Category: Physics

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.

“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

Journal information: Science Advances 

Provided by University of New South Wales 

Imagine going for an MRI scan of your knee. This scan measures the density of water molecules present in your knee, at a resolution of about one cubic millimeter—which is great for determining whether, for example, a meniscus in the knee is torn. But what if you need to investigate the structural data of a single molecule that’s five cubic nanometers, or about 10 trillion times smaller than the best resolution current MRI scanners are capable of producing?

That’s the goal for Dr. Amit Finkler of the Weizmann Institute of Science’s Chemical and Biological Physics Department. In a recent study, Finkler, Ph.D. student Dan Yudilevich and their collaborators from the University of Stuttgart, Germany, have managed to take a giant step in that direction, demonstrating a novel method for imaging individual electrons. The method, now in its initial stages, might one day be applicable to imaging various kinds of molecules, which could revolutionize the development of pharmaceuticals and the characterization of quantum materials.

Current magnetic resonance imaging (MRI) techniques have been instrumental in diagnosing a vast array of illnesses for decades, but while the technology has been groundbreaking for countless lives, there are some underlying issues that remain to be resolved. For example, MRI readout efficiency is very low, requiring a sample size of hundreds of billions of water molecules—if not more—in order to function. The side effect of that inefficiency is that the output is then averaged. For most diagnostic procedures, the averaging is optimal, but when you average out so many different components, some detail is lost—possibly concealing important processes that are happening on a smaller scale.

Whether that’s a problem or not depends on the question you’re asking: For example, there’s a lot of information that could be detected from a photograph of a crowd in a packed football stadium, but a photo probably wouldn’t be the best tool to use if we wish to know more about the mole on the cheek of the person sitting in the third seat of the fourteenth row. If we wanted to gather more data on the mole, getting closer would probably be the way to go.

Finkler and his collaborators are essentially suggesting a molecular close-up shot. Employing such a tool could grant researchers the ability to closely inspect the structure of important molecules, and perhaps lead the way to new discoveries. Furthermore, there are some cases in which a small “canvas” would be essential to the work itself—such as in the preliminary stages of pharmaceutical development.

So how can one achieve a more precise MRI equivalent that can work on small samples—right down to the individual molecule? Finkler, Yudilevich and Stuttgart’s Drs. Rainer Stöhr and Andrej Denisenko have developed a method that can pinpoint the precise location of an electron. It is based on a rotating magnetic field that is in the vicinity of a nitrogen-vacancy center—an atom-sized defect in a special synthetic diamond, which is used as a quantum sensor. Because of its atomic size, this sensor is particularly sensitive to nearby changes; because of its quantum nature, it can differentiate whether a single electron is present, or more, making it especially suited to measuring the location of an individual electron with incredible accuracy.

“This new method,” says Finkler, “could be harnessed to provide a complementary point of view to existing methods, in an effort to better understand the holy molecular trinity of structure, function and dynamics.”

For Finkler and his peers, this research is a pivotal step on the way to precise nanoimaging, and they envision a future in which we would be able to use this technique to image a diverse class of molecules, that will, hopefully, be ready for their close-up.

More information: Dan Yudilevich et al, Mapping Single Electron Spins with Magnetic Tomography, Physical Review Applied (2022). DOI: 10.1103/PhysRevApplied.18.054016

Provided by Weizmann Institute of Science

Physicists have been trying to identify reliable strategies to manipulate quantum states in solid-state materials, cold atoms and other systems, as this could inform the development of new technologies. One of these strategies is Floquet engineering, which entails the periodic driving of quantum states of matter.

Researchers at Tsinghua University, Beihang University and the Chinese Academy of Sciences in China have recently demonstrated the experimental realization of Floquet band engineering in a model semiconductor, namely black phosphorus. Their paper, published in Nature, could inform future research efforts exploring the Floquet engineering of semiconducting materials and trying to realize light-induced emerging phenomena, such as light-induced topological phase transitions.

“Light-matter interaction plays critical roles in experimental condensed matter physics and materials sciences, not only as experimental probes for revealing the underlying physics of low-dimensional quantum materials, but more importantly, as effective control knobs for manipulating the electronic structures and quantum states in the non-equilibrium state,” Shuyun Zhou, who initiated and directed this research, told Phys.org.

“Such nonequilibrium control provides the fascinating opportunities to induce new physical phenomena beyond those in the equilibrium state. Along this line, tailoring the quantum states of matter through time-periodic fields (i.e., Floquet engineering) has attracted extensive interests over the past few decades.”

Past studies have applied Floquet engineering to condensed matter systems, cold atoms and optical lattices. Theoretical works have also predicted intriguing phenomena based on Floquet engineering, such as light-induced topological phase transitions. Experimental evidence of Floquet engineering, however, is still relatively scarce.

“Many fundamental questions remain to be answered through experimental results,” Zhou said. “For example, can Floquet engineering be realized in a semiconductor under realistic experimental conditions? Addressing this question is important as semiconductors are widely used for electronic and optoelectronic devices.”

For several years, Zhou and his colleagues have been trying to identify favorable methods and experimental conditions for investigating light-induced emerging phenomena and realizing Floquet engineering in semiconductors. This can be particularly challenging, as Floquet engineering requires low photon energy and a strong peak electric field.

To meet these requirements, the researchers developed instruments that apply high-intensity mid-infrared pumping pulses. In their experiments, they combined these tools with a state-of-the-art measure known as time- and angle-resolved photoemission spectroscopy (TrARPES).

“We chose an almost-ideal semiconductor sample to start with—high-quality black phosphorus with a small band gap and high mobility, which could be favorable for realizing Floquet engineering,” Zhou said. “The most challenging aspect of our study is that this is still a widely unexplored area, and it is not clear what experimental conditions (pump photon energy, pump polarizations etc.) are favorable for inducing light-induced manipulation of the electronic structure. It is just like searching in the dark, and it has taken us quite a few years before we observed something.”

Zhou and his colleagues were ultimately able to observe the light-driven transient Floquet band structure modulation in black phosphorus by systematically fine tuning the photon energy, polarization and time delay in their sample. This is the first experimental demonstration of Floquet band engineering in a semiconductor.

“Our work provides important insights into the Floquet engineering of semiconductors, highlighting the importance of resonance pumping,” Zhou said. “While optical transitions have been conventionally viewed as detrimental for Floquet states, our work shows that indeed for a semiconductor, resonance pumping could be favorable and even critical for Floquet band engineering. This surprising finding provides a pathway for searching for Floquet engineering in quantum materials.”

The recent work by this team of researchers is an important step towards achieving of light-induced topological phase transition, a key goal in the field of quantum physics. Their findings could thus soon pave the way for new studies aimed at transiently manipulating topological states in ultrafast timescales.

The experimental methods used by Zhou and his colleagues are very promising for achieving lattice symmetry-enforced Floquet band engineering with a stronger pump polarization selectivity. These methods can be used to reliably turn on and off the Floquet band in semiconductors, which could support the development of new high-speed devices.

Peizhe Tang, one of the theorists who worked out the theory behind the pseudospin selection rules of the Floquet engineering in this work, commented, “this work clearly shows that the Floquet engineering physics can be further enriched by pseudospin, a quantum degree of freedom in analogy to spin.”

“This work paves an important step toward topological phase transition via Floquet engineering,” Zhou added. “The next step would be achieving light-induced topological phase transition or even inducing nontrivial topology in a topological trivial material on ultrafast timescales by Floquet engineering. In addition, we would like to extend Floquet engineering to many more solid-state materials.”

More information: Shaohua Zhou et al, Pseudospin-selective Floquet band engineering in black phosphorus, Nature (2023). DOI: 10.1038/s41586-022-05610-3

Journal information: Nature 

© 2023 Science X Network

Engineers from Caltech have discovered that Leonardo da Vinci’s understanding of gravity—though not wholly accurate—was centuries ahead of his time.

In an article published in the journal Leonardo, the researchers draw upon a fresh look at one of da Vinci’s notebooks to show that the famed polymath had devised experiments to demonstrate that gravity is a form of acceleration—and that he further modeled the gravitational constant to around 97 percent accuracy.

Da Vinci, who lived from 1452 to 1519, was well ahead of the curve in exploring these concepts. It wasn’t until 1604 that Galileo Galilei would theorize that the distance covered by a falling object was proportional to the square of time elapsed and not until the late 17th century that Sir Isaac Newton would expand on that to develop a law of universal gravitation, describing how objects are attracted to one another. Da Vinci’s primary hurdle was being limited by the tools at his disposal. For example, he lacked a means of precisely measuring time as objects fell.

Da Vinci’s experiments were first spotted by Mory Gharib, the Hans W. Liepmann Professor of Aeronautics and Medical Engineering, in the Codex Arundel, a collection of papers written by da Vinci that cover science, art, and personal topics. In early 2017, Gharib was exploring da Vinci’s techniques of flow visualization to discuss with students he was teaching in a graduate course when he noticed a series of sketches showing triangles generated by sand-like particles pouring out from a jar in the newly released Codex Arundel, which can be viewed online courtesy of the British Library.

“What caught my eye was when he wrote ‘Equatione di Moti’ on the hypotenuse of one of his sketched triangles—the one that was an isosceles right triangle,” says Gharib, lead author of the Leonardo paper. “I became interested to see what Leonardo meant by that phrase.”

To analyze the notes, Gharib worked with colleagues Chris Roh, at the time a postdoctoral researcher at Caltech and now an assistant professor at Cornell University, as well as Flavio Noca of the University of Applied Sciences and Arts Western Switzerland in Geneva. Noca provided translations of da Vinci’s Italian notes (written in his famous left-handed mirror writing that reads from right to left) as the trio pored over the manuscript’s diagrams.

In the papers, da Vinci describes an experiment in which a water pitcher would be moved along a straight path parallel to the ground, dumping out either water or a granular material (most likely sand) along the way. His notes make it clear that he was aware that the water or sand would not fall at a constant velocity but rather would accelerate—also that the material stops accelerating horizontally, as it is no longer influenced by the pitcher, and that its acceleration is purely downward due to gravity.

If the pitcher moves at a constant speed, the line created by falling material is vertical, so no triangle forms. If the pitcher accelerates at a constant rate, the line created by the collection of falling material makes a straight but slanted line, which then forms a triangle. And, as da Vinci pointed out in a key diagram, if the pitcher’s motion is accelerated at the same rate that gravity accelerates the falling material, it creates an equilateral triangle—which is what Gharib originally noticed that da Vinci had highlighted with the note “Equatione di Moti,” or “equalization (equivalence) of motions.”

Da Vinci sought to mathematically describe that acceleration. It is here, according to the study’s authors, that he didn’t quite hit the mark. To explore da Vinci’s process, the team used computer modeling to run his water vase experiment. Doing so yielded da Vinci’s error.

“What we saw is that Leonardo wrestled with this, but he modeled it as the falling object’s distance was proportional to 2 to the t power [with t representing time] instead proportional to t squared,” Roh says. “It’s wrong, but we later found out that he used this sort of wrong equation in the correct way.” In his notes, da Vinci illustrated an object falling for up to four intervals of time—a period through which graphs of both types of equations line up closely.

“We don’t know if da Vinci did further experiments or probed this question more deeply,” Gharib says. “But the fact that he was grappling with this problem in this way—in the early 1500s—demonstrates just how far ahead his thinking was.”

The paper is titled “Leonardo da Vinci’s Visualization of Gravity as a Form of Acceleration.”

More information: Morteza Gharib et al, Leonardo da Vinci’s Visualization of Gravity as a Form of Acceleration, Leonardo (2022). DOI: 10.1162/leon_a_02322

Provided by California Institute of Technology 

Researchers from North Carolina State University and the University of North Carolina at Chapel Hill used chiral phonons to convert wasted heat into spin information—without needing magnetic materials. The finding could lead to new classes of less expensive, energy-efficient spintronic devices for use in applications ranging from computational memory to power grids.

Spintronic devices are electronic devices that harness the spin of an electron, rather than its charge, to create current used for data storage, communication, and computing. Spin caloritronic devices—so-called because they utilize thermal energy to create spin current—are promising because they can convert waste heat into spin information, which makes them extremely energy efficient. However, current spin caloritronic devices must contain magnetic materials in order to create and control the electron’s spin.

“We used chiral phonons to create a spin current at room temperature without needing magnetic materials,” says Dali Sun, associate professor of physics and member of the Organic and Carbon Electronics Lab (ORaCEL) at North Carolina State University.

“By applying a thermal gradient to a material that contains chiral phonons, you can direct their angular momentum and create and control spin current,” says Jun Liu, associate professor of mechanical and aerospace engineering at NC State and ORaCEL member.

Both Liu and Sun are co-corresponding authors of the research, which appears in Nature Materials.

Chiral phonons are groups of atoms that move in a circular direction when excited by an energy source—in this case, heat. As the phonons move through a material, they propagate that circular motion, or angular momentum, through it. The angular momentum serves as the source of spin, and the chirality dictates the direction of the spin.

“Chiral materials are materials that cannot be superimposed on their mirror image,” Sun says. “Think of your right and left hands—they are chiral. You can’t put a left-handed glove on a right hand, or vice versa. This ‘handedness’ is what allows us to control the spin direction, which is important if you want to use these devices for memory storage.”

The researchers demonstrated chiral phonon-generated spin currents in a two-dimensional layered hybrid organic-inorganic perovskite by using a thermal gradient to introduce heat to the system.

“A gradient is needed because temperature difference in the material—from hot to cold—drives the motion of the chiral phonons through it,” says Liu. “The thermal gradient also allows us to use captured waste heat to generate spin current.”

The researchers hope that the work will lead to spintronic devices that are cheaper to produce and can be used in a wider variety of applications.

“Eliminating the need for magnetism in these devices means you’re opening the door wide in terms of access to potential materials,” Liu says. “And that also means increased cost-effectiveness.”

“Using waste heat rather than electric signals to generate spin current makes the system energy efficient—and the devices can operate at room temperature,” Sun says. “This could lead to a much wider variety of spintronic devices than we currently have available.”

More information: Lifa Zhang, Chiral-phonon-activated spin Seebeck effect, Nature Materials (2023). DOI: 10.1038/s41563-023-01473-9. www.nature.com/articles/s41563-023-01473-9

Journal information: Nature Materials 

Provided by North Carolina State University 

Research in fundamental science has revealed the existence of quark-gluon plasma (QGP)—a newly identified state of matter—as the constituent of the early universe. Known to have existed a microsecond after the Big Bang, the QGP, essentially a soup of quarks and gluons, cooled down with time to form hadrons like protons and neutrons—the building blocks of all matter.

One way to reproduce the extreme conditions prevailing when QGP existed is through relativistic heavy-ion collisions. In this regard, particle accelerator facilities like the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider have furthered our understanding of QGP with experimental data pertaining to such collisions.

Meanwhile, theoretical physicists have employed multistage relativistic hydrodynamic models to explain the data, since the QGP behaves very much like a perfect fluid. However, there has been a serious lingering disagreement between these models and data in the region of low transverse momentum, where both the conventional and hybrid models have failed to explain the particle yields observed in the experiments.

Against this backdrop, a team of researchers from Japan, led by theoretical physicist Professor Tetsufumi Hirano of Sophia University, undertook an investigation to account for the missing particle yields in the relativistic hydrodynamic models.

In their recent work, they proposed a novel “dynamical core-corona initialization framework” to comprehensively describe high-energy nuclear collisions. Their findings were published in the journal Physical Review C and involved contributions from Dr. Yuuka Kanakubo, doctoral student at Sophia University, (Present affiliation: postdoctoral research fellow at the University of Jyväskylä, Finland) and Assistant Professor Yasuki Tachibana from Akita International University, Japan.

“To find a mechanism that can account for the discrepancy between theoretical modeling and experimental data, we used a dynamical core-corona initialization (DCCI2) framework in which the particles generated during high-energy nuclear collisions are described using two components: the core, or equilibrated matter, and the corona, or nonequilibrated matter,” explains Prof. Hirano. “This picture allows us to examine the contributions of the core and corona components towards hadron production in the low transverse momentum region.”

The researchers conducted heavy-ion Pb-Pb collision simulations on PYTHIA (a computer simulation program) at an energy of 2.76 TeV to test their DCCI2 framework. Dynamical initialization of the QGP fluids allowed the separation of core and corona components, which were made to undergo hadronization through “switching hypersurface” and “string fragmentation,” respectively. These hadrons were then subjected to resonance decays to obtain the transverse momentum (p_T) spectra.

“We switched off the hadronic scatterings and performed only resonance decays to see a breakdown of the total yield into core and corona components, as hadronic scatterings mix up the two components in the late stage of reaction,” explains Dr. Kanakubo.

The researchers then investigated the fraction of core and corona components in the p_T spectra of charged pions, charged kaons, and protons and antiprotons for collisions at 2.76 TeV. Next, they compared these spectra with that obtained from experimental data (from the ALICE detector at LHC for Pb-Pb collisions at 2.76 TeV) to quantify the contributions from corona components. Lastly, they investigated the effects of contributions from corona components on the flow variables.

They found a relative increase in corona contributions in the spectral region of approximately 1 GeV for both 0-5% and 40-60% centrality classes. While this was true for all the hadrons, they found almost 50% corona contribution to particle production in the spectra of protons and antiprotons in the region of very low p_T (≈ 0 GeV) .

Furthermore, results from full DCCI2 simulations showed better agreement with the ALICE experimental data compared when only core components with hadronic scatterings (which neglect corona components) were compared. The corona contribution was found to be responsible for diluting the four-particle cumulants (a flow observable) obtained purely from core contributions, indicating more permutations of particles with corona contribution.

“These findings imply that the nonequilibrium corona components contribute to particle production in the region of very low transverse spectra,” says Prof. Hirano. “This explains the missing yields in hydrodynamic models, which extract only the equilibrated core components from experimental data. This clearly shows that it is necessary to extract the nonequilibrated components as well for a more precise understanding of the properties of QGP.”

More information: Yuuka Kanakubo et al, Nonequilibrium components in the region of very low transverse momentum in high-energy nuclear collisions, Physical Review C (2022). DOI: 10.1103/PhysRevC.106.054908

Provided by Sophia University

Particle accelerators are devices that use electromagnetic fields to speed up particles and collide them together or against a specific target. These devices are widely used by physicists to study particles, the forces that drive them and interactions between them.

The larger and most advanced particle accelerators in the world consume huge amounts of energy. In a paper published in Nature Physics, researchers at Technische Universität Darmstadt recently introduced a new measurement at their particle accelerator that can retain a high beam power while consuming significantly less energy.

“Particle accelerators are available in compact designs, such as those used for X-ray machines, and as large-scale facilities that are used for fundamental research,” Manuel Dutine, one of the researchers who carried out the study, told Phys.org. “The latter in particular consume an enormous amount of energy. This raises the question whether the added value justifies the realization of large research-accelerator facilities, especially from an economic and sustainable point of view.”

Future large-scale particle physics research will require accelerators with an increasingly high beam power. The key objective of the work by Dutine and his colleagues was to create an accelerator that could help to meet these needs while consuming less energy, using a technology known as energy recovery.

“The concept of energy recovery in particle accelerators is not new but under continuous development,” Dutine explained. “To achieve higher beam energies at reduced resource needs, the energy recovery concept should be combined with multi-turn acceleration, which was the purpose of our research.”

Accelerators accelerate beams, which are essentially a large set of particles, and this process unavoidably consumes energy. A high energy accelerated beam could collide with a resting target, another beam of particles or even laser light, potentially resulting in desired reactions that can then be studied. Reactions of interest, however, are very rare, and most particles in a beam end up not interacting with anything during an experiment.

“In a conventional accelerator, the remaining beam is dumped at high energy and the energy is wasted,” Dutine said. “In an ERL (energy recovery linac), the beam’s energy is recovered by decelerating the bunches. The energy is temporarily stored in the microwave fields in the accelerating structures and can afterwards be used to accelerate the next upcoming low-energy bunches.”

The ERL accelerator developed by Dutine and his colleagues has so far achieved very promising results. In initial tests, the team measured its energy recovery and recycling (i.e., the use of recovered energy for accelerating subsequent particle bunches) while accelerating an electron beam.

“By measuring energy savings, we demonstrated that the multi-turn energy recovery concept works,” Dutine said. “We measured a recovery of up to 87% of consumed beam power in the main accelerator. This power saving is financially relevant for beams with Gigawatt power or may help to reduce the footprint of energy consumption for large-scale facilities.”

The new multi-turn energy recovery accelerator created by this team of researchers could help to carry out physics studies more efficiently and sustainably, consuming less power while still achieving high beam energies. In the future, its unique design could inform the development of other accelerators with even lower fabrication costs and energy consumption, thus further promoting sustainable research. Dutine and his colleagues are now planning to upgrade their particle accelerator and also build a new one.

“The potential for energy saving is even larger for facilities with increased beam current, beam energies and number of turns,” Dutine added. “Here we are limited by the properties of our existing facility. We have started discussions with scientific partners, funding agencies, and our ministry to expand this research to a next-generation facility, in which we want to include our current findings, establish the energy recycling in a particle accelerator at larger scale, and thereby convince potential users from international science centers or industry to consider energy-recovering for their particle-accelerator needs.”

More information: Felix Schliessmann et al, Realization of a multi-turn energy recovery accelerator, Nature Physics (2023). DOI: 10.1038/s41567-022-01856-w.

Journal information: Nature Physics 

© 2023 Science X Network

In recent years, many physicists and computer scientists have been working on the development of quantum computing technologies. These technologies are based on qubits, the basic units of quantum information.

In contrast with classical bits, which have a value of 0 or 1, qubits can exist in superposition states, so they can have a value of 0 and 1 simultaneously. Qubits can be made of different physical systems, including electrons, nuclear spins (i.e., the spin state of a nucleus), photons, and superconducting circuits.

Electron spins confined in silicon quantum dots (i.e., tiny silicon-based structures) have shown particular promise as qubits, particularly due to their long coherence times, high gate fidelities and compatibility with existing semiconductor manufacturing methods. Coherently controlling multiple electron spin states, however, can be challenging.

Researchers at University of Rochester have recently introduced a new strategy to coherently manipulate either single or multiple electron spins in silicon quantum dots. This method, introduced in a paper published in Nature Physics, could open new possibilities for the development of reliable and highly performing quantum computers.

“As with many experiments in science, we were initially investigating an unrelated topic, when we started noticing all sorts of coherent oscillations popping up in our data.” John Nichol, one of the researchers who carried out the study, told Phys.org. “It took us a little while to come up with the theoretical explanation, but once we did, everything fell into place. Spin-valley coupling has been explored before many times, but never to directly mediate coherent transitions between different spin states.”

The strategy for controlling electron spins in silicon proposed by Nichol and his colleagues takes advantage of spin-valley coupling, the interaction between an electron’s spin and valley states. Electrons in silicon quantum dots have both spin and valley quantum numbers. Their spin state can be “up” or “down,” while their valley state can be + or −.

“At a certain magnetic field, the energy of the up,+ state, for example, can be almost equal in energy to the down,- state,” Nichol explained. “Because the energy difference between the + and − states depends on electric fields, we can use a voltage pulse to then bring up,+ exactly into resonance with down,−. When this happens, an electron initially prepared in an up,+ state will coherently oscillate to down,−, and back and forth. These are spin-valley oscillations.”

So far, the standard method to manipulate electron spins in silicon quantum dots entailed the use of time-varying magnetic fields. Nichol and his colleagues showed that their strategy enables the coherent manipulation of electron spins without the need to use oscillating electromagnetic fields.

“Oscillating magnetic fields can be especially difficult to generate at cryogenic temperatures, and spin-valley coupling eliminates this need,” Nichol said. “Another achievement is that the valley degree of freedom in silicon has often been regarded as a ‘bug’ rather than a feature of silicon qubits, but our work shows that it can be a very useful feature.”

The recent work by this team of researchers highlights the promise of utilizing spin-valley coupling to achieve the coherent control of qubits based on electron spins confined in silicon quantum dots. In their next papers, they hope to gain a better understanding of what characteristics of the growth, fabrication, and tuning of quantum dots can impact spin-valley coupling, as this could further inform the fabrication of electron-based quantum computing technologies.

“We would also like to explore how one could implement multi-qubit gates in this framework,” Nichol added. “One challenge is that the magnetic field needs to be tuned separately for each qubit, and we are looking into realistic ways to implement this.”

More information: Xinxin Cai et al, Coherent spin–valley oscillations in silicon, Nature Physics (2023). DOI: 10.1038/s41567-022-01870-y

Journal information: Nature Physics 

© 2023 Science X Network

Purdue University researchers found that by tailoring the film thickness of conducting nitrides and oxides, specifically plasmonic titanium nitride (TiN) and aluminum-doped zinc oxide (AZO), they can control the materials’ optical properties, most notably their epsilon near zero (ENZ) behaviors. The TiN and AZO materials developed at Purdue also feature the lowest reported optical losses. This provides novel applications for the telecommunications field and furthers the study of many optical nonlinearities.

Vladimir M. Shalaev and Alexandra Boltasseva, Purdue professors of electrical and computer engineering, and their team of researchers, led by then-postdoctoral researcher Soham Saha, investigated this method of controlling the ENZ point, the wavelength at which a material is neither dielectric nor metallic.

When light travels through an ENZ material, its group velocity slows to near zero, and it’s able to interact with the material for a longer period. This gives rise to many interesting nonlinearities. However, with most conventional materials, the ENZ point is fixed and difficult to move.

What the researchers demonstrated is that the thickness association of the optical properties is one of the easiest things to manipulate, enabling them to grow films with different ENZ properties using the same growth environment. By tailoring the thickness of material films and controlling light absorption near the ENZ regime, researchers can study absolute ENZ physics at many different wavelength ranges. The enables a wide range of nonlinear optical phenomena, including all-optical switching, time refraction and high-harmonic generation.

In their study, the researchers developed a low-loss, polycrystalline films of TiN on silicon by reactive magnetron sputtering. AZO was grown by pulsed laser deposition on the as-grown TiN. They investigated the cause of the thickness-dependent optical properties through spectroscopic ellipsometry, atomic force microscopy and transmission electron microscopy, connecting the optical properties to their structural properties.

As a proof of concept of a dynamic, tunable device, researchers demonstrated all-optical switching of the resonators using an interband pump, showing picosecond relaxation times.

The study is published in the journal Advanced Materials.

More information: Soham Saha et al, Tailoring the Thickness‐Dependent Optical Properties of Conducting Nitrides and Oxides for Epsilon‐Near‐Zero‐Enhanced Photonic Applications, Advanced Materials (2022). DOI: 10.1002/adma.202109546

Journal information: Advanced Materials 

Provided by Purdue University 

Granular materials contain large numbers of small, discrete particles, which collectively behave like uniform media. Their thermal conductivity is crucial to understanding their overall behavior—but so far, researchers haven’t considered how this value is affected by the surface roughness of their constituent particles.

Through new analysis published in The European Physical Journal B, Bo Persson at the Peter Grünberg Institute, part of the Jülich Research Centre in Germany, has discovered that when this roughness is considered, thermal conductivity in granular materials is heavily influenced by particle sizes. These findings could help physicists to better describe a wide array of granular materials: from sand and snow, to piles of rice, coffee beans, and fertilizer.

The thermal conductivity of a granular material will typically be much smaller than that of solid blocks of the same material. Its value is affected by several factors: including particle shapes and sizes; the nature of their contact between each other; and the temperature, pressure, and humidity of the gas surrounding the particles.

Most previous studies have only considered how heat is transferred between smooth, spherical particles—but in reality, particles have a surface roughness on many length scales, which heavily affects the thermal conductivity of the overall material.

In his study, Persson examines the thermal conductivity of granular materials in a humid atmosphere, where heat flows via air and water are especially relevant. Unlike previous studies, he considered particles with a mathematically-defined surface roughness—found in real particles produced through the crushing of brittle minerals.

For particles smaller than 10 microns, he found that heat transfer in humid air is dominated by water capillaries, which form in the gaps between particles. In contrast, for bigger particles, heat is mostly transferred through the air. These discoveries clarify the mechanisms driving thermal conductivity in granular materials, and will enable researchers in future studies to simulate their physical properties more closely.

The work is published in The European Physical Journal B.

More information: B. N. J. Persson, Heat transfer in granular media consisting of particles in humid air at low confining pressure, The European Physical Journal B (2023). DOI: 10.1140/epjb/s10051-023-00483-5

Journal information: European Physical Journal B 

Provided by Springer