QT/ Experiment opens door for millions of qubits on one chip
Quantum news biweekly vol. 73, 9th May — 23rd May
TL;DR
- Researchers have achieved the first controllable interaction between two hole spin qubits in a conventional silicon transistor. The breakthrough opens up the possibility of integrating millions of these qubits on a single chip using mature manufacturing processes.
- MIT physicists and colleagues have created a five-lane superhighway for electrons that could allow ultra-efficient electronics and more. The work is one of several important discoveries by the same team over the last year involving a material that is essentially a unique form of pencil lead.
- Researchers have used surface acoustic waves to overcome a significant obstacle in the quest to realize a quantum internet. In a new study scientists describe a technique for pairing particles of light and sound that could be used to faithfully convert information stored in qubits to optical fields, which can be transmitted over long distances.
- For the first time, the state of an atomic nucleus was switched with a laser. For decades, physicists have been looking for such a nuclear transition — now it has been found. Now, nuclei can be used for extremely precise measurements. For example, a nuclear clock could be built that could measure time more precisely than the best atomic clocks available today.
- Researchers propose a new approach to discovering the tauonium — the smallest and heaviest atom with pure electromagnetic interaction.
- A research team has created an innovative method to control tiny magnetic states within ultrathin, two-dimensional van der Waals magnets — a process akin to how flipping a light switch controls a bulb.
- A lead-vacancy (PbV) center in diamond has been developed as a quantum emitter for large-scale quantum networks by researchers. This innovative color center exhibits a sharp zero-phonon-line and emits photons with specific frequencies.
- Researchers have demonstrated that ferromagnetism, an ordered state of atoms, can be induced by increasing particle motility and that repulsive forces between atoms are sufficient to maintain it.
- A new technique can generate batches of certain entangled states in a quantum processor. This advance could help scientists study the fundamental quantum property of entanglement and enable them to build larger and more complex quantum processors.
- Researchers have successfully achieved robust superconductivity in high magnetic fields using a newly created one-dimensional (1D) system. This breakthrough offers a promising pathway to achieving superconductivity in the quantum Hall regime, a longstanding challenge in condensed matter physics.
- And more!
Quantum Computing Market
According to the recent market research report by MarketsandMarkets, the Quantum Computing market is expected to grow to USD 5,3 million by 2029, at a CAGR of 32.7%. The 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.
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
Anisotropic exchange interaction of two hole-spin qubits
by Simon Geyer, Bence Hetényi, Stefano Bosco, Leon C. Camenzind, Rafael S. Eggli, Andreas Fuhrer, Daniel Loss, Richard J. Warburton, Dominik M. Zumbühl, Andreas V. Kuhlmann in Nature Physics
Researchers from the University of Basel and the NCCR SPIN have achieved the first controllable interaction between two hole spin qubits in a conventional silicon transistor. The breakthrough opens up the possibility of integrating millions of these qubits on a single chip using mature manufacturing processes.
The race to build a practical quantum computer is well underway. Researchers around the world are working on a huge variety of qubit technologies. So far, there is no consensus on what type of qubit is most suitable for maximizing the potential of quantum information science.
Qubits are the foundation of a quantum computer: they handle the processing, transfer and storage of data. To work correctly, they have to both reliably store and rapidly process information. The basis for rapid information processing is stable and fast interactions between a large number of qubits whose states can be reliably controlled from the outside.
For a quantum computer to be practical, millions of qubits must be accommodated on a single chip. The most advanced quantum computers today have only a few hundred qubits, meaning they can only perform calculations that are already possible (and often more efficient) on conventional computers..
To solve the problem of arranging and linking thousands of qubits, researchers at the University of Basel and the NCCR SPIN rely on a type of qubit that uses the spin (intrinsic angular momentum) of an electron or a hole. A hole is essentially a missing electron in a semiconductor. Both holes and electrons possess spin, which can adopt one of two states: up or down, analogous to 0 and 1 in classical bits. Compared to an electron spin, a hole spin has the advantage that it can be entirely electrically controlled without needing additional components like micromagnets on the chip.
As early as 2022, Basel physicists were able to show that the hole spins in an existing electronic device can be trapped and used as qubits. These “FinFETs” (fin field-effect transistors) are built into modern smartphones and are produced in widespread industrial processes. Now, a team led by Dr. Andreas Kuhlmann has succeeded for the first time in achieving a controllable interaction between two qubits within this setup.
A quantum computer needs “quantum gates” to perform calculations. These represent operations that manipulate the qubits and couple them to each other. As the researchers report, they were able to couple two qubits and bring about a controlled flip of one of their spins, depending on the state of the other’s spin — known as a controlled spin-flip.
“Hole spins allow us to create two-qubit gates that are both fast and high-fidelity. This principle now also makes it possible to couple a larger number of qubit pairs,” says Kuhlmann.
The coupling of two spin qubits is based on their exchange interaction, which occurs between two indistinguishable particles that interact with each other electrostatically. Surprisingly, the exchange energy of holes is not only electrically controllable, but strongly anisotropic. This is a consequence of spin-orbit coupling, which means that the spin state of a hole is influenced by its motion through space.
To describe this observation in a model, experimental and theoretical physicists at the University of Basel and the NCCR SPIN combined forces. “The anisotropy makes two-qubit gates possible without the usual trade-off between speed and fidelity,” Dr. Kuhlmann says in summary.
Large quantum anomalous Hall effect in spin-orbit proximitized rhombohedral graphene
by Tonghang Han et al in Science
MIT physicists and colleagues have created a five-lane superhighway for electrons that could allow ultra-efficient electronics and more. The work is one of several important discoveries by the same team over the last year involving a material that is essentially a unique form of pencil lead.
“This discovery has direct implications for low-power electronic devices because no energy is lost during the propagation of electrons, which is not the case in regular materials where the electrons are scattered,” says Long Ju, an assistant professor in the MIT Department of Physics and corresponding author of the paper.
The phenomenon is akin to cars traveling down an open turnpike as opposed to those moving through neighborhoods. The neighborhood cars can be stopped or slowed by other drivers making abrupt stops or U-turns that disrupt an otherwise smooth commute.
The material behind this work, known as rhombohedral pentalayer graphene, was discovered two years ago by physicists led by Ju. “We found a goldmine, and every scoop is revealing something new,” says Ju, who is also affiliated with MIT’s Materials Research Laboratory.
In a paper last October Ju and colleagues reported the discovery of three important properties arising from rhombohedral graphene. For example, they showed that it could be topological, or allow the unimpeded movement of electrons around the edge of the material but not through the middle. That resulted in a superhighway, but required the application of a large magnetic field some tens of thousands times stronger than the Earth’s magnetic field. In the current work, the team reports creating the superhighway without any magnetic field.
Tonghang Han, an MIT graduate student in physics, is a co-first author of the paper. “We are not the first to discover this general phenomenon, but we did so in a very different system. And compared to previous systems, ours is simpler and also supports more electron channels,” explains Ju. “Other materials can only support one lane of traffic on the edge of the material. We suddenly bumped it up to five.”
Additional co-first authors of the paper who contributed equally to the work are Zhengguang Lu and Yuxuan Yao. Lu is a postdoctoral associate in the Materials Research Laboratory. Yao conducted the work as a visiting undergraduate student from Tsinghua University. Other authors are MIT Professor Liang Fu of physics; Jixiang Yang and Junseok Seo, both graduate students in MIT physics; Chiho Yoon and Fan Zhang of the University of Texas at Dallas; and Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Japan.
Pencil lead, or graphite, is composed of graphene, a single layer of carbon atoms arranged in hexagons resembling a honeycomb structure. Rhombohedral graphene is composed of five layers of graphene stacked in a specific overlapping order.
Ju and colleagues isolated rhombohedral graphene thanks to a novel microscope Ju built at MIT in 2021 that can quickly and relatively inexpensively determine a variety of important characteristics of a material at the nanoscale. Pentalayer rhombohedral stacked graphene is only a few billionths of a meter thick.
In the current work, the team tinkered with the original system, adding a layer of tungsten disulfide (WS2). “The interaction between the WS2 and the pentalayer rhombohedral graphene resulted in this five-lane superhighway that operates at zero magnetic field,” says Ju.
The phenomenon that the Ju group discovered in rhombohedral graphene that allows electrons to travel with no resistance at zero magnetic field is known as the quantum anomalous Hall effect. Most people are more familiar with superconductivity, a completely different phenomenon that does the same thing but happens in very different materials.
Ju notes that although superconductors were discovered in the 1910s, it took some 100 years of research to coax the system to work at the higher temperatures necessary for applications. “And the world record is still well below room temperature,” he notes.
Similarly, the rhombohedral graphene superhighway currently operates at about 2 Kelvin, or -456 Fahrenheit. “It will take a lot of effort to elevate the temperature, but as physicists, our job is to provide the insight; a different way for realizing this [phenomenon],” Ju says.
Coherent optical coupling to surface acoustic wave devices
by Arjun Iyer et al in Nature Communications
Researchers at the University of Rochester have used surface acoustic waves to overcome a significant obstacle in the quest to realize a quantum internet.
In a new study, scientists from Rochester’s Institute of Optics and Department of Physics and Astronomy describe a technique for pairing particles of light and sound that could be used to faithfully convert information stored in quantum systems — qubits — to optical fields, which can be transmitted over long distances.
Surface acoustic waves are vibrations that glide along the exterior of materials like a wave in the ocean or tremors along the ground during an earthquake. They are used for a variety of applications — many of the electrical components of our phones have surface acoustic wave filters — because they make very precise cavities that can be used for precise timing in uses like navigation. But scientists have begun using them in quantum applications as well.
“In the last 10 years, surface acoustic waves have emerged as a good resource for quantum applications because the phonon, or individual particle of sound, couples very well to different systems,” says William Renninger, associate professor of optics and physics.
Using existing methods, surface acoustic waves are accessed, manipulated, and controlled through piezoelectric materials to turn electricity into acoustic waves and vice versa. However, these electric signals must be applied to mechanical fingers inserted into the middle of the acoustic cavity, which cause parasitic effects by scattering phonons in ways that have to be compensated for.
Rather than coupling the phonons to electric fields, Renninger’s lab tried a less invasive approach, shining light on the cavities and eliminating the need for mechanical contact.
“We were able to strongly couple surface acoustic waves with light,” says Arjun Iyer, an optics Ph.D. student and first author of the paper. “We designed acoustic cavities, or tiny echo chambers, for these waves where sound could last for a long time, allowing for stronger interactions. Notably, our technique works on any material, not just the piezoelectric materials that can be electrically controlled.”
Renninger’s team partnered with the lab of Associate Professor of Physics John Nichol to make the surface acoustic wave devices described in the study. In addition to producing strong quantum coupling, the devices have the added benefits of simple fabrication, small size, and the ability to handle large amounts of power.
Beyond applications in hybrid quantum computing, the team says their techniques can be used for spectroscopy to explore the property of materials, as sensors, and to study condensed matter physics.
Laser Excitation of the Th-229 Nucleus
by J. Tiedau, M. V. Okhapkin, K. Zhang, J. Thielking, G. Zitzer, E. Peik, F. Schaden, T. Pronebner, I. Morawetz, L. Toscani De Col, F. Schneider, A. Leitner, M. Pressler, G. A. Kazakov, K. Beeks, T. Sikorsky, T. Schumm in Physical Review Letters
Physicists have been hoping for this moment for a long time: for many years, scientists all around the world have been searching for a very specific state of thorium atomic nuclei that promises revolutionary technological applications. It could be used, for example, to build an nuclear clock that could measure time more precisely than the best atomic clocks available today. It could also be used to answer completely new fundamental questions in physics — for example, the question of whether the constants of nature are actually constant or whether they change in space and time.
Now this hope has come true: the long-sought thorium transition has been found, its energy is now known exactly. For the first time, it has been possible to use a laser to transfer an atomic nucleus into a state of higher energy and then precisely track its return to its original state. This makes it possible to combine two areas of physics that previously had little to do with each other: classical quantum physics and nuclear physics. A crucial prerequisite for this success was the development of special thorium-containing crystals. A research team led by Prof. Thorsten Schumm from TU Wien (Vienna) has now published this success together with a team from the National Metrology Institute Braunschweig (PTB).
Manipulating atoms or molecules with lasers is commonplace today: if the wavelength of the laser is chosen exactly right, atoms or molecules can be switched from one state to another. In this way, the energies of atoms or molecules can be measured very precisely. Many precision measurement techniques are based on this, such as today’s atomic clocks, but also chemical analysis methods. Lasers are also often used in quantum computers to store information in atoms or molecules.
For a long time, however, it seemed impossible to apply these techniques to atomic nuclei. “Atomic nuclei can also switch between different quantum states. However, it usually takes much more energy to change an atomic nucleus from one state to another — at least a thousand times the energy of electrons in an atom or a molecule,” says Thorsten Schumm. “This is why normally atomic nuclei cannot be manipulated with lasers. The energy of the photons is simply not enough.”
This is unfortunate, because atomic nuclei are actually the perfect quantum objects for precision measurements: They are much smaller than atoms and molecules and are therefore much less susceptible to external disturbances, such as electromagnetic fields. In principle, they would therefore allow measurements with unprecedented accuracy.
Since the 1970s, there has been speculation that there might be a special atomic nucleus which, unlike other nuclei, could perhaps be manipulated with a laser, namely thorium-229. This nucleus has two very closely adjacent energy states — so closely adjacent that a laser should in principle be sufficient to change the state of the atomic nucleus.
For a long time, however, there was only indirect evidence of the existence of this transition. “The problem is that you have to know the energy of the transition extremely precisely in order to be able to induce the transition with a laser beam,” says Thorsten Schumm. “Knowing the energy of this transition to within one electron volt is of little use, if you have to hit the right energy with a precision of one millionth of an electron volt in order to detect the transition.” It is like looking for a needle in a haystack — or trying to find a small treasure chest buried on a kilometer-long island.
Some research groups have tried to study thorium nuclei by holding them individually in place in electromagnetic traps. However, Thorsten Schumm and his team chose a completely different technique. “We developed crystals in which large numbers of thorium atoms are incorporated,” explains Fabian Schaden, who developed the crystals in Vienna and measured them together with the PTB team. “Although this is technically quite complex, it has the advantage that we can not only study individual thorium nuclei in this way but can hit approximately ten to the power of seventeen thorium nuclei simultaneously with the laser — about a million times more than there are stars in our galaxy.” The large number of thorium nuclei amplifies the effect, shortens the required measurement time and increases the probability of actually finding the energy transition.
On November 21, 2023, the team was finally successful: the correct energy of the thorium transition was hit exactly, the thorium nuclei delivered a clear signal for the first time. The laser beam had actually switched their state. After careful examination and evaluation of the data, the result has now been published.
“For us, this is a dream coming true,” says Thorsten Schumm. Since 2009, Schumm had focused his research entirely on the search for the thorium transition. His group as well as competing teams from all over the world have repeatedly achieved important partial successes in recent years. “Of course we are delighted that we are now the ones who can present the crucial breakthrough: The first targeted laser excitation of an atomic nucleus,” says Schumm.
Novel method for identifying the heaviest QED atom
by Jing-Hang Fu et al in Science Bulletin
The hydrogen atom was once considered the simplest atom in nature, composed of a structureless electron and a structured proton. However, as research progressed, scientists discovered a simpler type of atom, consisting of structureless electrons, muons, or tauons and their equally structureless antiparticles. These atoms are bound together solely by electromagnetic interactions, with simpler structures than hydrogen atoms, providing a new perspective on scientific problems such as quantum mechanics, fundamental symmetry, and gravity.
To date, only two types of atoms with pure electromagnetic interactions have been discovered: the electron-positron bound state discovered in 1951 and the electron-antimuon bound state discovered in 1960. Over the past 64 years, there have been no other signs of such atoms with pure electromagnetic interactions, although there are some proposals to search for them in cosmic rays or high-energy colliders.
Tauonium, composed of a tauon and its antiparticle, has a Bohr radius of only 30.4 femtometers (1 femtometer = 10–15 meters), approximately 1/1,741 of the Bohr radius of a hydrogen atom. This implies that tauonium can test the fundamental principles of quantum mechanics and quantum electrodynamics at smaller scales, providing a powerful tool for exploring the mysteries of the micro-material world.
The study demonstrates that by collecting data of 1.5 ab-1 near the threshold of tauon pair production at an electron and positron collider and selecting signal events containing charged particles accompanied by the undetected neutrinos carrying away energy, the significance of observing tauonium will exceed 5σ. This indicates strong experimental evidence for the existence of tauonium.
The study also found that using the same data, the precision of measuring the tau lepton mass can be improved to an unprecedented level of 1 keV, two orders of magnitude higher than the highest precision achieved by current experiments. This achievement will not only contribute to the precise testing of the electroweak theory in the Standard Model but also have profound implications for fundamental physics questions such as lepton flavor universality.
This achievement serves as one of the most important physical objectives of the proposed Super Tau-Charm Facility (STCF) in China or the Super Charm-Tau Factory (SCTF) in Russia: to discover the smallest and heaviest atom with pure electromagnetic interactions by running the machine near the tauon pair threshold for one year and to measure the tau lepton mass with a high precision.
Tunneling current-controlled spin states in few-layer van der Waals magnets
by ZhuangEn Fu, Piumi I. Samarawickrama, John Ackerman, Yanglin Zhu, Zhiqiang Mao, Kenji Watanabe, Takashi Taniguchi, Wenyong Wang, Yuri Dahnovsky, Mingzhong Wu, TeYu Chien, Jinke Tang, Allan H. MacDonald, Hua Chen, Jifa Tian in Nature Communications
Imagine a future where computers can learn and make decisions in ways that mimic human thinking, but at a speed and efficiency that are orders of magnitude greater than the current capability of computers.
A research team at the University of Wyoming created an innovative method to control tiny magnetic states within ultrathin, two-dimensional (2D) van der Waals magnets — a process akin to how flipping a light switch controls a bulb.
“Our discovery could lead to advanced memory devices that store more data and consume less power or enable the development of entirely new types of computers that can quickly solve problems that are currently intractable,” says Jifa Tian, an assistant professor in the UW Department of Physics and Astronomy and interim director of UW’s Center for Quantum Information Science and Engineering.
Van der Waals materials are made up of strongly bonded 2D layers that are bound in the third dimension through weaker van der Waals forces. For example, graphite is a van der Waals material that isbroadly used in industry in electrodes, lubricants, fibers, heat exchangers and batteries. The nature of the van der Waals forces between layers allows researchers to use Scotch tape to peel the layers into atomic thickness.
The team developed a device known as a magnetic tunnel junction, which uses chromium triiodide — a 2D insulating magnet only a few atoms thick — sandwiched between two layers of graphene. By sending a tiny electric current — called a tunneling current — through this sandwich, the direction of the magnet’s orientation of the magnetic domains (around 100 nanometers in size) can be dictated within the individual chromium triiodide layers, Tian says.
Specifically, “this tunneling current not only can control the switching direction between two stable spin states, but also induces and manipulates switching between metastable spin states, called stochastic switching,” says ZhuangEn Fu, a graduate student in Tian’s research lab and now a postdoctoral fellow at the University of Maryland.
“This breakthrough is not just intriguing; it’s highly practical. It consumes three orders of magnitude smaller energy than traditional methods, akin to swapping an old lightbulb for an LED, marking it a potential game-changer for future technology,” Tian says. “Our research could lead to the development of novel computing devices that are faster, smaller and more energy-efficient and powerful than ever before. Our research marks a significant advancement in magnetism at the 2D limit and sets the stage for new, powerful computing platforms, such as probabilistic computers.”
Traditional computers use bits to store information as 0’s and 1’s. This binary code is the foundation of all classic computing processes. Quantum computers use quantum bits that can represent both “0” and “1” at the same time, increasing processing power exponentially.
“In our work, we’ve developed what you might think of as a probabilistic bit, which can switch between ‘0’ and ‘1’ (two spin states) based on the tunneling current controlled probabilities,” Tian says. “These bits are based on the unique properties of ultrathin 2D magnets and can be linked together in a way that is similar to neurons in the brain to form a new kind of computer, known as a probabilistic computer.
“What makes these new computers potentially revolutionary is their ability to handle tasks that are incredibly challenging for traditional and even quantum computers, such as certain types of complex machine learning tasks and data processing problems,” Tian continues. “They are naturally tolerant to errors, simple in design and take up less space, which could lead to more efficient and powerful computing technologies.”
Transform-Limited Photon Emission from a Lead-Vacancy Center in Diamond above 10 K
by Peng Wang, Lev Kazak, Katharina Senkalla, Petr Siyushev, Ryotaro Abe, Takashi Taniguchi, Shinobu Onoda, Hiromitsu Kato, Toshiharu Makino, Mutsuko Hatano, Fedor Jelezko, Takayuki Iwasaki in Physical Review Letters
Much like how electric circuits use components to control electronic signals, quantum networks rely on special components and nodes to transfer quantum information between different points, forming the foundation for building quantum systems. In the case of quantum networks, color centers in diamond, which are defects intentionally added to a diamond crystal, are crucial for generating and maintaining stable quantum states over long distances.
When stimulated by external light, these color centers in diamond emit photons carrying information about their internal electronic states, especially the spin states. The interaction between the emitted photons and the spin states of the color centers enables quantum information to be transferred between different nodes in quantum networks.
A well-known example of color centers in diamond is the nitrogen-vacancy (NV) center, where a nitrogen atom is added adjacent to missing carbon atoms in the diamond lattice. However, the photons emitted from NV color centers do not have well-defined frequencies and are affected by interactions with the surrounding environment, making it challenging to maintain a stable quantum system.
To address this, an international group of researchers, including Associate Professor Takayuki Iwasaki from Tokyo Institute of Technology, has developed a single negatively charged lead-vacancy (PbV) center in diamond, where a lead atom is inserted between neighboring vacancies in a diamond crystal. In the study, the researchers reveal that the PbV center emits photons of specific frequencies that are not influenced by the crystal’s vibrational energy. These characteristics make the photons dependable carriers of quantum information for large-scale quantum networks.
For stable and coherent quantum states, the emitted photon must be transform-limited, which means that it should have the minimum possible spread in its frequency. Additionally, it should have emission into zero-phonon-line (ZPL), meaning that the energy associated with the emission of photons is only used to change the electronic configuration of the quantum system, and not exchanged with the vibrational lattice modes (phonons) in the crystal lattice.
To fabricate the PbV center, the researchers introduced lead ions beneath the diamond surface through ion implantation. An annealing process was then carried out to repair any damage caused by the lead ion implantation. The resulting PbV center exhibits a spin 1/2 system, with four distinct energy states with the ground and the excited state split into two energy levels. On photoexciting the PbV center, electron transitions between the energy levels produced four distinct ZPLs, classified by the researchers as A, B, C, and D based on the decreasing energy of the associated transitions. Among these, the C transition was found to have a transform-limited linewidth of 36 MHz.
“We investigated the optical properties of single PbV centers under resonant excitation and demonstrated that the C-transition, one of the ZPLs, reaches the nearly transform-limit at 6.2 K without prominent phonon-induced relaxation and spectral diffusion,” says Dr. Iwasaki.
The PbV center stands out by being able to maintain its linewidth at approximately 1.2 times the transform-limit at temperatures as high as 16 K. This is important to achieve around 80% visibility in two-photon interference. In contrast, color centers like SiV, GeV, and SnV need to be cooled to much lower temperatures (4 K to 6 K) for similar conditions. By generating well-defined photons at relatively high temperatures compared to other color centers, the PbV center can function as an efficient quantum light-matter interface, which enables quantum information to be carried long distances by photons via optical fibers.
“These results can pave the way for the PbV center to become a building block to construct large-scale quantum networks,” concludes Dr. Iwasaki.
Activity-induced ferromagnetism in one-dimensional quantum many-body systems
by Kazuaki Takasan, Kyosuke Adachi, Kyogo Kawaguchi in Physical Review Research
Researchers Kazuaki Takasan and Kyogo Kawaguchi of the University of Tokyo with Kyosuke Adachi of RIKEN, Japan’s largest comprehensive research institution, have demonstrated that ferromagnetism, an ordered state of atoms, can be induced by increasing particle motility and that repulsive forces between atoms are sufficient to maintain it. The discovery not only extends the concept of active matter to quantum systems but also contributes to the development of novel technologies that rely on the magnetic properties of particles, such as magnetic memory and quantum computing.
Flocking birds, swarming bacteria, cellular flows. These are all examples of active matter, a state in which individual agents, such as birds, bacteria, or cells, self-organize. The agents change from a disordered to an ordered state in what is called a “phase transition.” As a result, they move together in an organized fashion without an external controller.
“Previous studies have shown that the concept of active matter can apply to a wide range of scales, from nanometers (biomolecules) to meters (animals),” says Takasan, the first author. “However, it has not been known whether the physics of active matter can be applied usefully in the quantum regime. We wanted to fill in that gap.”
To fill the gap, the researchers needed to demonstrate a possible mechanism that could induce and maintain an ordered state in a quantum system. It was a collaborative work between physics and biophysics. The researchers took inspiration from the phenomena of flocking birds because, due to the activity of each agent, the ordered state is more easily achieved than in other types of active matter. They created a theoretical model in which atoms were essentially mimicking the behavior of birds. In this model, when they increased the motility of the atoms, the repulsive forces between atoms rearranged them into an ordered state called ferromagnetism. In the ferromagnetic state, spins, the angular momentum of subatomic particles and nuclei, align in one direction, just like how flocking birds face the same direction while flying.
“It was surprising at first to find that the ordering can appear without elaborate interactions between the agents in the quantum model,” Takasan reflects on the finding. “It was different from what was expected based on biophysical models.”
The researcher took a multi-faceted approach to ensure their finding was not a fluke. Thankfully, the results of computer simulations, mean-field theory, a statistical theory of particles, and mathematical proofs based on linear algebra were all consistent. This strengthened the reliability of their finding, the first step in a new line of research.
“The extension of active matter to the quantum world has only recently begun, and many aspects are still open,” says Takasan. “We would like to further develop the theory of quantum active matter and reveal its universal properties.”
Probing entanglement in a 2D hard-core Bose–Hubbard lattice
by Amir H. Karamlou, Ilan T. Rosen, Sarah E. Muschinske, Cora N. Barrett, et al in Nature
Entanglement is a form of correlation between quantum objects, such as particles at the atomic scale. This uniquely quantum phenomenon cannot be explained by the laws of classical physics, yet it is one of the properties that explains the macroscopic behavior of quantum systems.
Because entanglement is central to the way quantum systems work, understanding it better could give scientists a deeper sense of how information is stored and processed efficiently in such systems.
Qubits, or quantum bits, are the building blocks of a quantum computer. However, it is extremely difficult to make specific entangled states in many-qubit systems, let alone investigate them. There are also a variety of entangled states, and telling them apart can be challenging. Now, MIT researchers have demonstrated a technique to efficiently generate entanglement among an array of superconducting qubits that exhibit a specific type of behavior.
Over the past years, the researchers at the Engineering Quantum Systems (EQuS) group have developed techniques using microwave technology to precisely control a quantum processor composed of superconducting circuits. In addition to these control techniques, the methods introduced in this work enable the processor to efficiently generate highly entangled states and shift those states from one type of entanglement to another — including between types that are more likely to support quantum speed-up and those that are not.
“Here, we are demonstrating that we can utilize the emerging quantum processors as a tool to further our understanding of physics. While everything we did in this experiment was on a scale which can still be simulated on a classical computer, we have a good roadmap for scaling this technology and methodology beyond the reach of classical computing,” says Amir H. Karamlou ’18, MEng ’18, PhD ’23, the lead author of the paper.
The senior author is William D. Oliver, the Henry Ellis Warren professor of electrical engineering and computer science and of physics, director of the Center for Quantum Engineering, leader of the EQuS group, and associate director of the Research Laboratory of Electronics. Karamlou and Oliver are joined by Research Scientist Jeff Grover, postdoc Ilan Rosen, and others in the departments of Electrical Engineering and Computer Science and of Physics at MIT, at MIT Lincoln Laboratory, and at Wellesley College and the University of Maryland.
In a large quantum system comprising many interconnected qubits, one can think about entanglement as the amount of quantum information shared between a given subsystem of qubits and the rest of the larger system. The entanglement within a quantum system can be categorized as area-law or volume-law, based on how this shared information scales with the geometry of subsystems. In volume-law entanglement, the amount of entanglement between a subsystem of qubits and the rest of the system grows proportionally with the total size of the subsystem.
On the other hand, area-law entanglement depends on how many shared connections exist between a subsystem of qubits and the larger system. As the subsystem expands, the amount of entanglement only grows along the boundary between the subsystem and the larger system. In theory, the formation of volume-law entanglement is related to what makes quantum computing so powerful.
“While have not yet fully abstracted the role that entanglement plays in quantum algorithms, we do know that generating volume-law entanglement is a key ingredient to realizing a quantum advantage,” says Oliver.
However, volume-law entanglement is also more complex than area-law entanglement and practically prohibitive at scale to simulate using a classical computer.
“As you increase the complexity of your quantum system, it becomes increasingly difficult to simulate it with conventional computers. If I am trying to fully keep track of a system with 80 qubits, for instance, then I would need to store more information than what we have stored throughout the history of humanity,” Karamlou says.
The researchers created a quantum processor and control protocol that enable them to efficiently generate and probe both types of entanglement. Their processor comprises superconducting circuits, which are used to engineer artificial atoms. The artificial atoms are utilized as qubits, which can be controlled and read out with high accuracy using microwave signals.
The device used for this experiment contained 16 qubits, arranged in a two-dimensional grid. The researchers carefully tuned the processor so all 16 qubits have the same transition frequency. Then, they applied an additional microwave drive to all of the qubits simultaneously. If this microwave drive has the same frequency as the qubits, it generates quantum states that exhibit volume-law entanglement. However, as the microwave frequency increases or decreases, the qubits exhibit less volume-law entanglement, eventually crossing over to entangled states that increasingly follow an area-law scaling.
“Our experiment is a tour de force of the capabilities of superconducting quantum processors. In one experiment, we operated the processor both as an analog simulation device, enabling us to efficiently prepare states with different entanglement structures, and as a digital computing device, needed to measure the ensuing entanglement scaling,” says Rosen.
To enable that control, the team put years of work into carefully building up the infrastructure around the quantum processor. By demonstrating the crossover from volume-law to area-law entanglement, the researchers experimentally confirmed what theoretical studies had predicted. More importantly, this method can be used to determine whether the entanglement in a generic quantum processor is area-law or volume-law.
In the future, scientists could utilize this technique to study the thermodynamic behavior of complex quantum systems, which is too complex to be studied using current analytical methods and practically prohibitive to simulate on even the world’s most powerful supercomputers.
“The experiments we did in this work can be used to characterize or benchmark larger-scale quantum systems, and we may also learn something more about the nature of entanglement in these many-body systems,” says Karamlou.
One-dimensional proximity superconductivity in the quantum Hall regime
by Julien Barrier, Minsoo Kim, Roshan Krishna Kumar, Na Xin, P. Kumaravadivel, Lee Hague, E. Nguyen, A. I. Berdyugin, Christian Moulsdale, V. V. Enaldiev, J. R. Prance, F. H. L. Koppens, R. V. Gorbachev, K. Watanabe, T. Taniguchi, L. I. Glazman, I. V. Grigorieva, V. I. Fal’ko, A. K. Geim in Nature
In a significant development in the field of superconductivity, researchers at The University of Manchester have successfully achieved robust superconductivity in high magnetic fields using a newly created one-dimensional (1D) system. This breakthrough offers a promising pathway to achieving superconductivity in the quantum Hall regime, a longstanding challenge in condensed matter physics.
Superconductivity, the ability of certain materials to conduct electricity with zero resistance, holds profound potential for advancements of quantum technologies. However, achieving superconductivity in the quantum Hall regime, characterised by quantised electrical conductance, has proven to be a mighty challenge.
The research details extensive work of the Manchester team led by Professor Andre Geim, Dr Julien Barrier and Dr Na Xin to achieve superconductivity in the quantum Hall regime. Their initial efforts followed the conventional route where counterpropagating edge states were brought into close proximity of each other. However, this approach proved to be limited.
“Our initial experiments were primarily motivated by the strong persistent interest in proximity superconductivity induced along quantum Hall edge states,” explains Dr Barrier, the paper’s lead author. “This possibility has led to numerous theoretical predictions regarding the emergence of new particles known as non-abelian anyons.”
The team then explored a new strategy inspired by their earlier work demonstrating that boundaries between domains in graphene could be highly conductive. By placing such domain walls between two superconductors, they achieved the desired ultimate proximity between counterpropagating edge states while minimising effects of disorder.
“We were encouraged to observe large supercurrents at relatively ‘balmy’ temperatures up to one Kelvin in every device we fabricated,” Dr Barrier recalls.
Further investigation revealed that the proximity superconductivity originated not from the quantum Hall edge states propagating along domain walls, but rather from strictly 1D electronic states existing within the domain walls themselves. These 1D states, proven to exist by the theory group of Professor Vladimir Fal’ko’s at the National Graphene Institute, exhibited a greater ability to hybridise with superconductivity as compared to quantum Hall edge states. The inherent one-dimensional nature of the interior states is believed to be responsible for the observed robust supercurrents at high magnetic fields.
This discovery of single-mode 1D superconductivity shows exciting avenues for further research. “In our devices, electrons propagate in two opposite directions within the same nanoscale space and without scattering,” Dr Barrier elaborates. “Such 1D systems are exceptionally rare and hold promise for addressing a wide range of problems in fundamental physics.”
The team has already demonstrated the ability to manipulate these electronic states using gate voltage and observe standing electron waves that modulated the superconducting properties.
“It is fascinating to think what this novel system can bring us in the future. The 1D superconductivity presents an alternative path towards realising topological quasiparticles combining the quantum Hall effect and superconductivity,” concludes Dr Xin. This is just one example of the vast potential our findings holds.”
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