Quantum computing has been touted as a revolutionary advance that uses our growing scientific understanding of the subatomic world to create a machine with powers far beyond those of conventional computers.
Google scientists said Wednesday they have passed a major milestone in their quest to develop effective quantum computing, with a new study showing they reduced the rate of errors—long an obstacle for the much-hyped technology.
Quantum computing has been touted as a revolutionary advance that uses our growing scientific understanding of the subatomic world to create a machine with powers far beyond those of today’s conventional computers.
However the technology remains largely theoretical, with many thorny problems still standing in the way—including stubbornly high error rates.
In new research published in the journal Nature, the Google Quantum AI lab described a system that can significantly decrease the error rate.
That could give the US tech giant a step up on its rivals such as IBM, which is also working on superconducting quantum processors.
While traditional computers process information in bits that can be represented by 0 or 1, quantum computers use qubits, which can be a combination of both at the same time.
This property, known as superposition, means that a quantum computer can crunch an enormous number of potential outcomes simultaneously.
The computers harness some of the most mind-boggling aspects of quantum mechanics, including a phenomenon known as “entanglement”—in which two members of a pair of bits can exist in a single state, even if far apart.
‘Magic’
But a problem called decoherence can cause the qubits to lose their information when they leave their quantum state and come into contact with the outside world.
This fragility causes high error rates, which also increase with the number of qubits, frustrating scientists wanting to ramp up their experiments.
However Google’s team said it had demonstrated for the first time in practice that a system using error-correcting code can detect and fix errors without affecting the information.
The system was first theorized in the 1990s, however previous attempts had just thrown up more errors, not less, said Google’s Hartmut Neven, a co-author of the study.
“But if all components of your system have sufficiently low error rates, then the magic of quantum error correction kicks in,” Neven told a press conference.
Julian Kelly, another study co-author, hailed the development as “a key scientific milestone”, saying that “quantum error correction is the single most important technology for the future of quantum computing“.
Neven said the result was still “not good enough, we need to get to an absolute low error rate”.
He added that “there are more steps to come” to achieve the dream of a useable quantum computer.
Google claimed in 2019 it had passed a milestone known as “quantum supremacy”, when the tech giant said its Sycamore machine executed a calculation in 200 seconds that would have taken a conventional supercomputer 10,000 years to complete.
However the achievement has since been disputed, with Chinese researchers saying last year that a supercomputer could have beaten Sycamore’s time.
More information: Suppressing quantum errors by scaling a surface code logical qubit, Nature (2023). DOI: 10.1038/s41586-022-05434-1
QTM imaging of the energy bands of TBG. Credit: Nature (2023). DOI: 10.1038/s41586-022-05685-y
One of the striking aspects of the quantum world is that a particle, say, an electron, is also a wave, meaning that it exists in many places at the same time. In a new study, reported today in Nature, researchers from the Weizmann Institute of Science make use of this property to develop a new type of tool—the quantum twisting microscope (QTM)—that can create novel quantum materials while simultaneously gazing into the most fundamental quantum nature of their electrons.
The study’s findings may be used to create electronic materials with unprecedented functionalities.
The QTM involves the “twisting,” or rotating, of two atomically-thin layers of material with respect to one another. In recent years, such twisting has become a major source of discoveries. It began with the discovery that placing two layers of graphene, one-atom-thick crystalline sheets of carbon, one atop the other with a slight relative twist angle, leads to a “sandwich” with unexpected new properties.
The twist angle turned out to be the most critical parameter for controlling the behavior of electrons: Changing it by merely one-tenth of a degree could transform the material from an exotic superconductor into an unconventional insulator. But critical as it is, this parameter is also the hardest to control in experiments. By and large, twisting two layers to a new angle requires building a new “sandwich” from scratch, a process that is very long and tedious.
“Our original motivation was to solve this problem by building a machine that could continuously twist any two materials with respect to one another, readily producing an infinite range of novel materials,” says team leader Prof. Shahal Ilani of Weizmann’s Condensed Matter Physics Department. “However, while building this machine, we discovered that it can also be turned into a very powerful microscope, capable of seeing quantum electronic waves in ways that were unimaginable before.”
Creating a quantum picture
Pictures have long played a central role in scientific discovery. Light microscopes and telescopes routinely provide images that allow scientists to gain a deeper understanding of biological and astrophysical systems. Taking pictures of electrons inside materials, on the other hand, has for many years been notoriously hard, owing to the small dimensions involved.
This was transformed some 40 years ago with the invention of the scanning tunneling microscope, which earned its developers the 1986 Nobel Prize in Physics. This microscope uses an atomically sharp needle to scan the surface of a material, measuring the electric current and gradually building an image of the distribution of electrons in the sample.
“Many different scanning probes have been developed since this invention, each measuring a different electronic property, but all of them measure these properties at one location at a time. So, they mostly see electrons as particles, and can only indirectly learn about their wave nature,” explains Prof. Ady Stern from the Weizmann Institute, who took part in the study along with three other theoretical physicists from the same department: Profs. Binghai Yan, Yuval Oreg and Erez Berg.
“As it turned out, the tool that we have built can visualize the quantum electronic waves directly, giving us a way to unravel the quantum dances they perform inside the material,” Stern says.
An animation showing the quantum twisting microscope in action. Electrons tunnel from the probe (inverted pyramid at the top) to the sample (bottom) as quantum mechanical waves (red). Credit: Weizmann Institute of Science
Spotting an electron in several places at once
“The trick for seeing quantum waves is to spot the same electron in different locations at the same time,” says Alon Inbar, a lead author on the paper. “The measurement is conceptually similar to the famous two-slit experiment, which was used a century ago to prove for the first time that electrons in quantum mechanics have a wave nature,” adds Dr. John Birkbeck, another lead author. “The only difference is that we perform such an experiment at the tip of our scanning microscope.”
To achieve this, the researchers replaced the atomically sharp tip of the scanning tunneling microscope with a tip that contains a flat layer of a quantum material, such as a single layer of graphene. When this layer is brought into contact with the surface of the sample of interest, it forms a two-dimensional interface across which electrons can tunnel at many different locations.
Quantum mechanically, they tunnel in all locations simultaneously, and the tunneling events at different locations interfere with each other. This interference allows an electron to tunnel only if its wave functions on both sides of the interface match exactly. “To see a quantum electron, we have to be gentle,” says Ilani. “If we don’t ask it the rude question ‘Where are you?’ but instead provide it with multiple routes to cross into our detector without us knowing where it actually crossed, we allow it to preserve its fragile wave-like nature.”
Twist and tunnel
Generally, the electronic waves in the tip and the sample propagate in different directions and therefore do not match. The QTM uses its twisting capability to find the angle at which matching occurs: By continuously twisting the tip with respect to the sample, the tool causes their corresponding wave functions to also twist with respect to one another. Once these wave functions match on both sides of the interface, tunneling can occur.
The twisting therefore allows the QTM to map how the electronic wave function depends on momentum, similarly to the way lateral translations of the tip enable the mapping of its dependence on position.
Merely knowing at which angles electrons cross the interface supplies the researchers with a great deal of information about the probed material. In this manner they can learn about the collective organization of electrons within the sample, their speed, energy distribution, patterns of interference and even the interactions of different waves with one another.
A new twist on quantum materials
“Our microscope will give scientists a new kind of ‘lens’ for observing and measuring the properties of quantum materials,” says Jiewen Xiao, another lead author.
The Weizmann team has already applied their microscope to studying the properties of several key quantum materials at room temperature and is now gearing up toward doing new experiments at temperatures of a few kelvins, where some of the most exciting quantum mechanical effects are known to take place.
Peering so deeply into the quantum world can help reveal fundamental truths about nature. In the future, it might also have a tremendous effect on emerging technologies. The QTM will provide researchers with access to an unprecedented spectrum of new quantum interfaces, as well as new “eyes” for discovering quantum phenomena within them.
(a) Electronic and magnetic phase diagram extracted from this study on films synthesized on NdGaO3 substrates. AFM and PM refer to antiferromagnetic and paramagnetic, respectively. The filled and empty symbols indicate the temperatures extracted for warming and cooling cycles, respectively, from electronic transport (◇), RXS (∘) and ARPES (□) measurements. The insets show the Fermi surfaces measured by ARPES for the AFM metallic phase and the PM metallic phase of a sample with x = 0.03. (b) Magnitude of PHE versus in-plane cooling field angle taken at 1.8 K for I // [100] (red) and I // [110] (blue). All data in this figure was taken on samples with x = 0.02–0.03. The light square points were taken with the field turned on (9 T), whereas the dark round points were taken after the removal of the field (0 T). The dashed lines serve as guides to the eye. The cartoon shows the sample geometry for I // [100]. Credit: Nature Physics (2023). DOI: 10.1038/s41567-022-01907-2
Researchers at Harvard University, the Lawrence Berkeley National Laboratory, Arizona State University, and other institutes in the United States have recently observed an antiferromagnetic metal phase in electron-doped NdNiO3 a material known to be a non-collinear antiferromagnet (i.e., exhibiting an onset of antiferromagnetic ordering that is concomitant with a transition into an insulating state).
“Previous works on the rare-earth nickelates (RNiO3) have found them to host a rather exotic phase of magnetism known as a ‘noncollinear antiferromagnet,'” Qi Song, Spencer Doyle, Luca Moreschini and Julia A. Mundy, Four of the researchers who carried out the study, told Phys.org.
“This type of magnet has unique potential applications in the field of spintronics, yet rare-earth nickelates famously change spontaneously from being metallic to insulating at the exact same temperature that this noncollinear antiferromagnet phase turns on. We wanted to see if we could somehow modify one of these materials in a way so that it remained metallic, but still had this interesting magnetic phase.”
Ensuring that rare-earth nickelates remain metallic at low temperatures, where their antiferromagnetic phase appears, would ultimately enable their use for the development of spintronic devices. In their experiments, Song, Doyle, Moreschini, Mundy and their colleagues tried to achieve this using the rare-earth nickelate NdNiO3.
To prompt the material to retain its antiferromagnet metal phase without eliciting its transition into an insulator, they used electron doping, a technique for changing the number of electrons in materials. Essentially, they grew a series of NdNiO3 samples in which they added varying amounts of cerium atoms in the place of neodymium atoms, to add more electrons to the system.
“Once we had these samples, we collected electrical transport measurements, where we applied a small current through each sample and measured the resistance,” Doyle said. “By performing this measurement as we changed the temperature of the sample, we were able to deduce whether or not the sample was metallic or insulating in the magnetic phase.”
Ultimately, to demonstrate the potential of the electron-doped antiferromagnetic metal sample they realized for the development of spintronic devices, the team also collected a further measurement, known as the “zero field planar Hall effect.” Simply put, this measurement can be used to verify whether a material can “remember” whether a magnetic field had been applied to it or not, even after this field was turned off.
These tests yielded very promising results, as the electron-doped samples produced by Doyle and his colleagues demonstrated this “memory effect.” Remarkably, the effect observed in these materials was very strong compared to those typically observed in antiferromagnets.
“We created a new phase in the family of nickelates, which was not seen before. The key property of these materials is that they have a metal-insulator transition, which comes together with a magnetic one, as it often happens,” Moreschini explained. “Above the transition, you have weak or no magnetic order, and below you do have one, in this case, antiferromagnetism. In other previous studies, this transition had been suppressed in some conditions, but for the first time we succeeded in decoupling them: the magnetic transition is still there, but the metal-insulator transition is gone.”
The electron doping-based strategy proposed by this team of researchers allowed them to elicit an antiferromagnetic metal phase in a rare-earth nickelate. Below the temperature at which the material transitions to an antiferromagnetic phase, it is still a metal, even if a less performing metal. Overall, their electron doped-samples are thus metals with an antiferromagnetic order, a phase that had not been observed before in nickelates.
In the future, the recent study by Song, Doyle, Moreschini, Mundy and their colleagues could open new and exciting possibilities for the development of spintronic devices based on rare-earth nickelates. In their next work, the team plans to explore strategies that would allow them to further increase the temperature at which the metal-metal transition of these rare-earth nickelates takes place.
“The ultimate goal of course is to push it all the way to room temperature, because that is where you want your devices to work. At the moment honestly speaking, the nickelates are still far from there, but in a broader perspective what we learned here from this new phase can hopefully guide the engineering of new phases in other families of oxides or other materials in general where these transitions happen a little closer to room temperature, and tweaking a bit the electronic correlations you can give them that last push to have them at room temperature,” say the researchers.
More information: Qi Song et al, Antiferromagnetic metal phase in an electron-doped rare-earth nickelate, Nature Physics (2023). DOI: 10.1038/s41567-022-01907-2
Schematic of the moiré superlattice formed between tungsten diselenide and tungsten disulfide, filled with one charge carrier per moiré unit cell. Credit: Lawrence Berkeley National Laboratory
The shifting, scintillating pattern you can see when you stack two slightly misaligned window screens is called moiré. A similar interference effect occurs when scientists stack two-dimensional crystals with mismatched atomic spacings. Moiré superlattices display exotic physical properties that are absent in the layers that make up the patterns. These properties are rooted in the quantum nature of electrons.
Researchers have discovered a new property in the moiré superlattices formed in crystals made of tungsten diselenide/tungsten disulfide(WSe2/WS2). In these two-dimensional crystals, the interactions between electrons become so strong that electrons “freeze” and form an ordered array.
WSe2/WS2 moiré superlattices turn out to be an optimal playground for tuning the interactions between electrons. The stronger these interactions, the more prominent the quantum mechanical nature of solid materials. This allows exotic states of matter like unconventional superconductivity to form.
Researchers used lasers to “observe” the electron motion without the artifacts that plague other measurement techniques. They uncovered a rare quantum state of matter, never before observed in moiré superlattices. Understanding and controlling the quantum motion of electrons will allow scientists to build microelectronic devices of the future and robust qubits for quantum computing.
In solids, the energy levels that electrons occupy form energy bands. Moiré superlattices alter the atomic periodicity seen by the electrons and thus the energy bands. Moiré effects can lead to “flat” bands, in which the energy levels are squeezed together, causing electrons to lower their kinetic energy and thus to feel their mutual repulsion more strongly.
A team of researchers at Lawrence Berkeley National Laboratory (LBNL) used a novel optical technique to observe electron motion, while changing the number of electrons injected in the sample. When only one carrier per moiré unit cell was injected, the electrons were expected to move freely and thus conduct electricity. Instead, the sample became insulating. This result illustrates the Mott insulator state, in which electrons interact so strongly that they avoid being in the same cell. If every cell is occupied, then the electrons stop moving.
The real surprise came when fewer electrons were injected so that only one half or one third of the cells were occupied. At these low densities, scientists expected the electrons to feel their presence less and have high mobility. However, the sample turned out an insulator. In WSe2/WS2, electrons interact so strongly that they even avoid sitting on neighboring sites. This rare phenomenon is known as Wigner electron crystal.
LBNL researchers also demonstrated that in WSe2/WS2, light with appropriate polarization interacts with spin-up and spin-down electrons separately, making it possible to selectively change the energy of electrons based on their spin. In doing so, they observed spin excitations persisting orders of magnitude longer than charge excitations. This opens the door for the future investigation of exotic spin states such as quantum spin liquidity.
Related research was previously published in 2020 in the journal Nature.
More information: Emma C. Regan et al, Mott and generalized Wigner crystal states in WSe2/WS2 moiré superlattices, Nature (2020). DOI: 10.1038/s41586-020-2092-4
The Laboratoire Sous-marin Provence Méditerranée (LSPM) lies 40 km off the coast of Toulon, at a depth of 2,450 m, inaccessible even to sunlight. Through this national research platform run by the CNRS in collaboration with Aix-Marseille University (AMU) and IFREMER, scientists will investigate undersea unknowns while scanning the skies for neutrinos. These elementary particles of extraterrestrial origin know few obstacles and can even traverse our planet without bumping into a single atom.
The main instrument at the LSPM is KM3NeT, a giant neutrino detector developed by a team of 250 researchers from 17 countries. In the pitch-black abyss, KM3NeT will study the trails of bluish light that neutrinos leave in the water. Capable of detecting dozens of these particles a day, it will help elucidate their quantum properties, which still defy our understanding.
The other LSPM instruments will permit the scientific community to study the life and chemistry of these depths. They will offer researchers insights into ocean acidification, deep-sea deoxygenation, marine radioactivity, and seismicity, and allow them to track cetacean populations as well as observe bioluminescent animals. This oceanographic instrumentation is integrated into the subsea observatory network of the EMSO European research infrastructure.
LSPM is structured around a series of titanium junction boxes and intelligent systems able to power multiple scientific instruments and retrieve their data in real time, through a 42-km-long electro-optical cable. The base currently has three junction boxes, but the future addition of a second cable could bring the number up to five.
An unusual form of cesium atom is helping a University of Queensland-led research team unmask unknown particles that make up the universe.
Dr. Jacinda Ginges, from UQ’s School of Mathematics and Physics, said the unusual atom—made up of an ordinary cesium atom and an elementary particle called a muon—may prove essential in better understanding the universe’s fundamental building blocks.
“Our universe is still such a mystery to us,” Dr. Ginges said.
“Astrophysical and cosmological observations have shown that the matter we know about—commonly referred to as ‘Standard Model’ particles in physics—makes up only five percent of the matter and energy content of the universe.”
“Most matter is ‘dark’, and we currently know of no particle or interaction within the Standard Model that explains it.”
“The search for dark matter particles lies at the forefront of particle physics research, and our work with cesium might prove essential in solving this mystery.”
The work may also one day improve technology.
“Atomic physics plays a major role in technologies we use every day, such as navigation with the Global Positioning System (GPS), and atomic theory will continue to be important in the advancement of new quantum technologies based on atoms,” Dr. Ginges said.
Through theoretical research, Dr. Ginges and her team have improved the understanding of the magnetic structure of cesium’s nucleus, its effects in atomic cesium and the effects of the weird and wonderful muon.
“A muon is basically a heavy electron—200 times more massive—and it orbits the nucleus 200 times closer than the electrons,” Dr. Ginges said.
“Because of this, it can pick up on details of the structure of the nucleus.”
“It sounds complicated, but in a nutshell, this work will help to improve atomic theory calculations that are used in the search for new particles.”
The researchers said the new approach can offer greater sensitivity and an alternative technique to finding new particles, through the use of precision atomic measurements.
“You may have heard of the Large Hadron Collider at CERN, the world’s largest and most powerful particle accelerator, which smashes together subatomic matter at high energies to find previously unseen particles,” Dr. Ginges said.
“But our research can offer greater sensitivity, with an alternative technique to find new particles—through precision atomic measurements.”
“It doesn’t need a giant collider, and instead uses precision instruments to look for atomic changes at low energy.”
“Rather than explosive, high-energy collisions, it’s the equivalent of creating an ultra-sensitive ‘microscope’ to witness the true nature of atoms.”
“This can be a more sensitive technique, unveiling particles that particle colliders simply can’t see.”
Cesium is having a moment, after being featured in the news recently, as the element in the radioactive capsule that went missing, and was subsequently found, in Western Australia’s outback.
This research, led by Dr. Ginges, was performed together with graduate student George Sanamyan and Dr. Benjamin Roberts, and has been published in Physical Review Letters.
More information: G. Sanamyan et al, Empirical Determination of the Bohr-Weisskopf Effect in Cesium and Improved Tests of Precision Atomic Theory in Searches for New Physics, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.053001
All-optical logic gates are essential elements for the optical processing of information, since they overcome the fundamental difficulties of their electronic counterparts, in particular the limited data transfer speed and bandwidth.
Recently, silicon has been used as a basic element in making passive and active photonic devices due to its high thermal and mechanical properties, stability, high quality, low loss, and large bandwidth extending from 1.55 μm to nearly 7 μm.
In a new study, Amer Kotb and Li Wei at Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS) and Kyriakos Zoiros at the Democritus University of Thrace in Greece have designed seven basic logic operations—NOT, XOR, AND, OR, NOR, NAND and XNOR—using silicon-on-silica waveguides operated at 1.55 μm.
These logic operations are realized based on the constructive and destructive interferences between the input beams. The operations’ performance is evaluated against the contrast ratio (CR).
With the convolutional perfectly matched layer as an absorbing boundary condition, a Lumerical finite-difference-time-domain is run to simulate and demonstrate the operation of the proposed logic operations.
The simulation results published in Physica Scripta suggest that a compact waveguide can be used to realize all-optical logic gates with higher CRs and a speed as high as 120 Gb/s compared with previous designs.
More information: Amer Kotb et al, Silicon-on-silica waveguides-based all-optical logic gates at 1.55 μm, Physica Scripta (2023). DOI: 10.1088/1402-4896/acbb40
Quantum dot with a spherical impurity. Credit: K Hasanirokh, L H Abbud
Quantum dots are semiconductor particles measuring just a few nanometers across, which are now widely studied for their intriguing electrical and optical properties. Through new research published in The European Physical Journal B, Kobra Hasanirokh at Azarbaijan Shahid Madani University in Iran, together with Luay Hashem Abbud at Al-Mustaqbal University College, Iraq, show how quantum dots containing spherical defects can significantly enhance their nonlinear optical properties. By fine-tuning these defects, researchers could tightly control the frequency and brightness of the light emitted by quantum dots.
The duo’s discoveries could lead to new advances in optoelectronic devices including LEDs and light-based computer circuits, which operate through the interaction between light and electricity. If achieved, improvements to this technology could significantly boost the speed of computing and communications systems.
Nonlinear optical properties can arise in some materials when they are illuminated with intense light, producing new photons with identical frequencies and waveforms to the original light. Increasingly, researchers are exploring the potential of third-order nonlinear optical processes, which generate photons with triple the frequency of the original light. Recent studies have shown that these processes are readily triggered in spherical quantum dots, containing spherical defects in the structure of their atomic lattices.
Building on this research, Hasanirokh and Abbud explored how third-order nonlinear susceptibility can be controlled by varying the numbers and sizes of these defects. In their study, the duo used mathematical techniques to consider multi-layered quantum dots containing a cadmium sulfide core and a zinc sulfide outer shell. These layers were separated by a spherical defect containing a carefully-adjusted mixture of cadmium, zinc, and sulfur.
By fine-tuning this structure, the researchers calculated that its third-order nonlinear optical properties could be considerably enhanced, allowing them to tightly control the brightness and frequency of the light produced. They now hope their results could inspire new techniques for quantum dot manufacturing, which could bring their advanced theoretical structure into reality.
More information: Kobra Hasanirokh et al, Third-order nonlinear susceptibility in CdS/Cdx1Zn1−x1S/ZnS multilayer spherical quantum dot, The European Physical Journal B (2023). DOI: 10.1140/epjb/s10051-022-00464-0
Polydisperse particles. Schematic of the experimental setup and microscopic image of polydisperse oil droplets confined in two-dimensional space. Credit: Yanagisawa and Shimamoto
The ways in which particles, such as sand or liquid droplets, behave during various mechanical processes is well studied. Typically, in situations where space is constrained, jams can occur, and understanding this can be useful in various industries. However, only instances where the particles in question are similar or have a limited range of sizes have been successfully modeled. For the first time, a model has been made that describes bodies of particles with highly diverse sizes, and in different jamming scenarios.
If you’ve ever put a bunch of balls in a box, you probably noticed the amount of wasted space between them all, especially if they’re all the same size. But the more varied the sizes of the balls, the more of that wasted space can be filled, due to the presence of smaller ones that can fill the gaps between larger ones. There is an intuitive obviousness to this situation, but as is often the case with such things, it’s surprisingly difficult to model the way this actually happens. And the less regular the packing objects, or particles, become, the harder it is to know how they will behave in a given physical scenario.
For the first time, researchers from the Komaba Institute for Science at the University of Tokyo have identified a structure that commonly appears when particles with extreme size variation are randomly packed together, regardless of their size distribution. This kind of model can be extremely useful to people whose work involves the movement, separation or mixing of particulate matter. For example, the construction industry works with stones, sand, cement; the medical industry uses biomolecules, powders, oil droplets; food manufacturers pack grains, seeds, fruits; and so on, the list is extensive.
Particle packing efficiency. Examples of packing patterns for polydisperse systems numerically produced by the researchers’ model. Credit: Yanagisawa and Shimamoto
“When I thought about what might be happening inside a packed collection of mixed particles, it made me want to explore this experimentally. One of the challenges to doing this, though, lies in how to make an idealized sample of packing particles,” said Associate Professor Miho Yanagisawa.
“In our experiment, droplets of oil in water were repeatedly fractured to break them up in an ordered way. This yielded particle sizes which followed a mathematical pattern called a power distribution. Essentially a very broad range of sizes, this was important so that no one size range would be overrepresented in our results. The droplets were gently compressed between two glass plates; this constrained them to a two-dimensional surface and prevented them from overlapping vertically, which was critical for our imaging analysis.”
When identical particles are constrained in two dimensions, they will form a hexagonal lattice. But if size varies, or are said to be polydisperse, this structural symmetry is broken. However, Yanagisawa and fellow researcher Daisuke Shimamoto found that there actually is a common structure of randomly packed particles with extreme size variation; it’s just not obvious by looking at it. This model is statistical rather than geometric and describes the distribution of particles of different sizes as they jam together. An extremely useful implication of this is that the right conditions of particle size variation that allow for denser packing can be modeled, which could mean less wasted space in applications where spatial efficiency is important.
“Although diversity and universality seem to be contradictory concepts, this study shows diversity can produce universality,” said Yanagisawa. “In fact, diverse size distributions are common in nature. Therefore, even phenomena that appear to be very diverse at first look may have a hidden universality, or universality can be revealed by considering the particle distribution of the system as a whole.”
More information: Daisuke S. Shimamoto et al, Common packing patterns for jammed particles of different power size distributions, Physical Review Research (2023). DOI: 10.1103/PhysRevResearch.5.L012014
Sketch of the silicon nanoelectronic device that hosts the “flip-flop” qubit. The nuclear spin (“n,” in orange) and the electron spin (“e,” in blue) flip-flop with respect to each other while always pointing in opposite directions. Credit: University of New South Wales
A team led by Professor Andrea Morello at UNSW Sydney has just demonstrated the operation of a new type of quantum bit, called “flip-flop” qubit, which combines the exquisite quantum properties of single atoms with easy controllability using electric signals, just as those used in ordinary computer chips.
A deliberate target: Electrical control of a single-atom quantum bit
“Sometimes new qubits, or new modes of operations, are discovered by lucky accident. But this one was completely by design,” says Prof. Morello. “Our group has had excellent qubits for a decade, but we wanted something that could be controlled electrically, for maximum ease of operation. So we had to invent something completely new.”
Prof. Morello’s group was the first in the world to demonstrate that using the spin of an electron as well as the nuclear spin of a single phosphorus atom in silicon could be used as “qubits”—units of information that are used to make quantum computing calculations. He explains that while both qubits perform exceptionally well on their own, they require oscillating magnetic fields for their operation.
“Magnetic fields are difficult to localize at the nanometer scale, which is the typical size of the individual quantum computer components. This is why the very first proposal for a silicon quantum bit envisaged having all the qubits immersed in a uniform oscillating magnetic field, applied across the whole chip, and then using local electric fields to select which qubit gets operated.”
A few years ago, Prof. Morello’s team had a realization: defining the qubit as the combined up-down / down-up orientation of the electron and the nucleus of the atom would permit controlling such a qubit using the electric fields alone. Today’s result is the experimental demonstration of that visionary idea.
“This new qubit is called ‘flip-flop’ because it’s made out of two spins belonging to the same atom—the electron and the nuclear spin—with the condition that they always point in opposite directions,” says Dr. Rostyslav Savytskyy, one of the lead experimental authors of the paper, published in Science Advances.
Experimental measurement of the probability of finding the nuclear and the electron spins pointing ‘up’. The data shows clearly the flip-flopping dynamics, where the two spins swap orientation multiple times as the electrical driving signal is progressively applied. Credit: University of New South Wales
“For example, if the ‘0’ state is ‘electron-down / nucleus -up’ and the ‘1’ state is ‘electron-up / nucleus-down,’ changing from ‘0’ to ‘1’ means that the electron ‘flips’ up and the nucleus ‘flops’ down. Hence the name.”
The theory predicted that by displacing the electron with respect to the nucleus, one could program arbitrary quantum states of the flip-flop qubit.
“Our experiment confirms that prediction with perfect accuracy,” says Dr. Tim Botzem, another lead experimental author.
“Most importantly, such electron displacement is obtained simply by applying a voltage to a small metallic electrode, instead of irradiating the chip with an oscillating magnetic field. It’s a method that much more closely resembles the type of electrical signal normally routed within conventional silicon computer chips, as we use every day in our computers and smartphones.”
A promising strategy to scale up to large quantum processors
The electrical control of the “flip-flop” qubit by displacing the electron from the nucleus is accompanied by a very important side effect. When a negative charge (the electron) is displaced away from a positive charge (the nucleus), an electric dipole is formed. Placing two (or more) electric dipoles in each other’s proximity gives rise to a strong electrical coupling between them, which can mediate multi-qubit quantum logic operations of the kind required to perform useful quantum computations.
“The standard way to couple spin qubits in silicon is by placing the electrons so close to each other that they effectively ‘touch,'” says Prof. Morello.
“This requires the qubits to be placed on a grid of a few 10s of nanometers in pitch. The engineering challenges in doing so are quite severe. In contrast, electric dipoles don’t need to ‘touch’ each other—they influence each other from the distance. Our theory indicates that 200 nanometers is the optimal distance for fast and high-fidelity quantum operations.
“This could be a game-changing development, because 200 nanometers is far enough to allow inserting various control and readout devices in between the qubits, making the processor easier to wire up and operate.”
More information: Rostyslav Savytskyy et al, An electrically driven single-atom “flip-flop” qubit, Science Advances (2023). DOI: 10.1126/sciadv.add9408
