ” Walk notes .
“ You can ’ t certify strings .
But you can certify processes.
” @ @ @ @ @ @ @ a quantum certification process – essentially an additional measurement – into their protocol for generating random numbers .
They also developed rigorous theoretical proofs of its effectiveness and demonstrated that it can be implemented in practice by performing experiments using quantum light .
According to Renato Renner , a physicist at ETH Zurich in Switzerland who was not affiliated with the study , such steps are important for creating a practical system .
“ You really want to have a device that produces some certificate , otherwise you don ’ t really profit @ @ @ @ @ @ @ says .
In their experiments , the researchers send laser light ( a photonic state ) into one input of a beam splitter while the other input is kept void , resulting in a zero signal ( a vacuum state ) .
The consequent pair of output beams is then measured using two separate detectors .
Because each photon that arrives at the beam splitter has an equal ( 50 % ) chance of being reflected or transmitted , the difference between the numbers of photons recorded by each detector is unpredictable .
@ @ @ @ @ @ @ number , Drahi explains .
To confirm that randomness generated in this way is reliable and useful , the researchers performed another measurement on the photonic state before it reaches the beam splitter .
In this certification measurement , the light signal is discarded if it would not generate the desired amount of randomness at the end of the experiment .
This can happen if the laser signal contains either too few photons or too many .
Too few , and the number of possible unpredictable events will be too low for the @ @ @ @ @ @ @ many , and the detectors will hit their maximum value , making the measurement fully predictable .
The inclusion of this certification measurement means that the researchers have theorized and built a device that not only produces randomness at a fast rate of 8.05 gigabits per second , but also ensures the quality of that randomness in real time .
According to Feihu Xu , a physicist at the University of Science and Technology of China who was not involved in the work , this “ development of a formal framework to monitor @ @ @ @ @ @ @ even though some other ideas in the study have been explored before .
The researchers ’ inclusion of real-time randomness certification in their experiment also has consequences for its possible future applications , because the study ’ s theoretical framework proves that the RNG protocol is partly independent of the devices used to implement it .
For instance , the light source used can be “ untrusted ” – the randomness analysis is independent of any information about it .
As long as the light signal passes the certification measurement , the properties @ @ @ @ @ @ @ not affect the quality of final randomness .
This flexibility means that the group ’ s quantum RNG could in principle be used by “ a person or a computer that knows nothing about quantum physics ” , Walk notes .
The numbers obtained in this fashion would be reliably random regardless of operator ’ s expertise , he adds , since the protocol automatically ensures its own performance .
As a bonus , the research team managed to construct their experiment using affordable off-the-shelf components – a feature that allowed Drahi to @ @ @ @ @ @ @ some of his co-authors in Moscow .
“ It got there and it worked , ” he says , noting that it would have been virtually impossible to ship all of the components for a more exotic quantum device across a continent , have a different researcher assemble them upon arrival , and then successfully generate random numbers at the same high speed .
This level of practicality , combined with the rigorous approach to confirming the randomness of their random numbers as well as generating them , sets up this study as @ @ @ @ @ @ @ of real-world quantum devices providing reliably true randomness .
“ It could have very broad applications , ” Renner concludes .
The new quantum RNG is described in Physical Review X .

Thumbs-up for randomness David Drahi in the quantum optics lab. (Courtesy: David Drahi)
While world events are often difficult to predict, true randomness is surprisingly hard to find. In recent years, physicists have turned to quantum mechanics for a solution, using the inherently unpredictable behavior of photons to generate the truly random numbers that underpin many modern cryptographic protocols. Now, a new study promises to make this process of quantum random number generation more accessible, by showing that it is possible to produce certifiably random numbers quickly using a system built with off-the-shelf components.
When numbers are used to securely encode information, the randomness of those numbers is crucial: a string of truly random numbers is one that a hacker can never guess. In classical physics, however, all processes – even chaotic ones – are deterministic, making true randomness impossible. To illustrate this, study lead author David Drahi, a physicist at the University of Oxford, UK, notes that classically, a simple coin flip is about as random as it gets. However, he continues, “if you know the mass of the coin, if you can see the coin, if you can look at the wind, you can predict where it is going to land”. Classical randomness is therefore limited by the existence of information about the physical process meant to produce it.
In the quantum world, in contrast, “there are these fundamentally non-deterministic processes,” says Nathan Walk, a physicist at Freie Universitat Berlin, Germany and a co-author on the study. The results of quantum measurements, he adds, are inherently unpredictable, because their outcome does not exist in any meaningful way until the measurement has been made and the wavefunction of the system has, famously, collapsed.
Certifiably random
In developing their random number generator (RNG), the study’s authors focused not only on producing randomness, but also on confirming that this randomness originates from a non-deterministic quantum process rather than some incidental classical noise in the experiment. “There is not a test you can do on a string of numbers to tell if it’s random,” Walk notes. “You can’t certify strings. But you can certify processes.”
Drahi, Walk and collaborators built such a quantum certification process – essentially an additional measurement – into their protocol for generating random numbers. They also developed rigorous theoretical proofs of its effectiveness and demonstrated that it can be implemented in practice by performing experiments using quantum light. According to Renato Renner, a physicist at ETH Zurich in Switzerland who was not affiliated with the study, such steps are important for creating a practical system. “You really want to have a device that produces some certificate, otherwise you don’t really profit from any quantum advantages,” he says.
In their experiments, the researchers send laser light (a photonic state) into one input of a beam splitter while the other input is kept void, resulting in a zero signal (a vacuum state). The consequent pair of output beams is then measured using two separate detectors. Because each photon that arrives at the beam splitter has an equal (50%) chance of being reflected or transmitted, the difference between the numbers of photons recorded by each detector is unpredictable. It is bound to be a random number, Drahi explains.
To confirm that randomness generated in this way is reliable and useful, the researchers performed another measurement on the photonic state before it reaches the beam splitter. In this certification measurement, the light signal is discarded if it would not generate the desired amount of randomness at the end of the experiment. This can happen if the laser signal contains either too few photons or too many. Too few, and the number of possible unpredictable events will be too low for the measurement to be sufficiently random. Too many, and the detectors will hit their maximum value, making the measurement fully predictable.
The inclusion of this certification measurement means that the researchers have theorized and built a device that not only produces randomness at a fast rate of 8.05 gigabits per second, but also ensures the quality of that randomness in real time. According to Feihu Xu, a physicist at the University of Science and Technology of China who was not involved in the work, this “development of a formal framework to monitor and certify the randomness” stands out even though some other ideas in the study have been explored before.
“Untrusted” light source
The researchers’ inclusion of real-time randomness certification in their experiment also has consequences for its possible future applications, because the study’s theoretical framework proves that the RNG protocol is partly independent of the devices used to implement it. For instance, the light source used can be “untrusted” – the randomness analysis is independent of any information about it. As long as the light signal passes the certification measurement, the properties of the device that produced it do not affect the quality of final randomness. This flexibility means that the group’s quantum RNG could in principle be used by “a person or a computer that knows nothing about quantum physics”, Walk notes. The numbers obtained in this fashion would be reliably random regardless of operator’s expertise, he adds, since the protocol automatically ensures its own performance.
As a bonus, the research team managed to construct their experiment using affordable off-the-shelf components – a feature that allowed Drahi to ship the experimental setup from Oxford to some of his co-authors in Moscow. “It got there and it worked,” he says, noting that it would have been virtually impossible to ship all of the components for a more exotic quantum device across a continent, have a different researcher assemble them upon arrival, and then successfully generate random numbers at the same high speed. This level of practicality, combined with the rigorous approach to confirming the randomness of their random numbers as well as generating them, sets up this study as a promising starting point for the development of real-world quantum devices providing reliably true randomness. “It could have very broad applications,” Renner concludes.
The new quantum RNG is described in Physical Review X.


physicsworld--2020-12-09--Acoustic metamaterial helps moths avoid being eaten by bats - Physics World

Natural acoustic metamaterials found on the wings of some moths could help the insects avoid being eaten by bats – according to Marc Holderied and colleagues at the UK ’ s University of Bristol .
By doing a combination of simulations and experiments , the team found that coupled vibrations of wing scales enable the moths to absorb ultrasound over a broad range of frequencies .
The discovery could lead to the development of bio-inspired sound proofing materials with the potential to perform far better than current designs .
As a key source of prey for echolocating bats , moths @ @ @ @ @ @ @ .
While many moth species are very good at hearing the ultrasound used by bats , many other species lack this ability .
Instead , these insects use acoustic camouflage to avoid detection .
This involves sound waves being scattered or absorbed by sub-wavelength size structures on the moth ’ s wings .
Scientists have studied similar metamaterial structures on butterfly wings , which can have special optical properties that produce vibrant iridescent colours .
However , these scales are not suitable for dampening sound , leading Holderied ’ s team to ask @ @ @ @ @ @ @ ?
In their study , the researchers investigated sound dampening mechanisms in two earless species of moth – Antheraea pernyi and Dactyloceras lucina .
They first used a combination of scanning electron microscopy and micro-computed tomography to study the shapes and arrangements of the wing scales of both species .
The images revealed overlapping tile arrangements of paddle-shaped scales , with stalks attached to stiff , lightweight membranes .
These layers were typically less than 0.3 mm thick , which is far shorter than the ultrasound wavelengths used by bats .
In subsequent @ @ @ @ @ @ @ these layers could significantly dampen sound waves within a broad range of ultrasound frequencies .
Sound at frequencies as low as 20 kHz is absorbed and the layers display a maximum absorption of 72 % at 78 kHz .
Using computer models , the researchers worked out the mechanisms responsible for these advanced absorption properties .
The scales vary significantly in terms of their sizes and shapes – depending on their location on the wing .
The team found that this leads to variations in the natural resonant frequencies of the scales .
@ @ @ @ @ @ @ coupling between the vibrations of neighbouring scales on shared membranes .
This means that collectively , the scales act to dampen acoustic waves over a broad range of frequencies .
This is wholly unlike the acoustic behaviour of butterfly wings , whose uniform scales can only resonate in small , localized clusters .
The wings of the two moth species are the first confirmed examples of natural acoustic metamaterials .
Their design could be mimicked to create lightweight , ultrathin soundproofing panels , with the potential to out-perform the thick , porous absorbers @ @ @ @ @ @ @ research is described in Proceedings of the National Academy of Science .

Ultrasound absorbers: false-colour 3D representation of a 0.21×0.28 mm wing section of the moth Lasiocampa quercus showing the structure, diversity, and arrangement of base scales (orange) and cover scales (blue and yellow). (Courtesy: Simon Reichel/Thomas Neil/Zhiyuan Shen/Marc Holderied)
Natural acoustic metamaterials found on the wings of some moths could help the insects avoid being eaten by bats – according to Marc Holderied and colleagues at the UK’s University of Bristol. By doing a combination of simulations and experiments, the team found that coupled vibrations of wing scales enable the moths to absorb ultrasound over a broad range of frequencies. The discovery could lead to the development of bio-inspired sound proofing materials with the potential to perform far better than current designs.
As a key source of prey for echolocating bats, moths are under evolutionary pressure to evade capture. While many moth species are very good at hearing the ultrasound used by bats, many other species lack this ability. Instead, these insects use acoustic camouflage to avoid detection. This involves sound waves being scattered or absorbed by sub-wavelength size structures on the moth’s wings.
Scientists have studied similar metamaterial structures on butterfly wings, which can have special optical properties that produce vibrant iridescent colours. However, these scales are not suitable for dampening sound, leading Holderied’s team to ask what is different about moths’ wings? In their study, the researchers investigated sound dampening mechanisms in two earless species of moth – Antheraea pernyi and Dactyloceras lucina.
Microscope and tomography
They first used a combination of scanning electron microscopy and micro-computed tomography to study the shapes and arrangements of the wing scales of both species. The images revealed overlapping tile arrangements of paddle-shaped scales, with stalks attached to stiff, lightweight membranes. These layers were typically less than 0.3 mm thick, which is far shorter than the ultrasound wavelengths used by bats.
In subsequent experiments, Holderied and colleagues showed that these layers could significantly dampen sound waves within a broad range of ultrasound frequencies. Sound at frequencies as low as 20 kHz is absorbed and the layers display a maximum absorption of 72% at 78 kHz.
Using computer models, the researchers worked out the mechanisms responsible for these advanced absorption properties. The scales vary significantly in terms of their sizes and shapes – depending on their location on the wing. The team found that this leads to variations in the natural resonant frequencies of the scales. Furthermore, their models revealed a strong coupling between the vibrations of neighbouring scales on shared membranes. This means that collectively, the scales act to dampen acoustic waves over a broad range of frequencies. This is wholly unlike the acoustic behaviour of butterfly wings, whose uniform scales can only resonate in small, localized clusters.
The wings of the two moth species are the first confirmed examples of natural acoustic metamaterials. Their design could be mimicked to create lightweight, ultrathin soundproofing panels, with the potential to out-perform the thick, porous absorbers currently used for sound insulation.
The research is described in Proceedings of the National Academy of Science.