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QT/ Electrons tango at the extreme quantum limit

Quantum news biweekly vol.13, 24th September — 8th October

TL;DR

  • A team led by the Department of Energy’s Oak Ridge National Laboratory has found a rare quantum material in which electrons move in coordinated ways, essentially “dancing.” Straining the material creates an electronic band structure that sets the stage for exotic, more tightly correlated behavior — akin to tangoing — among Dirac electrons, which are especially mobile electric charge carriers that may someday enable faster transistors. The results are published in the journal Science Advances.
  • Light has no mass, but Europe’s Large Hadron Collider (LHC) can convert light’s energy into massive particles. Physicists studied matter-generating collisions of light and showed the departure angle of their debris is subtly distorted by quantum interference patterns in the light prior to collision. Their findings will help physicists accurately interpret future experiments aimed at finding ‘new physics’ beyond the Standard Model.
  • Researchers have succeeded in developing an all-nitride superconducting qubit using epitaxial growth on a silicon substrate that does not use aluminum as the conductive material. This qubit uses niobium nitride (NbN) with a superconducting transition temperature of 16 K (-257 °C) as the electrode material, and aluminum nitride (AlN) for the insulating layer of the Josephson junction. It is a new type of qubit made of all-nitride materials grown epitaxially on a silicon substrate and free of any amorphous oxides, which are a major noise source. By realizing this new material qubit on a silicon substrate, long coherence times have been obtained: an energy relaxation time (T1) of 16 microseconds and a phase relaxation time (T2) of 22 microseconds as the mean values. This is about 32 times T1 and about 44 times T2 of nitride superconducting qubits grown on a conventional magnesium oxide substrate.
  • Quantum key distribution (QKD) is a method used for secure or secret key exchanges between two remote users. Using secure communication, cyberscientists ultimately aim to establish a global quantum network. Existing field tests suggest that such quantum networks are feasible. To achieve a practical quantum network, several challenges must be overcome including the realization of varied topologies at large scales, simple network maintenance and robustness to node failures. In a new report published on Science a research team in quantum physics, quantum information and interdisciplinary information sciences in China, presented a field operation of a quantum metropolitan area network with 46 nodes.
  • Quantum computers are gaining pace. They promise to provide exponentially more computing power for certain very tricky problems. They do this by exploiting the peculiar behaviour of quantum particles, such as photons of light. A team has now shown how to protect qubits from errors using photons in a silicon chip.
  • Researchers have found a way to make ‘single-crystal flake’ devices that are so thin and free of defects, they have the potential to outperform components used today in quantum computer circuits.
  • A new and much faster quantum cryptography protocol has been developed: Usually, quantum cryptography is done with photons that can be in two different states. Using eight different states, cryptographic keys can be generated much faster and with much more robustness against interference.
  • Quantum gases consisting of atoms are extremely suitable for observing quantum mechanical phenomena and making new types of quantum matter. In his Ph.D. research Mestrom was able to quantify the effects of three-particle collisions in those ultracold gases. With a new numerical method he was able to characterize and predict certain effects of these collisions.
  • 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

Correlated oxide Dirac semimetal in the extreme quantum limit

by Jong Mok Ok, Narayan Mohanta, Jie Zhang, Sangmoon Yoon, Satoshi Okamoto, Eun Sang Choi, Hua Zhou, Megan Briggeman, Patrick Irvin, Andrew R. Lupini, Yun-Yi Pai, Elizabeth Skoropata, Changhee Sohn, Haoxiang Li, Hu Miao, Benjamin Lawrie, Woo Seok Choi, Gyula Eres, Jeremy Levy, Ho Nyung Lee in Science Advances

A team led by the Department of Energy’s Oak Ridge National Laboratory has found a rare quantum material in which electrons move in coordinated ways, essentially “dancing.” Straining the material creates an electronic band structure that sets the stage for exotic, more tightly correlated behavior — akin to tangoing — among Dirac electrons, which are especially mobile electric charge carriers that may someday enable faster transistors. The results are published in the journal Science Advances.

“We combined correlation and topology in one system,” said co-principal investigator Jong Mok Ok, who conceived the study with principal investigator Ho Nyung Lee of ORNL. Topology probes properties that are preserved even when a geometric object undergoes deformation, such as when it is stretched or squeezed. “The research could prove indispensable for future information and computing technologies,” added Ok, a former ORNL postdoctoral fellow.

In conventional materials, electrons move predictably (for example, lethargically in insulators or energetically in metals). In quantum materials in which electrons strongly interact with each other, physical forces cause the electrons to behave in unexpected but correlated ways; one electron’s movement forces nearby electrons to respond.

To study this tight tango in topological quantum materials, Ok led the synthesis of an extremely stable crystalline thin film of a transition metal oxide. He and colleagues made the film using pulsed-laser epitaxy and strained it to compress the layers and stabilize a phase that does not exist in the bulk crystal. The scientists were the first to stabilize this phase.

Using theory-based simulations, co-principal investigator Narayan Mohanta, a former ORNL postdoctoral fellow, predicted the band structure of the strained material. “In the strained environment, the compound that we investigated, strontium niobate, a perovskite oxide, changes its structure, creating a special symmetry with a new electron band structure,” Mohanta said.

Different states of a quantum mechanical system are called “degenerate” if they have the same energy value upon measurement. Electrons are equally likely to fill each degenerate state. In this case, the special symmetry results in four states occurring in a single energy level.

“Because of the special symmetry, the degeneracy is protected,” Mohanta said. “The Dirac electron dispersion that we found here is new in a material.” He performed calculations with Satoshi Okamoto, who developed a model for discovering how crystal symmetry influences band structure.

Strain-induced Dirac metallic state in SrNbO3 thin films. (A and B) Octahedral distortion pattern for (A) cubic SrNbO3 (a0a0a0 in the Glazer notation) and (B) strained tetragonal SrNbO3 (a0a0c−). Epitaxial strain induces octahedral distortion. © Octahedral rotation-induced half-order superstructure diffraction peaks of (3/2 1/2 L/2) with L = 1, 3, 5 for fully strained (red, 7.2 nm, c− rotation) and fully relaxed (blue, 130 nm, c0 rotation) SrNbO3 thin films. r.l.u., reciprocal lattice unit. (D and E) Calculated electronic structure of (D) cubic SrNbO3 and (E) strained tetragonal SrNbO3. The red circle in plot (E) shows the Dirac point that appears near the Fermi level at the P point in the strained tetragonal phase. (F) Dirac dispersions near the P point within the tetragonal Brillouin zone. The larger Fermi velocity in the tetragonal phase, near the P point, would lead to higher carrier mobility and a favorable source of a nontrivial Berry phase in the presence of a magnetic field.

“Think of a quantum material under a magnetic field as a 10-story building with residents on each floor,” Ok posited. “Each floor is a defined, quantized energy level. Increasing the field strength is akin to pulling a fire alarm that drives all the residents down to the ground floor to meet at a safe place. In reality, it drives all the Dirac electrons to a ground energy level called the extreme quantum limit.”

Lee added:

“Confined here, the electrons crowd together. Their interactions increase dramatically, and their behavior becomes interconnected and complicated.” This correlated electron behavior, a departure from a single-particle picture, sets the stage for unexpected behavior, such as electron entanglement. In entanglement, a state Einstein called “spooky action at a distance,” multiple objects behave as one. It is key to realizing quantum computing.

“Our goal is to understand what will happen when electrons enter the extreme quantum limit, where we find phenomena we still don’t understand,” Lee said. “This is a mysterious area.”

Speedy Dirac electrons hold promise in materials including graphene, topological insulators and certain unconventional superconductors. ORNL’s unique material is a Dirac semimetal, in which electron valence and conduction bands cross and this topology yields surprising behavior. Ok led measurements of the Dirac semimetal’s strong electron correlations.

“We found the highest electron mobility in oxide-based systems,” Ok said. “This is the first oxide-based Dirac material reaching the extreme quantum limit.”

That bodes well for advanced electronics. Theory predicts that it should take about 100,000 tesla (a unit of magnetic measurement) for electrons in conventional semiconductors to reach the extreme quantum limit. The researchers took their strain-engineered topological quantum material to Eun Sang Choi of the National High Magnetic Field Laboratory at the University of Florida to see what it would take to drive electrons to the extreme quantum limit. There, he measured quantum oscillations showing the material would require only 3 tesla to achieve that.

Other specialized facilities allowed the scientists to experimentally confirm the behavior Mohanta predicted. The experiments occurred at low temperatures so that electrons could move around without getting bumped by atomic-lattice vibrations. Jeremy Levy’s group at the University of Pittsburgh and the Pittsburgh Quantum Institute confirmed quantum transport properties. With synchrotron x-ray diffraction, Hua Zhou at the Advanced Photon Source, a DOE Office of Science user facility at Argonne National Laboratory, confirmed that the material’s crystallographic structure stabilized in the thin-film phase yielded the unique Dirac band structure. Sangmoon Yoon and Andrew Lupini, both of ORNL, conducted scanning transmission electron microscopy experiments at ORNL that showed that the epitaxially grown thin films had sharp interfaces between layers and that the transport behaviors were intrinsic to strained strontium niobate.

“Until now, we could not fully explore the physics of the extreme quantum limit due to the difficulties in pushing all electrons to one energy level to see what would happen,” Lee said. “Now, we can push all the electrons to this extreme quantum limit by applying only a few tesla of magnetic field in a lab, accelerating our understanding of quantum entanglement.”

Observation of Forward Neutron Multiplicity Dependence of Dimuon Acoplanarity in Ultraperipheral Pb-Pb Collisions at sNN=5.02 TeV

by A. M. Sirunyan et al. in Physical Review Letters

Hot on the heels of proving an 87-year-old prediction that matter can be generated directly from light, Rice University physicists and their colleagues have detailed how that process may impact future studies of primordial plasma and physics beyond the Standard Model.

“We are essentially looking at collisions of light,” said Wei Li, an associate professor of physics and astronomy at Riceand co-author of the study published in Physical Review Letters.

“We know from Einstein that energy can be converted into mass,” said Li, a particle physicist who collaborates with hundreds of colleagues on experiments at high-energy particle accelerators like the European Organization for Nuclear Research’s Large Hadron Collider (LHC) and Brookhaven National Laboratory’s Relativistic Heavy Ion Collider(RHIC).

Accelerators like RHIC and LHC routinely turn energy into matter by accelerating pieces of atoms near the speed of light and smashing them into one another. The 2012 discovery of the Higgs particle at the LHC is a notable example. At the time, the Higgs was the final unobserved particle in the Standard Model, a theory that describes the fundamental forces and building blocks of atoms.

Impressive as it is, physicists know the Standard Model explains only about 4% of the matter and energy in the universe. Li said this week’s study, which was lead-authored by Rice postdoctoral researcher Shuai Yang, has implications for the search for physics beyond the Standard Model.

“There are papers predicting that you can create new particles from these ion collisions, that we have such a high density of photons in these collisions that these photon-photon interactions can create new physics beyond in the Standard Model,” Li said.

Yang said, “To look for new physics, one must understand Standard Model processes very precisely. The effect that we’ve seen here has not been previously considered when people have suggested using photon-photon interactions to look for new physics. And it’s extremely important to take that into account.”

The effect Yang and colleagues detailed occurs when physicists accelerate opposing beams of heavy ions in opposite directions and point the beams at one another. The ions are nuclei of massive elements like gold or lead, and ion accelerators are particularly useful for studying the strong force, which binds fundamental building blocks called quarks in the neutrons and protons of atomic nuclei. Physicists have used heavy ion collisions to overcome those interactions and observe both quarks and gluons, the particles quarks exchange when they interact via the strong force.

But nuclei aren’t the only things that collide in heavy ion accelerators. Ion beams also produce electric and magnetic fields that shroud each nuclei in the beam with its own cloud of light. These clouds move with the nuclei, and when clouds from opposing beams meet, individual particles of light called photons can meet head-on.

In a PRL study published in July, Yang and colleagues used data from RHIC to show photon-photon collisions produce matter from pure energy. In the experiments, the light smashups occurred along with nuclei collisions that created a primordial soup called quark-gluon plasma, or QGP.

“At RHIC, you can have the photon-photon collision create its mass at the same time as the formation of quark-gluon plasma,” Yang said. “So, you’re creating this new mass inside the quark-gluon plasma.”

Yang’s Ph.D. thesis work on the RHIC data published in PRL in 2018 suggested photon collisions might be affecting the plasma in a slight but measurable way. Li said this was both intriguing and surprising, because the photon collisions are an electromagnetic phenomena, and quark-gluon plasmas are dominated by the strong force, which is far more powerful than the electromagnetic force.

“To interact strongly with quark-gluon plasma, only having electric charge is not enough,” Li said. “You don’t expect it to interact very strongly with quark-gluon plasma.”

He said a variety of theories were offered to explain Yang’s unexpected findings.

“One proposed explanation is that the photon-photon interaction will look different not because of quark-gluon plasma, but because the two ions just get closer to each other,” Li said. “It’s related to quantum effects and how the photons interact with each other.”

If quantum effects had caused the anomalies, Yang surmised, they could create detectable interference patterns when ions narrowly missed one another but photons from their respective light clouds collided.

“So the two ions, they do not strike each other directly,” Yang said. “They actually pass by. It’s called an ultraperipheral collision, because the photons collide but the ions don’t hit each other.”

Theory suggested quantum interference patterns from ultraperipheral photon-photon collisions should vary in direct proportion to the distance between the passing ions. Using data from the LHC’s Compact Muon Solenoid (CMS)experiment, Yang, Li and colleagues found they could determine this distance, or impact parameter, by measuring something wholly different.

“The two ions, as they get closer, there’s a higher probability the ion can get excited and start to emit neutrons, which go straight down the beam line,” Li said. “We have a detector for this at CMS.”

Each ultraperipheral photon-photon collision produces a pair of particles called muons that typically fly from the collision in opposite directions. As predicted by theory, Yang, Li and colleagues found that quantum interference distorted the departure angle of the muons. And the shorter the distance between the near-miss ions, the greater the distortion.

Li said the effect arises from the motion of the colliding photons. Although each is moving in the direction of the beam with its host ion, photons can also move away from their hosts.

“The photons have motion in the perpendicular direction, too,” he said. “And it turns out, exactly, that that perpendicular motion gets stronger as the impact parameter gets smaller and smaller. “This makes it appear like something’s modifying the muons,” Li said. “It looks like one is going at a different angle from the other, but it’s really not. It’s an artifact of the way the photon’s motion was changing, perpendicular to the beam direction, before the collision that made the muons.”

Yang said the study explains most of the anomalies he previously identified. Meanwhile, the study established a novel experimental tool for controlling the impact parameter of photon interactions that will have far-reaching impacts.

“We can comfortably say that the majority came from this QED effect,” he said. “But that doesn’t rule out that there are still effects that relate to the quark-gluon plasma. This work gives us a very precise baseline, but we need more precise data. We still have at least 15 years to gather QGP data at CMS, and the precision of the data will get higher and higher.”

Enhanced coherence of all-nitride superconducting qubits epitaxially grown on silicon substrate

by Sunmi Kim, Hirotaka Terai, Taro Yamashita, Wei Qiu, Tomoko Fuse, Fumiki Yoshihara, Sahel Ashhab, Kunihiro Inomata, Kouichi Semba in Communications Materials

Researchers at the National Institute of Information and Communications Technology (NICT, President: TOKUDA Hideyuki, Ph.D.), in collaboration with researchers at the National Institute of Advanced Industrial Science and Technology (AIST, President: Dr. ISHIMURA Kazuhiko) and the Tokai National Higher Education and Research System Nagoya University (President: Dr. MATSUO Seiichi) have succeeded in developing an all-nitride superconducting qubit using epitaxial growth on a silicon substrate that does not use aluminum as the conductive material. This qubit uses niobium nitride (NbN) with a superconducting transition temperature of 16 K (-257 °C) as the electrode material, and aluminum nitride (AlN) for the insulating layer of the Josephson junction. It is a new type of qubit made of all-nitride materials grown epitaxially on a silicon substrate and free of any amorphous oxides, which are a major noise source. By realizing this new material qubit on a silicon substrate, long coherence times have been obtained: an energy relaxation time (T1) of 16 microseconds and a phase relaxation time (T2) of 22 microseconds as the mean values. This is about 32 times T1 and about 44 times T2 of nitride superconducting qubits grown on a conventional magnesium oxide substrate.

By using niobium nitride as a superconductor, it is possible to construct a superconducting quantum circuit that operates more stably, and it is expected to contribute to the development of quantum computers and quantum nodes as basic elements of quantum computation. Researchers will continue to work on optimizing the circuit structure and fabrication process, and will proceed with research and development to further extend the coherence time and realize large-scale integration.

Toward the coming future Society 5.0, there are limits to the performance improvement of semiconductor circuits that have supported the information society so far, and expectations for quantum computers are rising as a new information processing paradigm that breaks through such limits. However, the quantum superposition state, which is indispensable for the operation of a quantum computer, is easily destroyed by various disturbances (noise), and it is necessary to properly eliminate these effects.

All-nitride C-shunt flux qubit consisting of epitaxially grown NbN/AlN/NbN Josephson junctions on Si substrate. a a photograph of the qubit chip mounted into the sample package, b Laser scanning microscope image of the capacitively shunted flux qubit coupled to a half-wavelength (λ/2) CPW resonator made of NbN/TiN on a Si substrate. The inset shows a magnified image of a false-colored flux qubit structure with three NbN/AlN/NbN Josephson junctions (marked as JJ1, JJ2, and JJ3). c Scanning electron microscopy images corresponding to the three JJs taken after the qubit measurements. d The thickness profile of qubit taken from the laser scanning microscope system. The displayed scales are in μm. e Cross-sectional schematics of the part indicated by the blue star and dashed line in b. The JJ parts are marked by the orange dotted ellipses.

Since superconducting qubits are solid-state elements, they have excellent design flexibility, integration, and scalability, but they are easily affected by various disturbances in their surrounding environment. The challenge is how to extend the coherence time, which is the lifetime of quantum superposition states. Various efforts are being made by research institutes around the world to overcome this problem, and most of them use aluminum (Al) and aluminum oxide film (AlOx) as superconducting qubit materials. However, amorphous aluminum oxide, which is often used as an insulating layer, is a concern as a noise source, and it was essential to study materials that could solve this problem.

As an alternative to aluminum and amorphous aluminum oxide with a superconducting transition temperature TC of 1 K (-272 °C), epitaxially grown niobium nitride (NbN) with a TC of 16 K (-257 °C), NICT has been developing superconducting qubits using NbN / AlN / NbN all-nitride junctions, focusing on aluminum nitride (AlN) as an insulating layer.

In order to realize a NbN / AlN / NbN Josephson junction (epitaxial junction) in which the crystal orientation is aligned up to the upper electrode, it was necessary to use a magnesium oxide (MgO) substrate whose crystal lattice constants are relatively close to those of NbN. However, MgO has a large dielectric loss, and the coherence time of the superconducting quantum bit using the NbN / AlN / NbN junction on the MgO substrate was only about 0.5 microseconds.

NICT has succeeded in realizing NbN / AlN / NbN epitaxial Josephson junctions using titanium nitride (TiN) as a buffer layer on a silicon (Si) substrate with a smaller dielectric loss. This time, using this junction fabrication technology, we designed, fabricated, and evaluated a superconducting qubitthat uses NbN as the electrode material and AlN as the insulating layer of the Josephson junction.

The quantum circuit is fabricated on a silicon substrate so that the microwave cavity and the qubit can be coupled and interact with each other. From the transmission measurement of the microwave characteristics of the resonator weakly coupled to the qubit under small thermal fluctuation at the extremely low temperature of 10 mK, researchers achieved an energy relaxation time (T1) of 18 microseconds and a phase relaxation time (T2) of 23 microseconds. The mean values for 100 measurements are T1=16 microseconds and T2= 22 microseconds. This is an improvement of about 32 times for T1 and about 44 times for T2 compared to the case of superconducting qubits on MgO substrates.

For this result, they did not use conventional aluminum and aluminum oxide for the Josephson junction, which is the heart of superconducting qubits. Scientists have succeeded in developing a nitride superconducting qubit that has a high superconducting critical temperature TC and excellent crystallinity due to epitaxial growth. These two points have great significance. In particular, it is the first time that anyone in the world has succeeded in observing coherence times in the tens of microseconds from nitride superconducting qubits by reducing dielectric loss by epitaxially growing them on a Si substrate. The superconducting qubit of this nitride is still in the early stages of development, and they believe that it is possible to further improve the coherence time by optimizing the design and fabrication process of the qubit.

Error-protected qubits in a silicon photonic chip

by Caterina Vigliar, Stefano Paesani, Yunhong Ding, Jeremy C. Adcock, Jianwei Wang, Sam Morley-Short, Davide Bacco, Leif K. Oxenløwe, Mark G. Thompson, John G. Rarity, Anthony Laing in Nature Physics

A team of researchers from Bristol’s Quantum Engineering and Technology Labs (QETLabs) has shown how to protect qubits from errors using photons in a silicon chip.

Quantum computers are gaining pace. They promise to provide exponentially more computing power for certain very tricky problems. They do this by exploiting the peculiar behaviour of quantum particles, such as photons of light.

However, quantum states of particles are very fragile. The quantum bits, or qubits, that underpin quantum computing pick up errors very easily and are damaged by the environment of the everyday world. Fortunately, we know in principle how to correct for these errors.

Quantum error correcting codes are a method to protect, or to nurture, qubits, by embedding them in a more robust entangled state of many particles. Now a team led by researchers at Bristol’s Quantum Engineering and Technology Laboratories (QETLabs) has demonstrated this using a quantum photonic chip.

The team showed how large states of entangled photons can contain individual logical qubits and protect them from the harmful effects of the classical world. The Bristol-led team included researchers from DTU in Copenhagen who fabricated the chip.

Dr Caterina Vigliar, first author on the work, said: “The chip is really versatile. It can be programmed to deliver different kinds of entangled states called graphs. Each graph protects logical quantum bits of information from different environmental effects.”

Anthony Laing, co-Director of QETLabs, and an author on the work said: “Finding ways to efficiently deliver large numbers of error protected qubits is key to one day delivering quantum computers.”

Superconducting Quantum Interference in Twisted van der Waals Heterostructures

by Liam S. Farrar, Aimee Nevill, Zhen Jieh Lim, Geetha Balakrishnan, Sara Dale, Simon J. Bending in Nano Letters

Researchers at the University of Bath in the UK have found a way to make ‘single-crystal flake’ devices that are so thin and free of defects, they have the potential to outperform components used today in quantum computer circuits.

The team from the university’s Department of Physics made its discovery while exploring the junction between two layers of the superconductor niobium diselenide (NbSe?) after these layers had been cleaved apart, twisted about 30 degrees with respect to one another, then stamped back together. In cleaving, twisting and recombining the two layers, the researchers were able to build a Superconducting Quantum Interferometer Device (SQUID) — an extremely sensitive sensor used to measure incredibly tiny magnetic fields.

SQUIDs have a wide range of important applications in areas that include healthcare (as seen in cardiology and magnetoencephalography — a test that maps brain function) and mineral exploration.

SQUIDS are also the building blocks of today’s commercial quantum computers — machines that perform certain computational tasks much more rapidly than classical computers. Quantum computing is still in its infancy but in the next decade, it is likely to transform the problem-solving capacity of companies and organisations across many sectors — for instance by fast-tracking the discovery of new drugs and materials.

“Due to their atomically perfect surfaces, which are almost entirely free of defects, we see potential for our crystalline flakes to play a significant role in building quantum computers of the future,” said Professor Simon Bending, who carried out the research together with his PhD student Liam Farrar. “Also, SQUIDs are ideal for studies in biology — for instance, they are now being used to trace the path of magnetically-labelled drugs through the intestine — so we’re very excited to see how our devices could be developed in this field too.”

As Professor Bending is quick to point out, however, his work on SQUIDs made using NbSe? flakes is very much at the start of its journey.

“This is a completely new and unexplored approach to making SQUIDs and a lot of research will still have to done before these applications become a reality,” he said.

The flakes from which the Bath superconductors are fabricated are extremely thin single crystals (10,000 times thinner than a human hair) that bend easily, which also makes them suitable for incorporation into flexible electronics, as used in computer keyboards, optical displays, solar cells and various automotive components.

Because the bonds between layers of NbSe? are so weak, cleaved flakes — with their perfectly flat, defect-free surfaces — create atomically sharp interfaces when pushed back together again. This makes them excellent candidates for the components used in quantum computing.

While this is not the first time NbSe₂ layers have been stamped together to create a weak superconducting link, this is the first demonstration of quantum interference between two such junctions patterned in a pair of twisted flakes. This quantum interference has allowed the researchers to modulate the maximum supercurrent that can flow through their SQUIDs by applying a small magnetic field, creating an extremely sensitive field sensor. They were also able to show that the properties of their devices could be systematically tuned by varying the twist angle between the two flakes.

(a) Schematic of the device fabrication method. (i) Au contacts are deposited onto a Si/SiO2 substrate. (ii) A single exfoliation is made of a bulk NbSe2 crystal, and the resulting flakes are transferred onto a PDMS stamp. (iii) The PDMS is brought into contact with the substrate, which is tilted at a small angle. The PDMS is slowly pressed into the substrate until the boundary of the PDMS-substrate contact region lies beyond one of the flakes. (iv) The PDMS is then retracted, leaving the first flake deposited onto the Au contacts. The substrate is then rotated such that the crystallographic axes of the two NbSe2 flakes are now misaligned by an angle θ. (v) The second flake is positioned above the first, and the PDMS is brought into contact with the substrate. (vi) The PDMS is now retracted, leaving the second flake contacting both the first flake and the Au contacts. (b) Optical micrograph of two NbSe2 flakes on a PDMS stamp. © Optical micrograph of the two overlapping NbSe2 flakes on SiO2/Si. Here one of the flakes (labeled #2) has been rotated by 60°. (d) Temperature-dependence of the normalized resistance R(T)/R(294 K) for the two NbSe2 flakes and the overlapping junction region between them. The inset shows an expanded view of the superconducting transitions.

Pathways for Entanglement-Based Quantum Communication in the Face of High Noise

by Xiao-Min Hu, Chao Zhang, Yu Guo, Fang-Xiang Wang, Wen-Bo Xing, Cen-Xiao Huang, Bi-Heng Liu, Yun-Feng Huang, Chuan-Feng Li, Guang-Can Guo, Xiaoqin Gao, Matej Pivoluska, Marcus Huber in Physical Review Letters

Quantum cryptography is one of the most promising quantum technologies of our time: Exactly the same information is generated at two different locations, and the laws of quantum physics guarantee that no third party can intercept this information. This creates a code with which information can be perfectly encrypted.

The team of Prof. Marcus Huber from the Atomic Institute of TU Wien developed a new type of quantum cryptography protocol, which has now been tested in practice in cooperation with Chinese research groups: While up to now one normally used photons that can be in two different states, the situation here is more complicated: Eight different paths can be taken by each of the photons. As the team has now been able to show, this makes the generation of the quantum cryptographic key faster and also significantly more robust against interference.

“There are many different ways of using photons to transmit information,” says Marcus Huber. “Often, experiments focus on their photons’ polarisation. For example, whether they oscillate horizontally or vertically — or whether they are in a quantum-mechanical superposition state in which, in a sense, they assume both states simultaneously. Similar to how you can describe a point on a two-dimensional plane with two coordinates, the state of the photon can be represented as a point in a two-dimensional space.”

But a photon can also carry information independently of the direction of polarization. One can, for example, use the information about which path the photon is currently travelling on. This is exactly what has now been exploited: “A laser beam generates photon pairs in a special kind of crystal. There are eight different points in the crystal where this can happen,” explains Marcus Huber. Depending on the point at which the photon pair was created, each of the two photons can move along eight different paths — or along several paths at the same time, which is also permitted according to the laws of quantum theory.

These two photons can be directed to completely different places and analysed there. One of the eight possibilities is measured, completely at random — but as the two photons are quantum-physically entangled, the same result is always obtained at both places. Whoever is standing at the first measuring device knows what another person is currently detecting at the second measuring device — and no one else in the universe can get hold of this information.

“The fact that we use eight possible paths here, and not two different polarisation directions as it is usually the case, makes a big difference,” says Marcus Huber. “The space of possible quantum states becomes much larger. The photon can no longer be described by a point in two dimensions, mathematically it now exists in eight dimensions.”

This has several advantages: First, it allows more information to be generated: At 8307 bits per second and over 2.5 bits per photon pair, a new record has been set in entanglement-based quantum cryptography key generation. And secondly, it can be shown that this makes the process less susceptible to interference.

“With all quantum technologies, you have to deal with the problem of decoherence,” says Marcus Huber. “No quantum system can be perfectly shielded from disturbances. But if it comes into contact with disturbances, then it can lose its quantum properties very easily: The quantum entanglements are destroyed.” Higher-dimensional quantum states, however, are less likely to lose their entanglement even in the presence of disturbances.

Moreover, sophisticated quantum error-correction mechanisms can be used to compensate for the influence of external perturbations. “In the experiments, additional light was switched on in the laboratory to deliberately cause disturbances — and the protocol still worked,” says Marcus Huber. “But only if we actually used eight different paths. We were able to show that with a mere two-dimensional encoding a cryptographic key can no longer be generated in this case.”

In principle, it should be possible to improve the new, faster and more reliable quantum cryptography protocol further by using additional degrees of freedom or an even larger number of different paths. “However, this not only increases the space of possible states, it also becomes increasingly difficult at some point to read out the states correctly,” says Marcus Huber. “We seem to have found a good compromise here, at least within the range of what is currently technically possible.”

Implementation of a 46-node quantum metropolitan area network

by Teng-Yun Chen et al. in npj Quantum Information

Quantum key distribution (QKD) is a method used for secure or secret key exchanges between two remote users. Using secure communication, cyberscientists ultimately aim to establish a global quantum network. Existing field tests suggest that such quantum networks are feasible. To achieve a practical quantum network, several challenges must be overcome including the realization of varied topologies at large scales, simple network maintenance and robustness to node failures. In a new report now published on Science Advances, Teng-Yun Chen and a research team in quantum physics, quantum information and interdisciplinary information sciences in China, presented a field operation of a quantum metropolitan area network with 46 nodes. They realized diverse topological structures and ran the network for 31 months via standard equipment. They then realized QKD pairing and key management for secure communications including real-time voice telephone, text messaging and file transmission with one-time pad encryption to support 11 pairs of users to make simultaneous audio calls. The technique can be combined with an intercity quantum backbone and via ground-satellite links to form a global quantum network.

In this work, Chen et al. constructed a 46-node quantum metropolitan-area network throughout the city of Hefei. Quantum key distribution (QKD) ultimately aims to construct a global quantum network where communication traffics have information-theoretic security guarantees. A global QKD network can maintain two types of links including the ground network and satellite network, where the ground network can be further divided into a backbone, metropolitan and access networks to cover intercity distance and fiber-to-home distances. Researchers have studied the feasibility of the QKD between two users through long-distance free space, telecom fibers and simulated ground-satellite links. Examples of the field tests of QKD networks that are already realized include a three-user network by DARPA, a six-node network in Europe, the SwissQuantum network as well as a mesh-type six-node network in Tokyo. The satellite network provided a promising method to realize intercontinental, secure communication as a result of low transmission attenuation in space while serving as a trusted relay to connect remote user nodes or subnetworks. Scientists had recently implemented a large-scale satellite network containing four metropolitan-area networks, a backbone network and two satellite-ground links. However, these QKD experiments and networks are still preliminary, the team therefore addressed the challenges surrounding the realization of a large-scale practical QKD network.

A schematic for the QKD set-up. There are four laser sources in the transmitter emitting four corresponding polarization states in the BB84 protocol. The polarization is modulated via the PBS and the PC, and the average light intensity is modulated via the attenuator. Each laser produces three light pulses with different intensities including signal, decoy and vacuum states. The signal and decoy states contain mean photon numbers of 0.6 and 0.2 per pulse, respectively, and the ratio between the signal, decoy, and vacuum states is 6:1:1. The optical misalignment is less than 0.5%. In the detection side, a four-channel InGaAs single-photon detector is integrated with the following parameters. The detection efficiency is 10%, the dark count is 10−6, the dead time is 2 μs, the afterpulse probability is less than 0.5% and the effective gate width is 500 ps. The receiver detects the light signal with the PC as a polarization feedback. The Cir is used to realize transmission and reception of light signals simultaneously. BS: beam splitter; PBS: polarizing beam splitter; PC: polarization controller; Att: attenuator; Cir: circulator.

Chen et al. built a 46-node quantum metropolitan-area network to connect 40 user nodes, three trusted relays and three optical switches, throughout Hefei. The network covered the entire urban area and connected several organizations within the city districts including governments, banks, hospitals, and research universities. They first reviewed the basic topological structures in a network where the most robust method used a fully connected topology where each user was directly connected to every other user in the network. The type of network did not require the users to trust one another. User nodes can also be connected via a central switch in a star-like network, where two users can build secure keys with a sufficient number of trusted relays. For instance, the Shanghai-Beijing backbone used this technique; however, the disadvantage is that the users must trust the relay. Chen et al. constructed three subnetworks in USTC, QuantumCTek and the City Library that are distributed 15 km apart.

Twenty-two users simultaneously make calls with QKD protocols. The green areas represent the duration over which users make calls.

The researchers realized two basic types of topological connection structures including the full connection between three subnetworks and star-like connections for local access networks. During the experiments, the team used an optical switch known as a trusted node at the center of the star-like subnetwork. Using the trusted node, they assigned classical keys between users to function as a classical router, while the all-pass optical switches acted as quantum routers to redistribute quantum signals. Based on the setup, any two users could communicate directly without interfering with other users. Chen et al. further developed a type of switch module comprising four input and eight output ports, the other contained a 16-port switch that enabled eight pairs of users to communicate simultaneously. The team used a protocol to generate secret keys between directly connected users and trusted relays. If one user had a quantum transmitter and the other had a quantum receiver, they could generate keys. The platform therefore contained two types of users; those directly connected to a switch containing both a transmitter and receiver, and users directly connected to a trusted relay with only a quantum transmitter. As a result, the scientists used two types of equipment; one to transmit signals and another to transmit and receive signals at the same time. After basis reconciliation and error correction, they standardized the QKD equipment to greatly reduce the number of devices used.

Chen et al. developed a key management process to allow users to generate keys in high priority. To accomplish this, they designed a switching strategy based on the number of keys stored in the local memories for the users. They then connected a 16-port optical switch to 16 users to obtain a total of 120 possible key-pairing schemes by which two users could be connected for the QKD process for a switching time ranging from 10 to 60 minutes. To join the network, a new user first had to send a heartbeat frame from their QKD device to the key management server for authentication to then que the device to generate keys. For security, the team followed the standard decoy-state BB84 security analysis and generated the secret key rate of the BB84 quantum key distribution protocol. Based on the application of the network, the users made use of the generated secure keys to transfer information confidently. Using the network, Chen et al. transmitted encrypted information, including real-time voice telephone, instant messaging, and digital files with the one-time pad encryption method. The total delay in the encryption process was less than 50 µs. When the researchers tested the capacity of the network for 50 minutes, all 22 users could simultaneously make calls for six minutes, within the quantum network. To test the stability and robustness of the system, they continuously ran the network for 31 months.

The key rates versus time for some representative links. (a) The key rates between the three trusted relays. (b) The key rates between trusted relay and user. In the robustness test, 11 user nodes have continuously run for 31 months. The key rates are recorded every 30 s and taken average over a month. The detailed key rates are given in Supplementary Tables V and VI.

MISC

  • Quantum gases consisting of atoms are extremely suitable for observing quantum mechanical phenomena and making new types of quantum matter. In his Ph.D. research Mestrom was able to quantify the effects of three-particle collisions in those ultracold gases. With a new numerical method he was able to characterize and predict certain effects of these collisions. He defended his Ph.D. on September 27 at the department of Applied Physics.

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