Both the geometric and spectral information of the target are available with the hyperspectral light detection and ranging (LiDAR) in a single measurement. This advanced technique extends the scope of imaging spectroscopy to spectral three-dimensional (3D) sensing.
However, two major geometric radiative effects exist in hyperspectral LiDAR, namely, the distance effect and incidence angle effect, which seriously restrict its quantitative remote sensing applications.
A research team led by Prof. Niu Zheng from the Aerospace Information Research Institute (AIR), Chinese Academy of Sciences (CAS) has proposed correction algorithms while studying the geometric radiative effects existing in hyperspectral LiDAR.
The researchers discovered that the analysis and correction of the distance effect and incidence angle effect can be carried out independently. They proposed a piecewise function model that couples a quadratic function and an exponential decay function to analyze and correct distance effect and developed an improved Poullain algorithm to analyze and correct the incidence angle effect.
The results were published in ISPRS Journal of Photogrammetry and Remote Sensing and IEEE Transactions on Geoscience and Remote Sensing.
They found that distance effect originates from the system itself, and all wavelengths have a unified distance effect function. Based on this, they proposed a piecewise function model that couples a quadratic function and an exponential decay function to analyze and correct distance effect.
For different types of vegetation leaf targets, they usually exhibit different incidence angle effects due to their different surface microscopic physical structures and internal biochemical parameters. This effect is closely related to the bidirectional reflection characteristics of the measured target species under hyperspectral LiDAR conditions.
Therefore, the team pointed out that the more accurate expression of incidence angle effect of hyperspectral LiDAR should be “the incidence angle effect of a certain target under hyperspectral LiDAR.” They developed a new improved Poullain algorithm to correct the incidence angle effect of the target.
Compared with the traditional Lambert cosine law based on the assumption of isotropic scattering and the original Poullain algorithm, this algorithm considers the heterogeneity of the target roughness factor and diffuse reflection coefficient under different incidence angles and wavelengths, which is more in line with the reflection characteristics of natural target echoes.
The results of different vegetation leaf experiments showed that the standard deviations of the correction results were reduced by 30% to 60%, compared with the echo intensity and reflectivity under the standard 0-degree incidence angle.
The algorithm provides an important theoretical basis and technical support for accurate inversion of 3D biochemical parameters of vegetation in the future.
Currently, the research team has completed the design and development of the second-generation hyperspectral LiDAR system with high-speed acquisition capability, which is undergoing performance testing and is expected to be put into use by the end of 2023.
More information: Jie Bai et al, A Novel Algorithm for Leaf Incidence Angle Effect Correction of Hyperspectral LiDAR, IEEE Transactions on Geoscience and Remote Sensing (2021). DOI: 10.1109/TGRS.2021.3070652
Jie Bai et al, An exploration, analysis, and correction of the distance effect on terrestrial hyperspectral LiDAR data, ISPRS Journal of Photogrammetry and Remote Sensing (2023). DOI: 10.1016/j.isprsjprs.2023.03.001
Ulrich Busk Hoff is a researcher at DTU. Credit: Mikal Schlosser
How far along are the quantum technologies? And what do we really mean when we use the word quantum? Senior Adviser Ulrich Busk Hoff has been conducting research into and communicating about quantum physics for several years. Here he provides an overview of a rapidly developing field.
What is quantum technology?
It is the field of technologies in which quantum physics is actively utilized. That is technologies that are only possible using quantum physical phenomena. These are typically phenomena such as superposition, where an object, such as an atom, can assume more than one value or be in more than one place at the same time, and entanglement, where an object, such as an atom, is physically separated from another atom, but where they are nevertheless connected so that an impact of one atom will also affect the other.
Quantum physics is more than 100 years old, so what’s new?
The ideas are indeed old—from the time of Niels Bohr and Einstein—but we have now reached a stage where the theories have been demonstrated and we can begin to exploit them in practice. We can do this because today we can control quantum physical systems such as atoms, electrons, or photons in such a way that they can be used for technological solutions like encryption, sensors, and computers—although most of it is still only possible in laboratories.
Which quantum technology is most mature?
We are quite advanced in encryption and sensors. In the past year, DTU has participated in several demonstration experiments with quantum-encrypted data being sent between two geographical locations.
In quantum sensors, there are already many different types that can measure physical quantities with extreme precision. Some can measure tiny variations in the gravitational field. This can, for example, be utilized in construction works for subsoil mapping before construction, or to predict earthquakes. Other sensors measure magnetic fields from, for example, muscle activity and nerve paths and have great potential in fields such as medical diagnosis. Magnetic field sensors can also be used for military purposes like navigation.
The quantum computer is probably the most immature technology.
Why is there so much hype about the quantum computer?
There are several reasons. What started the hype, and is in some way still driving it, is an algorithm for quantum computers that Peter Shor, an American mathematician and professor at MIT, developed in 1994. Shor’s algorithm makes it possible for a powerful quantum computer to break RSA encryption. This is the encryption that is widely used when we send data on the Internet.
But there is also a clear assumption that quantum computers will eventually be able to handle many other calculations that are impossible using an ordinary computer. Therefore, a huge market potential is predicted for quantum computers. In other words, there is a lot of money to be made for those who can realize the quantum computer.
And the hype is also partly due to the quantum computer being so difficult to develop and to its exploitation of quantum phenomena that are incomprehensible to many. The fascination with the technology itself contributes to the hype.
When will we have quantum computers?
We are still very far from having a fully developed quantum computer. It is still unclear which physical system will constitute the quantum bits that can be utilized quantum-mechanically inside the quantum computer. Some are testing photons, other atoms, or ions—and yet others electrons in superconducting material. In some places, mechanical oscillations are used. Research and development is being conducted within all these platforms worldwide.
Calculations show that it takes a quantum computer of 10–20 million quantum bits to break an RSA encryption. Right now, the largest quantum computer is in the region of 430 quantum bits. So there is still some way to go. So, at the risk of becoming a laughing stock for posterity, I would guess that it will take another 20 years before we have a quantum computer that meets these expectations.
Will I get a quantum computer at home?
I will answer this based on where the technology is today and what we think a quantum computer will be good at—and with this in mind—my guess would be that quantum computers will not be something we will have at home. It will be meant for very specific and large-scale calculations. It will not be a computer we can use to go on Facebook or watch YouTube videos.
I would think that the quantum computer will have a role like HPC (high performance computing), which you can buy access to today if you need to perform large-scale calculations. But I could be completely wrong. Technology often develops completely differently than we predict, so we may all be walking around with a quantum computer in our pockets in 30 years.
Do you need to be interested in quantum physics as a non-physicist?
Quantum phenomena such as superposition and entanglement are a highly fascinating part of nature that can cause wonder and inspire completely new thoughts in us as humans. Instead of racing through the world with blinkers on, it makes us stop up and recognize that there is much more to nature than what immediately meets the eye. It is like the firmament. We could basically be indifferent to black holes and “dark matter,” but I think that it is about fascination, general education, and being aware of what nature actually contains.
Right now, we do not encounter the applications of quantum technology in our everyday lives. This means that when we talk about them, we have to talk about quantum phenomena. They challenge us, because we have no experience with, for example, superposition in our visible world, which is governed by conventional physics, where objects such as a chair can only be in one place in the room.
But I think that when we start using quantum technology, we will stop focusing on the underlying quantum phenomena. This is, in fact, the case with other technologies such as PCs and mobile phones. Most of us do not think about how they work. But we do know how to use them.
Schematic representation of the notation showing the three-body terms to order ωD−1, which are responsible for opening the topological bulk gap. The letters refer to the spin operator applied to each vertex, and the bond colors correspond to the bonds in the Kitaev honeycomb model. Credit: PRX Quantum (2023). DOI: 10.1103/PRXQuantum.4.020329
Chiral spin liquids are one of the most fascinating phases of matter ever imagined by physicists. These exotic liquids exhibit quasi-particles known as non-Abelian anyons that are neither bosons nor fermions, and whose manipulation could allow for the realization of a universal quantum computer. Despite intense efforts in condensed matter physics, discovering such a phase in nature remains an outstanding challenge at the forefront of modern research.
From a theoretical point of view, chiral spin liquids emerge in a simple model that was imagined by Kitaev in 2006, and which allows researchers to reveal their properties using analytical tools. Remarkably, recent advances in the design of quantum simulators open a possible path for the first experimental realization of the original Kitaev model, hence suggesting that chiral spin liquids (including their exotic quasi-particles) can be studied and manipulated in a highly-controlled experimental environment.
In a new study published in PRX Quantum, BoYe Sun and Nathan Goldman (ULB, Brussels), Monika Aidelsburger (LMU, Munich), and Marin Bukov (MPI-PKS Dresden, Sofia University) propose a realistic implementation of the Kitaev model in quantum simulators.
Based on a precise pulse sequence, their system is shown to host a chiral spin liquid with non-Abelian anyons. The authors describe practical methods to probe the striking properties of these exotic states. In particular, their methods unambiguously reveal the topological heat current that flows on the edge of the system: a hallmark signature of the non-Abelian anyons that emerge on the edge of chiral spin liquids.
This work paves the way for the quantum simulation of chiral spin liquids, offering an appealing alternative to their experimental investigation in quantum materials.
More information: Bo-Ye Sun et al, Engineering and Probing Non-Abelian Chiral Spin Liquids Using Periodically Driven Ultracold Atoms, PRX Quantum (2023). DOI: 10.1103/PRXQuantum.4.020329
PhD researcher is holding a PCB with two connectors and tweezers holding the quantum chip. Credit: QuTech
Researchers from QuTech improved the so-called “Andreev spin qubit” in a critical way and believe it can become a prime candidate in the pursuit of a perfect qubit. The new type of qubit is created in a more reliable and intrinsically stable way, compared to previous versions, by combining the advantages of two other types of qubits. The team has published their work in Nature Physics.
Unlike the world of conventional computers, where bits are based on very well established and reliable technologies, the perfect qubit has not been invented yet. Will the quantum computer of the future contain qubits that are based on superconducting transmon qubits, spin qubits in silicon, NV centers in diamond, or perhaps some other quantum phenomenon? Each type of qubit has their own advantages—and disadvantages. One is more stable, the second has a higher fidelity, and others are more easily mass-produced. The perfect qubit does not exist. Yet.
Best of both worlds
In this work, the researchers from QuTech—a collaboration between the Delft University of Technology and TNO—and together with international collaborators made a smart combination of existing techniques to store quantum information.
Marta Pita-Vidal, co-first author explains, “Two of the most promising types are spin qubits in semiconductors and transmon qubits in superconducting circuits. However, each type has its own challenges. For example, spin qubits are small and compatible with current industrial technology, but they struggle with interacting over long distances. On the other hand, transmon qubits can be controlled and read out efficiently over long distances, but they have a built-in speed limit for operations and are relatively large. The researchers in this study aim to harness the advantages of both types of qubits by developing a hybrid architecture that combines them.”
Andreev spin qubits
“In our experiment, we managed to directly manipulate the spin of the qubit using a microwave signal,” says Arno Bargerbos, the other co-first author. “We achieved very high ‘Rabi frequencies,’ which is a measure of how fast they can control the qubit. Next, they embedded this ‘Andreev spin qubit’ within a superconducting transmon qubit which enables fast measurement of the qubit state.”
The researchers characterized the coherence time of the Andreev spin qubit, a measure of how long the qubit can stay alive. They observed that its “longevity” is affected by the magnetic field from the surrounding materials.
“Finally,” says Bargerbos, “we demonstrated the first direct strong coupling between a spin qubit and a superconducting qubit, meaning that they could get the two qubits to interact in a controlled way. This suggests that the Andreev spin qubit can become a key element to interconnect quantum processors based on radically different qubit technologies: semiconducting spin qubits and superconducting qubits.”
Principal investigator Christian Andersen says, “The current Andreev spin qubit is not perfect yet. It still needs to demonstrate multi-qubit operations, which is needed for universal quantum computers. The coherence time is also sub-optimal. That can be improved by using another material. Fortunately, the scalability of the qubits is on par with semiconductor qubits, raising the hope that we can get to the point where making the quantum algorithms becomes the limiting factor and not the quantum hardware.”
Controlling sound with subwavelength plasmacoustic metalayers. The metalayer is composed of two electrodes made of thin conducting wires and a grid, spaced by a deep-subwavelength distance. For illustration purposes, the acoustic wavelength is not to scale. a Plasmacoustic metalayer placed in front of a rigid wall, when used for controlling acoustic reflection. When no voltage is applied between the collector and the emitter, the metalayer is essentially acoustically transparent. b When applying a biased sinusoidal voltage to the electrodes, the induced corona discharge forms a complex acoustic source consisting of a monopolar heat source H centered around the emitter (in red), and a dipolar force source F located between the electrodes (in blue). By controlling these two sources, synchronizing them with the incident wave, it is possible to control the total impedance of the wall in a broadband way, for example turning it into a perfect sound absorber. Credit: Nature Communications (2023). DOI: 10.1038/s41467-023-38522-5
Did you know that wires can be used to ionize air to make a loudspeaker? Simply put, it’s possible to generate sound by creating an electric field in a set of parallel wires, aka a plasma transducer, strong enough to ionize the air particles. The charged ions are then accelerated along the magnetic field lines, pushing the residual non-ionized air in a way to produce sound.
If a loudspeaker can generate sound, it can also absorb it.
While this plasma loudspeaker concept is not new, EPFL scientists went ahead and built a demonstration of the plasma transducer, with the aim to study noise reduction. They came up with a new concept, what they call the active “plasmacoustic metalayer” that can be controlled to cancel out noise. Their results are published in Nature Communications.
The scientists were intrigued by the idea of using plasma to reduce noise, since it gets rid of one of the most important aspects of conventional loudspeakers: the membrane. Loudspeakers equipped with membranes, like the ones in your car or at home, are some of the most studied solution for active noise reduction. It’s active because the membrane can be controlled to cancel out different sounds, as opposed to a wall that does the job passively.
The problem with using the conventional loudspeaker as a sound absorber is that its membrane limits the frequency range of operation. For sound absorption, the membrane behaves mechanically, vibrating to cancel out the sound waves in the air. The fact that the membrane is relatively heavy, i.e. the inertia of the membrane, limits its ability to interact efficiently with fast changing sounds or at high frequencies.
“We wanted to reduce the effect of the membrane as much as possible, since it’s heavy. But what can be as light as air? The air itself,” explains Stanislav Sergeev, postdoc at EPFL’s Acoustic Group and first author. “We first ionize the thin layer of air between the electrodes that we call a plasmacoustic metalayer. The same air particles, now electrically charged, can instantaneously respond to external electrical field commands and effectively interact with sound vibrations in the air around the device to cancel them out.”
Sergeev continues, “As expected, the communication between the electrical control system of the plasma and the acoustic environment is much faster than with a membrane.”
Not only is the plasma efficient at high frequencies, but it is also versatile since it can be tuned to work at low frequencies as well. Indeed, the scientists show that the dynamics of thin layers of air plasma can be controlled to interact with sound over deep-subwavelength distances, to actively respond to noise and cancel it out over a broad bandwidth. The fact that their device is active is key, since passive noise reduction technologies are limited in the band of frequencies that can be controlled.
The plasma absorber is also more compact that most conventional solutions. Exploiting the unique physics of plasmacoustic metalayers, the scientists experimentally demonstrate perfect sound absorption: “100% of the incoming sound intensity is absorbed by the metalayer and nothing is reflected back,” says EPFL’s Acoustic Group’s senior scientist Hervé Lissek. They also show tunable acoustic reflection from several Hz to the kHz range, with transparent plasma layers of thicknesses down to only a thousandth of a given wavelength, much smaller than conventional noise reduction solutions.
To give an idea of how much more compact the plasma absorber is, consider a low, audible sound frequency of 20 Hz, where the sonic wavelength is 17m meters long. The plasma layer would only need to be 17 mm thick to absorb the noise, whereas most conventional noise reduction solutions, like absorbing walls, would need to be at least 4 m thick which often limits its feasibility.
“The most fantastic aspect in this concept is that, unlike conventional sound absorbers relying on porous bulk materials or resonant structures, our concept is somehow ethereal. We have unveiled a completely new mechanism of sound absorption, that can be made as thin and light as possible, opening new frontiers in terms of noise control where space and weight matter, especially at low frequencies,” says Hervé Lissek.
EPFL has partnered with Sonexos SA, a Swiss-based audio technology company, to develop cutting-edge active sound absorbers that use the plasmacoustic metalayer concept. Together, they aim to provide novel and efficient solutions for reducing noise in a wide range of applications, including the automotive, consumer, commercial, and industrial sectors.
“This strategic collaboration leverages EPFL’s expertise in material science and acoustics, as well as Sonexos’ proven track record in delivering high-performance audio solutions,” explains Mark Donaldson, CEO and Founder of Sonexos.
More information: Stanislav Sergeev et al, Ultrabroadband sound control with deep-subwavelength plasmacoustic metalayers, Nature Communications (2023). DOI: 10.1038/s41467-023-38522-5
a) Photograph of a fully fabricated 300 mm wafer. (b) Close-up of a chip die. (c) Infrared micrograph with the LED turned on. (d) Holographic microscope setup. (e) Close-up of a reconstructed holographic image compared with the (f) ground truth. Credit: Singapore-MIT Alliance for Research and Technology (SMART)
Researchers in Singapore have developed the world’s smallest LED (light-emitting diode) that enables the conversion of existing mobile phone cameras into high-resolution microscopes. Smaller than the wavelength of light, the new LED was used to build the world’s smallest holographic microscope, paving the way for existing cameras in everyday devices such as mobile phones to be converted into microscopes via only modifications to the silicon chip and software. This technology also represents a significant step forward in the miniaturization of diagnostics for indoor farmers and sustainable agriculture.
This breakthrough was supplemented by the researchers’ development of a revolutionary neural networking algorithm that is able to reconstruct objects measured by the holographic microscope, thus enabling enhanced examination of microscopic objects such as cells and bacteria without the need for bulky conventional microscopes or additional optics. The research also paves the way for a major advancement in photonics—the building of a powerful on-chip emitter that is smaller than a micrometer, which has long been a challenge in the field.
The light in most photonic chips originates from off-chip sources, which leads to low overall energy efficiency and fundamentally limits the scalability of these chips. To address this issue, researchers have developed on-chip emitters using various materials such as rare-earth-doped glass, Ge-on-Si, and heterogeneously integrated III–V materials. While emitters based on these materials have shown promising device performance, integrating their fabrication processes into standard complementary metal-oxide-semiconductor (CMOS) platforms remains challenging.
While silicon (Si) has shown potential as a candidate material for nanoscale and individually controllable emitters, Si emitters suffer from low quantum efficiency because of the indirect bandgap, and this fundamental disadvantage combined with the limitations set by the available materials and fabrication tools has hindered the realization of a small native Si emitter in CMOS.
In a recently published Nature Communications paper titled “A sub-wavelength Si LED integrated in a CMOS platform,” SMART researchers described their development of the smallest reported Si emitter with a light intensity comparable to that of state-of-the-art Si emitters with much larger emission areas. In a related breakthrough, SMART researchers also unveiled their construction of a novel, untrained deep neural network architecture capable of reconstructing images from a holographic microscope in a paper titled “Simultaneous spectral recovery and CMOS micro-LED holography with an untrained deep neural network” recently published in the journal Optica.
The novel LED developed by SMART researchers is a CMOS-integrated sub-wavelength scale LED at room temperature exhibiting high spatial intensity (102 ± 48 mW/cm2) and possessing the smallest emission area (0.09 ± 0.04 μm2) among all known Si emitters in scientific literature. In order to demonstrate a potential practical application, the researchers then integrated this LED into an in-line, centimeter-scale, all-silicon holographic microscope requiring no lens or pinhole, integral to a field known as lensless holography.
Illustration of the process of image reconstruction using the LED, holographic microscope, and neural network. Credit: Singapore-MIT Alliance for Research and Technology (SMART)
A commonly faced obstacle in lensless holography is computational reconstruction of the imaged object. Traditional reconstruction methods require detailed knowledge of the experimental setup for accurate reconstruction and are sensitive to difficult-to-control variables such as optical aberrations, the presence of noise, and the twin image problem.
The research team also developed a deep neural network architecture to improve the quality of image reconstruction. This novel, untrained deep neural network incorporates total variation regularization for increased contrast and takes into account the wide spectral bandwidth of the source.
Unlike traditional methods of computational reconstruction that require training data, this neural network eliminates the need for training by embedding a physics model within the algorithm. In addition to holographic image reconstruction, the neutral network also offers blind source spectrum recovery from a single diffracted intensity pattern, which marks a groundbreaking departure from all previous supervised learning techniques.
The untrained neural network demonstrated in this study allows researchers to use novel light sources without prior knowledge of the source spectrum or beam profile, such as the novel and smallest known Si LED described above, fabricated via a fully commercial, unmodified bulk CMOS microelectronics.
The researchers envision that this synergetic combination of CMOS micro-LEDs and the neural network can be used in other computational imaging applications, such as a compact microscope for live-cell tracking or spectroscopic imaging of biological tissues such as living plants. This work also demonstrates the feasibility of next-generation on-chip imaging systems. Already, in-line holography microscopes have been employed for a variety of applications, including particle tracking, environmental monitoring, biological sample imaging, and metrology. Further applications include arraying these LEDs in CMOS to generate programmable coherent illumination for more complex systems in the future.
Iksung Kang, lead author of the Optica paper and research assistant at MIT at the time of this research, said, “Our breakthrough represents a proof of concept that could be hugely impactful for numerous applications requiring the use of micro-LEDs. For instance, this LED could be combined into an array for higher levels of illumination needed for larger-scale applications. In addition, due to the low cost and scalability of microelectronics CMOS processes, this can be done without increasing the system’s complexity, cost, or form factor. This enables us to convert, with relative ease, a mobile phone camera into a holographic microscope of this type. Furthermore, control electronics and even the imager could be integrated into the same chip by exploiting the available electronics in the process, thus creating an all-in-one micro-LED that could be transformative for the field.”
“On top of its immense potential in lensless holography, our new LED has a wide range of other possible applications. Because its wavelength is within the minimum absorption window of biological tissues, together with its high intensity and nanoscale emission area, our LED could be ideal for bio-imaging and bio-sensing applications, including near-field microscopy and implantable CMOS devices,” added Rajeev Ram, principal investigator at SMART CAMP and DiSTAP, Professor of Electrical Engineering at MIT and co-author of both papers. “Also, it is possible to integrate this LED with on-chip photodetectors, and it could then find further applications in on-chip communication, NIR proximity sensing, and on-wafer testing of photonics.”
More information: Zheng Li et al, A sub-wavelength Si LED integrated in a CMOS platform, Nature Communications (2023). DOI: 10.1038/s41467-023-36639-1
Iksung Kang et al, Simultaneous spectral recovery and CMOS micro-LED holography with an untrained deep neural network, Optica (2022). DOI: 10.1364/OPTICA.470712
Researchers developed a lidar system that uses quantum detection technology that can capture 3D images while submerged underwater. They demonstrated the system by using it to capture a 3D image (left) of a pipe (right). The scan was obtained in low scattering conditions with the single-photon system submerged in a tank. Credit: Aurora Maccarone, Heriot-Watt University
For the first time, researchers have demonstrated a prototype lidar system that uses quantum detection technology to acquire 3D images while submerged underwater. The high sensitivity of this system could allow it to capture detailed information even in extremely low-light conditions found underwater.
“This technology could be useful for a wide range of applications,” said research team member Aurora Maccarone, a Royal Academy of Engineering research fellow from Heriot-Watt University in the United Kingdom. “For example, it could be used to inspect underwater installations, such as underwater wind farm cables and the submerged structure of the turbines. Underwater lidar can also be used for monitoring or surveying submerged archaeology sites and for security and defense applications.”
Obtaining 3D images through ocean water can be challenging because it is light-limited, and any particles in the water will scatter light and distort the image. However, single-photon detection, which is a quantum-based technique, allows very high penetration and works even in low-light conditions.
In Optics Express, researchers from Heriot-Watt University and the University of Edinburgh describe experiments in which an entire single-photon lidar system was submerged in a large water tank. The new demonstrations bring the technology closer to practical applications compared to the research team’s earlier experiments with underwater single-photon detection, which were performed in carefully controlled laboratory conditions with the optical setup placed outside the water tank and data analysis performed offline.
They also implemented new hardware and software developments that allow the 3D images acquired by the system to be reconstructed in real time.
This animation shows a 3D profile of a pipe that was captured during the experiments with the system. Credit: Aurora Maccarone, Heriot-Watt University
“This work aims to make quantum detection technologies available for underwater applications, which means that we will be able to image the scene of interest in very low light conditions,” said Maccarone. “This will impact the use of offshore cable and energy installations, which are used by everyone. This technology could also allow monitoring without the presence of humans, which would mean less pollution and a less invasive presence in the marine environment.”
Faster low-light detection
Lidar systems create images by measuring how long it takes laser light to be reflected from objects in the scene and travel back to the system’s receiver, known as the “time of flight.” In the new work, the researchers sought to develop a way to acquire 3D images of targets that are obscured by turbid water and thus not visible to conventional lidar imaging systems.
They designed a lidar system that uses a green pulsed laser source to illuminate the scene of interest. The reflected pulsed illumination is detected by an array of single-photon detectors, which allows ultrafast low light detection and greatly reduces measurement time in photon-starved environments such as highly attenuating water.
“By taking time-of-flight measurements with picosecond timing resolution, we can routinely resolve millimeter details of the targets in the scene,” said Maccarone. “Our approach also allows us to distinguish the photons reflected by the target from those reflected by particles in the water, making it particularly suitable to performing 3D imaging in highly turbid waters where optical scattering can ruin image contrast and resolution.”
The fact that this approach requires thousands of single-photon detectors, all producing many hundreds of events per second, makes it extremely challenging to retrieve and process the data necessary to reconstruct the 3D image in a short time, especially for real-time applications. To solve this problem, the researchers developed algorithms specifically for imaging in highly scattering conditions and applied them in conjunction with widely available graphics processing unit (GPU) hardware.
The new technique builds on some important technological advances. “Heriot-Watt University has a long track record in single‑photon detection techniques and image processing of single-photon data, which allowed us to demonstrate advanced single‑photon imaging in extremely challenging conditions,” said Maccarone.
“The University of Edinburgh has achieved fundamental advances in the design and fabrication of single-photon avalanche diode detector arrays, which allowed us to build compact and robust imaging systems based on quantum detection technologies.”
Underwater testing
After optimizing the optical setup on a laboratory optical bench, the researchers connected the lidar system to a GPU to achieve real-time processing of the data while also implementing a number of image processing approaches for three-dimensional imaging. Once the system was working properly, they moved it to a tank that was 4 meters long, 3 meters wide, and 2 meters deep.
With the system submerged in the water, the researchers added a scattering agent in a controlled manner to make the water more turbid. Experiments at three different turbidity levels demonstrated successful imaging in controlled highly scattering scenarios at distances of 3 meters.
“Single-photon technologies are rapidly developing, and we have demonstrated very promising results in underwater environments,” said Maccarone. “The approach and image processing algorithms could also be used in a wider range of scenarios for improved vision in free space such as in fog, smoke or other obscurants.”
The researchers are now working to reduce the size of the system so that it could be integrated into an underwater vehicle. Through the UK Quantum Technology Hub Network and InnovateUK, the researchers are partnering with industry to make the technology accessible for a range of underwater applications.
More information: Aurora Maccarone et al, Submerged single-photon LiDAR imaging sensor used for real-time 3D scene reconstruction in scattering underwater environments, Optics Express (2023). DOI: 10.1364/OE.487129
The alexandrite crystals from the GALACTIC project were grown by Optomaterials and coated by Altechna to withstand the harsh conditions in space. Credit: LZH
Alexandrite laser crystals are well suited for use in Earth observation satellites. They are robust and enable laser systems with a tunable output wavelength. In the European Horizon 2020 project GALACTIC, the partners Laser Zentrum Hannover e.V. (LZH), Optomaterials S.r.l. (Italy) and Altechna (Lithuania) have now succeeded in establishing a solely European supply chain for alexandrite laser crystals, which can be used for space applications.
The Italian partner Optomaterials produces competitive crystals, which the Lithuanian company Altechna provides with a special coating. To achieve the necessary properties for space-applications, Altechna has developed special coating designs and processes based on ion beam and magnetron sputtering processes.
Proven: Space suitability
Scientists at the LZH thoroughly tested the crystals in special laser systems. They have designed these systems with future applications in mind. The demonstrators could lay the foundation for new laser-based measuring instruments. The LZH’s scientists exposed the Alexandrite crystals to proton and gamma radiation and ran them through several temperature cycles typical for space applications.
Before and after these environmental tests, they have characterized the crystals in terms of their transmission properties and laser performance, among other things. Since the environmental tests did not significantly change the measured parameters, the space suitability could be demonstrated. In addition, the researchers showed that the laser-induced damage threshold (LIDT) of the crystals is equal to—or even exceeds—top products on the world market.
The GALACTIC team has thus successfully raised the Technology Readiness Level (TRL) of space-qualified alexandrite crystals from Europe from 4 to 6 and have made the crystals ready for the market.
Special properties for more precise data
Alexandrite crystals have very good thermal conductivity and fracture strength. Therefore, they are suitable for use in high power laser systems and are robust enough to withstand the harsh conditions in space.
Since the crystals can be used to tune the output wavelength of laser systems, they could form the basis of new types of laser-based measuring instruments for Earth observation satellites. Such instruments could be used to collect more precise climate-relevant data on the state of the atmosphere or vegetation.
The findings are published in the journal Optics Express.
More information: S. Unland et al, High-performance cavity-dumped Q-switched Alexandrite laser CW diode-pumped in double-pass configuration, Optics Express (2022). DOI: 10.1364/OE.478628
In this experiment, a voltage (battery) is applied to a microwave amplifier (peach-colored block). It drives the photon (yellow) in the microwave cavity (grey strip) to strongly interact with a magnon (violet) and form a gain-driven polariton, which in turn emits coherent microwaves. Credit: B. M. Yao (first author of the paper), Chinese Academy of Sciences.
When light strongly interacts with matter, it can produce unique quasi-particles called polaritons, which are half light and half matter. In recent decades, physicists explored the realization of polaritons in optical cavities and their value for the development of highly performing lasers or other technologies.
Researchers at University of Manitoba recently developed a highly performing device based on cavity magnon polaritons that can emit and amplify microwaves. This device, introduced in Physical Review Letters, was found to significantly outperform previously proposed solid-state devices for coherent microwave emission and amplification at room temperature.
“In 1992, Claude Weisbush, a French semiconductor physicist working in Japan, discovered cavity exciton polariton by confining light in a quantum microcavity to interact with semiconductors,” Can-Ming Hu, the researcher who directed the study, told Phys.org.
“This led to the invention of polariton lasers with superior performance that have transformed solid-state laser technology. Two decades later, the magnetism community re-discovered cavity magnon polariton by confining microwaves in a cavity to interact with magnetic materials, such a half photon and half magnon quasi-particle was first discovered by Joe Artman and Peter Tannenwald in 1955 at MIT, which went largely unnoticed until recently.”
Wireless communication and quantum information technologies require coherent on-chip microwave sources. Motivated by this need, Hu and his colleagues set out to explore the potential use of cavity magnon polaritons to achieve high-quality microwave emission and amplification.
“Intrigued by the resemblance between cavity magnon polariton and cavity exciton polariton, I became curious whether the cavity magnon polariton might help us to make better solid-state microwave sources,” Hu said. “So, in 2015, my group launched a study to explore microwave emission of cavity magnon polaritons.”
The researchers initially set out to create a light–matter coupled system based on cavity magnon polaritons for coherent microwave emission. They ultimately hoped to achieve a higher performance than those reported in previous works, while retaining their device’s stability and controllability as a hybrid light–matter coupled system.
“First, we follow the principle proposed in 1920 by Dutch physicist van der Pol: using nonlinear damping to balance gain in an amplified oscillatory system, one can design and optimize a stable gain-driven cavity,” Bimu Yao, an associate professor from the Chinese Academy of Sciences who carried out this study at the University of Manitoba, told Phys.org. “Then, we set a magnetic material into such a gain-driven microwave cavity, letting the amplified microwaves to strongly interact with magnons.”
The strong interaction between amplified microwaves and magnons in the researchers’ system produces a new type of polariton, which they dubbed a “gain-driven” polariton. Compared to conventional polaritons realized in previous studies, this gain-driven polariton has a stable phase, which in turn enables the coherent emission of microwave photons.
“For decades, the magnetism community has been working on spin-toque oscillator (STO), which is a solid-state device that utilizes magnons to produce coherent microwaves,” Yongsheng Gui, a research associate at the University of Manitoba who carried out the study, told Phys.org. “The major hurdle is that the emission power of the STO is typically limited to less than 1 nW. Our device’s output is a million times more powerful, and the emission quality factor is a thousand times better.”
In initial evaluations, a proof-of-principle device created by this team of researchers achieved remarkable results, outperforming both STOs and solid-state masers developed in the past. Masers are devices that use the stimulated emission of radiation by atoms to amplify or generate microwave radiation.
“Outside of the magnetism community, there have been divers efforts for developing masers,” Gui said. “Compared with the best solid-state maser, our device’s output is a billion times more powerful, with a comparable emission quality factor.”
The new gain-driven polariton realized by Hu and his colleagues could open exciting new possibilities for the development of highly performing solid-state microwave sources that can be integrated on-chip. In addition to their compact sizes, these polariton microwave sources are frequency tunable due to the fabulous controllability of light-matter interaction. They could ultimately be integrated in a broad range of technologies and devices, including wireless communication systems and quantum computers.
“As the physics of gain-driven light-matter interaction is new, our study may also lead to new discoveries beyond microwave applications,” Hu added. “We have now submitted a patent application, and my students are working on developing prototype devices together with industry partners.”
More information: Bimu Yao et al, Coherent Microwave Emission of Gain-Driven Polaritons, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.146702
Existing magnetic field imaging equipment tends to be large and expensive, but this research marks the next step in the development of quantum sensing. Credit: Shutterstock/ PopTika
Smartphones could one day become portable quantum sensors thanks to a new chip-scale approach that uses organic light-emitting diodes (OLEDs) to image magnetic fields, with significant implications for use in health care and industry settings.
UNSW researchers from the ARC Center of Excellence in Exciton Science have demonstrated that OLEDs, a type of semiconductor material commonly found in flat-screen televisions, smartphone screens and other digital displays, can be harnessed to map magnetic fields.
The latest research, led by Dr. Rugang Geng and Professor Dane McCamey from the UNSW School of Physics, has been detailed in Nature Communications.
“Our findings show that OLEDs, a commercially available technology, can be used not only for displays and lighting, but also for quantum sensing and magnetic field imaging by integrating a small piece of microwave electronics,” says first author of the study, Dr. Rugang Geng.
“If this technology is properly developed, people could simply use their smartphones to map the magnetic fields around them, for example to spot defects in diamonds or jewelry. This also has applications in industry, such as finding defects in construction materials or as a biomedical sensor.”
How does this technology work?
OLEDs are a new display technology that provides some of the best quality screens in smartphones and TVs.
“The basic working principle of an OLED device is that when a voltage is applied, electrons and holes are injected into different layers of the device,” says Dr. Geng. “When the electrons and holes meet in the central layer, they form ‘excitons,’ which emit visible light when they decay, and that’s what makes OLEDs useful as displays and lighting sources.”
This light emission process exploits the charge characteristics of electrons, which have a negative charge, and holes, which have a positive charge. They both also have another intrinsic property called spin.
The spin property of electrons and holes, which meet in the middle layer of the device, either point up or down and are very sensitive to external magnetic fields. Credit: University of New South Wales
This spin either points up or down and is very sensitive to external magnetic fields. In fact, it can “flip-flop” (or switch direction) under magnetic resonance conditions.
“By measuring the signal change, both in electric current and emission light, induced by such a flip-flop, we are able to detect the strength of any magnetic field the device is exposed to,” says Dr. Geng.
By integrating an OLED with a microwave resonator, Dr. Geng, Prof. McCamey and their colleagues have generated a tiny oscillating magnetic field across the OLED device, allowing each individual pixel of the OLED screen to act as a small magnetic field sensor.
“We weren’t surprised at the result—we have been pursuing this for a few years,” says Prof. McCamey. “But we were surprised at the resolution of the images we could make—we can see details on sub-micron length scales, similar to the size of a bacteria or neuron.”
Commercialization and everyday uses
This latest research represents the next step in the development of magnetic field imaging equipment. Existing quantum sensing and magnetic field imaging equipment tends to be large and expensive, requiring either additional power from a high-powered laser, or extremely low temperatures. Under these conditions, the device integration potential and commercial scalability is limited.
However, the new technique developed by the team can function at a microchip scale and doesn’t require input from a laser, showing great potential for applications in scientific research, industry and medicine.
“Next, we hope to improve the overall performance of the device including optimizing the device architecture and exploring other techniques that can significantly increase the field sensitivity,” says Dr. Geng.
“We are also exploring collaborations with OLED technology companies as their experience at moving devices from the lab to commercial products will help accelerate translation of this technology.”
More information: Rugang Geng et al, Sub-micron spin-based magnetic field imaging with an organic light emitting diode, Nature Communications (2023). DOI: 10.1038/s41467-023-37090-y
