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QT/ Exotic quantum particles — less magnetic field required

Quantum news biweekly vol.18, 14th December — 28th December

TL;DR

  • Exotic quantum particles and phenomena are like the world’s most daring elite athletes. Like the free solo climbers who scale impossibly steep cliff faces without a rope or harness, only the most extreme conditions will entice them to show up. For exotic phenomena like superconductivity or particles that carry a fraction of the charge of an electron, that means extremely low temperatures or extremely high magnetic fields.
  • After the “first quantum revolution” — the development of devices such as lasers and the atomic clock — the “second quantum revolution” is currently in full swing. Experts from all over the world are developing fundamentally new technologies based on quantum physics. One key application is quantum communication, where information is written and sent in light. For many applications making use of quantum effects, the light has to be in a certain state — namely a single-photon state. But what is the best way of generating such single-photon states? In the PRX Quantum journal, researchers have now proposed an entirely new way of preparing quantum systems in order to develop components for quantum technology.
  • Technologies that take advantage of novel quantum mechanical behaviors are likely to become commonplace in the near future. These may include devices that use quantum information as input and output data, which require careful verification due to inherent uncertainties. The verification is more challenging if the device is time-dependent when the output depends on past inputs. For the first time, researchers using machine learning dramatically improved the efficiency of verification for time-dependent quantum devices by incorporating a certain memory effect present in these systems.
  • Frequency microcombs are specialized light sources that can function as light-based clocks, rulers and sensors to measure time, distance and molecular composition with high precision. New research presents a novel tool for investigating the quantum characteristics of these sources.
  • Quantum effects in superconductors could give semiconductor technology a new twist. Researchers have identified a composite material that could integrate quantum devices into semiconductor technology, making electronic components significantly more powerful.
  • Which factors determine how fast a quantum computer can perform its calculations? Physicists at the University of Bonn and the Technion — Israel Institute of Technology have devised an elegant experiment to answer this question. The results of the study are published in the journal Science Advances.
  • What does a quantum computer have in common with a top draft pick in sports? Both have attracted lots of attention from talent scouts. Quantum computers, experimental machines that can perform some tasks faster than supercomputers, are constantly evaluated, much like young athletes, for their potential to someday become game-changing technology.
  • Researchers found it’s possible to design a sensor, based on quantum physics, that could detect the SARS-CoV-2 virus. The approach may offer faster, cheaper, and more accurate detection of Covid-19, including new variants.
  • Scientists have experimentally confirmed an unusual quantum phenomenon for the motion of luminescent electronic quasiparticles in atomically-thin semiconductors.
  • Peers dispute the claim that tardigrades were entangled with qubits.
  • 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

Fractional Chern insulators in magic-angle twisted bilayer graphene

by Xie, Y., Pierce, A.T., Park, J.M. et al. in Nature

Exotic quantum particles and phenomena are like the world’s most daring elite athletes. Like the free solo climbers who scale impossibly steep cliff faces without a rope or harness, only the most extreme conditions will entice them to show up. For exotic phenomena like superconductivity or particles that carry a fraction of the charge of an electron, that means extremely low temperatures or extremely high magnetic fields.

But what if you could get these particles and phenomena to show up under less extreme conditions? Much has been made of the potential of room-temperature superconductivity, but generating exotic fractionally charged particles at low-to-zero magnetic field is equally important to the future of quantum materials and applications, including new types of quantum computing.

Now, a team of researchers from Harvard University led by Amir Yacoby, Professor of Physics and of Applied Physics at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and Ashvin Vishwanath, Professor of Physics in the Department of Physics, in collaboration with Pablo Jarillo-Herrero at the Massachusetts Institute of Technology, have observed exotic fractional states at low magnetic field in twisted bilayer graphene for the first time.

“One of the holy grails in the field of condensed matter physics is getting exotic particles with low to zero magnetic field,” said Yacoby, senior author of the study. “There have been theoretical predictions that we should be able to see these bizarre particles with low to zero magnetic field, but no one has been able to observe it until now.”

Incompressible states with fractional quantum numbers in MATBG. a, Local inverse compressibility dµ/dn measured as a function of magnetic field B and electrons per moiré unit cell ν. b, Wannier diagram identifying the incompressible peaks present in a. Black lines correspond to ChIs and integer quantum Hall (IQH) states; green lines correspond to correlated insulators (CIs) emanating with nonzero integer s and t = 0; blue lines correspond to CDWs with integer t = 0 and fractional s; yellow lines correspond to SBCIs with nonzero integer t and fractional s; and orange lines correspond to FCIs with fractional t and fractional s. Grey shaded regions correspond to the gaps to the remote bands.

The researchers were interested in a specific exotic quantum state known as fractional Chern insulators. Chern insulators are topological insulators, meaning they conduct electricity on their surface or edge, but not in the middle.

In a fractional Chern insulator, electron interactions form what’s known as quasiparticles, a particle that emerges from complex interactions between large numbers of other particles. Sound, for example, can be described as a quasiparticle because it emerges from the complex interactions of particles in a material. Like fundamental particles, quasiparticles have well defined properties like mass and charge.

In fractional Chern insulators, electron interactions are so strong within the material that quasiparticles are forced to carry a fraction of the charge of normal electrons. These fractional particles have bizarre quantum properties that could be used to create robust quantum bits that are extremely resilient to outside interference.

To build their insulator, the researchers used two sheets of graphene twisted together at the so-called magic angle. Twisting unlocks new and different properties in graphene, including superconductivity, as first discovered by Jarillo-Herrero’s group at MIT, and states known as Chern bands, which hold great potential to generate fractional quantum states, as shown theoretically by Vishwanath’s group at Harvard.

Think of these Chern bands like buckets that fill up with electrons.

“In previous studies, you needed a large magnetic field in order to generate these buckets, which are the topological building blocks you need to get these exotic fractional particles,” said Andrew T. Pierce, a graduate student in Yacoby’s group and co-first author of the paper. “But magic-angle twist bilayer graphene already has these useful topological units built in at zero magnetic field.”

To generate fractional states, the researchers need to fill the buckets a fraction of the way with electrons. But here’s the hitch: for this to work, all the electrons in a bucket must have nearly the same properties. In twisted bilayer graphene, they don’t. In this system, electrons have different levels of a property known as the Berry curvature, which causes each electron to experience a magnetic field tied to its particular momentum. (It’s more complicated than that, but what isn’t in quantum physics?)

When filling up the buckets, the electrons’ Berry curvature needs to be evened out for the fractional Chern insulator state to appear.

That’s where a small applied magnetic field comes in.

“We showed that we can apply a very small magnetic field to evenly distribute Berry curvature among electrons in the system, allowing us to observe a fractional Chern insulator in the twisted bilayer graphene,” said Yonglong Xie, a postdoctoral fellow at SEAS and co-first author of the paper. “This research sheds light on the importance of the Berry curvature to realize fractionalized exotic states and could point to alterative platforms where Berry curvature isn’t as heterogeneous as it is in twisted graphene.”

“Twisted bilayer graphene is the gift that keeps on giving and this discovery of fractional Chern insulators is arguably one of the most significant advances in the field,” said Vishwanath, senior author of the study. “It is astonishing to think that this wonder material is ultimately made of the same stuff as your pencil tip. “

“The discovery of low magnetic field fractional Chern insulators in magic angle twisted bilayer graphene opens a new chapter in the field of topological quantum matter,” said Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at MIT and senior author of the study. “It offers the realistic prospect of coupling these exotic states with superconductivity, possibly enabling the creation and control of even more exotic topological quasiparticles known as anyons.”

Swing-Up of Quantum Emitter Population Using Detuned Pulses

by Thomas K. Bracht, Michael Cosacchi, Tim Seidelmann, Moritz Cygorek, Alexei Vagov, V. Martin Axt, Tobias Heindel, Doris E. Reiter in PRX Quantum

After the “first quantum revolution” — the development of devices such as lasers and the atomic clock — the “second quantum revolution” is currently in full swing. Experts from all over the world are developing fundamentally new technologies based on quantum physics. One key application is quantum communication, where information is written and sent in light. For many applications making use of quantum effects, the light has to be in a certain state — namely a single photon state. But what is the best way of generating such single photon states? In the PRX Quantum journal, researchers from Münster, Bayreuth and Berlin (Germany) have now proposed an entirely new way of preparing quantum systems in order to develop components for quantum technology.

In the experts’ view it is highly promising to use quantum systems for generating single photon states. One well-known example of such a quantum system is a quantum dot. This is a semiconductor structure, just a few nanometres in size. Quantum dots can be controlled using laser pulses. Although quantum dots have properties similar to those of atoms, they are embedded in a crystal matrix, which is often more practical for applications.

“Quantum dots are excellent for generating single photons, and that is something we are already doing in our labs almost every day,” says Dr. Tobias Heindel, who runs an experimental lab for quantum communication at the Technical University of Berlin. “But there is still much room for improvement, especially in transferring this technology from the lab to real applications,” he adds.

The Bloch-vector dynamics for (a) off-resonant Rabi oscillations and (b) the SUPER scheme. © The dynamics of the excited-state occupation for the SUPER scheme under (d) a time-dependent detuning switching between Δhigh=−5.47Ω0 and Δlow=−2.74Ω0. The pulse has a duration of 4.95π/Ω0.

One difficulty that has to be overcome is to separate the generated single photons from the exciting laser pulse. In their work, the researchers propose an entirely new method of solving this problem.

“The excitation exploits a swing-up process in the quantum system,” explains Münster University’s Thomas Bracht, the lead author of the study. “For this, we use one or more laser pulses which have frequencies which differ greatly from those in the system. This makes spectral filtering very easy.”

Scientists define the “swing-up process” as a particular behaviour of the particles excited by the laser light in the quantum system — the electrons or, to be more precise, electron-hole pairs (excitons). Here, laser light from two lasers is used which emit light pulses almost simultaneously. As a result of the interaction of the pulses with one another, a rapid modulation occurs, and in each modulation cycle, the particle is always excited a little, but then dips towards the ground state again. In this process, however, it does not fall back to its previous level, but is excited more strongly with each swing up until it reaches the maximum state. The advantage of this method is that the laser light does not have the same frequency as the light emitted by the excited particles. This means that photons generated from the quantum dot can be clearly assigned.

The team simulated this process in the quantum system, thus providing guidelines for experimental implementation.

“We also explain the physics of the swing-up process, which helps us to gain a better understanding of the dynamics in the quantum system,” says associate professor Dr. Doris Reiter, who led the study.

In order to be able to use the photons in quantum communication, they have to possess certain properties. In addition, any preparation of the quantum system should not be negatively influenced by environmental processes or disruptive influences. In quantum dots, especially the interaction with the surrounding semiconductor material is often a big problem for such preparation schemes.

“Our numerical simulations show that the properties of the photons generated after the swing-up process are comparable with the results of established methods for generating single photons, which are less practical,” adds Prof. Martin Axt, who heads the team of researchers from Bayreuth.

The study constitutes theoretical work. As a result of the collaboration between theoretical and experimental groups, however, the proposal is very close to realistic experimental laboratory conditions, and the authors are confident that an experimental implementation of the scheme will soon be possible. With their results, the researchers are taking a further step towards developing the quantum technologies of tomorrow.

Learning Temporal Quantum Tomography

by Quoc Hoan Tran, Kohei Nakajima in Physical Review Letters

Technologies that take advantage of novel quantum mechanical behaviors are likely to become commonplace in the near future. These may include devices that use quantum information as input and output data, which require careful verification due to inherent uncertainties. The verification is more challenging if the device is time dependent when the output depends on past inputs. For the first time, researchers using machine learning dramatically improved the efficiency of verification for time-dependent quantum devices by incorporating a certain memory effect present in these systems.

Quantum computers make headlines in the scientific press, but these machines are considered by most experts to still be in their infancy. A quantum internet, however, may be a little closer to the present. This would offer significant security advantages over our current internet, amongst other things. But even this will rely on technologies that have yet to see the light of day outside the lab. While many fundamentals of the devices that can create our quantum internet may have been worked out, there are many engineering challenges in order to realize these as products. But much research is underway to create tools for the design of quantum devices.

Postdoctoral researcher Quoc Hoan Tran and Associate Professor Kohei Nakajima from the Graduate School of Information Science and Technology at the University of Tokyo have pioneered just such a tool, which they think could make verifying the behavior of quantum devices a more efficient and precise undertaking than it is at present. Their contribution is an algorithm that can reconstruct the workings of a time-dependent quantum device by simply learning the relationship between the quantum inputs and outputs. This approach is actually commonplace when exploring a classical physical system, but quantum information is generally tricky to store, which usually makes it impossible.

“The technique to describe a quantum system based on its inputs and outputs is called quantum process tomography,” said Tran. “However, many researchers now report that their quantum systems exhibit some kind of memory effect where present states are affected by previous ones. This means that a simple inspection of input and output states cannot describe the time-dependent nature of the system. You could model the system repeatedly after every change in time, but this would be extremely computationally inefficient. Our aim was to embrace this memory effect and use it to our advantage rather than use brute force to overcome it.”

Tran and Nakajima turned to machine learning and a technique called quantum reservoir computing to build their novel algorithm. This learns patterns of inputs and outputs that change over time in a quantum system and effectively guesses how these patterns will change, even in situations the algorithm has not yet witnessed. As it does not need to know the inner workings of a quantum system as a more empirical method might, but only the inputs and outputs, the team’s algorithm can be simpler and produce results faster as well.

“At present, our algorithm can emulate a certain kind of quantum system, but hypothetical devices may vary widely in their processing ability and have different memory effects. So the next stage of research will be to broaden the capabilities of our algorithms, essentially making something more general purpose and thus more useful,” said Tran. “I am excited by what quantum machine learning methods could do, by the hypothetical devices they might lead to.”

Quantum optics of soliton microcombs

by Melissa A. Guidry, Daniil M. Lukin, Ki Youl Yang, Rahul Trivedi, Jelena Vučković in Nature Photonics

Unlike the jumble of frequencies produced by the light that surrounds us in daily life, each frequency of light in a specialized light source known as a “soliton” frequency comb oscillates in unison, generating solitary pulses with consistent timing.

Each “tooth” of the comb is a different color of light, spaced so precisely that this system is used to measure all manner of phenomena and characteristics. Miniaturized versions of these combs — called microcombs — that are currently in development have the potential to enhance countless technologies, including GPS systems, telecommunications, autonomous vehicles, greenhouse gas tracking, spacecraft autonomy and ultra-precise timekeeping.

The lab of Stanford University electrical engineer Jelena Vučković only recently joined the microcomb community. “Many groups have demonstrated on-chip frequency combs in a variety of materials, including recently in silicon carbide by our team. However, until now, the quantum optical properties of frequency combs have been elusive,” said Vučković, the Jensen Huang Professor of Global Leadership in the School of Engineering and professor of electrical engineering at Stanford. “We wanted to leverage the quantum optics background of our group to study the quantum properties of the soliton microcomb.”

While soliton microcombs have been made in other labs, the Stanford researchers are among the first to investigate the system’s quantum optical properties, using a process that they outline in a paper . When created in pairs, microcomb solitons are thought to exhibit entanglement — a relationship between particles that allows them to influence each other even at incredible distances, which underpins our understanding of quantum physics and is the basis of all proposed quantum technologies. Most of the “classical” light we encounter on a daily basis does not exhibit entanglement.

“This is one of the first demonstrations that this miniaturized frequency comb can generate interesting quantum light — non-classical light — on a chip,” said Kiyoul Yang, a research scientist in Vučković’s Nanoscale and Quantum Photonics Lab and co-author of the paper. “That can open a new pathway toward broader explorations of quantum light using the frequency comb and photonic integrated circuits for large-scale experiments.”

Proving the utility of their tool, the researchers also provided convincing evidence of quantum entanglement within the soliton microcomb, which has been theorized and assumed but has yet to be proven by any existing studies.

“I would really like to see solitons become useful for quantum computing because it’s a highly studied system,” said Melissa Guidry, a graduate student in the Nanoscale and Quantum Photonics Lab and co-author of the paper. “We have a lot of technology at this point for generating solitons on chips at low power, so it would be exciting to be able to take that and show that you have entanglement.”

Former Stanford physics professor Theodor W. Hänsch won the Nobel Prize in 2005 for his work on developing the first frequency comb. To create what Hänsch studied requires complicated, tabletop-sized equipment. Instead, these researchers chose to focus on the newer, “micro” version, where all of the parts of the system are integrated into a single device and designed to fit on a microchip. This design saves on cost, size and energy.

To create their miniature comb, the researchers pump laser light through a microscopic ring of silicon carbide (which was painstakingly designed and fabricated using the resources of the Stanford Nano Shared Facilities and Stanford Nanofabrication Facilities). Traveling around the ring, the laser builds up intensity and, if all goes well, a soliton is born.

“It’s fascinating that, instead of having this fancy, complicated machine, you can just take a laser pump and a really tiny circle and produce the same sort of specialized light,” said Daniil Lukin, a graduate student in the Nanoscale and Quantum Photonics Lab and co-author of the paper.

He added that generating the microcomb on a chip enabled a wide spacing between the teeth, which was one step toward being able to look at the comb’s finer details.

The next steps involved equipment capable of detecting single particles of the light and packing the micro-ring with several solitons, creating a soliton crystal.

“With the soliton crystal, you can see there are actually smaller pulses of light in between the teeth, which is what we measure to infer the entanglement structure,” explained Guidry. “If you park your detectors there, you can get a good look at the interesting quantum behavior without drowning it out with the coherent light that makes up the teeth.”

Seeing as they were performing some of the first experimental studies of the quantum aspects of this system, the researchers decided to try to confirm a theoretical model, called the linearized model, which is commonly used as a shortcut to describe complex quantum systems. When they ran the comparison, they were astonished to find that the experiment matched the theory very well. So, while they have not yet directly measured that their microcomb has quantum entanglement, they have shown that its performance matches a theory that implies entanglement.

“The take-home message is that this opens the door for theorists to do more theory because now, with this system, it’s possible to experimentally verify that work,” said Lukin.

Microcombs in data centers could boost the speed of data transfer; in satellites, they could provide more precise GPS or analyze the chemical composition of far-away objects. The Vučković team is particularly interested in the potential for solitons in certain types of quantum computing because solitons are predicted to be highly entangled as soon as they are generated.

With their platform, and the ability to study it from a quantum perspective, the Nanoscale and Quantum Photonics Lab researchers are keeping an open mind about what they could do next. Near the top of their list of ideas is the possibility of performing measurements on their system that definitively prove quantum entanglement.

Momentum-resolved electronic structure and band offsets in an epitaxial NbN/GaN superconductor/semiconductor heterojunction

by Tianlun Yu, John Wright, Guru Khalsa, Betül Pamuk, Celesta S. Chang, Yury Matveyev, Xiaoqiang Wang, Thorsten Schmitt, Donglai Feng, David A. Muller, Huili Grace Xing, Debdeep Jena, Vladimir N. Strocov in Science Advances

Quantum effects in superconductors could give semiconductor technology a new twist. Researchers at the Paul Scherrer Institute PSI and Cornell University in New York State have identified a composite material that could integrate quantum devices into semiconductor technology, making electronic components significantly more powerful.

Our current electronic infrastructure is based primarily on semiconductors. This class of materials emerged around the middle of the 20th century and has been improving ever since. Currently, the most important challenges in semiconductor electronics include further improvements that would increase the bandwidth of data transmission, energy efficiency and information security. Exploiting quantum effects is likely to be a breakthrough.

Quantum effects that can occur in superconducting materials are particularly worthy of consideration. Superconductors are materials in which the electrical resistance disappears when they are cooled below a certain temperature. The fact that quantum effects in superconductors can be utilised has already been demonstrated in the first quantum computers.

To find possible successors for today’s semiconductor electronics, some researchers — including a group at Cornell University — are investigating so-called heterojunctions, i.e. structures made of two different types of materials. More specifically, they are looking at layered systems of superconducting and semiconducting materials.

“It has been known for some time that you have to select materials with very similar crystal structures for this, so that there is no tension in the crystal lattice at the contact surface,” explains John Wright, who produced the heterojunctions for the new study at Cornell University.

FIG. 1. Characterization of the MBE-grown NbN/GaN heterojunction.(A) STEM image showing high-quality NbN film and sharp interface of NbN/ GaN. A thin oxidized layer (~2 nm) is observed on top of NbN. (B) Simultaneously acquired HAADF-STEM and ABF-STEM images showing the interface quality and the polarity. Nb, Ga, and N from the atomic ball-and-stick model correspond to blue, green, and red spheres, respectively. © Bulk crystal lattice of NbN and GaN. The NbN [111] direction is aligned with the GaN [0001] direction in (A) and (B). (D) Surface BZs of (0001) GaN and (111) cubic NbN. (E and F) Measured band structure of the NbN/GaN heterojunction along the Γ-K and Γ-M directions. GaN data were taken at hv = 1064 eV, while NbN data were taken at hv = 570 eV. Calculated band structure for GaN (blue dashed lines) and NbN (green dashed lines) has been superimposed on the intensity data. (G) Density of electronic states in bulk GaN and NbN calculated using DFT indicating a barrier height of 0.93 eV.

Two suitable materials in this respect are the superconductor niobium nitride (NbN) and the semiconductor gallium nitride (GaN). The latter already plays an important role in semiconductor electronics and is therefore well researched. Until now, however, it was unclear exactly how the electrons behave at the contact interface of these two materials — and whether it is possible that the electrons from the semiconductor interfere with the superconductivity and thus obliterate the quantum effects.

“When I came across the research of the group at Cornell, I knew: here at PSI we can find the answer to this fundamental question with our spectroscopic methods at the ADRESS beamline,” explains Vladimir Strocov, researcher at the Synchrotron Light Source SLS at PSI.

This is how the two groups came to collaborate. In their experiments, they eventually found that the electrons in both materials “keep to themselves.” No unwanted interaction that could potentially spoil the quantum effects takes place.

The PSI researchers used a method well-established at the ADRESS beamline of the SLS: angle-resolved photoelectron spectroscopy using soft X-rays — or SX-ARPES for short.

“With this method, we can visualise the collective motion of the electrons in the material,” explains Tianlun Yu, a postdoctoral researcher in Vladimir Strocov’s team, who carried out the measurements on the NbN/GaN heterostructure. Together with Wright, Yu is the first author of the new publication.

The SX-ARPES method provides a kind of map whose spatial coordinates show the energy of the electrons in one direction and something like their velocity in the other; more precisely, their momentum.

“In this representation, the electronic states show up as bright bands in the map,” Yu explains. The crucial research result: at the material boundary between the niobium nitride NbN and the gallium nitride GaN, the respective “bands” are clearly separated from each other. This tells the researchers that the electrons remain in their original material and do not interact with the electrons in the neighbouring material.

“The most important conclusion for us is that the superconductivity in the niobium nitride remains undisturbed, even if this is placed atom by atom to match a layer of gallium nitride,” says Vladimir Strocov. “With this, we were able to provide another piece of the puzzle that confirms: This layer system could actually lend itself to a new form of semiconductor electronics that embeds and exploits the quantum effects that happen in superconductors.”

Observing crossover between quantum speed limits

by Gal Ness, Manolo R. Lam, Wolfgang Alt, Dieter Meschede, Yoav Sagi, Andrea Alberti in Science Advances

Which factors determine how fast a quantum computer can perform its calculations? Physicists at the University of Bonn and the Technion — Israel Institute of Technology have devised an elegant experiment to answer this question. The results of the study are published in the journal Science Advances.

Quantum computers are highly sophisticated machines that rely on the principles of quantum mechanics to process information. This should enable them to handle certain problems in the future that are completely unsolvable for conventional computers. But even for quantum computers, fundamental limits apply to the amount of data they can process in a given time.

QUANTUM-SPEED-LIMIT CROSSOVER. Measured orthogonalization times, τML and τMT, displayed through their reciprocals, with n the quantum number characterizing the initial wave packet shape. Shades in color identify the ML regime, where a crossover manifests at time τc, as opposed to the MT regime, where no crossover occurs. Inset highlights data points in the ML regime. The three points marked with arrows correspond to (A), (B), and © of Fig. 2. Solid lines show the expected curves computed with no free fitting parameters by numerical diagonalization of Hˆ(see “Spin-dependent optical lattice setup” section in Methods). The limiting case of a qubit (dashed line) and a coherent excitation (dotted line) are also shown. Values are expressed in units of the reciprocal of the trap oscillation period, which is around 16 μs.

The information stored in conventional computers can be thought of as a long sequence of zeros and ones, the bits. In quantum mechanics it is different: The information is stored in quantum bits (qubits), which resemble a wave rather than a series of discrete values. Physicists also speak of wave functions when they want to precisely represent the information contained in qubits.

In a traditional computer, information is linked together by so-called gates. Combining several gates allows elementary calculations, such as the addition of two bits. Information is processed in a very similar way in quantum computers, where quantum gates change the wave function according to certain rules.

Quantum gates resemble their traditional relatives in another respect: “Even in the quantum world, gates do not work infinitely fast,” explains Dr. Andrea Alberti of the Institute of Applied Physics at the University of Bonn. “They require a minimum amount of time to transform the wave function and the information this contains.”

More than 70 years ago, Soviet physicists Leonid Mandelstam and Igor Tamm deduced theoretically this minimum time for transforming the wave function. Physicists at the University of Bonn and the Technion have now investigated this Mandelstam-Tamm limit for the first time with an experiment on a complex quantum system. To do this, they used cesium atoms that moved in a highly controlled manner.

“In the experiment, we let individual atoms roll down like marbles in a light bowl and observe their motion,” explains Alberti, who led the experimental study.

Atoms can be described quantum mechanically as matter waves. During the journey to the bottom of the light bowl, their quantum information changes. The researchers now wanted to know when this “deformation” could be identified at the earliest. This time would then be the experimental proof of the Mandelstam-Tamm limit. The problem with this, however, is: that in the quantum world, every measurement of the atom’s position inevitably changes the matter wave in an unpredictable way. So it always looks like the marble has deformed, no matter how quickly the measurement is made.

“We therefore devised a different method to detect the deviation from the initial state,” Alberti says.

For this purpose, the researchers began by producing a clone of the matter wave, in other words an almost exact twin.

“We used fast light pulses to create a so-called quantum superposition of two states of the atom,” explains Gal Ness, a doctoral student at the Technion and first author of the study. “Figuratively speaking, the atom behaves as if it had two different colors at the same time.” Depending on the color, each atom twin takes a different position in the light bowl: One is high up on the edge and “rolls” down from there. The other, conversely, is already at the bottom of the bowl. This twin does not move — after all, it cannot roll up the walls and so does not change its wave function.

The physicists compared the two clones at regular intervals. They did this using a technique called quantum interference, which allows differences in waves to be detected very precisely. This enabled them to determine after what time a significant deformation of the matter wave first occurred.

By varying the height above the bottom of the bowl at the start of the experiment, the physicists were also able to control the average energy of the atom. Average because, in principle, the amount cannot be determined exactly. The “position energy” of the atom is therefore always uncertain.

“We were able to demonstrate that the minimum time for the matter wave to change depends on this energy uncertainty,” says Professor Yoav Sagi, who led the partner team at Technion: “The greater the uncertainty, the shorter the Mandelstam-Tamm time.”

This is exactly what the two Soviet physicists had predicted. But there was also a second effect: If the energy uncertainty was increased more and more until it exceeded the average energy of the atom, then the minimum time did not decrease further — contrary to what the Mandelstam-Tamm limit would actually suggest. The physicists thus proved a second speed limit, which was theoretically discovered about 20 years ago. The ultimate speed limit in the quantum world is therefore determined not only by the energy uncertainty, but also by the mean energy.

“It is the first time that both quantum speed boundaries could be measured for a complex quantum system, and even in a single experiment,” Alberti enthuses. Future quantum computers may be able to solve problems rapidly, but they too will be constrained by these fundamental limits.

Measuring the capabilities of quantum computers

by Timothy Proctor, Kenneth Rudinger, Kevin Young, Erik Nielsen, Robin Blume-Kohout in Nature Physics

What does a quantum computer have in common with a top draft pick in sports? Both have attracted lots of attention from talent scouts. Quantum computers, experimental machines that can perform some tasks faster than supercomputers, are constantly evaluated, much like young athletes, for their potential to someday become game-changing technology.

Now, scientist-scouts have their first tool to rank a prospective technology’s ability to run realistic tasks, revealing its true potential and limitations.

A new kind of benchmark test, designed at Sandia National Laboratories, predicts how likely it is that a quantum processor will run a specific program without errors.

The so-called mirror-circuit method, published in Nature Physics, is faster and more accurate than conventional tests, helping scientists develop the technologies that are most likely to lead to the world’s first practical quantum computer, which could greatly accelerate research for medicine, chemistry, physics, agriculture and national security.

Until now, scientists have been measuring performance on obstacle courses of random operations.

But according to the new research, conventional benchmark tests underestimate many quantum computing errors. This can lead to unrealistic expectations of how powerful or useful a quantum machine is. Mirror-circuits offer a more accurate testing method, according to the paper.

A mirror circuit is a computer routine that performs a set of calculations and then reverses it.

“It is standard practice in the quantum computing community to use only random, disordered programs to measure performance, and our results show that this is not a good thing to do,” said computer scientist Timothy Proctor, a member of Sandia’s Quantum Performance Laboratory who participated in the research.

The new testing method also saves time, which will help researchers evaluate increasingly sophisticated machines. Most benchmark tests check for errors by running the same set of instructions on a quantum machine and a conventional computer. If there are no errors, the results should match.

However, because quantum computers perform certain calculations much faster than conventional computers, researchers can spend a long time waiting for the regular computers to finish.

With a mirror circuit, however, the output should always be the same as the input or some intentional modification. So instead of waiting, scientists can immediately check the quantum computer’s result.

The research was funded by the Department of Energy’s Office of Science and Sandia’s Laboratory Directed Research and Development program. Sandia is a leading member of the Quantum Systems Accelerator, a Department of Energy national quantum research center.

Proctor and his colleagues found that randomized tests miss or underestimate the compound effects of errors. When an error is compounded it grows worse as the program runs, like a wide receiver who runs the wrong route, straying farther and farther from where they are supposed to be as the play goes on.

By mimicking functional programs, Sandia found final results often had larger discrepancies than randomized tests showed.

“Our benchmarking experiments revealed that the performance of current quantum computers is much more variable on structured programs” than was previously known, Proctor said.

The mirror-circuit method also gives scientists greater insight into how to improve current quantum computers.

“By applying our method to current quantum computers, we were able to learn a lot about the errors that these particular devices suffer — because different types of errors affect different programs a different amount,” Proctor said. “This is the first time these effects have been observed in many-qubit processors. Our method is the first tool for probing these error effects at scale.”

SARS-CoV-2 Quantum Sensor Based on Nitrogen-Vacancy Centers in Diamond

by Changhao Li, Rouhollah Soleyman, Mohammad Kohandel, Paola Cappellaro in Nano Letters

A novel approach to testing for the presence of the virus that causes Covid-19 may lead to tests that are faster, less expensive, and potentially less prone to erroneous results than existing detection methods. Though the work, based on quantum effects, is still theoretical, these detectors could potentially be adapted to detect virtually any virus, the researchers say.

The new approach is described in a paper published in the journal Nano Letters, by Changhao Li, an MIT doctoral student; Paola Cappellaro, a professor of nuclear science and engineering and of physics; and Rouholla Soleyman and Mohammad Kohandel of the University of Waterloo.

Existing tests for the SARS-CoV-2 virus include rapid tests that detect specific viral proteins, and polymerase chain reaction (PCR) tests that take several hours to process. Neither of these tests can quantify the amount of virus present with high accuracy. Even the gold-standard PCR tests might have false-negative rates of more than 25 percent. In contrast, the team’s analysis shows the new test could have false negative rates below 1 percent. The test could also be sensitive enough to detect just a few hundred strands of the viral RNA, within just a second.

The new approach makes use of atomic-scale defects in tiny bits of diamond, known as nitrogen vacancy (NV) centers. These tiny defects are extremely sensitive to minute perturbations, thanks to quantum effects taking place in the diamond’s crystal lattice, and are being explored for a wide variety of sensing devices that require high sensitivity.

The new method would involve coating the nanodiamonds containing these NV centers with a material that is magnetically coupled to them and has been treated to bond only with the specific RNA sequence of the virus. When the virus RNA is present and bonds to this material, it disrupts the magnetic connection and causes changes in the diamond’s fluorescence that are easily detected with a laser-based optical sensor.

The sensor uses only low-cost materials (the diamonds involved are smaller than specks of dust), and the devices could be scaled up to analyze a whole batch of samples at once, the researchers say. The gadolinium-based coating with its RNA-tuned organic molecules can be produced using common chemical processes and materials, and the lasers used to read out the results are comparable to cheap, widely available commercial green laser pointers.

While this initial work was based on detailed mathematical simulations that proved the system can work in principle, the team is continuing to work on translating that into a working lab-scale device to confirm the predictions.

“We don’t know how long it will take to do the final demonstration,” Li says. Their plan is first to do a basic proof-of-principle lab test, and then to work on ways to optimize the system to make it work on real virus diagnosis applications.

The multidisciplinary process requires a combination of expertise in quantum physics and engineering, for producing the detectors themselves, and in chemistry and biology, for developing the molecules that bind with the viral RNA and for finding ways to bond these to the diamond surfaces.

Even if complications arise in translating the theoretical analysis into a working device, Cappellaro says, there is such a large margin of lower false negatives predicted from this work that it will likely still have a strong advantage over standard PCR tests in that regard. And even if the accuracy were the same, this method would still have a major advantage in producing its results with a matter of minutes, rather than requiring several hours, she says.

The basic method can be adapted to any virus, she says, including any new ones that may arise, simply by adapting the compounds that are attached to the nanodiamond sensors to match the generic material of the specific target virus.

He adds that for his company, “we’re very excited about using diamond-based quantum sensors to build powerful tools for biomedical diagnostics. Needless to say, we will be following along with great interest as the ideas presented in this work are translated to the lab.”

Nonclassical Exciton Diffusion in Monolayer WSe2

by Koloman Wagner, Jonas Zipfel, Roberto Rosati, Edith Wietek, Jonas D. Ziegler, Samuel Brem, Raül Perea-Causín, Takashi Taniguchi, Kenji Watanabe, Mikhail M. Glazov, Ermin Malic, Alexey Chernikov in Physical Review Letters

A highly unusual movement of light emitting particles in atomically-thin semiconductors was experimentally confirmed by scientists from the Würzburg-Dresden Cluster of Excellence ct.qmat-Complexity and Topology in Quantum Matter. Electronic quasiparticles, known as excitons, seemed to move in opposite directions at the same time. Professor Alexey Chernikov-newly appointed physicist at the Technische Universität Dresden-and his team were able to reveal the consequences of this quantum phenomenon by monitoring light emission from mobile excitons using ultrafast microscopy at extremely low temperatures. These findings move the topic of quantum transport of excitonic many-body states into the focus of modern research.

Quantum materials studied by Alexey Chernikov and his team are only a few atoms thin. Due to extremely strong interactions in these systems, electrons come together to form new states known as excitons. Excitons behave like independent particles and are able to absorb and emit light with high efficiency. In atomically-thin layers they are stable from lowest temperatures such as minus 268 degree Celsius up to room temperature.

Regarding the current research project that focuses on the movement of excitons in ultra-thin matter, the physicist Chernikov explains:

“Excitons can be understood as a kind of moving light sources. Like other quantum mechanical objects, they combine both wave and particle properties, propagating through atomically-thin crystals. It means that they can store and transport both energy and information, but also convert them again to light. That makes them particularly interesting for us.”

Rapid movement of excitons in atomically-thin semiconductors was visualized using highly sensitive optical microscopy:

“First we applied a short laser pulse to the material that generated the excitons. Then we used an ultrafast detector to observe when and where the light was reemitted. When we repeated these experiments at very low temperatures, however, the movement of quasiparticles appeared rather astonishing,” says Chernikov.

So far, two general types of exciton movement were broadly known to the scientific community: either the excitons “jump” from one molecule to another (process known as hopping)-or they move rather “classically” like billiard balls that change their direction after random scattering events.

“In the ultra-thin semiconductors, however, the excitons behaved in a way that we have never seen before. In the end, the only possible explanation was that the excitons would occasionally move through closed loops in opposite directions at the same time. Such behavior was in fact known from individual electrons. However, to observe this experimentally for luminescent excitons-that was quite unusual,” notes Chernikov.

After all control experiments confirmed the result, the scientists looked for the cause of their unusual observation. A recently published theoretical work by the Russian researcher Mikhail M. Glazov from the Ioffe Institute in Saint Petersburg provided the key insight: Glazov describes how excitons in atomically-thin semiconductors can indeed move through closed, ring-like paths and enter superimposed states. This means that the excitons seem to essentially move both clockwise and counterclockwise at the same time. This effect is a purely quantum mechanical phenomenon, which does not occur for classical particles. Together with the team of Ermin Malic from the Philipps University of Marburg, who provided additional insights into the exciton dynamics, the scientists were finally able to track down this unusual behavior.

MISC

  • Peers dispute the claim that tardigrades were entangled with qubits: Scientists and journalists alike are disputing claims made by an international team of researchers that they had entangled a tardigrade with superconducting qubits. Their paper is published on the arXiv preprint server. Virtually all of those with an opinion pointed out that the work by the researchers in this new effort did not involve entanglement.

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