Quantum news biweekly vol.50, 13th April — 27th April
TL;DR
- In the quantum world particles can instantaneously know about each other’s state, even when separated by large distances. This is known as nonlocality. Now, A research group has produced some interesting findings on the Hardy nonlocality that have important ramifications for understanding quantum mechanics and its potential applications in communications.
- A team of physicists has illuminated certain properties of quantum systems by observing how their fluctuations spread over time. The research offers an intricate understanding of a complex phenomenon that is foundational to quantum computing.
- Hydrogen, the most abundant element in the universe, is found everywhere from the dust filling most of outer space to the cores of stars to many substances here on Earth. This would be reason enough to study hydrogen, but its individual atoms are also the simplest of any element with just one proton and one electron.
- Recently quantum computers started to work with more than just the zeros and ones we know from classical computers. Now a team demonstrates a way to efficiently create entanglement of such high-dimensional systems to enable more powerful calculations.
- The ‘spooky action at a distance’ that once unnerved Einstein may be on its way to being as pedestrian as the gyroscopes that currently measure acceleration in smartphones, according to a new study.
- There are high expectations that quantum computers may deliver revolutionary new possibilities for simulating chemical processes. This could have a major impact on everything from the development of new pharmaceuticals to new materials. Researchers have now used a quantum computer to undertake calculations within a real-life case in chemistry.
- Researchers report having achieved quantum teleportation from a photon to a solid-state qubit over a distance of 1km, with a novel approach using multiplexed quantum memories.
- A team has shown in the laboratory the unique and practical function of newly created materials, which they called quantum composites, that may advance electrical, optical, and computer technologies.
- Perturbing electron spins in a magnet usually results in excitations called ‘spin waves’ that ripple through the magnet like waves moving across the surface of a pond that’s been struck by a pebble. Physicists have now discovered dramatically different excitations called ‘spin excitons’ that can also ‘ripple’ through a nickel-based magnet as a coherent wave.
- Solids can be melted by heating, but in the quantum world it can also be the other way around: An experimental team has shown how a quantum liquid forms supersolid structures by heating. The scientists obtained a first phase diagram for a supersolid at finite temperature.
- 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
Increased success probability in Hardy’s nonlocality: Theory and demonstration
by Duc Minh Tran, Van-Duy Nguyen, Le Bin Ho, Hung Q. Nguyen in Physical Review A
The 2022 Nobel Prize in Physics was awarded to Alain Aspect, John Clauser, and Anton Zeilinger for their works on “quantum nonlocality” in quantum mechanics. Quantum nonlocality is a phenomenon where connected particles can affect each other instantly, regardless of the distance separated.
Imagine you owned a pair of gloves. These gloves are a pair and therefore correlated in some way, no matter how far apart they are. One day, you place one of the gloves into your backpack and hop on a flight to travel to another country, while the other glove remains at home. According to quantum nonlocality, if you changed the color of the glove you brought with you, the color of the glove back home would instantaneously change too, despite being separated by a large distance.
Nonlocality violates many of the concepts predicted by classical physics, where particles’ properties are predetermined and change occurs only through direct physical interaction or fields propagated at a finite speed. Nonlocality has a wide array of implications for understanding the future of reality, quantum mechanics, and the development of quantum technologies.
There exist several ways to define and interpret nonlocality. For instance, a set of mathematical expressions called the Bell and CHSH inequalities demonstrates nonlocality by violating inequalities. Meanwhile, Lucien Hardy proposed an alternative interpretation of quantum nonlocality in 1992 when he developed the Hardy Paradox.
Suppose there are three quantities A, B, and C, where A is greater than B and B is greater than C. Intuitively, and according to a fundamental mathematical property known as the transitive property (or local hidden variable theories in physics), this would render A greater than C. However, Hardy noted that there is still room for a situation where C is greater than A. This violates the transitive property, and such violations are possible in the quantum world when particles are entangled with each other. In other words, this is nonlocality.
We can use “rock-paper-scissors” to imagine this. While it is evident that rock beats scissors and scissors beat paper, it is impossible for the rock to beat paper. Paper beating rock does not align with any mathematical reasoning, hence why it is a paradox.
A recent study has made interesting revelations about the Hardy nonlocality. The study was co-authored by Dr. Le Bin Ho from Tohoku University’s Frontier Research Institute for Interdisciplinary Sciences (FRIS).
“The Hardy nonlocality has significant implications for understanding fundamental quantum mechanics, and it is vital for strengthening the probability of nonlocal,” said Le. “We used quantum computers and methods to investigate the measurement of Hardy nonlocality to improve its probability.”
Le and his colleagues did this by proposing a theoretical framework for attaining a higher nonlocal probability. They verified this by using a theoretical model and a quantum simulation. Despite previous studies showing the opposite, they discovered that nonlocal probability increases as the number of particles grows. This suggests that quantum effects persist even at larger scales, further challenging classical theories of physics.
Le says these findings have important ramifications for understanding quantum mechanics and its potential applications in communications. “Understanding quantum nonlocality can lead to groundbreaking technological advancements, such as the secure transmission of information through quantum communication via nonlocality resources.”
Verification of the area law of mutual information in a quantum field simulator
by Mohammadamin Tajik, Ivan Kukuljan, Spyros Sotiriadis, et al in Nature Physics
A team of physicists has illuminated certain properties of quantum systems by observing how their fluctuations spread over time. The research offers an intricate understanding of a complex phenomenon that is foundational to quantum computing — a method that can perform certain calculations significantly more efficiently than conventional computing.
“In an era of quantum computing it’s vital to generate a precise characterization of the systems we are building,” explains Dries Sels, an assistant professor in New York University’s Department of Physics and an author of the paper. “This work reconstructs the full state of a quantum liquid, consistent with the predictions of a quantum field theory — similar to those that describe the fundamental particles in our universe.”
Sels adds that the breakthrough offers promise for technological advancement.
“Quantum computing relies on the ability to generate entanglement between different subsystems, and that’s exactly what we can probe with our method,” he notes. “The ability to do such precise characterization could also lead to better quantum sensors — another application area of quantum technologies.”
The research team, which included scientists from Vienna University of Technology, ETH Zurich, Free University of Berlin, and the Max-Planck Institute of Quantum Optics, performed a tomography of a quantum system — the reconstruction of a specific quantum state with the aim of seeking experimental evidence of a theory.
The studied quantum system consisted of ultracold atoms — slow-moving atoms that make the movement easier to analyze because of their near-zero temperature — trapped on an atom chip.
In their work, the scientists created two “copies” of this quantum system — cigar-shaped clouds of atoms that evolve over time without influencing each other. At different stages of this process, the team performed a series of experiments that revealed the two copies’ correlations.
“By constructing an entire history of these correlations, we can infer what is the initial quantum state of the system and extract its properties,” explains Sels. “Initially, we have a very strongly coupled quantum liquid, which we split into two so that it evolves as two independent liquids, and then we recombine it to reveal the ripples that are in the liquid.
“It’s like watching the ripples in a pond after throwing a rock in it and inferring the properties of the rock, such as its size, shape, and weight.”
Stable Solid Molecular Hydrogen above 900 K from a Machine-Learned Potential Trained with Diffusion Quantum Monte Carlo
by Hongwei Niu, Yubo Yang, Scott Jensen, Markus Holzmann, Carlo Pierleoni, David M. Ceperley in Physical Review Letters
Hydrogen, the most abundant element in the universe, is found everywhere from the dust filling most of outer space to the cores of stars to many substances here on Earth. This would be reason enough to study hydrogen, but its individual atoms are also the simplest of any element with just one proton and one electron. For David Ceperley, a professor of physics at the University of Illinois Urbana-Champaign, this makes hydrogen the natural starting point for formulating and testing theories of matter.
Ceperley, also a member of the Illinois Quantum Information Science and Technology Center, uses computer simulations to study how hydrogen atoms interact and combine to form different phases of matter like solids, liquids, and gases. However, a true understanding of these phenomena requires quantum mechanics, and quantum mechanical simulations are costly. To simplify the task, Ceperley and his collaborators developed a machine learning technique that allows quantum mechanical simulations to be performed with an unprecedented number of atoms. They reported that their method found a new kind of high-pressure solid hydrogen that past theory and experiments missed.
“Machine learning turned out to teach us a great deal,” Ceperley said. “We had been seeing signs of new behavior in our previous simulations, but we didn’t trust them because we could only accommodate small numbers of atoms. With our machine learning model, we could take full advantage of the most accurate methods and see what’s really going on.”
Hydrogen atoms form a quantum mechanical system, but capturing their full quantum behavior is very difficult even on computers. A state-of-the-art technique like quantum Monte Carlo (QMC) can feasibly simulate hundreds of atoms, while understanding large-scale phase behaviors requires simulating thousands of atoms over long periods of time.
To make QMC more versatile, two former graduate students, Hongwei Niu and Yubo Yang, developed a machine learning model trained with QMC simulations capable of accommodating many more atoms than QMC by itself. They then used the model with postdoctoral research associate Scott Jensen to study how the solid phase of hydrogen that forms at very high pressures melts.
The three of them were surveying different temperatures and pressures to form a complete picture when they noticed something unusual in the solid phase. While the molecules in solid hydrogen are normally close-to-spherical and form a configuration called hexagonal close packed — Ceperley compared it to stacked oranges — the researchers observed a phase where the molecules become oblong figures — Ceperley described them as egg-like.
“We started with the not-too-ambitious goal of refining the theory of something we know about,” Jensen recalled. “Unfortunately, or perhaps fortunately, it was more interesting than that. There was this new behavior showing up. In fact, it was the dominant behavior at high temperatures and pressures, something there was no hint of in older theory.”
To verify their results, the researchers trained their machine learning model with data from density functional theory, a widely used technique that is less accurate than QMC but can accommodate many more atoms. They found that the simplified machine learning model perfectly reproduced the results of standard theory. The researchers concluded that their large-scale, machine learning-assisted QMC simulations can account for effects and make predictions that standard techniques cannot.
This work has started a conversation between Ceperley’s collaborators and some experimentalists. High-pressure measurements of hydrogen are difficult to perform, so experimental results are limited. The new prediction has inspired some groups to revisit the problem and more carefully explore hydrogen’s behavior under extreme conditions.
Ceperley noted that understanding hydrogen under high temperatures and pressures will enhance our understanding of Jupiter and Saturn, gaseous planets primarily made of hydrogen. Jensen added that hydrogen’s “simplicity” makes the substance important to study.
“We want to understand everything, so we should start with systems that we can attack,” he said. “Hydrogen is simple, so it’s worth knowing that we can deal with it.”
Native qudit entanglement in a trapped ion quantum processor
by Pavel Hrmo, Benjamin Wilhelm, Lukas Gerster, Martin W. van Mourik, Marcus Huber, Rainer Blatt, Philipp Schindler, Thomas Monz, Martin Ringbauer in Nature Communications
In the world of computing, we typically think of information as being stored as ones and zeros — also known as binary encoding. However, in our daily life we use ten digits to represent all possible numbers. In binary the number 9 is written as 1001 for example, requiring three additional digits to represent the same thing.
The quantum computers of today grew out of this binary paradigm, but in fact the physical systems that encode their quantum bits (qubit) often have the potential to also encode quantum digits (qudits), as recently demonstrated by a team led by Martin Ringbauer at the Department of Experimental Physics at the University of Innsbruck. According to experimental physicist Pavel Hrmo at ETH Zurich: “The challenge for qudit-based quantum computers has been to efficiently create entanglement between the high-dimensional information carriers.”
In a study the team at the University of Innsbruck now reports, how two qudits can be fully entangled with each other with unprecedented performance, paving the way for more efficient and powerful quantum computers. The example of the number 9 shows that, while humans are able calculate 9 x 9 = 81 in one single step, a classical computer (or calculator) has to take 1001 x 1001 and perform many steps of binary multiplication behind the scenes before it is able to display 81 on the screen. Classically, we can afford to do this, but in the quantum world where computations are inherently sensitive to noise and external disturbances, we need to reduce the number of operations required to make the most of available quantum computers.
Crucial to any calculation on a quantum computer is quantum entanglement. Entanglement is one of the unique quantum features that underpin the potential for quantum to greatly outperform classical computers in certain tasks. Yet, exploiting this potential requires the generation of robust and accurate higher-dimensional entanglement.
The researchers at the University of Innsbruck were now able to fully entangle two qudits, each encoded in up to 5 states of individual Calcium ions. This gives both theoretical and experimental physicists a new tool to move beyond binary information processing, which could lead to faster and more robust quantum computers.
Martin Ringbauer explains: “Quantum systems have many available states waiting to be used for quantum computing, rather than limiting them to work with qubits.” Many of today’s most challenging problems, in fields as diverse as chemistry, physics or optimisation, can benefit from this more natural language of quantum computing.
Entanglement-enhanced optomechanical sensing
by Yi Xia, Aman R. Agrawal, Christian M. Pluchar, Anthony J. Brady, Zhen Liu, Quntao Zhuang, Dalziel J. Wilson, Zheshen Zhang in Nature Photonics
The “spooky action at a distance” that once unnerved Einstein may be on its way to being as pedestrian as the gyroscopes that currently measure acceleration in smartphones.
Quantum entanglement significantly improves the precision of sensors that can be used to navigate without GPS, according to a new study.
“By exploiting entanglement, we improve both measurement sensitivity and how quickly we can make the measurement,” said Zheshen Zhang, associate professor of electrical and computer engineering at the University of Michigan and co-corresponding author of the study. The experiments were done at the University of Arizona, where Zhang was working at the time.
Optomechanical sensors measure forces that disturb a mechanical sensing device that moves in response. That motion is then measured with light waves. In this experiment, the sensors were membranes, which act like drum heads that vibrate after experiencing a push. Optomechanical sensors can function as accelerometers, which can be used for inertial navigation on a planet that doesn’t have GPS satellites or within a building as a person navigates different floors.
Quantum entanglement could make optomechanical sensors more accurate than inertial sensors currently in use. It could also enable optomechanical sensors to look for very subtle forces, such as identifying the presence of dark matter. Dark matter is invisible matter believed to account for five times more of the mass in the universe than what we can sense with light. It would tug on the sensor with gravitational force.
Here’s how entanglement improves optomechanical sensors: Optomechanical sensors rely on two synchronized laser beams. One of them is reflected from a sensor, and any movement in the sensor changes the distance that the light travels on its way to the detector. That difference in distance traveled shows up when the second wave overlaps with the first. If the sensor is still, the two waves are perfectly aligned. But if the sensor is moving, they create an interference pattern as the peaks and troughs of their waves cancel each other out in places. That pattern reveals the size and speed of vibrations in the sensor.
Usually in interferometry systems, the further the light travels, the more accurate the system becomes. The most sensitive interferometry system on the planet, the Laser Interferometer Gravitational-Wave Observatory, sends light on 8-kilometer journeys. But that’s not going to fit in a smartphone.
To enable high accuracy in miniaturized optomechanical sensors, Zhang’s team explored quantum entanglement. Rather than splitting the light once so that it bounced off a sensor and a mirror, they split each beam a second time so that the light bounced off two sensors and two mirrors. Dalziel Wilson, an assistant professor of optical sciences at the University of Arizona, along with his doctoral students Aman Agrawal and Christian Pluchar, built the membrane devices. These membranes, just 100 nanometers — or 0.0001 millimeters — thick, move in response to very small forces.
Doubling the sensors improves the accuracy, as the membranes should be vibrating in sync with each other, but the entanglement adds an extra level of coordination. Zhang’s group created the entanglement by “squeezing” the laser light. In quantum mechanical objects, such as the photons that make up light, there is a fundamental limit on how well the position and momentum of a particle can be known. Because photons are also waves, this translates to the phase of the wave (where it is in its oscillation) and its amplitude (how much energy it carries).
“Squeezing redistributes the uncertainty, so that the squeezed component is known more precisely, and the anti-squeezed component carries more of the uncertainty. We squeezed the phase because that is what we needed to know for our measurement,” said Yi Xia, a recent Ph.D. graduate from Zhang’s lab at the University of Arizona and co-corresponding author of the paper.
In squeezed light, the photons are more closely related to one another. Zhang contrasted what happens when the photons go through a beam splitter with cars coming to a fork in the freeway.
“You have three cars going one way and three cars going the other way. But in quantum superposition, each car goes both ways. Now the cars on the left are entangled with the cars on the right,” he said.
Because the fluctuations in the two entangled beams are linked, the uncertainties in their phase measurements are correlated. As a result, with some mathematical wizardry, the team was able to get measurements that are 40% more precise than with two unentangled beams, and they can do it 60% faster. What’s more, the precision and speed is expected to rise in proportion to the number of sensors.
“It is envisioned that an array of entanglement-enhanced sensors will offer orders-of-magnitude performance gain over existing sensing technology to enable the detection of particles beyond the present physical model, opening the door to a new world that is yet to be observed,” said Zhang.
The team’s next steps are to miniaturize the system. Already, they can put a squeezed-light source on a chip that is just half a centimeter to a side. They expect to have a prototype chip with the squeezed-light source, beam splitters, waveguides and inertial sensors within a year or two.
Reference-State Error Mitigation: A Strategy for High Accuracy Quantum Computation of Chemistry
by Phalgun Lolur, Mårten Skogh, Werner Dobrautz, Christopher Warren, Janka Biznárová, Amr Osman, Giovanna Tancredi, Göran Wendin, Jonas Bylander, Martin Rahm in Journal of Chemical Theory and Computation
There are high expectations that quantum computers may deliver revolutionary new possibilities for simulating chemical processes. This could have a major impact on everything from the development of new pharmaceuticals to new materials. Researchers at Chalmers University have now, for the first time in Sweden, used a quantum computer to undertake calculations within a real-life case in chemistry.
“Quantum computers could in theory be used to handle cases where electrons and atomic nuclei move in more complicated ways. If we can learn to utilise their full potential, we should be able to advance the boundaries of what is possible to calculate and understand,” says Martin Rahm, Associate Professor in Theoretical Chemistry at the Department of Chemistry and Chemical Engineering, who has led the study.
Within the field of quantum chemistry, the laws of quantum mechanics are used to understand which chemical reactions are possible, which structures and materials can be developed, and what characteristics they have. Such studies are normally undertaken with the help of super computers, built with conventional logical circuits. There is however a limit for which calculations conventional computers can handle. Because the laws of quantum mechanics describe the behaviour of nature on a subatomic level, many researchers believe that a quantum computer should be better equipped to perform molecular calculations than a conventional computer.
“Most things in this world are inherently chemical. For example, our energy carriers, within biology as well as in old or new cars, are made up of electrons and atomic nuclei arranged in different ways in molecules and materials. Some of the problems we solve in the field of quantum chemistry are to calculate which of these arrangements are more likely or advantageous, along with their characteristics,” says Martin Rahm.
There is still a way to go before quantum computers can achieve what the researchers are aiming for. This field of research is still young and the small model calculations that are run are complicated by noise from the quantum computer’s surroundings. However, Martin Rahm and his colleagues have now found a method that they see as an important step forward. The method is called Reference-State Error Mitigation (REM) and works by correcting for the errors that occur due to noise by utilising the calculations from both a quantum computer and a conventional computer.
“The study is a proof-of-concept that our method can improve the quality of quantum-chemical calculations. It is a useful tool that we will use to improve our calculations on quantum computers moving forward,” says Martin Rahm.
The principle behind the method is to first consider a reference state by describing and solving the same problem on both a conventional and a quantum computer. This reference state represents a simpler description of a molecule than the original problem intended to be solved by the quantum computer. A conventional computer can solve this simpler version of the problem quickly. By comparing the results from both computers, an exact estimate can be made for the amount of error caused by noise. The difference between the two computers’ solutions for the reference problem can then be used to correct the solution for the original, more complex, problem when it is run on the quantum processor. By combining this new method with data from Chalmers’ quantum computer Särimner* the researchers have succeeded in calculating the intrinsic energy of small example molecules such as hydrogen and lithium hydride. Equivalent calculations can be carried out more quickly on a conventional computer, but the new method represents an important development and is the first demonstration of a quantum chemical calculation on a quantum computer in Sweden.
“We see good possibilities for further development of the method to allow calculations of larger and more complex molecules, when the next generation of quantum computers are ready,” says Martin Rahm.
The research has been conducted in close collaboration with colleagues at the Department of Microtechnology and Nanoscience. They have built the quantum computers that are used in the study, and helped perform the sensitive measurements that are needed for the chemical calculations.
“It is only by using real quantum algorithms that we can understand how our hardware really works and how we can improve it. Chemical calculations are one of the first areas where we believe that quantum computers will be useful, so our collaboration with Martin Rahm’s group is especially valuable,” says Jonas Bylander, Associate Professor in Quantum Technology at the Department of Microtechnology and Nanoscience.
Long distance multiplexed quantum teleportation from a telecom photon to a solid-state qubit
by Dario Lago-Rivera, Jelena V. Rakonjac, Samuele Grandi, Hugues de Riedmatten in Nature Communications
Quantum teleportation is a technique allowing the transfer of quantum information between two distant quantum objects, a sender and a receiver, using a phenomenon called quantum entanglement as a resource. The unique feature of this process is that the actual information is not transferred by sending quantum bits (qubits) through a communication channel connecting the two parties; instead, the information is destroyed at one location and appears at the other one without physically travelling between the two. This surprising property is enabled by quantum entanglement, accompanied by the transmission of classical bits.
There is a deep interest in quantum teleportation nowadays within the field of quantum communications and quantum networks because it would allow the transfer of quantum bits between network nodes over very long distances, using previously shared entanglement. This would help the integration of quantum technologies into current telecommunication networks and extend the ultra-secure communications enabled by these systems to very long distances. In addition, quantum teleportation permits the transfer of quantum information between different kinds of quantum systems, e.g. between light and matter or between different kinds of quantum nodes.
Quantum teleportation was theoretically proposed in the early 90s and experimental demonstrations were carried out by several groups around the world. While the scientific community has gained extensive experience on how to perform these experiments, there is still an open question on how to teleport information in a practical way, allowing reliable and fast quantum communication over an extended network. It seems clear that such an infrastructure should be compatible with the current telecommunications network. In addition, the protocol of quantum teleportation requires a final operation to be applied on the teleported qubit, conditioned on the result of the teleportation measurement (transmitted by classical bits), in order to transfer the information faithfully and at a higher rate, a feature called active feed-forward. This means that the receiver requires a device known as a quantum memory that can store the qubit without degrading it until the final operation can be implemented. Finally, this quantum memory should be able to operate in a multiplexed fashion to maximize the speed of teleporting information when the sender and the receiver are far away. To date, no implementation had incorporated these three requirements in the same demonstration.
In a recent study, ICFO researchers Dario Lago-Rivera, Jelena V. Rakonjac, Samuele Grandi, led by ICREA Prof. at ICFO Hugues de Riedmatten have reported achieving long distance teleportation of quantum information from a photon to a solid-state qubit, a photon stored in a multiplexed quantum memory. The technique involved the use of an active feed-forward scheme, which, together with the multimodality of the memory, has allowed maximization of the teleportation rate. The proposed architecture was compatible with the telecommunications channels, and thus enabling future integration and scalability for long-distance quantum communication.
The team built two experimental setups, that in the jargon of the community are usually called Alice and Bob. The two setups were connected by a 1km optical fiber spun up in a spool, to emulate a physical distance between the parties. Three photons were involved in the experiment. In the first setup, Alice, the team used a special crystal to create two entangled photons: the first photon at 606 nm, called signal photon, and the second photon called idler photon, compatible with the telecommunications infrastructure. Once created, “we kept the first 606 nm photon at Alice and stored it in a multiplexed solid-state quantum memory, holding it in the memory for future processing. At the same time, we took the telecom photon created at Alice and sent it through the 1km of optical fiber to reach the second experimental setup, called Bob,” Dario Lago recalls.
In this second setup, Bob, the scientists had another crystal where they created a third photon, where they had encoded the quantum bit they wanted to teleport. Once the third photon was created, the second photon had arrived to Bob from Alice, and this is where the core of the teleportation experiment takes place.
The second and third photons interfered with each other through what is known as a Bell State measurement (BSM). The effect of this measurement was to mix the state of the second and third photon. Thanks to the fact that the first and second photons were entangled to begin with, i.e. their joint state was highly correlated, the result of the BSM was that of transferring the information encoded in the third photon to the first one, stored by Alice in the quantum memory, 1 km away. As Dario Lago and Jelena Rakonjac mention, “we are capable of transferring information between two photons that were never in contact before, but connected through a third photon that was indeed entangled with the first. The uniqueness of this experiment lies in the fact that we employed a multiplexed quantum memory capable of storing the first photon for long enough such that by the time Alice found out that the interaction had happened, we were still able to process the teleported information as the protocol requires.”
This processing that Dario and Jelena mention was the active feed-forward technique mentioned earlier. Depending on the outcome of the BSM, a phase-shift was applied to the first photon after storage in the memory. In this way, the same state would always be encoded in the first photon. Without this, half of the teleportation events would have to be discarded. Moreover, the multimodality of the quantum memory allowed them to increase the teleportation rate beyond the limits imposed by the 1 km separation between them without degrading the quality of the teleported qubit. Overall, this resulted in a teleportation rate three times higher than for a single-mode quantum memory, only limited by the speed of the classical hardware.
The experiment carried out by this group in 2021, where they achieved for the first time entanglement of two multimode quantum memories separated by 10 meters and heralded by a photon at the telecommunication wavelength, has been the precursor of this experiment. As Hugues de Riedmatten emphasizes, “Quantum teleportation will be crucial for enabling high-quality long-distance communication for the future quantum internet. Our goal is to implement quantum teleportation in more and more complex networks, with previously distributed entanglement. The solid-state and multiplexed nature of our quantum nodes as well as their compatibility with the telecom network make them a promising approach to deploy the technology over long distance in the installed fiber network.”
Further improvements are already being planned. On the one hand, the team is focused on developing and improving the technology in order to extend the setup to much longer distances while maintaining the efficiency and rates. On the other hand, they also aim at studying and using this technique in the transfer of information between different types of quantum nodes, for a future quantum Internet that will be able to distribute and process quantum information between remote parties.
Quantum Composites with Charge‐Density‐Wave Fillers
by Zahra Barani, Tekwam Geremew, Megan Stokey, Nicholas Sesing, Maedeh Taheri, Matthew J. Hilfiker, Fariborz Kargar, Mathias Schubert, Tina T. Salguero, Alexander A. Balandin in Advanced Materials
A team of UCR electrical engineers and material scientists demonstrated a research breakthrough that may result in wide-ranging advancements in electrical, optical, and computer technologies.
The Marlan and Rosemary Bourns College of Engineering research group, led by distinguished professor Alexander Balandin, has shown in the laboratory the unique and practical function of newly created materials, which they called quantum composites.
These composites consist of small crystals of called “charge density wave quantum materials” incorporated within a polymer (large molecules with repeating structures) matrix. Upon heating or light exposure, charge density wave material undergoes a phase transition that leads to an unusual electrical response of the composites.
Compared to other materials that reveal quantum phenomena, the quantum composites created by Balandin’s group exhibited functionality at a much wider range of temperatures and had a greatly increased ability to store electricity, giving them an excellent potential for utility. The University of California, Riverside, researchers describe the unique properties. The lead authors of the paper are Zahra Barani and Tekwam Geremew, UCR graduate students with the college’s Department of Electrical and Computer Engineering, who synthesized and tested the composites. Another UCR graduate student Maedeh Taheri is a co-author who helped with electrical measurements. Balandin and Fariborz Kargar, an assistant adjunct professor and project scientist, are the corresponding authors.
The term quantum refers to materials and devices where electrons behave more like waves than particles. The wave nature of electrons can give materials unusual properties that are used in a new generation of computer, electronic and optic technologies.
Materials that reveal quantum phenomena are sought for building quantum computers that go beyond the limitations of most computing that is now based on chips that use binary bits for computations. Such materials are also sought for super-sensitive sensors used for various electronic and optic applications. But the materials with quantum phenomena have major drawbacks, Balandin said.
“The problem with these materials is that the quantum phenomena are fragile and typically observed only at extremely low temperatures,” he said. “The defects and impurities destroy the electron wave function.”
Remarkably, the charge density wave material in the quantum composites created by Balandin’s lab exhibited functionality as high as 50º C above room temperature. This transition temperature is close to the temperature of the operation of computers and other electronic gadgets, which heat up when they operate. This temperature tolerance opens a possibility for a wide range of applications of quantum composites in electronics and energy storage.
The researchers also found that quantum composites have an unusually high dielectric constant — a metric that characterizes the material’s ability to store electricity. The dielectric constant of the electrically insulating composites increased by more than two orders of magnitude, which allows for smaller and more powerful capacitors used for energy storage.
“Energy storage capacitors can be found in battery-powered applications,” Balandin said. “Capacitors can be used to deliver peak power and provide energy for computer memory during an unexpected shut-off. Capacitors can charge and discharge faster compared to batteries. In order to broaden the use of capacitors for energy storage, one needs to increase the energy per volume. Our quantum composite material may help to achieve this goal.”
Another possible application for quantum composites is reflective coating. The change in the dielectric constant induced by heating, light exposure, or application of an electrical field can be used to change the light reflection from the glasses and windows coated with such composites.
“We hope that our ability to preserve the quantum condensate phases in the charge-density-wave materials even inside disordered composites and even above room temperature can become a game changer for many applications. It is a conceptually different approach for tuning the properties of composites that we use in everyday life,” Balandin added.
Diffusive excitonic bands from frustrated triangular sublattice in a singlet-ground-state system
by Bin Gao, Tong Chen, Xiao-Chuan Wu, Michael Flynn, Chunruo Duan, et al in Nature Communications
Perturbing electron spins in a magnet usually results in excitations called “spin waves” that ripple through the magnet like waves on a pond that’s been struck by a pebble. In a new study, Rice University physicists and their collaborators have discovered dramatically different excitations called “spin excitons” that can also “ripple” through a nickel-based magnet as a coherent wave.
In a study, the researchers reported finding unusual properties in nickel molybdate, a layered magnetic crystal. Subatomic particles called electrons resemble miniscule magnets, and they typically orient themselves like compass needles in relation to magnetic fields. In experiments where neutrons were scattered from magnetic nickel ions inside the crystals, the researchers found that two outermost electrons from each nickel ion behaved differently. Rather than aligning their spins like compass needles, the two canceled one another in a phenomenon physicists call a spin singlet.
“Such a substance should not be a magnet at all,” said Rice’s Pengcheng Dai, corresponding author of the study. “And if a neutron scatters off a given nickel ion, the excitations should remain local and not propagate through the sample.”
Dai and his collaborators were therefore surprised when instruments in the neutron-scattering experiments detected not one, but two families of propagating waves, each at dramatically different energies. To understand the waves’ origins, it was necessary to delve into the atomic details of the magnetic crystals. For instance, electromagnetic forces from atoms in crystals can compete with the magnetic field and affect electrons inside neighboring atoms. This is called the crystal field effect, and it can force electron spins to orient themselves along directions distinct from the orientation of the magnetic field. Probing crystal field effects in the nickel molybdate crystals required additional experiments and theoretical interpretation of the data from the experiments.
“The collaboration between experimental groups and theory is paramount to painting a full picture and understanding the unusual spin excitations observed in this compound,” said Rice co-author Emilia Morosan.
Morosan’s group probed the thermal response of the crystals to changes in temperature using specific heat measurements. From those experiments, the researchers concluded that two kinds of crystal field environments occurred in the layered nickel molybdate, and the two affected nickel ions very differently.
“In one, the field effect is rather weak and corresponds to a thermal energy of about 10 Kelvin,” said study co-author Andriy Nevidomskyy, a theoretical physicist at Rice who helped interpret the experimental data. “It is perhaps not surprising to see, at few-Kelvin temperatures, that neutrons can excite magnetic spin waves from nickel atoms that are subject to this first type of crystal field effect. But it is most puzzling to see them coming from nickel atoms that are subject to the second type. Those atoms have a tetrahedral arrangement of oxygens around them, and the electric field effect is nearly 20-fold stronger, meaning the excitations are that much harder to create.”
Nevidomskyy said this can be understood as if the spins on the corresponding nickel ions had different “mass.”
“The analogy is that of heavy basketballs that are intermixed with tennis balls,” he said. “To excite the spins of the second type, the heavier basketballs, one must administer a stronger ‘kick’ by shining more energetic neutrons at the material.”
The resulting effect on the nickel spin is called a spin exciton, and one would normally expect the effect of the exciton-producing “kick” to be confined to a single atom. But measurements from the experiments indicated “basketballs” were moving in unison, creating an unexpected sort of wave. Even more surprising, the waves appeared to persist at relatively high temperatures where the crystals no longer behaved as magnets.
The explanation offered by Nevidomskyy and theorist co-author Leon Balents from the University of California, Santa Barbara was: Heavier spin excitons — basketballs in the analogy — bob in response to the fluctuations of the surrounding, lighter magnetic excitons — the analogous tennis balls — and if the interactions between the two types of balls are sufficiently strong, the heavier spin excitons participate in a coherent motion akin to a wave.
“What is particularly interesting,” Dai said, “is that the two kinds of nickel atoms each form a triangular lattice, and the magnetic interactions within this lattice are therefore frustrated.”
In magnetism on triangular lattices, frustration refers to the difficulty in aligning all the magnetic moments anti-parallel (up-down) with respect to their three immediate, nearest neighbors. Understanding the role of magnetic frustrations in triangular lattices is one of the long-standing challenges that Dai and Nevidomskyy have both been working to address for a number of years.
“It is very exciting to find a puzzle, against one’s expectations, and then feel a sense of satisfaction of having understood its origin,” said Nevidomskyy.
Heating a dipolar quantum fluid into a solid
by J. Sánchez-Baena, C. Politi, F. Maucher, F. Ferlaino, T. Pohl. in Nature Communications
Supersolids are a relatively new and exciting area of research. They exhibit both solid and superfluid properties simultaneously. In 2019, three research groups were able to demonstrate this state for the first time beyond doubt in ultracold quantum gases, among them the research group led by Francesca Ferlaino from the Department of Experimental Physics at the University of Innsbruck and the ÖAW Institute for Quantum Optics and Quantum Information (IQOQI) in Innsbruck.
In 2021, Francesca Ferlaino’s team studied in detail the life cycle of supersolid states in a dipolar gas of dysprosium atoms. They observed something unexpected: “Our data suggested that an increase in temperature promotes the formation of supersolid structures,” recounts Claudia Politi of Francesca Ferlaino’s team. “This surprising behaviour was an important boost to theory, which had previously paid little attention to thermal fluctuations in this context.”
The Innsbruck scientists joined the force with the danish theoretical group led by Thomas Pohl to explore the effect of thermal fluctuation. They developed and published a theoretical model that can explain the experimental results and underlines the thesis that heating the quantum liquid can lead to the formation of a quantum crystal. The theoretical model shows that as the temperature rises, these structures can form more easily.
“With the new model, we now have a phase diagram for the first time that shows the formation of a supersolid state as a function of temperature,” Francesca Ferlaino is delighted to say. “The surprising behavior, which contradicts our everyday observation, arises from the anisotropic nature of the dipole-dipole interaction of the strongly magnetic atoms of dysprosium.”
MISC
Subscribe to Paradigm!
Medium. Twitter. Telegram. Telegram Chat. Reddit. LinkedIn.
Main sources
Research articles
