QT/ What coffee with cream can teach us about quantum physics
Quantum news biweekly vol.67, 16th January — 2nd February
TL;DR
- A new advancement in theoretical physics could, one day, help engineers develop new kinds of computer chips that might store information for longer in very small objects.
- New research conducted by nuclear physicists is using a method that connects theories of gravitation to interactions among the smallest particles of matter. The result is insight into the strong force, a powerful mediator of particle interactions in the subatomic realm. The research has revealed, for the first time, a snapshot of the distribution of the shear strength of the strong force inside the proton — or how strong an effort must be to overcome the strong force to move an object it holds in its grasp.
- Scientists have discovered a first-of-its-kind material, a 3D crystalline metal in which quantum correlations and the geometry of the crystal structure combine to frustrate the movement of electrons and lock them in place.
- An international team of researchers has disproved a previously held assumption about the impact of multiphoton components in interference effects of thermal fields (e.g. sunlight) and parametric single photons (generated in non-linear crystals).
- Researchers found evidence that bilayer graphene quantum dots may host a promising new type of quantum bit based on so-called valley states.
- In a new study, a group of researchers reports that they have achieved quantum coherence at room temperature: the ability of a quantum system to maintain a well-defined state over time without getting affected by surrounding disturbances.
- Certain materials have desirable properties that are hidden and scientists can use light to uncover these properties. Researchers have used an advanced optical technique, based on terahertz time-domain spectroscopy, to learn more about a quantum material called Ta2NiSe5 (TNS).
- A team of scientists has succeeded in cooling traveling sound waves in wave-guides considerably further than has previously been possible using laser light. This achievement represents a significant move towards the ultimate goal of reaching the quantum ground state of sound in wave-guides.
- Researchers grew a twisted multilayer crystal structure for the first time and measured the structure’s key properties. The twisted structure could help researchers develop next-generation materials for solar cells, quantum computers, lasers and other devices.
- Non-Heisenberg-type approximant crystals have many interesting properties and are intriguing for researchers of condensed matter physics. However, their magnetic phase diagrams, which are crucial for realizing their potential, remain completely unknown. Now, a team of researchers has constructed the magnetic phase diagram of a non-Heisenberg Tsai-type 1/1 gold-gallium-terbium approximant crystal. This development marks a significant step forward for quasicrystal research and for the realization of magnetic refrigerators and spintronic devices.
- 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
Ergodicity Breaking Provably Robust to Arbitrary Perturbations
by David T. Stephen, Oliver Hart, Rahul M. Nandkishore in Physical Review Letters
Add a dash of creamer to your morning coffee, and clouds of white liquid will swirl around your cup. But give it a few seconds, and those swirls will disappear, leaving you with an ordinary mug of brown liquid.
Something similar happens in quantum computer chips — devices that tap into the strange properties of the universe at its smallest scales — where information can quickly jumble up, limiting the memory capabilities of these tools. That doesn’t have to be the case, said Rahul Nandkishore, associate professor of physics at the University of Colorado Boulder.
In a new coup for theoretical physics, he and his colleagues have used math to show that scientists could create, essentially, a scenario where the milk and coffee never mix — no matter how hard you stir them. The group’s findings may lead to new advances in quantum computer chips, potentially providing engineers with new ways to store information in incredibly tiny objects.
“Think of the initial swirling patterns that appear when you add cream to your morning coffee,” said Nandkishore, senior author of the new study. “Imagine if these patterns continued to swirl and dance no matter how long you watched.”
Researchers still need to run experiments in the lab to make sure that these never-ending swirls really are possible. But the group’s results are a major step forward for physicists seeking to create materials that remain out of balance, or equilibrium, for long periods of time — a pursuit known as “ergodicity breaking.”
The study, which includes co-authors David Stephen and Oliver Hart, postdoctoal researchers in physics at CU Boulder, hinges on a common problem in quantum computing.
Normal computers run on “bits,” which take the form of zeros or ones. Nandkishore explained that quantum computers, in contrast, employ “qubits,” which can exist as zero, one or, through the strangeness of quantum physics, zero and one at the same time. Engineers have made qubits out of a wide range of things, including individual atoms trapped by lasers or tiny devices called superconductors. But just like that cup of coffee, qubits can become easily mixed up. If you flip, for example, all of your qubits to one, they’ll eventually flip back and forth until the entire chip becomes a disorganized mess.
In the new research, Nandkishore and his colleagues may have figured a way around that tendency toward mixing. The group calculated that if scientists arrange qubits into particular patterns, these assemblages will retain their information — even if you disturb them using a magnetic field or a similar disruption. That could, the physicist said, allow engineers to build devices with a kind of quantum memory.
“This could be a way of storing information,” he said. “You would write information into these patterns, and the information couldn’t be degraded.”
In the study, the researchers used mathematical modeling tools to envision an array of hundreds to thousands of qubits arranged in a checkerboard-like pattern. The trick, they discovered, was to stuff the qubits into a tight spot. If qubits get close enough together, Nadkishore explained, they can influence the behavior of their neighbors, almost like a crowd of people trying to squeeze themselves into a telephone booth. Some of those people might be standing upright or on their heads, but they can’t flip the other way without pushing on everyone else.
The researchers calculated that if they arranged these patterns in just the right way, those patterns might flow around a quantum computer chip and never degrade — much like those clouds of cream swirling forever in your coffee.
“The wonderful thing about this study is that we discovered that we could understand this fundamental phenomenon through what is almost simple geometry,” Nandkishore said.
The team’s findings could influence a lot more than just quantum computers. Nandkishore explained that almost everything in the universe, from cups of coffee to vast oceans, tends to move toward what scientists call “thermal equilibrium.” If you drop an ice cube into your mug, for example, heat from your coffee will melt the ice, eventually forming a liquid with a uniform temperature. His new findings, however, join a growing body of research that suggests that some small organizations of matter can resist that equilibrium — seemingly breaking some of the most immutable laws of the universe.
“We’re not going to have to redo our math for ice and water,” Nandkishore said. “The field of mathematics that we call statistical physics is incredibly successful for describing things we encounter in everyday life. But there are settings where maybe it doesn’t apply.”
Colloquium: Gravitational form factors of the proton
by V. D. Burkert, L. Elouadrhiri, F. X. Girod, C. Lorcé, P. Schweitzer, P. E. Shanahan in Reviews of Modern Physics
The power of gravity is writ large across our visible universe. It can be seen in the lock step of moons as they circle planets; in wandering comets pulled off-course by massive stars; and in the swirl of gigantic galaxies. These awesome displays showcase gravity’s influence at the largest scales of matter. Now, nuclear physicists are discovering that gravity also has much to offer at matter’s smallest scales.
New research conducted by nuclear physicists at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility is using a method that connects theories of gravitation to interactions among the smallest particles of matter to reveal new details at this smaller scale. The research has now revealed, for the first time, a snapshot of the distribution of the strong force inside the proton. This snapshot details the shear stress the force may exert on the quark particles that make up the proton.
According to the lead author on the study, Jefferson Lab Principal Staff Scientist Volker Burkert, the measurement reveals insight into the environment experienced by the proton’s building blocks. Protons are built of three quarks that are bound together by the strong force.
“At its peak, this is more than a four-ton force that one would have to apply to a quark to pull it out of the proton,” Burkert explained. “Nature, of course, does not allow us to separate just one quark from the proton because of a property of quarks called ‘color.’ There are three colors that mix quarks in the proton to make it appear colorless from the outside, a requirement for its existence in space. Trying to pull a colored quark out of the proton will produce a colorless quark/anti-quark pair, a meson, using the energy you put in to attempt to separate the quark, leaving a colorless proton (or neutron) behind. So, the 4-tons is an illustration of the strength of the force that is intrinsic in the proton.”
The result is only the second of the proton’s mechanical properties to be measured. The proton’s mechanical properties include its internal pressure (measured in 2018), its mass distribution (physical size), its angular momentum, and its shear stress (shown here). The result was made possible by a half-century-old prediction and two-decade-old data.
In the mid 1960s, it was theorized that if nuclear physicists could see how gravity interacts with subatomic particles, such as the proton, such experiments could reveal the proton’s mechanical properties directly.
“But at that time, there was no way. If you compare gravity with the electromagnetic force, for instance, there is 39 orders of magnitude of difference — So it’s completely hopeless, right?” explained Latifa Elouadhriri, a Jefferson Lab staff scientist and co-author on the study.
The decades-old data came from experiments conducted with Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF), a DOE Office of Science user facility. A typical CEBAF experiment would entail an energetic electron interacting with another particle by exchanging a packet of energy and a unit of angular momentum called a virtual photon with the particle. The energy of the electron dictates which particles it interacts with in this way and how they respond.
In the experiment, a force even much greater than the four tons needed to pull out a quark/antiquark pair was applied to the proton by the highly energetic electron beam interacting with the proton in a target of liquified hydrogen gas.
“We developed the program to study deeply virtual Compton scattering. This is where you have an electron exchanging a virtual photon with the proton. And at the final state, the proton remained the same but recoiled, and you have one real very highly energetic photon produced, plus the scattered electron,” said Elouadhriri. “At the time we took the data, we were not aware that beyond the 3-dimensional imaging we intended with this data, we were also collecting the data needed for accessing the mechanical properties of the proton.”
It turns out that this specific process — deeply virtual Compton scattering (DVCS) — could be connected to how gravity interacts with matter. The general version of this connection was stated in the 1973 textbook on Einstein’s general theory of relativity titled ‘Gravitation’ by Charles W. Misner, Kip S. Thorne and John Archibald Wheeler.
In it, they wrote, “Any mass-less spin-2 field would give rise to a force indistinguishable from gravitation, because a mass-less spin-2 field would couple to the stress-energy tensor in the same way that gravitational interactions do.”
Three decades later, theorist Maxim Polyakov followed up on this idea by establishing the theoretical foundation that connects the DVCS process and gravitational interaction.
“This breakthrough in theory established the relationship between the measurement of deeply virtual Compton scattering to the gravitational form factor. And we were able to use that for the first time and extract the pressure that we did, and now the normal force and the shear force,” Burkert explained.
A more detailed description of the connections between he DVCS process and the gravitational interaction can be found in this article describing the first result obtained from this research.
The researchers say their next step is to work on extracting the information they need from the existing DVCS data to enable the first determination of the proton’s mechanical size. They also hope to take advantage of newer, higher-statistics and higher-energy experiments that are continuing the DVCS research in the proton.
In the meantime, the study co-authors have been amazed at the plethora of new theoretical efforts, detailed in hundreds of theoretical publications, that have begun to exploit this newly discovered avenue for exploring the mechanical properties of the proton.
Non-Fermi liquid behaviour in a correlated flat-band pyrochlore lattice
by Jianwei Huang, Lei Chen, Yuefei Huang, Chandan Setty, Bin Gao, Yue Shi, Zhaoyu Liu, Yichen Zhang, Turgut Yilmaz, Elio Vescovo, Makoto Hashimoto, Donghui Lu, Boris I. Yakobson, Pengcheng Dai, Jiun-Haw Chu, Qimiao Si, Ming Yi in Nature Physics
Rice University scientists have discovered a first-of-its-kind material, a 3D crystalline metal in which quantum correlations and the geometry of the crystal structure combine to frustrate the movement of electrons and lock them in place.
The paper also describes the theoretical design principle and experimental methodology that guided the research team to the material. One part copper, two parts vanadium and four parts sulfur, the alloy features a 3D pyrochlore lattice consisting of corner-sharing tetrahedra.
“We look for materials where there are potentially new states of matter or new exotic features that haven’t been discovered,” said study co-corresponding author Ming Yi, a Rice experimental physicist.
Quantum materials are a likely place to look, especially if they host strong electron interactions that give rise to quantum entanglement. Entanglement leads to strange electronic behaviors, including frustrating the movement of electrons to the point where they become locked in place.
“This quantum interference effect is analogous to waves rippling across the surface of a pond and meeting head-on,” Yi said. “The collision creates a standing wave that does not move. In the case of geometrically frustrated lattice materials, it’s the electronic wave functions that destructively interfere.”
Electron localization in metals and semimetals produces flat electronic bands, or flat bands. In recent years, physicists have found that the geometric arrangement of atoms in some 2D crystals, like Kagome lattices, can also produce flat bands. The new study provides empirical evidence of the effect in a 3D material.
Using an experimental technique called angle-resolved photoemission spectroscopy, or ARPES, Yi and study lead author Jianwei Huang, a postdoctoral researcher in her lab, detailed the band structure of the copper-vanadium-sulfur material and found it hosted a flat band that is unique in several ways.
“It turns out that both types of physics are important in this material,” Yi said. “The geometric frustration aspect was there, as theory had predicted. The pleasant surprise was that there were also correlation effects that produced the flat band at the Fermi level, where it can actively participate in determining the physical properties.”
In solid-state matter, electrons occupy quantum states that are divided in bands. These electronic bands can be imagined as rungs on a ladder, and electrostatic repulsion limits the number of electrons that can occupy each rung. Fermi level, an inherent property of materials and a crucial one for determining their band structure, refers to the energy level of the highest occupied position on the ladder.
Rice theoretical physicist and study co-corresponding author Qimiao Si, whose research group identified the copper-vanadium alloy and its pyrochlore crystal structure as being a possible host for combined frustration effects from geometry and strong electron interactions, likened the discovery to finding a new continent.
“It’s the very first work to really show not only this cooperation between geometric- and interaction-driven frustration, but also the next stage, which is getting electrons to be in the same space at the top of the (energy) ladder, where there’s a maximal chance of their reorganizing into interesting and potentially functional new phases,” Si said.
He said the predictive methodology or design principle that his research group used in the study may also prove useful to theorists who study quantum materials with other crystal lattice structures.
“The pyrochlore is not the only game in town,” Si said. “This is a new design principle that allows theorists to predictively identify materials in which flat bands arise due to strong electron correlations.”
Yi said there is also plenty of room for further experimental exploration of pyrochlore crystals. “This is just the tip of the iceberg,” she said. “This is 3D, which is new, and just given how many surprising findings there have been on Kagome lattices, I’m envisioning that there could be equally or maybe even more exciting discoveries to be made in the pyrochlore materials.”
Spectral Hong-Ou-Mandel Effect between a Heralded Single-Photon State and a Thermal Field: Multiphoton Contamination and the Nonclassicality Threshold
by Anahita Khodadad Kashi, Lucia Caspani, Michael Kues in Physical Review Letters
An international team of researchers from Leibniz University Hannover (Germany) and the University of Strathclyde in Glasgow (United Kingdom) has disproved a previously held assumption about the impact of multiphoton components in interference effects of thermal fields (e.g. sunlight) and parametric single photons (generated in non-linear crystals).
“We experimentally proved that the interference effect between thermal light and parametric single photons also leads to quantum interference with the background field. For this reason, the background cannot simply be neglected and subtracted from calculations, as has been the case up to now,” says Prof. Dr. Michael Kues, Head of the Institute of Photonics and member of the Board of the PhoenixD Cluster of Excellence at Leibniz University Hannover.
The leading scientist was PhD student Anahita Khodadad Kashi, who performs research on photonic quantum information processing at the Institute of Photonics. She investigated how the visibility of the so-called Hong-Ou-Mandel effect, a quantum interference effect, is affected by multiphoton contamination. “With our experiment, we have disproved the previously valid assumption that multiphoton components would only impair visibility and can therefore be subtracted in the calculation,” says Khodadad Kashi and continues: “We have discovered a new fundamental characteristic that was not considered in previous calculations. Our newly developed model can predict the quantum interference and we can measure this effect in an experiment.”
The scientists came across their discovery while carrying out an experiment in the laser laboratory. They obtained a negative result when they initially followed the original calculation method. “But the result would have been physically impossible,” says Khodadad Kashi. Together, the team began troubleshooting the experimental setup and the calculation model.
“When an experiment turns out very different from what is expected, scientists start questioning previous assumptions and look for new explanations,” says Kues. They jointly developed their new theory of quantum interference of thermal fields with parametric single photons. Quantum researcher Lucia Caspani from the University of Strathclyde in Glasgow was the first to test the approach. As the next step, Khodadad Kashi presented her theory and the experimental results at international conferences, including Photonics West in San Francisco, the world’s largest specialist conference for optics and photonics, attracting around 22,000 participants. There, she discussed her model with other scientists and received confirmation of her results. The journal Physical Review Letters has now published the team’s research.
With the new theory and the experimental verification, Kues’ team has made an important contribution to a better understanding of quantum phenomena.
“The findings could be important for quantum key distribution, which is necessary for secure communications in the future, specifically how quantum interference effects are interpreted for the generation of secret keys,” says Khodadad Kashi. However, many questions remain unanswered, says Kues: “Little research has been done into multiphoton effects, so a lot of work is still needed.”
Long-lived valley states in bilayer graphene quantum dots
by Rebekka Garreis, Chuyao Tong, Jocelyn Terle, Max Josef Ruckriegel, Jonas Daniel Gerber, Lisa Maria Gächter, Kenji Watanabe, Takashi Taniguchi, Thomas Ihn, Klaus Ensslin, Wei Wister Huang in Nature Physic
In quantum computing, the question as to what physical system, and which degrees of freedom within that system, may be used to encode quantum bits of information — qubits, in short — is at the heart of many research projects carried out in physics and engineering laboratories. Superconducting qubits, spin qubits, and qubits encoded in the motion of trapped ions are already recognised widely as prime candidates for future practical applications of quantum computers; other systems need to be better understood and thus offer a stimulating ground for fundamental investigation.
Rebekka Garreis, Chuyao Tong, Wister Huang and their colleagues in the group of Professors Klaus Ensslin and Thomas Ihn from the Department of Physics at ETH Zurich have been looking into bilayer graphene (BLG) quantum dots, known as a potential platform for spin qubits, to find out if another degree of freedom of BLG can be used to encode quantum information. Their latest findings, show that the so-called valley degree of freedom in BLG is associated with quantum states that are extremely long-lived and are thus worth considering further as an additional resource for solid-state quantum computing.
Graphene is a two-dimensional material given by a single layer of carbon atoms bound in a hexagonal lattice structure. Its sheet-like appearance is deceitful, as graphene is among the strongest materials on Earth; its mechanical and electronic properties are of great interest to many industry sectors. In bilayer graphene, the system used by the researchers, two sheets of carbon atoms lie on top of each other. Both graphene and BLG are semimetals, as they lack the characteristic energy band gap found in semiconductors and, most notably, insulators. Nevertheless, a tunable band gap can be engineered in BLG by applying an electric field perpendicularly to the plane of the sheets.
Opening a band gap is necessary to use BLG as a host material for quantum dots, which are nanometer-scale ‘boxes’ capable of confining single or few electrons. Usually fabricated in semiconductor host materials, quantum dots offer excellent control over individual electrons. For this reason, they are an important platform for spin qubits, systems where quantum information is encoded in the electron spin degree of freedom.
Because quantum information is much more prone to being corrupted — and therefore become unsuitable for computational tasks — by the surrounding environment than its classical counterpart, researchers who study different qubit candidates must characterise their coherence properties: these tell them how well and for how long quantum information can survive in their qubit system. In most traditional quantum dots, electron spin decoherence can be caused by the spin-orbit interaction, which introduces an unwanted coupling between the electron spin and the vibrations of the host lattice, and the hyperfine interaction between the electron spin and the surrounding nuclear spins. In graphene as well as in other carbon-based materials, spin-orbit coupling and hyperfine interaction are both weak: this makes graphene quantum dots especially appealing for spin qubits. The results reported by Garreis, Tong and co-authors add one more promising facet to the picture.
The hexagonal lattice of BLG can be imaged with specific microscopy techniques. The hexagonal symmetry observed in this so-called real space is also present in momentum space, where the vertices of the lattice don’t correspond to the spatial locations of carbon atoms but to values of momentum associated with the free electrons on the lattice. In momentum space, free electrons are found in the local minima and maxima of the energy landscape, namely at points where the conduction and valence bands meet. These energy extrema are called valleys. In BLG, the hexagonal symmetry dictates the existence of two degenerate energy valleys (that is, characterised by the same electron energy) corresponding to opposite electron momentum values. This valley degree of freedom can be treated in much the same way as electron spin in BLG — in fact, valleys in graphene are commonly called pseudo-spins. While valley states in bilayer graphene were known before, their suitability as practical qubits remained unclear until now.
Garreis, Tong and co-workers considered a double quantum dot — that is, two dots with tunable coupling — in BLG and measured the relaxation time for valley and spin states. The relaxation time sets the temporal scale over which the system makes a transition from one valley or spin state to another and, as a result of the relaxation process, loses its energy and becomes unsuitable for further qubit operations. The research team finds that valley states have relaxation times exceeding half a second, a result that points to promising coherence properties for future valley qubits. The spin relaxation time measurement in the BLG double quantum dot gives a value below 25 ms, which is much shorter than the relaxation time for valley states but is in good agreement with spin relaxation times measured in semiconductor quantum dots. Importantly, both values are acceptable for high-quality qubit manipulation and readout.
In the paper, the researchers also highlight aspects that call for further experimental and theoretical investigation. They present data showing the dependence of the relaxation times for spin and valley states on two parameters that are expected to play a role in the states’ relaxation dynamics. One parameter is the energy detuning: this is the energy difference between the ground states of two distinct configurations for the double quantum dot. Varying the detuning means acting on the energy difference between the states involved in the relaxation process. The other parameter is known as inter-dot coupling and determines how easily an electron in one quantum dot can ‘trespass’ into the territory of the other dot. The authors report behaviours that cannot be explained through the mechanisms that are usually at play in quantum-dot spin qubits. The relaxation time is shown to increase with higher energy detuning, which doesn’t match observations in other systems. Remarkably, varying the inter-dot coupling leaves the valley relaxation time unaffected.
It’s clear that a more complete understanding of the mechanisms affecting valley and spin relaxation times is needed to identify which variables may work best for manipulating future valley qubits. Meanwhile, the findings presented by Garreis, Tong and collaborators make the case for adding valley states in BLG quantum dots to the landscape of solid-state quantum computing.
Room-temperature quantum coherence of entangled multiexcitons in a metal-organic framework
by Akio Yamauchi, Kentaro Tanaka, Masaaki Fuki, Saiya Fujiwara, Nobuo Kimizuka, Tomohiro Ryu, Masaki Saigo, Ken Onda, Ryota Kusumoto, Nami Ueno, Harumi Sato, Yasuhiro Kobori, Kiyoshi Miyata, Nobuhiro Yanai in Science Advances
In a study, a group of researchers led by Associate Professor Nobuhiro Yanai from Kyushu University’s Faculty of Engineering, in collaboration with Associate Professor Kiyoshi Miyata from Kyushu University and Professor Yasuhiro Kobori of Kobe University, reports that they have achieved quantum coherence at room temperature: the ability of a quantum system to maintain a well-defined state over time without getting affected by surrounding disturbances.
This breakthrough was made possible by embedding a chromophore, a dye molecule that absorbs light and emits color, in a metal-organic framework, or MOF, a nanoporous crystalline material composed of metal ions and organic ligands.
Their findings mark a crucial advancement for quantum computing and sensing technologies. While quantum computing is positioned as the next major advancement of computing technology, quantum sensing is a sensing technology that utilizes the quantum mechanical properties of qubits (quantum analogs of bits in classical computing that can exist in a superposition of 0 and 1).
Various systems can be employed to implement qubits, with one approach being the utilization of intrinsic spin — a quantum property related to a particle’s magnetic moment — of an electron. Electrons have two spin states: spin up and spin down. Qubits based on spin can exist in a combination of these states and can be “entangled,” allowing the state of one qubit to be inferred from another.
By leveraging the extremely sensitive nature of a quantum entangled state to environmental noise, quantum sensing technology is expected to enable sensing with higher resolution and sensitivity compared to traditional techniques. However, so far, it has been challenging to entangle four electrons and make them respond to external molecules, that is, achieve quantum sensing using a nanoporous MOF.
Notably, chromophores can be used to excite electrons with desirable electron spins at room temperatures through a process called singlet fission. However, at room temperature causes the quantum information stored in qubits to lose quantum superposition and entanglement. As a result, it is usually only possible to achieve quantum coherence at liquid nitrogen level temperatures.
To suppress the molecular motion and achieve room-temperature quantum coherence, the researchers introduced a chromophore based on pentacene (polycyclic aromatic hydrocarbon consisting of five linearly fused benzene rings) in a UiO-type MOF. “The MOF in this work is a unique system that can densely accumulate chromophores. Additionally, the nanopores inside the crystal enable the chromophore to rotate, but at a very restrained angle,” says Yanai.
The MOF structure facilitated enough motion in the pentacene units to allow the electrons to transition from the triplet state to a quintet state, while also sufficiently suppressing motion at room temperature to maintain quantum coherence of the quintet multiexciton state. Upon photoexciting electrons with microwave pulses, the researchers could observe the quantum coherence of the state for over 100 nanoseconds at room temperature. “This is the first room-temperature quantum coherence of entangled quintets,” remarks an excited Kobori.
While the coherence was observed only for nanoseconds, the findings will pave the way for designing materials for the generation of multiple qubits at room temperatures.
“It will be possible to generate quintet multiexciton state qubits more efficiently in the future by searching for guest molecules that can induce more such suppressed motions and by developing suitable MOF structures,” speculates Yanai. “This can open doors to room-temperature molecular quantum computing based on multiple quantum gate control and quantum sensing of various target compounds.”
Terahertz parametric amplification as a reporter of exciton condensate dynamics
by Sheikh Rubaiat Ul Haque, Marios H. Michael, Junbo Zhu, Yuan Zhang, Lukas Windgätter, Simone Latini, Joshua P. Wakefield, Gu-Feng Zhang, Jingdi Zhang, Angel Rubio, Joseph G. Checkelsky, Eugene Demler, Richard D. Averitt in Nature Materials
Certain materials have desirable properties that are hidden, and just as you would use a flashlight to see in the dark, scientists can use light to uncover these properties. Researchers at the University of California San Diego have used an advanced optical technique to learn more about a quantum material called Ta2NiSe5 (TNS).
Materials can be perturbed through different external stimuli, often with changes in temperature or pressure; however, because light is the fastest thing in the universe, materials will respond very quickly to optical stimuli, revealing properties that would otherwise remain hidden.
“In essence, we shine a laser on a material and it’s like stop-action photography where we can incrementally follow a certain property of that material,” said Professor of Physics Richard Averitt, who led the research and is one of the paper’s authors. “By looking at how constituent particles move around in that system, we can tease out these properties that are really tricky to find otherwise.”
The experiment was conducted by lead author Sheikh Rubaiat Ul Haque, who graduated from UC San Diego in 2023 and is now a postdoctoral scholar at Stanford University. He, along with Yuan Zhang, another graduate student in Averitt’s lab, improved upon a technique called terahertz time-domain spectroscopy. This technique allows scientists to measure a material’s properties over a range of frequencies, and Haque’s improvements allowed them access to a broader range of frequencies.
The work was based on a theory created by another of the paper’s authors, Eugene Demler, a professor at ETH Zürich. Demler and his graduate student Marios Michael developed the idea that when certain quantum materials are excited by light, they may turn into a medium that amplifies terahertz frequency light. This led Haque and colleagues to look closely into the optical properties of TNS.
When an electron is excited to a higher level by a photon, it leaves behind a hole. If the electron and hole are bound, an exciton is created. Excitons may also form a condensate — a state that occurs when particles come together and behave as a single entity.
Haque’s technique, backed by Demler’s theory and using density functional calculations by Angel Rubio’s group at Max Planck Institute for the Structure and Dynamics of Matter, the team was able to observe anomalous terahertz light amplification, which uncovered some of the hidden properties of the TNS exciton condensate.
Condensates are a well-defined quantum state and using this spectroscopic technique could allow some of their quantum properties to be imprinted onto light. This may have implications in the emerging field of entangled light sources (where multiple light sources have interconnected properties) utilizing quantum materials.
“I think it’s a wide-open area,” stated Haque. “Demler’s theory can be applied to a suite of other materials with nonlinear optical properties. With this technique, we can discover new light-induced phenomena that haven’t been explored before.”
Optoacoustic Cooling of Traveling Hypersound Waves
by Laura Blázquez Martínez, Philipp Wiedemann, Changlong Zhu, Andreas Geilen, Birgit Stiller in Physical Review Letters
The quantum ground state of an acoustic wave of a certain frequency can be reached by completely cooling the system. In this way, the number of quantum particles, the so-called acoustic phonons, which cause disturbance to quantum measurements, can be reduced to almost zero and the gap between classical and quantum mechanics bridged.
Over the past decade, major technological advances have been made, making it possible to put a wide variety of systems into this state. Mechanical vibrations oscillating between two mirrors in a resonator can be cooled to very low temperatures as far as the quantum ground state. This has not yet been possible for optical fibers in which high-frequency sound waves can propagate. Now researchers from the Stiller Research Group have taken a step closer to this goal.
In their study, recently published in Physical Review Letters, they report that they were able to lower the temperature of a sound wave in an optical fiber initially at room temperature by 219 K using laser cooling, ten times further than had previously been reported. Ultimately, the initial phonon number was reduced by 75%, at a temperature of 74 K, -194 Celsius. Such a drastic reduction in temperature was made possible by the use of laser light. Cooling of the propagating sound waves was achieved via the nonlinear optical effect of stimulated Brillouin scattering, in which light waves are efficiently coupled to sound waves. Through this effect, the laser light cools the acoustic vibrations and creates an environment with less thermal noise which is, to an extent, “disturbing” noise for a quantum communication system, for example.
“An interesting advantage of glass fibers, in addition to this strong interaction, is the fact that they can conduct light and sound excellently over long distances,” says Laura Blázquez Martínez, one of the lead authors of the article and a doctoral student in the Stiller research group.
Most physical platforms previously brought to the quantum ground state were microscopic. However, in this experiment, the length of the optical fiber was 50 cm and a sound wave extending over the full 50 cm of the core of the fiber was cooled to extremely low temperatures.
“These results are a very exciting step towards the quantum ground state in waveguides and the manipulation of such long acoustic phonons opens up possibilities for broadband applications in quantum technology,” according to Dr. Birgit Stiller, head of the quantum optoacoustics group.
Sound, in the day-to-day classical world, can be understood as a density wave in a medium. However, from the perspective of quantum mechanics, sound can also be described as a particle: the phonon. This particle, the sound quantum, represents the smallest amount of energy which occurs as an acoustic wave at a certain frequency. In order to see and study single quanta of sound, the number of phonons must be minimized. The transition from the classical to quantum behavior of sound is often more easily observed in the quantum ground state, where the number of phonons is close to zero on average, such that the vibrations are almost frozen and quantum effects can be measured. Stiller: “This opens the door to a new landscape of experiments that allow us to gain deeper insights into the fundamental nature of matter.” The advantage of using a waveguide system is that light and sound are not bound between two mirrors, but propagating along the waveguide. The acoustic waves exist as a continuum — not only for certain frequencies — and can have a broad bandwidth, making them promising for applications such as high-speed communication systems.
“We are very enthusiastic about the new insights that pushing these fibers into the quantum ground state will bring,” emphasizes the research group leader. “Not only from the fundamental research point of view, allowing us to peek into the quantum nature of extended objects, but also because of the applications this could have in quantum communications schemes and future quantum technologies.”
Twisted epitaxy of gold nanodisks grown between twisted substrate layers of molybdenum disulfide
by Yi Cui, Jingyang Wang, Yanbin Li, Yecun Wu, Emily Been, Zewen Zhang, Jiawei Zhou, Wenbo Zhang, Harold Y. Hwang, Robert Sinclair, Yi Cui in Science
Researchers with the Department of Energy’s SLAC National Accelerator Laboratory, Stanford University and the DOE’s Lawrence Berkeley National Laboratory (LBNL) grew a twisted multilayer crystal structure for the first time and measured the structure’s key properties. The twisted structure could help researchers develop next-generation materials for solar cells, quantum computers, lasers and other devices.
“This structure is something that we have not seen before — it was a huge surprise to me,” said Yi Cui, a professor at Stanford and SLAC and paper co-author. “A new quantum electronic property could appear within this three-layer twisted structure in future experiments.”
The crystals the team designed extended the concept of epitaxy, a phenomenon that occurs when one type of crystal material grows on top of another material in an ordered way — kind of like growing a neat lawn on top of soil, but at the atomic level. Understanding epitaxial growth has been critical to the development of many industries for more than 50 years, particularly the semiconductor industry. Indeed, epitaxy is part of many of the electronic devices that we use today, from cell phones to computers to solar panels, allowing electricity to flow, and not flow, through them.
To date, epitaxy research has focused on growing one layer of material onto another, and the two materials have the same crystal orientation at the interface. This approach has been successful for decades in many applications, such as transistors, light-emitting diodes, lasers and quantum devices. But to find new materials that perform even better for more demanding needs, like quantum computing, researchers are searching for other epitaxial designs — ones that might be more complex, yet better performing, hence the “twisted epitaxy” concept demonstrated in this study.
In their experiment, detailed this month in Science, researchers added a layer of gold between two sheets of a traditional semiconducting material, molybdenum disulfide (MoS2). Because the top and bottom sheets were oriented differently, the gold atoms could not align with both simultaneously, which allowed the Au structure to twist, said Yi Cui, Professor Cui’s graduate student in materials science and engineering at Stanford and co-author of the paper.
“With only a bottom MoS2 layer, the gold is happy to align with it, so no twist happens,” said Cui, the graduate student. “But with two twisted MoS2 sheets, the gold isn’t sure to align with the top or bottom layer. We managed to help the gold solve its confusion and discovered a relationship between the orientation of Au and the twist angle of bilayer MoS2.”
To study the gold layer in detail, the researcher team from the Stanford Institute for Materials and Energy Sciences (SIMES) and LBNL heated a sample of the whole structure to 500 degrees Celsius. Then they sent a stream of electrons through the sample using a technique called transmission electron microscopy (TEM), which revealed the morphology, orientation and strain of the gold nanodiscs after annealing at the different temperatures. Measuring these properties of the gold nanodiscs was a necessary first step toward understanding how the new structure could be designed for real world applications in the future.
“Without this study, we would not know if twisting an epitaxial layer of metal on top of a semiconductor was even possible,” said Cui, the graduate student. “Measuring the complete three-layer structure with electron microscopy confirmed that it was not only possible, but also that the new structure could be controlled in exciting ways.”
Next, researchers want to further study the optical properties of the gold nanodiscs using TEM and learn if their design alters physical properties like band structure of Au. They also want to extend this concept to try to build three-layer structures with other semiconductor materials and other metals.
“We’re beginning to explore whether only this combination of materials allows this or if it happens more broadly,” said Bob Sinclair, the Charles M. Pigott Professor in Stanford’s school of Materials Science and Engineering and paper co-author. “This discovery is opening a whole new series of experiments that we can try. We could be on our way to finding brand new material properties that we could exploit.”
Unveiling exotic magnetic phase diagram of a non-Heisenberg quasicrystal approximant
by Farid Labib, Kazuhiro Nawa, Shintaro Suzuki, Hung-Cheng Wu, Asuka Ishikawa, Kazuki Inagaki, Takenori Fujii, Katsuki Kinjo, Taku J. Sato, Ryuji Tamura in Materials Today Physics
Quasicrystals are intermetallic materials that have garnered significant attention from researchers aiming to advance condensed matter physics understanding. Unlike normal crystals, in which atoms are arranged in an ordered repeating pattern, quasicrystals have non-repeating ordered patterns of atoms. Their unique structure leads to many exotic and interesting properties, which are particularly useful for practical applications in spintronics and magnetic refrigeration.
A unique quasicrystal variant, known as the Tsai-type icosahedral quasicrystal (iQC) and their cubic approximant crystals (ACs), display intriguing characteristics. These include long-range ferromagnetic (FM) and anti-ferromagnetic (AFM) orders, as well as unconventional quantum critical phenomenon, to name a few. Through precise compositional adjustments, these materials can also exhibit intriguing features like aging, memory, and rejuvenation, making them suitable for the development of next-generation magnetic storage devices. Despite their potential, however, the magnetic phase diagram of these materials remains largely unexplored.
To uncover more, a team of researchers, led by Professor Ryuji Tamura from the Department of Materials Science and Technology at Tokyo University of Science (TUS) in collaboration with researchers fromTohoku University recently conducted magnetization and powder neutron diffraction (PND) experiments on the non-Heisenberg Tsai-type 1/1 gold-gallium-terbium AC.
“For the first time, the phase diagrams of the non-Heisenberg Tsai-type AC have been unravelled. This will boost applied physics research on magnetic refrigeration and spintronics,” remarks Professor Tamura.
Through several experiments, the researchers developed the first comprehensive magnetic phase diagram of the non-Heisenberg Tsai-type AC, covering a broad range of electron-per-atom (e/a) ratios (a parameter crucial for understanding the fundamental nature of QCs). Additionally, measurements using the powder neutron diffraction (PND) revealed the presence of a noncoplanar whirling AFM order at an e/a ratio of 1.72 and a noncoplanar whirling FM order at the e/a ratio of 1.80. The team further elucidated the ferromagnetic and anti-ferromagnetic phase selection rule of magnetic interactions by analyzing the relative orientation of magnetic moments between nearest-neighbour and next-nearest neighbour sites.
Professor Tamura adds that their findings open up new doors for the future of condensed matter physics. “These results offer important insights into the intricate interplay between magnetic interactions in non-Heisenberg Tsai-type ACs. They lay the foundation for understanding the intriguing properties of not only non-Heisenberg ACs but also non-Heisenberg iQCs that are yet to be discovered.”
In summary, the present breakthrough propels condensed matter physics and quasicrystal research into uncharted territories, paving the way for advanced electronic devices and next-generation refrigeration technologies.
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