QT/ Breakthrough in optical information transmission
Quantum news biweekly vol.39, 26th October — 7th November
TL;DR
- Scientists have managed for the first time to create a unidirectional device that significantly increases the quality of a special class of transmitted signals in optical communications: optical vortices. By transmitting selective optical vortex modes exclusively unidirectionally, the developed device largely reduces detrimental backscattering to a minimum. The scientists emphasize the great practical utility of their discovery in many optical systems, with applications ranging from mode division multiplexed communications, optical tweezers, vortex lasers to quantum manipulation systems.
- A team of quantum physicists has established a new method to observe vortices in dipolar quantum gases. These quantum vortices are considered a strong indication of superfluidity, the frictionless flow of a quantum gas, and have now been experimentally detected for the first time in dipolar gases.
- Quantum dots are clusters of some 1,000 atoms which act as one large ‘super-atom’. It is possible to accurately design the electronic properties of these dots just by changing their size. A team has succeeded in making a highly conductive optoelectronic metamaterial through self-organization.
- Researchers have found an efficient way to identify ‘topological’ materials, whose surfaces can have different electrical or functional properties than their interiors. The approach should make it easier uncover materials that could be the basis of next-generation computer chips or quantum devices.
- A new way to combine two materials with special electrical properties — a monolayer superconductor and a topological insulator — provides the best platform to date to explore an unusual form of superconductivity called topological superconductivity. The combination could provide the basis for topological quantum computers that are more stable than their traditional counterparts.
- The comparisons of power consumptions or luminosity delivered for a given power for future Higgs-producing colliders have been widely considered, but a new article considers the environmental impact of future ‘Higgs factories’ that could replace the Large Hadron Collider.
- Scientists have created a Bose-Einstein condensate out of excitons — quasiparticles that combine electrons and positively charged ‘holes’ — in a semiconductor. Quasiparticle Bose-Einstein condensates have for six decades been something of a holy grail of low-temperature physics.
- Extreme miniaturization of infrared (IR) detectors is critical for their integration into next-generation consumer electronics, wearables and ultra-small satellites. Thus far, however, IR detectors have relied on bulky (and expensive) materials and technologies. A team of scientists has now succeeded in developing a cost-effective miniaturization process for IR spectrometers based on a quantum dot photodetector, which can be integrated on a single chip.
- Computing power of quantum machines is currently still very low. Increasing it is still proving to be a major challenge. Physicists now present a new architecture for a universal quantum computer that overcomes such limitations and could be the basis of the next generation of quantum computers soon.
- As climate change intensifies summer heat, demand is growing for technologies to cool buildings. Now, researchers report that they have used advanced computing technology and artificial intelligence to design a transparent window coating that could lower the temperature inside buildings, without expending a single watt of energy.
- And more!
Quantum Computing Market
According to the recent market research report ‘Quantum Computing Market with COVID-19 impact by Offering (Systems and Services), Deployment (On Premises and Cloud Based), Application, Technology, End-use Industry and Region — Global Forecast to 2026’, published by MarketsandMarkets, the Quantum Computing market is expected to grow from USD 472 million in 2021 to USD 1,765 million by 2026, at a CAGR of 30.2%. The early adoption of quantum computing in the banking and finance sector is expected to fuel the growth of the market globally. Other key factors contributing to the growth of the quantum computing market include rising investments by governments of different countries to carry out research and development activities related to quantum computing technology. Several companies are focusing on the adoption of QCaaS post-COVID-19. This, in turn, is expected to contribute to the growth of the quantum computing market. However, stability and error correction issues are expected to restrain the growth of the market.
According to ‘Quantum Computing Market Research Report: By Offering, Deployment Type, Application, Technology, Industry — Industry Share, Growth, Drivers, Trends and Demand Forecast to 2030’ report, the quantum computing market is projected to reach $64,988 million by 2030. Machine learning (ML) is expected to progress at the highest CAGR, during the forecast period, among all application categories, owing to the fact that quantum computing is being integrated in ML for improving the latter’s use case.
Latest Research
Nonreciprocal vortex isolator via topology-selective stimulated Brillouin scattering
by Xinglin Zeng, Philip St.J. Russell, Christian Wolff, Michael H. Frosz, Gordon K. L. Wong, Birgit Stiller in Science Advances
Scientists at the Max Planck Institute for the Science of Light have managed for the first time to create a unidirectional device that significantly increases the quality of a special class of transmitted signals in optical communications: optical vortices. By transmitting selective optical vortex modes exclusively unidirectionally, the developed device largely reduces detrimental backscattering to a minimum. The scientists emphasize the great practical utility of their discovery in many optical systems, with applications ranging from mode division multiplexed communications, optical tweezers, vortex lasers to quantum manipulation systems.
Optical communication can be improved by increasing the amount of optical information transmitted. This can be achieved by using multiplexed channels such as using many optical wavelengths, different polarization states or multiple time slots. In the last decade, optical spatial modes, which are the eigenfields in the waveguides, are widely exploited to further improve the communication capacity due to the little crosstalk between orthogonal spatial modes.
In classical communication as well as in quantum communication, the use of vortex modes in multiplexing methods has proven to be advantageous. This special mode set possesses a helical optical phase distribution and allows an additional degree of freedom for multiplexing optical signals. Devices like vortex generators, lasers and signal amplifiers were demon-strated and are in great demand.
A limiting effect on the applicability is that there has not yet been a device that permits transmission of certain vortex modes in one direction but not the opposite one. However, just this kind of device — a so-called optical vortex isolator — is of crucial importance for the improvement of signal quality and purity. The particular difficulty in developing such a device is a fundamental principle of optics: reciprocity. It requires a symmetrical response of a transmission channel when the source and observation points are interchanged.
Now, a team at the Max Planck Institute for the Science of Light led by Xinglin Zeng, Philip Russell and Birgit Stiller, achieved a breakthrough that makes this possible: They used sound waves that propagate only in one direction to break the light transmission reciprocity for chosen vortex modes. The effect of so-called topology-selective Brillouin-Mandelstam scattering in chiral photonic crystal fibre allows for a unidirectional interaction of vortex-carrying light waves with traveling sound waves. A specific optical vortex can be strongly suppressed or amplified with a well-designed control light. The experimental results show a significant vortex isolation rate, preventing random backscattering and signal degradation in the system.
“This is the first nonreciprocal system for vortex modes, which opens up a new perspective in nonreciprocal optics — the same physical effect can happen not only on the fundamental modes but also on higher-order modes” says Xinglin Zeng, the first author of this paper.
“The light-driven optical vortex isolator will have great impact on the applications such as optical communications, quantum information processing, optical tweezers, and fiber lasers. I find the possibility of selective manipulation of vortex modes solely by light and sound waves a very fascinating concept” says Birgit Stiller, the leader of the Quantum Optoacoustics Research Group.
Observation of vortices and vortex stripes in a dipolar condensate
by Lauritz Klaus, Thomas Bland, Elena Poli, Claudia Politi, Giacomo Lamporesi, Eva Casotti, Russell N. Bisset, Manfred J. Mark, Francesca Ferlaino in Nature Physics
A team of quantum physicists from Innsbruck, Austria, led by three-time ERC laureate Francesca Ferlaino has established a new method to observe vortices in dipolar quantum gases. These quantum vortices are considered a strong indication of superfluidity, the frictionless flow of a quantum gas, and have now been experimentally detected for the first time in dipolar gases.
Vortices are ubiquitous in nature: Whirling up water can create swirls. When the atmosphere is stirred up, huge tornadoes can form. This is also the case in the quantum world, except that there many identical vortices are being formed simultaneously — the vortex is quantized. In many quantum gases, such quantized vortices have already been demonstrated.
“This is interesting because such vortices are a clear indication of the frictionless flow of a quantum gas — the so-called superfluidity,” says Francesca Ferlaino from the Department of Experimental Physics at the University of Innsbruck and the Institute of Quantum Optics and Quantum Information at the Austrian Academy of Sciences.
Ferlaino and her team are researching quantum gases made of strongly magnetic elements. For such dipolar quantum gases, in which atoms are highly connected to each other, quantum vortices could not be demonstrated so far. Scientists have developed a new method: “We use the directionality of our quantum gas of dysprosium, whose atoms behave like many small magnets, to stir the gas,” explains Manfred Mark from Francesca Ferlaino’s team. To do this, the scientists apply a magnetic field to their quantum gas in such a way that this initially round, pancake-shaped gas becomes elliptically deformed due to magnetostriction. This idea, as simple as it is powerful, originated from a theoretical proposal a few years ago by the Newcastle University theoretical team, led by Nick Parker and of which Thomas Bland, the paper’s second author, was a member.
“By rotating the magnetic field, we can rotate the quantum gas,” explains Lauritz Klaus, first author of the current paper. “If it spins fast enough, then small vortices form in the quantum gas. This is how the gas tries to balance the angular momentum.”
At sufficiently high rotational speeds, peculiar stripes of vortices form along the magnetic field. These are a special characteristic of dipolar quantum gases and have now been observed for the first time at the University of Innsbruck, Austria.
The new method will be used to study superfluidity in supersolid states in which quantum matter is simultaneously solid and liquid. “It is indeed still a major open question the degree of superfluid character in the newly discovered supersolid states, and this question remains still very little studied today.”
Approaching Bulk Mobility in PbSe Colloidal Quantum dots 3D Superlattices
by Jacopo Pinna, Razieh Mehrabi K., Dnyaneshwar S. Gavhane, Majid Ahmadi, Suhas Mutalik, Muhammad Zohaib, Loredana Protesescu, Bart J. Kooi, Giuseppe Portale, Maria Antonietta Loi in Advanced Materials
Quantum dots are clusters of some 1,000 atoms which act as one large ‘super-atom’. It is possible to accurately design the electronic properties of these dots just by changing their size. However, to create functional devices, a large number of dots have to be combined into a new material. During this process, the properties of the dots are often lost. Now, a team led by University of Groningen professor of Photophysics and Optoelectronics, Maria Antonietta Loi, has succeeded in making a highly conductive optoelectronic metamaterial through self-organization.
Quantum dots of PbSe (lead selenide) or PbS (lead sulphide) can convert shortwave infrared light into an electrical current. This is a useful property for making detectors, or a switch for telecommunications.
‘However, a single dot does not make a device. And when dots are combined, the assembly often loses the unique optical properties of individual dots, or, if they do maintain them, their capacity to transport charge carriers becomes very poor’, explains Loi. ‘This is because it is difficult to create an ordered material from the dots.’
Working with colleagues from the Zernike Institute for Advanced Materials at the Faculty of Science and Engineering, University of Groningen, Loi experimented with a method that allows the production of a metamaterial from a colloidal solution of quantum dots. These dots, each about five to six nanometres in size, show a very high conductivity when assembled in an ordered array, while maintaining their optical properties.
‘We knew from the literature that dots can self-organize into a two-dimensional, ordered layer. We wanted to expand this to a 3D material’, says Loi. To achieve this, they filled small containers with a liquid that acted as a ‘mattress’ for the colloidal quantum dots. ‘By injecting a small amount onto the surface of the liquid, we created a 2D material. Then, adding a bigger volume of quantum dots turned out to produce an ordered 3D material.’
The dots are not submersed in the liquid, but self-orient on the surface to achieve a low energy state. ‘The dots have a truncated cubic shape, and when they are put together, they form an ordered structure in three dimensions; a superlattice, where the dots act like atoms in a crystal’, explains Loi. This superlattice that is composed by the quantum dot super atoms displays the highest electron mobility reported for quantum dot assemblies.
It took special equipment to ascertain what the new metamaterial looks like. The team used an electron microscope which has atomic resolution to show the details of the material. They also ‘imaged’ the large-scale structure of the material using a technique called Grazing-incidence small-angle X-ray scattering. ‘Both techniques are available at the Zernike Institute, thanks to my colleagues Bart Kooi and Giuseppe Portale, respectively, which was a great help’, says Loi.
Measurements of the electronic properties of the material show that it closely resembles that of a bulk semiconductor, but with the optical properties of the dots. Thus, the experiment paves the way to create new metamaterials based on quantum dots. The sensitivity of the dots used in the present study to infrared light could be used to create optical switches for telecommunication devices. ‘And they might also be used in infrared detectors for night-vision and autonomous driving.’
Machine‐Learning Spectral Indicators of Topology
by Nina Andrejevic, Jovana Andrejevic, B. Andrei Bernevig, Nicolas Regnault, Fei Han, Gilberto Fabbris, Thanh Nguyen, Nathan C. Drucker, Chris H. Rycroft, Mingda Li in Advanced Materials
Topological materials, an exotic class of materials whose surfaces exhibit different electrical or functional properties than their interiors, have been a hot area of research since their experimental realization in 2007 — a finding that sparked further research and precipitated a Nobel Prize in Physics in 2016. These materials are thought to have great potential in a variety of fields, and might someday be used in ultraefficient electronic or optical devices, or key components of quantum computers.
But there are many thousands of compounds that may theoretically have topological characteristics, and synthesizing and testing even one such material to determine its topological properties can take months of experiments and analysis. Now a team of researchers at MIT and elsewhere have come up with a new approach that can rapidly screen candidate materials and determine with more than 90 percent accuracy whether they are topological.
Using this new method, the researchers have produced a list candidate materials. A few of these were already known to have topological properties, but the rest are newly predicted by this approach. The findings are reported in a paper by Mingda Li, the Class ’47 Career Development Professor at MIT, graduate students (and twin sisters) Nina Andrejevic at MIT and Jovana Andrejevic at Harvard University, and seven others at MIT, Harvard, Princeton University, and Argonne National Laboratory.
Topological materials are named after a branch of mathematics that describes shapes based on their invariant characteristics, which persist no matter how much an object is continuously stretched or squeezed out of its original shape. Topological materials, similarly, have properties that remain constant despite changes in their conditions, such as external perturbations or impurities.
There are several varieties of topological materials, including semiconductors, conductors, and semimetals, among others. Initially, it was thought that there were only a handful of such materials, but recent theory and calculations have predicted that in fact thousands of different compounds may have at least some topological characteristics. The hard part is figuring out experimentally which compounds may be topological.
Applications for such materials span a wide range, including devices that could perform computational and data storage functions similarly to silicon-based devices but with far less energy loss, or devices to harvest electricity efficiently from waste heat, for example in thermal power plants or in electronic devices. Topological materials can also have superconducting properties, which could potentially be used to build the quantum bits for topological quantum computers. But all of this relies on developing or discovering the right materials.
“To study a topological material, you first have to confirm whether the material is topological or not,” Li says, “and that part is a hard problem to solve in the traditional way.”
A method called density functional theory is used to perform initial calculations, which then need to be followed with complex experiments that require cleaving a piece of the material to atomic-level flatness and probing it with instruments under high-vacuum conditions.
“Most materials cannot even be measured due to various technical difficulties,” Nina Andrejevic says. But for those that can, the process can take a long time. “It’s a really painstaking procedure,” she says.
Whereas the traditional approach relies on measuring the material’s photoemissions or tunneling electrons, Li explains, the new technique he and his team developed relies on absorption, specifically, the way the material absorbs X-rays. Unlike the expensive apparatus needed for the conventional tests, X-ray absorption spectrometers are readily available and can operate at room temperature and atmospheric pressure, with no vacuum needed. Such measurements are widely conducted in biology, chemistry, battery research, and many other applications, but they had not previously been applied to identifying topological quantum materials. X-ray absorption spectroscopy provides characteristic spectral data from a given sample of material. The next challenge is to interpret that data and how it relates to the topological properties. For that, the team turned to a machine-learning model, feeding in a collection of data on the X-ray absorption spectra of known topological and nontopological materials, and training the model to find the patterns that relate the two. And it did indeed find such correlations.
“Surprisingly, this approach was over 90 percent accurate when tested on more than 1500 known materials,” Nina Andrejevic says, adding that the predictions take only seconds. “This is an exciting result given the complexity of the conventional process.”
Though the model works, as with many results from machine learning, researchers don’t yet know exactly why it works or what the underlying mechanism is that links the X-ray absorption to the topological properties. “While the learned function relating X-ray spectra to topology is complex, the result may suggest that certain attributes the measurement is sensitive to, such as local atomic structures, are key topological indicators,” Jovana Andrejevic says.
The team has used the model to construct a periodic table that displays the model’s overall accuracy on compounds made from each of the elements. It serves as a tool to help researchers home in on families of compounds that may offer the right characteristics for a given application. The researchers have also produced a preliminary study of compounds that they have used this X-ray method on, without advance knowledge of their topological status, and compiled a list of 100 promising candidate materials — a few of which were already known to be topological.
Crossover from Ising- to Rashba-type superconductivity in epitaxial Bi2Se3/monolayer NbSe2 heterostructures
by Hemian Yi, Lun-Hui Hu, Yuanxi Wang, Run Xiao, et al in Nature Materials
A new way to combine two materials with special electrical properties — a monolayer superconductor and a topological insulator — provides the best platform to date to explore an unusual form of superconductivity called topological superconductivity. The combination could provide the basis for topological quantum computers that are more stable than their traditional counterparts.
Superconductors — used in powerful magnets, digital circuits, and imaging devices — allow the electric current to pass without resistance, while topological insulators are thin films only a few atoms thick that restrict the movement of electrons to their edges, which can result in unique properties. A team led by researchers at Penn State describe how they have paired the two materials.
“The future of quantum computing depends on a kind of material that we call a topological superconductor, which can be formed by combining a topological insulator with a superconductor, but the actual process of combining these two materials is challenging,” said Cui-Zu Chang, Henry W. Knerr Early Career Professor and Associate Professor of Physics at Penn State and leader of the research team. “In this study, we used a technique called molecular beam epitaxy to synthesize both topological insulator and superconductor films and create a two-dimensional heterostructure that is an excellent platform to explore the phenomenon of topological superconductivity.”
In previous experiments to combine the two materials, the superconductivity in thin films usually disappears once a topological insulator layer is grown on top. Physicists have been able to add a topological insulator film onto a three-dimensional “bulk” superconductor and retain the properties of both materials. However, applications for topological superconductors, such as chips with low power consumption inside quantum computers or smartphones, would need to be two-dimensional.
In this paper, the research team stacked a topological insulator film made of bismuth selenide (Bi2Se3) with different thicknesses on a superconductor film made of monolayer niobium diselenide (NbSe2), resulting in a two-dimensional end-product. By synthesizing the heterostructures at very lower temperature, the team was able to retain both the topological and superconducting properties.
“In superconductors, electrons form ‘Cooper pairs’ and can flow with zero resistance, but a strong magnetic field can break those pairs,” said Hemian Yi, a postdoctoral scholar in the Chang Research Group at Penn State and the first author of the paper. “The monolayer superconductor film we used is known for its ‘Ising-type superconductivity,’ which means that the Cooper pairs are very robust against the in-plane magnetic fields. We would also expect the topological superconducting phase formed in our heterostructures to be robust in this way.”
By subtly adjusting the thickness of the topological insulator, the researchers found that the heterostructure shifted from Ising-type superconductivity — where the electron spin is perpendicular to the film — to another kind of superconductivity called “Rashba-type superconductivity” — where the electron spin is parallel to the film. This phenomenon is also observed in the researchers’ theoretical calculations and simulations.
This heterostructure could also be a good platform for the exploration of Majorana fermions, an elusive particle that would be a major contributor to making a topological quantum computer more stable than its predecessors.
“This is an excellent platform for the exploration of topological superconductors, and we are hopeful that we will find evidence of topological superconductivity in our continuing work,” said Chang. “Once we have solid evidence of topological superconductivity and demonstrate Majorana physics, then this type of system could be adapted for quantum computing and other applications.”
The carbon footprint of proposed e+ e− Higgs factories
by Patrick Janot, Alain Blondel in The European Physical Journal Plus
In 2012 CERN’s Large Hadron Collider (LHC) revolutionised particle physics when it was announced that the Higgs boson had been created and detected by the world’s most powerful particle accelerator.
Yet, the work of the LHC isn’t done. It is currently in its third run and being prepared for a high luminosity upgrade that will lead to more collisions and thus the creation of more Higgs particles. But eventually the accelerator will need to be retired and replaced.
The comparisons of power consumptions or luminosity delivered for a given power for future Higgs-producing colliders have been widely considered, but a new paper in EPJ Plus by CERN researcher Patrick Janot and the University of Geneva’s Alain Blondel considers the environmental impact of future ‘Higgs factories’ that could replace the LHC.
“What is new about this research and motivated by our personal interest in conservation, is the attitude with respect to environmental concerns,” Janot says. “We are placing the environmental future of our planet as one of the top-level decision criteria when it comes to the choice, the design and the optimisation of a collider.”
In the paper, Janot says that he and co-author Blondel express the environmental impact in terms of carbon footprint per Higgs boson produced, suggesting that this figure of merit should be minimised when choosing the future Higgs factory.
The paper suggests that of five currently proposed replacement accelerator models — all of which have a ‘Higgs factory’ stage — circular colliders have a fantastic physics capability and also the best energy efficiency in the case of Higgs boson studies.
“This advantage gets multiplied for a CERN facility by the better carbon emission property of the electricity it uses,” Janot adds. “This difference reaches a factor of 100 in the case of the projects that are being considered and should definitely have a strong weight in the choice.” Janot concludes: “We believe it is important to send the message that scientists are sensitive to it and propose that this is taken into account in the choice of facilities.”
Observation of Bose-Einstein condensates of excitons in a bulk semiconductor
by Yusuke Morita, Kosuke Yoshioka, Makoto Kuwata-Gonokam in Nature Communications
Physicists have created the first Bose-Einstein condensate — the mysterious “‘fifth state” of matter — made from quasiparticles, entities that do not count as elementary particles but that can still have elementary-particle properties like charge and spin. For decades, it was unknown whether they could undergo Bose-Einstein condensation in the same way as real particles, and it now appears that they can. The finding is set to have a significant impact on the development of quantum technologies including quantum computing.
Bose-Einstein condensates are sometimes described as the fifth state of matter, alongside solids, liquids, gases and plasmas. Theoretically predicted in the early 20th century, Bose-Einstein condensates, or BECs, were only created in a lab as recently as 1995. They are also perhaps the oddest state of matter, with a great deal about them remaining unknown to science. BECs occur when a group of atoms is cooled to within billionths of a degree above absolute zero. Researchers commonly use lasers and “magnet traps” to steadily reduce the temperature of a gas, typically composed of rubidium atoms. At this ultracool temperature, the atoms barely move and begin to exhibit very strange behavior. They experience the same quantum state — almost like coherent photons in a laser — and start to clump together, occupying the same volume as one indistinguishable “super atom.” The collection of atoms essentially behaves as a single particle.
Currently, BECs remain the subject of much basic research, and for simulating condensed matter systems, but in principle, they have applications in quantum information processing. Quantum computing, still in early stages of development, makes use of a number of different systems. But they all depend upon quantum bits, or qubits, that are in the same quantum state. Most BECs are fabricated from dilute gases of ordinary atoms. But until now, a BEC made out of exotic atoms has never been achieved. Exotic atoms are atoms in which one subatomic particle, such as an electron or a proton, is replaced by another subatomic particle that has the same charge. Positronium, for example, is an exotic atom made of an electron and its positively charged anti-particle, a positron.
An “exciton” is another such example. When light hits a semiconductor, the energy is sufficient to “excite” electrons to jump up from the valence level of an atom to its conduction level. These excited electrons then flow freely in an electric current — in essence transforming light energy into electrical energy. When the negatively charged electron performs this jump, the space left behind, or “hole,” can be treated as if it were a positively charged particle. The negative electron and positive hole are attracted and thus bound together.
Combined, this electron-hole pair is an electrically neutral “quasiparticle” called an exciton. A quasiparticle is a particle-like entity that does not count as one of the 17 elementary particles of the standard model of particle physics, but that can still have elementary-particle properties like charge and spin. The exciton quasiparticle can also be described as an exotic atom because it is in effect a hydrogen atom that has had its single positive proton replaced by a single positive hole.
Excitons come in two flavors: orthoexcitons, in which the spin of the electron is parallel to the spin of its hole, and paraexcitons, in which the electron spin is anti-parallel (parallel but in the opposite direction) to that of its hole. Electron-hole systems have been used to create other phases of matter such as electron-hole plasma and even exciton liquid droplets. The researchers wanted to see if they could make a BEC out of excitons.
“Direct observation of an exciton condensate in a three-dimensional semiconductor has been highly sought after since it was first theoretically proposed in 1962. Nobody knew whether quasiparticles could undergo Bose-Einstein condensation in the same way as real particles,” said Makoto Kuwata-Gonokami, a physicist at the University of Tokyo and co-author of the paper. “It’s kind of the holy grail of low-temperature physics.”
The researchers thought that hydrogen-like paraexcitons created in cuprous oxide (Cu2O), a compound of copper and oxygen, were one of the most promising candidates for fabricating exciton BECs in a bulk semiconductor because of their long lifetime. Attempts at creating paraexciton BEC at liquid helium temperatures of around 2 K had been made in the 1990s, but failed because, in order to create a BEC out of excitons, temperatures far lower than that are needed. Orthoexcitons cannot reach such a low temperature as they are too short-lived. Paraexcitons, however, are experimentally well known to have an extremely long lifetime of over several hundred nanoseconds, sufficiently long to cool them down to the desired temperature of a BEC.
The team managed to trap paraexcitons in the bulk of Cu2O below 400 millikelvins using a dilution refrigerator, a cryogenic device that cools by mixing two isotopes of helium together and which is commonly used by scientists attempting to realize quantum computers. They then directly visualized the exciton BEC in real space by the use of mid-infrared induced absorption imaging, a type of microscopy making use of light in the middle of the infrared range. This allowed the team to take precision measurements, including the density and temperature of the excitons, that in turn enabled them to mark out the differences and similarities between exciton BEC and regular atomic BEC.
The group’s next step will be to investigate the dynamics of how the exciton BEC forms in the bulk semiconductor, and to investigate collective excitations of exciton BECs. Their ultimate goal is to build a platform based on a system of exciton BECs, for further elucidation of its quantum properties, and to develop a better understanding of the quantum mechanics of qubits that are strongly coupled to their environment.
Integrated photodetectors for compact Fourier-transform waveguide spectrometers
by Matthias J. Grotevent, Sergii Yakunin, Dominik Bachmann, Carolina Romero, Javier R. Vázquez de Aldana, Matteo Madi, Michel Calame, Maksym V. Kovalenko, Ivan Shorubalko in Nature Photonics
Extreme miniaturization of infrared (IR) detectors is critical for their integration into next-generation consumer electronics, wearables and ultra-small satellites. Thus far, however, IR detectors have relied on bulky (and expensive) materials and technologies. A team of scientists lead by Empa researcher Ivan Shorubalko now succeeded in developing a cost-effective miniaturization process for IR spectrometers based on a quantum dot photodetector, which can be integrated on a single chip.
Miniaturization of infrared spectrometers will lead to their wider use in consumer electronics, such as smartphones enabling food control, the detection of hazardous chemicals, air pollution monitoring and wearable electronics. They can be used for the quick and easy detection of certain chemicals without using laboratory equipment. Moreover, they can be useful for the detection of counterfeit medical drugs as well as of greenhouse gases such as methane and CO2.
A team of scientists at Empa, ETH Zurich, EPFL, the University of Salamanca, Spain, the European Space Agency (ESA) and the University of Basel now built a proof-of-concept miniaturized Fourier-transform waveguide spectrometer that incorporates a subwavelength photodetector as a light sensor, consisting of colloidal mercury telluride quantum dot (Hg Te) and compatible with complementary metal-oxide-semiconductor (CMOS) technology, as they report.
The resulting spectrometer exhibits a large spectral bandwidth and moderate spectral resolution of 50 cm−1 at a total active spectrometer volume below 100 μm × 100 μm × 100 μm. This ultra-compact spectrometer design allows the integration of optical-analytical measurement instruments into consumer electronics and space devices.
“The monolithic integration of subwavelength IR photodetectors has a tremendous effect on the scaling of Fourier-transform waveguide spectrometers,” says Empa researcher Ivan Shorubalko. “But this may also be of great interest for miniaturized Raman spectrometers, biosensors and lab-on-a-chip devices as well as the development of high-resolution snapshot hyperspectral cameras.”
High-Performance Transparent Radiative Cooler Designed by Quantum Computing
by Seongmin Kim, Wenjie Shang, Seunghyun Moon, Trevor Pastega, Eungkyu Lee, Tengfei Luo in ACS Energy Letters
As climate change intensifies summer heat, demand is growing for technologies to cool buildings. Now, researchers report that they have used advanced computing technology and artificial intelligence to design a transparent window coating that could lower the temperature inside buildings, without expending a single watt of energy.
Studies have estimated that cooling accounts for about 15% of global energy consumption. That demand could be lowered with a window coating that could block the sun’s ultraviolet and near-infrared light — the parts of the solar spectrum that typically pass through glass to heat an enclosed room. Energy use could be reduced even further if the coating radiates heat from the window’s surface at a wavelength that passes through the atmosphere into outer space. However, it’s difficult to design materials that can meet these criteria simultaneously and can also transmit visible light, meaning they don’t interfere with the view. Eungkyu Lee, Tengfei Luo and colleagues set out to design a “transparent radiative cooler” (TRC) that could do just that.
The team constructed computer models of TRCs consisting of alternating thin layers of common materials like silicon dioxide, silicon nitride, aluminum oxide or titanium dioxide on a glass base, topped with a film of polydimethylsiloxane. They optimized the type, order and combination of layers using an iterative approach guided by machine learning and quantum computing, which stores data using subatomic particles. This computing method carries out optimization faster and better than conventional computers because it can efficiently test all possible combinations in a fraction of a second. This produced a coating design that, when fabricated, beat the performance of conventionally designed TRCs in addition to one of the best commercial heat-reduction glasses on the market.
In hot, dry cities, the researchers say, the optimized TRC could potentially reduce cooling energy consumption by 31% compared with conventional windows. They note their findings could be applied to other applications, since TRCs could also be used on car and truck windows. In addition, the group’s quantum computing-enabled optimization technique could be used to design other types of composite materials.
Universal Parity Quantum Computing
by Michael Fellner, Anette Messinger, Kilian Ender, Wolfgang Lechner in Physical Review Letters
Computing power of quantum machines is currently still very low. Increasing it is still proving to be a major challenge. Physicists now present a new architecture for a universal quantum computer that overcomes such limitations and could be the basis of the next generation of quantum computers soon.
Quantum bits (qubits) in a quantum computer serve as a computing unit and memory at the same time. Because quantum information cannot be copied, it cannot be stored in a memory as in a classical computer. Due to this limitation, all qubits in a quantum computer must be able to interact with each other. This is currently still a major challenge for building powerful quantum computers. In 2015, theoretical physicist Wolfgang Lechner, together with Philipp Hauke and Peter Zoller, addressed this difficulty and proposed a new architecture for a quantum computer, now named LHZ architecture after the authors.
“This architecture was originally designed for optimization problems,” recalls Wolfgang Lechner of the Department of Theoretical Physics at the University of Innsbruck, Austria. “In the process, we reduced the architecture to a minimum in order to solve these optimization problems as efficiently as possible.”
The physical qubits in this architecture do not represent individual bits but encode the relative coordination between the bits. “This means that not all qubits have to interact with each other anymore,” explains Wolfgang Lechner. With his team, he has now shown that this parity concept is also suitable for a universal quantum computer.
Parity computers can perform operations between two or more qubits on a single qubit.
“Existing quantum computers already implement such operations very well on a small scale,” Michael Fellner from Wolfgang Lechner’s team explains. “However, as the number of qubits increases, it becomes more and more complex to implement these gate operations.”
In two publications the Innsbruck scientists now show that parity computers can, for example, perform quantum Fourier transformations — a fundamental building block of many quantum algorithms — with significantly fewer computation steps and thus more quickly.
“The high parallelism of our architecture means that, for example, the well-known Shor algorithm for factoring numbers can be executed very efficiently,” Fellner explains.
The new concept also offers hardware-efficient error correction. Because quantum systems are very sensitive to disturbances, quantum computers must correct errors continuously. Significant resources must be devoted to protecting quantum information, which greatly increases the number of qubits required.
“Our model operates with a two-stage error correction, one type of error (bit flip error or phase error) is prevented by the hardware used,” say Anette Messinger and Kilian Ender, also members of the Innsbruck research team. There are already initial experimental approaches for this on different platforms.
“The other type of error can be detected and corrected via the software,” Messinger and Ender say.
This would allow a next generation of universal quantum computers to be realized with manageable effort. The spin-off company ParityQC, co-founded by Wolfgang Lechner and Magdalena Hauser, is already working in Innsbruck with partners from science and industry on possible implementations of the new model.
MISC
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