Tag Archives: #quantum

A Quantum Step To A Heat Switch With No Moving Parts (Material Science)

Study confirms unusual electron behavior in a quantum material

Researchers have discovered a new electronic property at the frontier between the thermal and quantum sciences in a specially engineered metal alloy – and in the process identified a promising material for future devices that could turn heat on and off with the application of a magnetic “switch.”

Joseph Heremans © OSU

In this material, electrons, which have a mass in vacuum and in most other materials, move like massless photons or light – an unexpected behavior, but a phenomenon theoretically predicted to exist here. The alloy was engineered with the elements bismuth and antimony at precise ranges based on foundational theory.

Under the influence of an external magnetic field, the researchers found, these oddly behaving electrons manipulate heat in ways not seen under normal conditions. On both the hot and cold sides of the material, some of the electrons generate heat, or energy, while others absorb energy, effectively turning the material into an energy pump. The result: a 300% increase in its thermal conductivity.

Take the magnet away, and the mechanism is turned off.

“The generation and absorption form the anomaly,” said study senior author Joseph Heremans, professor of mechanical and aerospace engineering and Ohio Eminent Scholar in Nanotechnology at The Ohio State University. “The heat disappears and reappears elsewhere – it is like teleportation. It only happens under very specific circumstances predicted by quantum theory.”

This property, and the simplicity of controlling it with a magnet, makes the material a desirable candidate as a heat switch with no moving parts, similar to a transistor that switches electrical currents or a faucet that switches water, that could cool computers or increase the efficiency of solar-thermal power plants.

“Solid-state heat switches without moving parts are extremely desirable, but they don’t exist,” Heremans said. “This is one of the possible mechanisms that would lead to one.”

The research is published today (June 7, 2021) in the journal Nature Materials.

Nandini Trivedi © OSU

The bismuth-antimony alloy is among a class of quantum materials called Weyl semimetals – whose electrons don’t behave as expected. They are characterized by properties that include negatively and positively charged particles, electrons and holes, respectively, that behave as “massless” particles. Also part of a group called topological materials, their electrons react as if the material contains internal magnetic fields that enable the establishment of new pathways along which those particles move.

In physics, an anomaly – the electrons’ generation and absorption of heat discovered in this study – refers to certain symmetries that are present in the classical world but are broken in the quantum world, said study co-author Nandini Trivedi, professor of physics at Ohio State.

Bismuth alloys and other similar materials also feature classical conduction like most metals, by which vibrating atoms in a crystal lattice and the movement of electrons carry heat. Trivedi described the new pathway along which light-like electrons manipulate heat among themselves as a highway that seems to appear out of nowhere.

“Imagine if you were living in a small town that had tiny roads, and suddenly there’s a highway that opens up,” she said. “This particular pathway only opens up if you apply a thermal gradient in one direction and a magnetic field in the same direction. So you can easily close the highway by putting the magnetic field in a perpendicular direction.

“No such highways exist in ordinary metals.”

When a metal like copper is heated and electrons flow from the hot end to the cold end, both the heat and the charge move together. Because of the way this highway opens in the experimental Weyl semimetal material, there’s no net charge motion – only energy movement. The absorption of heat by certain electrons represents a break in chirality, or directionality, meaning that it’s possible to pump energy between two particles that wouldn’t be expected to interact – another characteristic of Weyl semimetals.

The theoretical physicists and engineers collaborating on this study predicted that these properties existed in specific bismuth alloys and other topological materials. For these experiments, the scientists constructed the specialized alloy to test their predictions.

“We worked hard to synthesize the correct material, which was designed from the ground up by us to show this effect. It was important to purify it way below the levels of impurities that you find in nature,” Heremans said. As composed, the alloy minimized background conduction so the researchers could detect the behavior of the massless electrons, known as Weyl Fermions.

“In ordinary materials, electrons drag around with them a small magnet. However, the peculiar electronic structure of these bismuth alloys means the electrons drag around a magnet almost 50 times bigger than normal,” said Michael Flatté, professor of physics and astronomy at the University of Iowa and a study co-author. “These huge subatomic magnets allowed the novel electronic state to be formed using laboratory magnetic fields.

“These results show that theories developed for high-energy physics and subatomic particle theories can often be realized in specially designed electronic materials.”

Like everything quantum, Heremans said, “what we observed looks a little like magic, but that is what our equations say it should do and that is what we proved experimentally that it does.”

One catch: The mechanism in this material works only at a low temperature, below minus 100 degrees Fahrenheit. With the fundamentals now understood, the researchers have lots of options as they work toward potential applications.

“Now we know what materials to look for and what purity we need,” Heremans said. “That is how we get from discovery of a physical phenomenon to an engineering material.”

This work was supported by Ohio State’s Center for Emergent Materials, which is a National Science Foundation Materials Research Science and Engineering Center.

Additional co-authors include Dung Vu and Wenjuan Zhang of Ohio State and Cüneyt Şahin of the University of Iowa. Flatté and Şahin were also affiliated with the University of Chicago at the time this work was done.

Featured image: The cones in this image illustrate the equations of motion of electrons when an external magnetic field is applied to the bismuth alloy engineered for the study. Green lines and purple lines represent electrons that generate and absorb energy, respectively.Illustration by Renee Ripley

Reference: Vu, D., Zhang, W., Şahin, C. et al. Thermal chiral anomaly in the magnetic-field-induced ideal Weyl phase of Bi1−xSbx. Nat. Mater. (2021). https://doi.org/10.1038/s41563-021-00983-8

Provided by Ohio State University

Quantum Holds The Key To Secure Conference Calls (Quantum)

The world is one step closer to ultimately secure conference calls, thanks to a collaboration between Quantum Communications Hub researchers and their German colleagues, enabling a quantum-secure conversation to take place between four parties simultaneously.

The demonstration, led by Hub researchers based at Heriot-Watt University and published in Science Advances, is a timely advance, given the global reliance on remote collaborative working, including conference calls, since the start of the C19 pandemic.

There have been reports of significant escalation of cyber-attacks on popular teleconferencing platforms in the last year. This advance in quantum secured communications could lead to conference calls with inherent unhackable security measures, underpinned by the principles of quantum physics.

Senior author, Professor Alessandro Fedrizzi, who led the team at Heriot-Watt, said: “We’ve long known that quantum entanglement, which Albert Einstein called ‘spooky action at a distance’ can be used for distributing secure keys. Our work is the first example where this was achieved via ‘spooky action’ between multiple users at the same time — something that a future quantum internet will be able to exploit.”

Secure communications rely upon the sharing of cryptographic keys. The keys used in most systems are relatively short and can therefore be compromised by hackers, and the key distribution procedure is under increasing threat from quickly advancing quantum computers. These growing threats to data security require new, secure methods of key distribution.

A mature quantum technology called Quantum Key Distribution (QKD), deployed in this demonstration in a network scenario for the first time, harnesses the properties of quantum physics to facilitate guaranteed secure distribution of cryptographic keys.

QKD has been used to secure communications for over three decades, facilitating communications of over 400km over terrestrial optical fibre and recently even through space, however, crucially, these communications have only ever occurred exclusively between two parties, limiting the practicality of the technology used to facilitate secure conversations between multiple users.

The system demonstrated by the team here utilises a key property of quantum physics, entanglement, which is the property of quantum physics that gives correlations – stronger than any with which we are familiar in everyday life – between two or more quantum systems, even when these are separated by large distances.

By harnessing multi-party entanglement, the team were able to share keys simultaneously between the four parties, through a process known as ‘Quantum Conference Key Agreement’, overcoming the limitations of traditional QKD systems to share keys between just two users, and enabling the first quantum conference call to occur with an image of a Cheshire cat shared between the four parties, separated by up to 50 km of optical fibre.

Entanglement-based quantum networks are just one part of a large programme of work that the Quantum Communications Hub is undertaking to deliver future quantum secured networks.

The technology demonstrated here has potential to drastically reduce the resource costs for conference calls in quantum networks when compared to standard two-party QKD methods. It is one of the first examples of the expected benefits of a future quantum internet, which is expected to supply entanglement to a system of globally distributed nodes.

Reference: Massimiliano Proietti, Joseph Ho, Federico Grasselli, Peter Barrow, Mehul Malik, Alessandro Fedrizzi, “Experimental quantum conference key agreement”, Science Advances  04 Jun 2021: Vol. 7, no. 23, eabe0395 DOI: https://doi.org/10.1126/sciadv.abe0395

Provided by Heriot-Watt University

Quantum Hall Effect and the Third Dimension (Physics)

The quasi-quantized Hall effect is a three-dimensional relative of the Quantum Hall effect in two-dimensional systems.

The quantum Hall effect traditionally only plays a role in two-dimensional electron systems. Recently, however, a three-dimensional version of the quantum Hall effect was described in the Dirac semimetal ZrTe5. It has been suggested that this version results from a magnetic field-induced Fermi surface instability that transforms the original three-dimensional electron system into a stack of two-dimensional electron systems. Now scientists at the Max Planck Institute for Chemical Physics of Solids in Dresden, at the Technical University of Dresden, at the Brookhaven National Laboratory in New York, at the Helmholtz Center Dresden-Rossendorf, the Max Planck Institute for Microstructure Physics in Halle and at the Würzburg-Dresden Cluster of Excellence ct.qmat were able to show that the electron system of ZrTe5, contrary to the original explanation, remains three-dimensional even in strong magnetic fields and that the quasi-quantization of the Hall effect is nevertheless directly linked to quantum-Hall physics.

Universal also for conventional metals.

The findings from the study of quantum Hall physics in the third dimension can be universally applied to conventional metals and promise a unified explanation of the plateaus that have been observed in Hall measurements in many three-dimensional materials, which were often puzzling in the past. In addition, the concept can be directly applied to generalize the two-dimensional quantum anomalous Hall effect to generic three-dimensional magnets.

The results were published in Nature Communications.

Reference: Galeski, S., Ehmcke, T., Wawrzyńczak, R. et al. Origin of the quasi-quantized Hall effect in ZrTe5. Nat Commun 12, 3197 (2021). https://doi.org/10.1038/s41467-021-23435-y

Provided by CPFS MPG

USTC Constructs A Multiplexed Quantum Repeater Based On Absorptive Quantum Memories (Quantum Physics)

Chinese researchers realized an elementary link of a quantum repeater based on absorptive quantum memories (QMs) and demonstrated the multiplexed quantum repeater for the first time. On June 2nd the work is published in Nature.

The fundamental task of a quantum network is to distribute quantum entanglement between two remote locations. However, the transmission loss of optical fiber has limited the distance of entanglement distribution to approximately 100 km on the ground. Quantum repeaters can overcome this difficulty by dividing long-distance transmission into several short-distance elementary links. The entanglement of two end nodes of each link is created firstly. Then the entanglement distance is gradually expanded through entanglement swapping between each link.

Previously, an elementary link of a quantum repeater has been realized in cold atomic ensembles and single quantum systems. These demonstrations are all based on emissive QMs, in which the entangled photons are emitted from QMs. Quantum repeaters constructed by emissive QMs have simple structures, but poor compatibilities. It is of great challenge to support deterministic entanglement sources and multiplexed operations simultaneously, which are two key technologies to enhance the entanglement distribution rate. Quantum repeaters based on absorptive QMs can overcome such limitation because they separate the quantum memories and the entangled photon sources.

The research team, led by Prof. LI Chuanfeng and Prof. ZHOU Zongquan from University of Science and Technology of China (USTC), focuses on the research of absorptive QMs based on rare-earth-ion-doped crystals. For this kind of QMs, the entanglement source can be flexibly selected, including deterministic entanglement sources, while remaining the capability of multiplexed operations, and therefore should be more efficient for quantum repeater applications. In this work, they used external entangled photon-pair sources (EPPSs) based on spontaneous parametric down-conversion and achieved heralded entanglement distribution between two absorptive QMs for the first time.

They built an elementary link with an intermediate station and two nodes at the ends. Each node contains an absorptive QM with a bandwidth of 1GHz and a bandwidth-matched EPPS. In each node, one entangled photon of each photon pair was stored in the “Sandwich-like” QM while the other was transmitted to the middle station for joint Bell-state measurement (BSM). A successful entanglement swapping operation was heralded by the successful click of BSM. The entanglement between two QMs 3.5 meters apart was established with a fidelity of approximately 80.4%, although there weren’t any direct interactions between two remote QMs. Four temporal modes were employed in this demonstration of an elementary link of a quantum repeater, accelerating the entanglement distribution rate by four times.

Prof. ZHOU Zongquan said: “The use of absorptive quantum memory is expected to achieve high efficiency quantum repeater and quantum network in the future, and further promote the communication between ‘Cowherd and Weaver Girl’ in the quantum world.”

This work provides a feasible roadmap for the development of practical quantum repeaters and lays the foundation for the construction of high-speed quantum networks. Reviewers pointed out”The present work focuses on the ensemble approach, which has a number of advantages in the context of quantum repeater applications, multiplexing for instance”. They highly recommend this work as”a significant accomplishment that will form the basis for further research” and “a major step forward in the development of a practical quantum repeater”.

Prof. LI Chuanfeng said that the team will continue to improve the indicators of absorptive QM, ” we will use deterministic entanglement source to greatly improve the entanglement distribution rate, and to achieve practical quantum repeaters beyond direct transmission of optical fiber.”

LIU Xiao and HU Jun from CAS Key Laboratory of Quantum Information and CAS Center for Excellence in Quantum Information and Quantum Physics are the co-first authors. The corresponding authors are Prof. LI Chuanfeng and Prof. ZHOU Zongquan.

For future developments, the research team will continue to improve the performances of the absorptive QMs, and adopt deterministic entanglement sources, so as to greatly enhance the entanglement distribution rate, and to achieve a practical quantum repeater that outperform the direct transmission of photons.

Featured image: An elementary link of a quantum repeater based on two absorptive QMs with the Sandwich-like structure © WANG Guoyan and MA Yanbing

Reference: Ma, Y., Ma, YZ., Zhou, ZQ. et al. One-hour coherent optical storage in an atomic frequency comb memory. Nat Commun 12, 2381 (2021). https://doi.org/10.1038/s41467-021-22706-y

Provided by USTC

Harmonious Electronic Structure Leads to Enhanced Quantum Materials (Material Science)

The electronic structure of metallic materials determines the behavior of electron transport. Magnetic Weyl semimetals have a unique topological electronic structure – the electron’s motion is dynamically linked to its spin. These Weyl semimetals have come to be the most exciting quantum materials that allow for dissipationless transport, low power operation, and exotic topological fields that can accelerate the motion of the electrons in new directions. The compounds Co3Sn2S2 and Co2MnGa [1-4], recently discovered by the Felser group, have shown some of the most prominent effects due to a set of two topological bands.

Researchers at the Max Planck Institute for Chemical Physics of Solids in Dresden, the University of South Florida in the USA, and co-workers have discovered a new mechanism in magnetic compounds that couples multiple topological bands. The coupling can significantly enhance the effects of quantum phenomena. One such effect is the anomalous Hall effect that arises with spontaneous symmetry breaking time-reversal fields that cause a transverse acceleration to electron currents. The effects observed and predicted in single crystals of Co3Sn2S2 and Co2MnGa display a sizable increase compared to conventional magnets.

In the current publication, we explored the compounds XPt3, where we predicted an anomalous Hall effect nearly twice the size of the previous compounds. The large effect is due to sets of entangled topological bands with the same chirality that synergistically accelerates charged particles. Interestingly, the chirality of the bands couple to the magnetization direction and determine the direction of the acceleration of the charged particles. This chirality can be altered by chemical substitution. Our theoretical results of CrPt3 show the maximum effect, where MnPt3 significantly reduced the effect due to the change in the order of the chiral bands.

Advanced thins films of the CrPt3 were grown at the Max Planck Institute. We found in various films a pristine anomalous Hall effect, robust against disorder and variation of temperature. The result is a strong indication that the topological character dominates even at finite temperatures. The results show to be near twice as large as any intrinsic effect measured in thin films. The advantage of thin films is the ease of integration into quantum devices with an interplay of other freedoms, such as charge, spin, and heat.  XPt3 films show possible utilization for Hall sensors, charge-to-spin conversion in electronic devices, and charge-to-heat conversion in thermoelectric devices with such a strong response.

Featured image: Schematic of a single set of band interactions, where E is the band energy and EF the Fermi energy. A change in chirality or magnetization would cause a change in the anomalous Hall conductivity.  Schematic of multiple sets of band interactions, where E is the band energy and EF the Fermi energy. Comparison of off stoichiometric CrPt3 with elemental metals and magnetic Weyl Semimetals. © MPI CPfS

References: [1] Enke Liu et al., Nat. Phys. 14, 1125 (2018).
[2] Kaustuv Manna et al., Phys. Rev. X 8, 041045 (2018).
[3] D. F. Liu, et al. Science 365, 1282–1285 (2019).
[4] Noam Morali et al. Science 365, 1286–1291 (2019).
[5] Anastasios Markou et al., Hard magnet topological semimetals in XPt3 compounds with the harmony of Berry curvature, Communications Physics 4, 104 (2021) DOI: https://doi.org/10.1038/s42005-021-00608-1

Provided by MPI CPfS

New Quantum Material Discovered (Material Science)

A research team from TU Wien together with US research institutes came across a surprising form of ‘quantum criticality’; this could lead to a design concept for new materials

In everyday life, phase transitions usually have to do with temperature changes – for example, when an ice cube gets warmer and melts. But there are also different kinds of phase transitions, depending on other parameters such as magnetic field. In order to understand the quantum properties of materials, phase transitions are particularly interesting when they occur directly at the absolute zero point of temperature. These transitions are called “quantum phase transitions” or a “quantum critical points”.

Such a quantum critical point has now been discovered by an Austrian-American research team in a novel material, and in an unusually pristine form. The properties of this material are now being further investigated. It is suspected that the material could be a so-called Weyl-Kondo semimetal, which is considered to have great potential for quantum technology due to special quantum states (so-called topological states). If this proves to be true, a key for the targeted development of topological quantum materials would have been found. The results were found in a cooperation between TU Wien, Johns Hopkins University, the National Institute of Standards and Technology (NIST) and Rice University and has now been published in the journal Science Advances.

Quantum criticality – simpler and clearer than ever before

“Usually quantum critical behaviour is studied in metals or insulators. But we have now looked at a semimetal,” says Prof. Silke Bühler-Paschen from the Institute of Solid State Physics at TU Wien. The material is a compound of cerium, ruthenium and tin – with properties that lie between those of metals and semiconductors.

Usually, quantum criticality can only be created under very specific environmental conditions – a certain pressure or an electromagnetic field. “Surprisingly, however, our semimetal turned out to be quantum critical without any external influences at all,” says Wesley Fuhrman, a PhD student in Prof. Collin Broholm’s team at Johns Hopkins University, who made an important contribution to the result with neutron scattering measurements. “Normally you have to work hard to produce the appropriate laboratory conditions, but this semimetal provides the quantum criticality all by itself.”

This surprising result is probably related to the fact that the behaviour of electrons in this material has some special features. “It is a highly correlated electron system. This means that the electrons interact strongly with each other, and that you cannot explain their behaviour by looking at the electrons individually,” says Bühler-Paschen. “This electron interaction leads to the so-called Kondo effect. Here, a quantum spin in the material is shielded by electrons surrounding it, so that the spin no longer has any effect on the rest of the material.”

If there are only relatively few free electrons, as is the case in a semimetal, then the Kondo effect is unstable. This could be the reason for the quantum critical behavior of the material: the system fluctuates between a state with and a state without the Kondo effect, and this has the effect of a phase transition at zero temperature.

Quantum fluctuations could lead to Weyl particles

The main reason why the result is of such central importance is that it is suspected to be closely connected to the phenomenon of “Weyl fermions”. In solids, Weyl fermions can appear in the form of quasiparticles – i.e. as collective excitations such as waves in a pond. According to theoretical predictions, such Weyl fermions should exist in this material,” says theoretical physicist Qimiao Si of Rice University. Experimental proof, however, is yet to be found. “We suspect that the quantum criticality we observed favours the occurrence of such Weyl fermions,” says Silke Bühler-Paschen. “Quantum critical fluctuations could therefore have a stabilising effect on Weyl fermions, in a similar way to quantum critical fluctuations in high-temperature superconductors holding superconducting Cooper pairs together. This is a very fundamental question that is the subject of a lot of research around the world, and we’ve discovered a hot new lead here.”

It seems to us that certain quantum effects – namely quantum critical fluctuations, the Kondo effect and Weyl fermions – are tightly intertwined in the newly discovered material and, together, give rise to exotic Weyl-Kondo states. These are “topological” states of great stability that, unlike other quantum states, cannot be easily destroyed by external disturbances. This makes them particularly interesting for quantum computers.

To verify all this, further measurements under different external conditions are to be carried out. The team expects that a similar interplay of the various quantum effects should also be found in other materials. “This could lead to the establishment of a design concept with which such materials can be specifically improved, tailored and used for concrete applications,” says Bühler-Paschen.

Featured image: A compound of cerium, ruthenium and tin — with surprising properties. © TU Wien

Reference: Wesley T. Fuhrman et al., “Pristine quantum criticality in a Kondo semimetal”, Science Advances  19 May 2021: Vol. 7, no. 21, eabf9134 DOI: 10.1126/sciadv.abf9134

Provided by Vienna University of Technology

Researchers First Achieve Quantum Information Masking Experimentally (Physics / Quantum)

The research team, led by Academician GUO Guangcan from University of Science and Technology of China (USTC) of the Chinese Academy of Sciences, collaborating with LI Bo from Shangrao Normal University and CHEN Jingling from Nankai University, achieved the masking of optical quantum information. The researchers concealed quantum information into non-local quantum entangled states. The study was published in the journal Physical Review Letters.

Quantum information masking as one of the new information processing protocol transfers quantum information from a single quantum carrier to the quantum entangled state between multiple carriers avoiding the information decode from single quantum carrier. Not all the kind of quantum states can achieve masking, but the variety of that helps people to select.

The quantum information masking can be used in a wide situation, not only in actual quantum information tasks such as quantum secret sharing but also the further understanding in the conservation of quantum information.

In this research, the team realized quantum information masking for the first time based on the linear optics research platform.

Compared with the theoretical value, the fidelity of the entangled state can be 97.7%, meaning that the secure transmission of simple images can be complete for the three-party quantum secret sharing based on quantum information masking.

This study has great significance for theoretical research and practical application of secure quantum communication. Based on it, the feasibility of quantum information masking as a brand-new quantum information processing protocol is improved.

Reference: Zheng-Hao Liu, Xiao-Bin Liang (梁晓斌), Kai Sun, Qiang Li, Yu Meng, Mu Yang, Bo Li, Jing-Ling Chen, Jin-Shi Xu, Chuan-Feng Li, and Guang-Can Guo, “Photonic Implementation of Quantum Information Masking”, Phys. Rev. Lett. 126, 170505 – Published 30 April 2021. DOI: https://doi.org/10.1103/PhysRevLett.126.170505

Machine Learning Algorithm Helps Unravel the Physics Underlying Quantum Systems (Quantum)

Protocol to reverse engineer Hamiltonian models advances automation of quantum devices

Scientists from the University of Bristol’s Quantum Engineering Technology Labs (QETLabs) have developed an algorithm that provides valuable insights into the physics underlying quantum systems – paving the way for significant advances in quantum computation and sensing, and potentially turning a new page in scientific investigation.

In physics, systems of particles and their evolution are described by mathematical models, requiring the successful interplay of theoretical arguments and experimental verification. Even more complex is the description of systems of particles interacting with each other at the quantum mechanical level, which is often done using a Hamiltonian model. The process of formulating Hamiltonian models from observations is made even harder by the nature of quantum states, which collapse when attempts are made to inspect them.

In the paper, Learning models of quantum systems from experiments, published in Nature Physics, quantum mechanics from Bristol’s QET Labs describe an algorithm which overcomes these challenges by acting as an autonomous agent, using machine learning to reverse engineer Hamiltonian models.

The team developed a new protocol to formulate and validate approximate models for quantum systems of interest. Their algorithm works autonomously, designing and performing experiments on the targeted quantum system, with the resultant data being fed back into the algorithm. It proposes candidate Hamiltonian models to describe the target system, and distinguishes between them using statistical metrics, namely Bayes factors.

Excitingly, the team were able to successfully demonstrate the algorithm’s ability on a real-life quantum experiment involving defect centres in a diamond, a well-studied platform for quantum information processing and quantum sensing.

The algorithm could be used to aid automated characterisation of new devices, such as quantum sensors. This development therefore represents a significant breakthrough in the development of quantum technologies.

“Combining the power of today’s supercomputers with machine learning, we were able to automatically discover structure in quantum systems. As new quantum computers/simulators become available, the algorithm becomes more exciting: first it can help to verify the performance of the device itself, then exploit those devices to understand ever-larger systems,” said Brian Flynn from the University of Bristol’s QETLabs and Quantum Engineering Centre for Doctoral Training.

“This level of automation makes it possible to entertain myriads of hypothetical models before selecting an optimal one, a task that would be otherwise daunting for systems whose complexity is ever increasing,” said Andreas Gentile, formerly of Bristol’s QETLabs, now at Qu & Co.

“Understanding the underlying physics and the models describing quantum systems, help us to advance our knowledge of technologies suitable for quantum computation and quantum sensing,” said Sebastian Knauer, also formerly of Bristol’s QETLabs and now based at the University of Vienna’s Faculty of Physics.

Anthony Laing, co-Director of QETLabs and Associate Professor in Bristol’s School of Physics, and an author on the paper, praised the team: “In the past we have relied on the genius and hard work of scientists to uncover new physics. Here the team have potentially turned a new page in scientific investigation by bestowing machines with the capability to learn from experiments and discover new physics. The consequences could be far reaching indeed.”

The next step for the research is to extend the algorithm to explore larger systems, and different classes of quantum models which represent different physical regimes or underlying structures.

Featured image: The nitrogen vacancy centre set-up, that was used for the first experimental demonstration of QMLA. © Gentile et al.

Paper: Gentile, A.A., Flynn, B., Knauer, S. et al. Learning models of quantum systems from experiments. Nat. Phys. (2021). Link when paper is live: https://dx.doi.org/10.1038/s41567-021-01201-7

Provided by University of Bristol

UChicago Scientists Harness Molecules Into Single Quantum State (Quantum)

Discovery could open new fields in quantum chemistry and technology

Researchers have big ideas for the potential of quantum technology, from unhackable networks to earthquake sensors. But all these things depend on a major technological feat: being able to build and control systems of quantum particles, which are among the smallest objects in the universe.

That goal is now a step closer with the publication of a new method by University of Chicago scientists. Published April 28 in Nature, the paper shows how to bring multiple molecules at once into a single quantum state–one of the most important goals in quantum physics.

“People have been trying to do this for decades, so we’re very excited,” said senior author Cheng Chin, a professor of physics at UChicago who said he has wanted to achieve this goal since he was a graduate student in the 1990s. “I hope this can open new fields in many-body quantum chemistry. There’s evidence that there are a lot of discoveries waiting out there.”

One of the essential states of matter is called a Bose-Einstein condensate: When a group of particles cooled to nearly absolute zero share a quantum state, the entire group starts behaving as though it were a single atom. It’s a bit like coaxing an entire band to march entirely in step while playing in tune–difficult to achieve, but when it happens, a whole new world of possibilities can open up.

Scientists have been able to do this with atoms for a few decades, but what they’d really like to do is to be able to do it with molecules. Such a breakthrough could serve as the underpinning for many forms of quantum technology.

But because molecules are larger than atoms and have many more moving parts, most attempts to harness them have dissolved into chaos. “Atoms are simple spherical objects, whereas molecules can vibrate, rotate, carry small magnets,” said Chin. “Because molecules can do so many different things, it makes them more useful, and at the same time much harder to control.”

Chin’s group wanted to take advantage of a few new capabilities in the lab that had recently become available. Last year, they began experimenting with adding two conditions.

The first was cooling the entire system down even further–down to 10 nanokelvins, a split hair above absolute zero. Then they packed the molecules into a crawl space so that they were pinned flat. “Typically, molecules want to move in all directions, and if you allow that, they are much less stable,” said Chin. “We confined the molecules so that they are on a 2D surface and can only move in two directions.”

The result was a set of virtually identical molecules–lined up with exactly the same orientation, the same vibrational frequency, in the same quantum state.

The scientists described this molecular condensate as like a pristine sheet of new drawing paper for quantum engineering. “It’s the absolute ideal starting point,” Chin said. “For example, if you want to build quantum systems to hold information, you need a clean slate to write on before you can format and store that information.”

So far, they’ve been able to link up to a few thousand molecules together in such a state, and are beginning to explore its potential.

“In the traditional way to think about chemistry, you think about a few atoms and molecules colliding and forming a new molecule,” Chin said. “But in the quantum regime, all molecules act together, in collective behavior. This opens a whole new way to explore how molecules can all react together to become a new kind of molecule.

“This has been a goal of mine since I was a student,” he added, “so we’re very, very happy about this result.”

The first author on the study was graduate student Zhendong Zhang; the other two authors were Shanxi University’s Liangchao Chen (formerly a visiting scholar at UChicago) and graduate student Kaixuan Yao.

Funding: National Science Foundation, Army Research Office, University of Chicago Materials Research Science and Engineering Center.

Featured image: Image of the molecules successfully pooled into a Bose-Einstein condensate. © Chin lab

Reference: Zhang, Z., Chen, L., Yao, KX. et al. Transition from an atomic to a molecular Bose–Einstein condensate. Nature 592, 708–711 (2021). https://doi.org/10.1038/s41586-021-03443-0

Provided by University of Chicago