New superconducting state could be significant for quantum computing future

 

Scientists using one of the world's most powerful quantum microscopes have made a discovery that could have significant consequences for the future of computing.

Researchers at the Macroscopic Quantum Matter Group laboratory in University College Cork (UCC) have discovered a spatially modulating superconducting state in a new and unusual superconductor, uranium ditelluride (UTe2). This new superconductor may provide a solution to one of quantum computing's greatest challenges. Their findings have been published in Nature.

Lead author Joe Carroll, a Ph.D. researcher working with UCC Prof. of Quantum Physics Séamus Davis, explains the subject of the paper.

"Superconductors are amazing materials which have many strange and unusual properties. Most famously they allow electricity to flow with zero resistance. That is, if you pass a current through them they don't start to heat up, in fact, they don't dissipate any energy despite carrying a huge current. They can do this because instead of individual electrons moving through the metal we have pairs of electrons which bind together. These pairs of electrons together form macroscopic quantum mechanical fluid."

"What our team found was that some of the electron pairs form a new crystal structure embedded in this background fluid. These types of states were first discovered by our group in 2016 and are now called Electron Pair-Density Waves. These Pair Density Waves are a new form of superconducting matter the properties of which we are still discovering."

"What is particularly exciting for us and the wider community is that UTe2 appears to be a new type of superconductor. Physicists have been searching for a material like it for nearly 40 years. The pairs of electrons appear to have intrinsic angular momentum. If this is true, then what we have detected is the first Pair-Density Wave composed of these exotic pairs of electrons."

When asked about the practical implications of this work Mr. Carroll explained, "There are indications that UTe2 is a special type of superconductor that could have huge consequences for quantum computing."

"Typical, classical, computers use bits to store and manipulate information. Quantum computers rely on quantum bits or qubits to do the same. The problem facing existing quantum computers is that each qubit must be in a superposition with two different energies—just as Schrödinger's cat could be called both 'dead' and 'alive.' This quantum state is very easily destroyed by collapsing into the lowest energy state—'dead'—thereby cutting off any useful computation."

"This places huge limits on the application of quantum computers. However, since its discovery five years ago there has been a huge amount of research on UTe2 with evidence pointing to it being a superconductor which may be used as a basis for topological quantum computing. In such materials there is no limit on the lifetime of the qubit during computation opening up many new ways for more stable and useful quantum computers. In fact, Microsoft have already invested billions of dollars into topological quantum computing so this is a well-established theoretical science already." he said.

"What the community has been searching for is a relevant topological superconductor; UTe2 appears to be that."

"What we've discovered then provides another piece to the puzzle of UTe2. To make applications using materials like this we must understand their fundamental superconducting properties. All of modern science moves step by step. We are delighted to have contributed to the understanding of a material which could bring us closer to much more practical quantum computers."

Professor John F. Cryan, Vice President Research and Innovation said, "This important discovery will have significant consequences for the future of quantum computing. In the coming weeks, the University will launch UCC Futures—Future Quantum and Photonics and research led by Professor Seamus Davis and the Macroscopic Quantum Matter Group, with the use of one of the world's most powerful microscopes, will play a crucial role in this exciting initiative."

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Electron tunneling associated with ferritin was proposed as early as 1988, but it is still viewed skeptically despite substantial evidence that it occurs. In our recent paper published in IEEE Transactions on Molecular, Biological and Multi-Scale Communications, my co-authors and I review the evidence of electron tunneling in ferritin, as well as the evidence that such electron tunneling may be used by biological systems that include the retina, the cochlea, macrophages, glial cells, mitochondria and magnetosensory systems.

While these diverse systems fall in different fields of study, we hope that this article will raise awareness of the mechanism of electron tunneling associated with ferritin and encourage further research into that phenomenon in biological systems that incorporate ferritin, particularly where there is no apparent need for the iron storage functions of ferritin in those systems.
A brief history of ferritin and ferritin research

Ferritin is an iron storage protein that self-assembles into a 12-nanometer diameter spherical shell that is 2 nanometers thick, and it can store up to ~4,500 iron atoms in an 8-nanometer diameter core. With an evolutionary history that appears to stretch back more than 1.2 billion years, it might seem rather old, but it should be kept in mind that single-celled organisms are believed to have first evolved ~3.5 billion years ago. As such, it may have taken more than 2 billion years for ferritin to evolve. When the first multicellular organisms evolved ~600 million years ago, members of the ferritin family of proteins were likely present, and they can be found today in almost all plants and animals.

The first suggestion that ferritin might have some quantum mechanical properties was made as early as 1988, 88 years after the discovery of quantum mechanics and eight years after the discovery of quantum dots, semiconductor nanoparticles that behave like artificial atoms and which are similar in size to ferritin. The quantum mechanical properties include magnetic behavior that arises from the way iron forms crystalline structures in the core of the ferritin shell, and electron tunneling.

Subsequent studies discussed in the paper provide substantial evidence that such quantum mechanical properties exist. However, those properties in this billion-year-old biostructure appear to have been mostly considered to be a curiosity or artifact, and not a quantum biological agent. Quantum biology as a study has been viewed with skepticism by biologists and many other scientists (although many of the scientists who discovered quantum mechanics more than 100 years ago believed it could be applicable to biology), but it is a growing field with research being conducted at many of the top universities, such as Caltech, Yale, the University of Chicago and UCLA.
What is electron tunneling?

Quantum mechanics proposes that the physical properties of electrons, protons, neutrons and other things referred to as subatomic "particles" are defined in terms of probability waves. Experimental evidence of the wave-like behavior of these particles has been obtained and is generally accepted. Those waves are described by those experiments as a probability of detecting a physical property of the particle at a location in time and space, which is sometimes referred to as "collapse" of the wave function.

However, nothing changes about the particle when it collapses, other than the behavior of the wave function. When the wave function behaves in accordance with the Schrödinger wave function, it may be called "coherent," and when it interacts with other particles and no longer behaves in accordance with that wave function, it may be called "incoherent."

The spatial wave-like properties of electrons in a vacuum can have a wavelength of around 5 nanometers at room temperature, which is significant for molecular interactions. Electrons can move between molecules when they "touch" each other (recognizing that the wave functions of the atoms and sub-atomic particles in the molecules are what is actually interacting), which may be referred to as adiabatic or classical behavior, but under the right conditions, an electron can "tunnel" between molecules, which means that it can appear to move from one molecule to another molecule in a way that is not permitted by adiabatic or classical behavior. There is nothing mysterious about this, it is just a physical property of electrons, but because the wave function is a probability wave, it can seem mysterious.

Some of my co-authors have shown that electrons appear to tunnel over distances of up to 12 nm through ferritin in sequential tunneling events, and that the unusual magnetic properties of the core materials of the ferritin might be associated with this unusually long electron tunneling distance. That work has been based on what are referred to as "solid state" experiments, which do not involve living biological systems. Because electron tunneling cannot be directly observed, it has to be inferred from other evidence such as measured currents and voltages. In biological systems, it can be more difficult to obtain evidence of such electron tunneling, but it is not impossible.
Electron tunneling in biological systems containing ferritin

There are several proposed cellular reactions associated with electron tunneling in ferritin. The first is electron storage. In laboratory tests outside of cells, which are sometimes referred to as "in vitro" for the Latin term meaning "in glass," the ability of ferritin in solution with water to store electrons for several hours has been demonstrated. This is unusual, because it would be expected that the iron stored inside the ferritin would be released as soon as an electron is received, but that does not happen quickly. This observation indicates that electrons are not readily conducted through the insulating protein shell by classical conduction, and that instead they move electrochemically or by tunneling.

Evidence also indicates that electrons can tunnel distances of up to 8 nanometers in a single tunneling event through the ferritin in solid state tests, so it is possible that the electrons stored inside the ferritin core can tunnel to molecules outside of the 2-nanometer-thick protein shell, such as free radicals that have energy levels that allow them to receive electrons. These free radicals can take electrons from other molecules and cause cellular damage, and neutralizing free radicals by donating an electron is one of the functions of antioxidants.

Ferritin interacts with antioxidants like ascorbic acid (known more commonly as vitamin C) in a cellular environment in a way that stabilizes the stored iron, and it is also overexpressed in response to free radicals. If ferritin is able to store electrons from antioxidants to make them available to free radicals through electron tunneling, it could improve the efficiency of that neutralization reaction by allowing the electrons to reach free radicals that are farther away and by storing the electrons until they are needed.

If the only function of ferritin is iron storage, that would make no sense in situations where the source of the free radicals, inflammation and ROS is not excess iron, which is often the case. The complexity of the way that cells use iron, known as iron homeostasis, has made it difficult to identify electron tunneling associated with ferritin.

Another proposed quantum biological function for electron tunneling in ferritin is electron transport across cellular distances. In a type of cell called an M2 macrophage, ferritin can form somewhat regularly ordered structures that the macrophages appear to use to provide the ferritin to a cell that the macrophage is aiding. For example, macrophages are associated with increased ferritin levels associated with some cancers and appear to help the cancer cells to neutralize inflammation.

Antioxidants may also help some cancer cells to survive by providing electrons to neutralize free radicals and ROS, but in the absence of antioxidants in those cells, is it possible for electrons to tunnel through the ferritin structures in M2 macrophages into ferritin in other cells? Evidence of that function also exists.

In small angle neutron scattering (SANS) tests performed by Dr. Olga Mykhaylyk on placental tissue that included macrophages, increased neutron scattering was measured that was absent in bulk ferritin extracted from the tissues. Neutron scattering can occur in solids that contain nanoparticles with aligned magnetic moments, and these tests indicate that the ferritin in placental tissue with macrophages has aligned magnetic moments.

SANS tests were also conducted on self-assembled monolayers (SAMs) of ferritin by Prof. Heinz Nakotte that demonstrated neutron scattering, and tests that I conducted with Prof. Cai Shen showed that self-assembled multilayers of ferritin similar to those in M2 macrophages were not only able to conduct electrons over distances as great as 80 microns in vitro but were also able to route those electrons using a physical mechanism known as a Coulomb blockade.

Routing electrons to ferritin where they are need for elimination of free radicals, inflammation and ROS in cells is another proposed quantum biological function, but because electron tunneling cannot be directly observed, further research to investigate that hypothesis is needed.
Conclusion and next steps

This new paper in IEEE Transactions provides more details on how these building blocks of electron tunneling functions could be used in different biological systems that contain ferritin, but it will be up to researchers in the different fields of study for those biological systems to design tests and to investigate whether electron tunneling is occurring.

Many researchers in biology do not understand electron tunneling and are skeptical of quantum biology, so it might take decades before these questions are answered and used to develop treatments for cancer, blindness, deafness and other maladies. Hopefully, this paper will help to raise awareness and foster additional research into whether and how biological systems utilize the proven phenomenon of electron tunneling in ferritin.

#ElectronTunneling #FerritinInVivo #Retina #Cochlea #Macrophages #Tissues

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Researchers report on a new technique for cooling membranes with lasers

 

Using a new technique, researchers at the University of Basel have succeeded in cooling a small membrane down to temperatures close to absolute zero using only laser light. Such extremely cooled membranes could, for instance, find applications in highly sensitive sensors.

As far back as 400 years ago, the German astronomer Johannes Kepler came up with the idea of solar sails that could be used by ships to sail through the universe. He suspected that light exerted a force when reflected by an object. This concept also allowed him to explain why the tails of comets point away from the sun.

Nowadays, scientists use the light force, among other things, to slow down and cool atoms and other particles. Typically, one needs a complex apparatus to do this. A team of researchers at the University of Basel, led by Prof. Dr. Philipp Treutlein and Prof. Dr. Patrick Potts, have now succeeded in cooling a wafer-thin membrane down to a temperature close to the absolute zero of minus 273.15 degrees Celsius using nothing but laser light. They recently published their results in the journal Physical Review X.
Feedback without measurement


"What makes our method special is that we achieve this cooling effect without making any kind of measurement," says physicist Maryse Ernzer, a Ph.D. student and first author of the research paper. According to the laws of quantum mechanics, a measurement, as is typically required in a feedback loop, leads to a change of the quantum state and hence to disturbances.

To avoid that, the Basel scientists developed a so-called coherent feedback loop in which the laser light acts both as a sensor and as a damper. In this way they dampened and cooled the thermal vibrations of a membrane made of silicon nitrate around half a millimeter in size.

In their experiment, the researchers directed a laser beam onto the membrane and fed the light reflected by the membrane into a fiber optic cable. In that process, the vibrations of the membrane caused small changes in the oscillation phase of the reflected light. The information on the instantaneous motional state of the membrane contained in that oscillation phase was then used, with a time delay, to apply just the right amount of force on the membrane at the right moment with the same laser light.

"That's a bit like slowing down a swing by briefly touching the ground with one's feet at the right time," Ernzer explains. To achieve the optimal delay of around 100 nanoseconds, the researchers used a 30-meter-long fiber optic cable.
Close to absolute zero


"Professor Potts and his collaborators developed a theoretical description of the new technique and calculated the settings for which we could expect to achieve the lowest temperatures; that was then confirmed by the experiment," says Dr. Manel Bosch Aguilera, who contributed to the study as a postdoc. He and his colleagues were able to cool the membrane down to 480 micro-Kelvin—less than a thousandth of a degree above the absolute zero of temperature.

In a next step, the researchers want to improve their experiment to the point that the membrane reaches the lowest possible temperature—the quantum mechanical ground state of the membrane's oscillations, that is. After that, it should also be possible to create so-called squeezed states of the membrane. Such states are particularly interesting for building sensors as they allow for a higher measurement accuracy. Possible applications of such sensors include atomic force microscopes, which are used to scan surfaces with nanometer resolution.

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Microsoft claims to have achieved first milestone in creating a reliable and practical quantum computer

 

A team of researchers at Microsoft Quantum has reportedly achieved a first milestone toward creating a reliable and practical quantum computer. In their paper, published in the journal Physical Review B, the group describes the milestone and their plans to build a reliable quantum computer over the next 25 years.


Physicists and computer engineers are working toward building a reliable, useful quantum computer. Such efforts have been hampered, however, by error rates. In this new effort, the team at Microsoft suggests that quantum computer development is following a trajectory similar to that of traditional computers.

In the beginning, new concepts were followed by a series of hardware upgrades that have led to the machines of today. Likewise, they suggest that while current approaches used to represent logical qubits, such as a spin transmon, or a gatemon, have been useful as learning devices, none of them are scalable. They suggest a new approach must be found that allows for scaling.

They now report that they have engineered a new way to represent a logical qubit with hardware stability. The device can reportedly induce a phase of matter characterized by Majorana zero modes—types of fermions. They also report that such devices have demonstrated low enough disorder to pass the topological gap protocol, proving the technology is viable. They believe this represents a first step toward the creation of not just a quantum computer, but a quantum supercomputer.

In its announcement, Microsoft also stated that it has created a new measure to gauge the performance of a quantum supercomputer: reliable quantum operations per second (rQOPS), a figure that describes how many reliable operations a computer can execute in a single second. They suggest that for a machine to qualify as a quantum supercomputer, its rQOPS needs to be at least 1 million. They note that such machines could reach a billion rQOPS, making them truly useful.

#QuantumComputing #Microsoft #TechnologicalAdvancements #QuantumMilestone #Innovation


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New device opens door to storing quantum information as sound waves

 

Quantum computing, just like traditional computing, needs a way to store the information it uses and processes. On the computer you're using right now, information, whether it be photos of your dog, a reminder about a friend's birthday, or the words you're typing into browser's address bar, must be stored somewhere. Quantum computing, being a new field, is still working out where and how to store quantum information.

In a paper published in the journal Nature Physics, Mohammad Mirhosseini, assistant professor of electrical engineering and applied physics, shows a new method his lab has developed for efficiently translating electrical quantum states into sound and vice versa. This type of translation may allow for storing quantum information prepared by future quantum computers, which are likely to made from electrical circuits.

This method makes use of what are known as phonons, the sound equivalent of a light particle called a photon. (Remember that in quantum mechanics, all waves are particles and vice versa). The experiment investigates phonons for storing quantum information because it's relatively easy to build small devices that can store these mechanical waves.

To understand how a sound wave can store information, imagine an extremely echoey room. Now, let's say you need to remember your grocery list for the afternoon, so you open the door to that room and shout, "Eggs, bacon, and milk!" and shut the door. An hour later, when it's time to go to the grocery store, you open the door, poke your head inside, and hear your own voice still echoing, "Eggs, bacon, and milk!" You've just used sound waves to store information.

Of course, in the real world, an echo like that wouldn't last very long, and your voice might end up so distorted you can no longer make out your own words, not to mention that using an entire room for storing a little bit of data would be ridiculous. The research team's solution is a tiny device consisting of flexible plates that are vibrated by sound waves at extremely high frequencies. When an electric charge is placed on those plates, they become able to interact with electrical signals carrying quantum information. This allows that information to be piped into the device for storage, and be piped out for later use—not unlike the door to the room into which you were shouting earlier in this story.

According to Mohammad Mirhosseini, previous studies had investigated a special type of materials known as piezoelectrics as a means of converting mechanical energy to electrical energy in quantum applications.

"These materials, however, tend to cause energy loss for electrical and sound waves, and loss is a big killer in the quantum world," Mirhosseini says. In contrast, the new method developed by Mirhosseini and his team is independent on the properties of specific materials, making it compatible with established quantum devices, which are based on microwaves.

Creating effective storage devices with small footprints has been another practical challenge for researchers working on quantum applications, says Alkim Bozkurt, a graduate student in Mirhosseini's group and the lead author of the paper.

"However, our method enables the storage of quantum information from electrical circuits for durations two orders of magnitude longer than other compact mechanical devices," he adds.

Study co-authors include Chaitali Joshi and Han Zhao, both postdoctoral scholars in electrical engineering and applied physics; and Peter Day and Henry LeDuc, who are scientists at the Jet Propulsion Laboratory, which Caltech manages for NASA.

#QuantumStorage #QuantumInformation #SoundWaves #QuantumTechnology #QuantumComputing

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Quantum interference can protect and enhance photoexcitation

 

When a photon interacts with a material, an interaction occurs that causes its atoms to change their quantum state (a description of the physical properties of nature at the atomic level). The resulting state is called, aptly, photoexcitation. These photoexcitations are conventionally assumed to kill one another when they come near each other, radically limiting their density and mobility. This in turn limits how efficient tools that rely on photoexcitation such as solar cells and light-emitting devices can be.


But in a study published June 19 in the journal Nature Chemistry, scientists at Northwestern University and Purdue University challenge this assumption with evidence that annihilation depends on the quantum phase relationships of photoexcitations. This means that at times photoexcitations do not annihilate each other when such quantum phases interfere destructively.

"Quantum interference is often believed to be fragile," Northwestern's Roel Tempelaar said.

"This is an exciting new direction for the use of quantum interference enabled by detailed chemical control of molecular crystals. Our team is advancing the field by experimentally demonstrating control of annihilation by quantum interference, the principle of which was previously predicted theoretically by one of the present study's authors. This contrasts with the currently prevailing viewpoint that annihilation proceeds as a 'classical' (non-quantum) process."

Tempelaar is an assistant professor of chemistry in the Weinberg College of Arts and Sciences. He is a member of the Center for Molecular Quantum Transduction at Northwestern.

The study, led by Tempelaar and Libai Huang at Purdue University, demonstrate that quantum interference sensitively governs a photoexcitation's behavior. By adding different chemical side-groups to identical molecules, the team made perylene diimide molecules—an industrial dye—crystallize in unique ways with different motifs. The photoexcitations inside each crystal differed starkly in their quantum phase relationships, which in turn yielded orders-of-magnitude differences in their rate of annihilation.

The team performed quantum chemical calculations to predict the difference in annihilation rates among the molecular crystals and corroborated the estimates with spectroscopic measurements.

The researchers took special care to disentangle the spectroscopic contribution of excitation mobility—which allows photoexcitations to meet one another—from that of the annihilation process itself. This was achieved by time-resolved microscopy-spectroscopy, which allows mobility rates to be determined, and laser-intensity control, which allows the likelihood of annihilation to be varied.

The researchers hope their work can be used to create new devices like solar cells with photoexcitations that have high densities and mobilities. Such enhanced devices would need detailed control of photoexcitations' quantum phases, achievable through crystals with uniquely designed packing motifs. Applications could range from optoelectronics to quantum information science.

"This research helps pave the way for more advanced molecular material design by harnessing quantum interference as a principal ingredient," Tempelaar said.

#QuantumInterference #Photoexcitation #QuantumPhysics #QuantumMechanics #QuantumEffects #QuantumCoherence #QuantumSystems #QuantumInterferometry #QuantumTechnology #QuantumEnhancement

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Directly imaging quantum states in two-dimensional materials

 

When some semiconductors absorb light, excitons (or particle pairs made of an electron bound to an electron hole) can form. Two-dimensional crystals of tungsten disulfide (WS2) have unique exciton states that are not found in other materials. However, these states are short lived and can change from one to another very quickly.


Scientists have developed a new approach to create separate images of these individual quantum states. By tracking the individual quantum states, researchers showed that the coupling mechanisms that lead to mixing of the states may not fully match current theories.

Scientists are excited about transition metal dichalcogenides, the family of crystals that includes tungsten disulfide, because they absorb light very strongly despite being only a few atoms thick. Researchers could use these crystals to build new nanoscale solar cells or electronic sensors. Using a new technique called time-resolved momentum microscopy, researchers can now better track the transitions between different exciton quantum states. This technique is broadly applicable, so scientists can now put other next-generation materials and devices under this momentum microscope to see how they work.

A variety of light-induced exciton states can form in monolayer transition metal dichalcogenides (TMDs) like WS2 under different conditions. Varying the wavelength or power of the exciting light or the temperature of the crystal allows different exciton states to form or persist. Light that is circularly polarized, where the direction of the electric field rotates around the direction the light wave travels, can selectively create excitons with a given quantum spin configuration in a specific set of energy bands.

Researchers at Stony Brook University have developed a unique instrument to directly visualize this effect under different ultrafast light excitation conditions and disentangle the complex mixture of quantum states that can form.

Published in Physical Review Letters, these new findings show how the force that binds the electron and electron hole together in the exciton also contributes to very rapid coupling, or mixing, of different exciton states. The researchers demonstrated that this effect leads to mixing of excitons with different spin configurations while still conserving both energy and momentum in the coupling process.

Surprisingly, the results showed that the rate of exciton mixing did not depend on the exciton energies as researchers had previously predicted. This study provides crucial experimental support for some current theories of exciton coupling in TMDs, but also sheds light on important discrepancies. Understanding the interplay between these excitonexciton states is a key step towards the harnessing potential of TMDs for nanotechnology and quantum sensing.

#QuantumStates #TwoDimensionalMaterials #ImagingTechniques #QuantumPhysics #QuantumMechanics

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New research on self-locking light sources presents opportunities for quantum technologies

 

In a paper published today in Nature Communications, researchers from the Paul Drude Institute in Berlin, Germany, and the Instituto Balseiro, Bariloche, Argentina, demonstrated that light emitters with different resonance frequencies can asynchronously self-lock their relative energies by exchanging mechanical energy. This finding paves the way for increased control of light sources and GHz-to-THz interconversion relevant to quantum technology.

Oscillators with slightly different resonance frequencies tend to lock their frequency to a common value when they start to interact with one another. This phenomenon was originally observed in a system of two pendula sharing the same support by Christiaan Huygens in the 17th century. Huygens first noticed that it was difficult to make two pendula with the same oscillation frequency—a necessary condition to make precise clocks. If, however, he would hang them on a common support, the clocks would slowly synchronize their motion and, after some time, oscillate at the same frequency.

The synchronization process described above is a general property of oscillating systems known as mode locking or entrainment. It appears in a wide range of oscillators from the very precise time synchronization required for GPS to the synchronization of the human biological clock regulating its daily rhythms.

The mechanism behind synchronization is far from trivial. To understand it, we must first consider that the amount of energy stored in a pendulum depends on its frequency and amplitude of motion. In addition, a pendulum can oscillate with frequency within a narrow band, whose width depends on the rate by which the pendulum loses energy (i.e., how quickly the pendulum comes to rest).

The frequency locking between the two Huygens' pendula relies on the exchange of energy via the bar supporting the pendula. This process requires that the narrow band of frequencies of the two pendula overlap, and that the energy transfer rate is much faster than the decay time of their oscillations. If these conditions are fulfilled, energy will be transferred back and forth between the pendula until their vibrations lock to a single frequency. In the locked regime, the net energy exchange between them vanishes.

In Huygens's experiments, the pendula have almost equal frequencies. A new study from the Paul Drude Institute and the Instituto Balseiro set out to demonstrate how one can synchronize the motion of pendula with very different resonance frequencies, i.e., with differences Δω between their resonance frequencies far larger than the frequency band of each pendulum.

This scenario occurs if the pendula have different lengths and therefore different resonance frequencies, as is illustrated in the upper part of Figure 1. This process–asynchronous locking of frequencies–is relevant for several applications including the precise frequency locking using phase-lock-loops (PLL) in electronic circuitry as well as the generation of radio waves or light beams with a well-defined frequency difference.

In their publication, Chafatinos and colleagues demonstrated an integrated array of asynchronously locked laser-like emitters irradiating at frequencies differing by multiples of a well-defined amount/quantity ωₘ. (cf. Figure 1, lower panel). The laser-like light is generated by an array of μm-sized emitters inserted in a hybrid semiconductor opto-mechanical resonator with a mechanical resonance frequency ωₘ of approximately 20 GHz. The emitters are excited by an external continuous wave laser beam.

The researchers show that the emitters can self-adjust their individual energies under laser excitation until they fulfill the conditions for asynchronous locking. At this point, the relative energy separations between the emitters automatically lock to multiples of ωₘ via the exchange of quanta of mechanical energy. The whole array then starts to self-oscillate at the mechanical frequency ωₘ.

Asynchronous locking in coupled pendula can be achieved by coupling them to a mechanical resonator with frequency ωₘ close to a multiple of Δω (i.e., Δω = n ωₘ , where n is an integer). Such a mechanical resonator is illustrated by the spring-bar system in the upper panel of Figure 1. Energy exchange via the mechanical oscillator provides the frequency offset required for locking. In fact, it can be shown that the requirements for asynchronous locking are the same as the ones for conventional locking when this frequency offset is taken into account.

An analogous process occurs for the array of light emitters in the lower panel of the figure. Here, the optomechanical interaction excites vibrations and, simultaneously, induces the energy offset required for asynchronous locking. Interestingly, a quite similar asynchronous locking behavior has been reported recently in a completely different context: the Pitangus sulphuratus, a bird from the Americas, manages to lock the frequency difference between its two vocal cords.

The work of Chafatinos and colleagues demonstrates a new concept for optomechanical materials based on arrays of μm-sized centers strongly interacting with confined GHz vibrations. These results pave the way for ultrafast GHz coherent mechanical control of light sources and interstate transitions relevant for quantum technologies.

#QuantumTechnologies #SelfLockingLightSources #CuttingEdgeResearch #QuantumComputing #QuantumCommunication #QuantumInformation #Photonics #QuantumOptics #ScientificAdvancements #QuantumRevolution

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The 'breath' between atoms—a new building block for quantum technology

University of Washington researchers have discovered they can detect atomic "breathing," or the mechanical vibration between two layers of atoms, by observing the type of light those atoms emitted when stimulated by a laser. The sound of this atomic "breath" could help researchers encode and transmit quantum information.

The researchers also developed a device that could serve as a new type of building block for quantum technologies, which are widely anticipated to have many future applications in fields such as computing, communications and sensor development.


The researchers published these findings June 1 in Nature Nanotechnology.

"This is a new, atomic-scale platform, using what the scientific community calls 'optomechanics,' in which light and mechanical motions are intrinsically coupled together," said senior author Mo Li, a UW professor of both electrical and computer engineering and physics. "It provides a new type of involved quantum effect that can be utilized to control single photons running through integrated optical circuits for many applications."

Previously, the team had studied a quantum-level quasiparticle called an "exciton." Information can be encoded into an exciton and then released in the form of a photon—a tiny particle of energy considered to be the quantum unit of light. Quantum properties of each photon emitted —such as the photon's polarization, wavelength and/or emission timing—can function as a quantum bit of information, or "qubit," for quantum computing and communication. And because this qubit is carried by a photon, it travels at the speed of light.

"The bird's-eye view of this research is that to feasibly have a quantum network, we need to have ways of reliably creating, operating on, storing and transmitting qubits," said lead author Adina Ripin, a UW doctoral student of physics. "Photons are a natural choice for transmitting this quantum information because optical fibers enable us to transport photons long distances at high speeds, with low losses of energy or information."

The researchers were working with excitons in order to create a single photon emitter, or "quantum emitter," which is a critical component for quantum technologies based on light and optics. To do this, the team placed two thin layers of tungsten and selenium atoms, known as tungsten diselenide, on top of each other.

When the researchers applied a precise pulse of laser light, they knocked a tungsten diselenide atom's electron away from the nucleus, which generated an exciton quasiparticle. Each exciton consisted of a negatively charged electron on one layer of the tungsten diselenide and a positively charged hole where the electron used to be on the other layer. And because opposite charges attract each other, the electron and the hole in each exciton were tightly bonded to each other. After a short moment, as the electron dropped back into the hole it previously occupied, the exciton emitted a single photon encoded with quantum information—producing the quantum emitter the team sought to create.

But the team discovered that the tungsten diselenide atoms were emitting another type of quasiparticle, known as a phonon. Phonons are a product of atomic vibration, which is similar to breathing. Here, the two atomic layers of the tungsten diselenide acted like tiny drumheads vibrating relative to each other, which generated phonons. This is the first time phonons have ever been observed in a single photon emitter in this type of two-dimensional atomic system.

When the researchers measured the spectrum of the emitted light, they noticed several equally spaced peaks. Every single photon emitted by an exciton was coupled with one or more phonons. This is somewhat akin to climbing a quantum energy ladder one rung at a time, and on the spectrum, these energy spikes were represented visually by the equally spaced peaks.

"A phonon is the natural quantum vibration of the tungsten diselenide material, and it has the effect of vertically stretching the exciton electron-hole pair sitting in the two layers," said Li, who is also is a member of the steering committee for the UW's QuantumX, and is a faculty member of the Institute for Nano-Engineered Systems. "This has a remarkably strong effect on the optical properties of the photon emitted by the exciton that has never been reported before."

The researchers were curious if they could harness the phonons for quantum technology. They applied electrical voltage and saw that they could vary the interaction energy of the associated phonons and emitted photons. These variations were measurable and controllable in ways relevant to encoding quantum information into a single photon emission and this was all accomplished in one integrated system—a device that involved only a small number of atoms.

Next the team plans to build a waveguide—fibers on a chip that catch single photonphoton emissions and direct them where they need to go—and then scale up the system. Instead of controlling only one quantum emitter at a time, the team wants to be able to control multiple emitters and their associated phonon states. This will enable the quantum emitters to "talk" to each other, a step toward building a solid base for quantum circuitry.

"Our overarching goal is to create an integrated system with quantum emitters that can use single photons running through optical circuits and the newly discovered phonons to do quantum computing and quantum sensing," Li said. "This advance certainly will contribute to that effort, and it helps to further develop quantum computing which, in the future, will have many applications."

Other co-authors are Ruoming Peng, Xiaowei Zhang, Srivatsa Chakravarthi, Minhao He, Xiaodong Xu, Kai-Mei Fu and Ting Cao.z

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Exploring gravity's effect on quantum spins

 

A joint research group led by Prof. Sheng Dong and Prof. Lu Zhengtian from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS), investigated the coupling effect between neutron spin and gravitational force via employing a high-precision xenon isotope magnetometer. This work was published in Physical Review Letters.



This research aims to uncover the coupling strength between neutron spin and gravity by measuring the weight difference between the neutron's spin-up and spin-down states. The experimental results revealed that the weight difference between these two states was less than two sextillionths (<2×10-21), setting a new upper limit on the coupling strength of this effect.

An article titled "Testing Gravity's Effect on Quantum Spins," reported in Physics, highlights this precise measurement research as a novel exploration of the intersection of quantum theory and gravity.

There are four fundamental physical interactions in nature, among which only gravity has not been experimentally found to be related to a particle's intrinsic spin. If the spin is coupled with gravity, particles in different spin states will exhibit extremely small differences in energy and force within the Earth's gravitational field.

Since the 1970s, researchers have been developing various classical or quantum measurement methods to search for the coupling phenomenon between spin and gravity, continuously improving the precision of measurement. These experiments have also investigated the fundamental spacetime symmetry in gravitational interactions and seek to identify axion-like particles that mediate monopole-dipole interactions.

The USTC team developed a highly stable and sensitive 129Xe-131Xe-Rb co-magnetometer, combining their self-developed atomic devices and spectroscopic measurement techniques, precision measurement methods developed to suppress systematic errors in co-magnetometer systems.

The co-magnetometer is used as a quantum compass, measuring the coherent effects between the two quantum spin states pointing upward and downward. The quantum axis of the system aligns with the Earth's rotation axis (i.e., the direction of the North Star) with a precision better than 0.6 degrees, significantly reducing systematic experimental errors caused by the rotation of the Earth.

The experimental results compress the upper limit of the neutron's spin-gravity coupling strength by a factor of 17, and improve the precision of various fundamental physical effects by an order of magnitude.

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New technique in error-prone quantum computing makes classical computers sweat

 

Despite steady improvements in quantum computers, they're still noisy and error-prone, which leads to questionable or wrong answers. Scientists predict that they won't truly outcompete today's "classical" supercomputers for at least five or ten years, until researchers can adequately correct the errors that bedevil entangled quantum bits, or qubits.

But a new study shows that even lacking good error correction, there are ways to mitigate errors that could make quantum computers useful today.

Researchers at IBM Quantum in New York and their collaborators at the University of California, Berkeley, and Lawrence Berkeley National Laboratory report today (June 14) in the journal Nature that they pitted a 127-qubit quantum computer against a state-of-the-art supercomputer, and for at least one type of calculation, bested the supercomputer.

The calculation wasn't chosen because it was difficult for classical computers, the researchers say, but because it's similar to ones that physicists make all the time. Crucially, the calculation could be made increasingly complex in order to test whether today's noisy, error-prone quantum computers can produce accurate results for certain types of common calculations.

The fact that the quantum computer produced the verifiably correct solution as the calculation became more complex, while the supercomputer algorithm produced an incorrect answer, provides hope that quantum computing algorithms with error mitigation, instead of the more difficult error correction, could tackle cutting-edge physics problems, such as understanding the quantum properties of superconductors and novel electronic materials.

"We're entering the regime where the quantum computer might be able to do things that current algorithms on classical computers cannot do," said UC Berkeley graduate student and study co-author Sajant Anand.


"We can start to think of quantum computers as a tool for studying problems that we wouldn't be able to study otherwise," added Sarah Sheldon, senior manager for Quantum Theory and Capabilities at IBM Quantum.

Conversely, the quantum computer's trouncing of the classical computer could also spark new ideas to improve the quantum algorithms now used on classical computers, according to co-author Michael Zaletel, UC Berkeley associate professor of physics and holder of the Thomas and Alison Schneider Chair in Physics.

"Going into it, I was pretty sure that the classical method would do better than the quantum one," he said. "So, I had mixed emotions when IBM's zero-noise extrapolated version did better than the classical method. But thinking about how the quantum system is working might actually help us figure out the right classical way to approach the problem. While the quantum computer did something that the standard classical algorithm couldn't, we think it's an inspiration for making the classical algorithm better so that the classical computer performs just as well as the quantum computer in the future."

Boost the noise to suppress the noise

One key to the seeming advantage of IBM's quantum computer is quantum error mitigation, a novel technique for dealing with the noise that accompanies a quantum computation. Paradoxically, IBM researchers controllably increased the noise in their quantum circuit to get even noisier, less accurate answers and then extrapolated backward to estimate the answer the computer would have gotten if there had been no noise. This relies on having a good understanding of the noise that affects quantum circuits and predicting how it affects the output.

The problem of noise comes about because IBM's qubits are sensitive superconducting circuits that represent the zeroes and ones of a binary computation. When the qubits are entangled for a calculation, unavoidable annoyances such as heat and vibration can alter the entanglement, introducing errors. The greater the entanglement, the worse the effects of noise.


Quantum computers have the potential to solve some of the world’s biggest problems, but they are limited by their extreme sensitivity to errors caused by environmental noise. New research from IBM Quantum and UC Berkeley shows that a family of computational techniques called quantum error mitigation could allow quantum computers to solve useful problems at a scale far beyond the capability of even the most sophisticated classical supercomputing methods. Credit: IBM Research

In addition, computations that act on one set of qubits can introduce random errors in other, uninvolved qubits. Additional computations then compound these errors. Scientists hope to use extra qubits to monitor such errors so they can be corrected—so-called fault-tolerant error correction. But achieving scalable fault-tolerance is a huge engineering challenge, and whether it will work in practice for ever greater numbers of qubits remains to be shown, Zaletel said.

Instead, IBM engineers came up with a strategy of error mitigation they called zero noise extrapolation (ZNE), which uses probabilistic methods to controllably increase the noise on the quantum device. Based on a recommendation from a former intern, IBM researchers approached Anand, postdoctoral researcher Yantao Wu and Zaletel to ask their help in assessing the accuracy of the results obtained using this error mitigation strategy. Zaletel develops supercomputer algorithms to solve difficult calculations involving quantum systems, such as the electronic interactions in new materials. These algorithms, which employ tensor network simulations, can be directly applied to simulate interacting qubits in a quantum computer.

Over a period of several weeks, Youngseok Kim and Andrew Eddins at IBM Quantum ran increasingly complex quantum calculations on the advanced IBM Quantum Eagle processor, and then Anand attempted the same calculations using state-of-the-art classical methods on the Cori supercomputer and Lawrencium cluster at Berkeley Lab and the Anvil supercomputer at Purdue University. When Quantum Eagle was rolled out in 2021, it had the highest number of high-quality qubits of any quantum computer, seemingly beyond the ability of classical computers to simulate.

In fact, exactly simulating all 127 entangled qubits on a classical computer would require an astronomical amount of memory. The quantum state would need to be represented by 2 to the power of 127 separate numbers. That's 1 followed by 38 zeroes; typical computers can store around 100 billion numbers, 27 orders of magnitude too small. To simplify the problem, Anand, Wu and Zaletel used approximation techniques that allowed them to solve the problem on a classical computer in a reasonable amount of time, and at a reasonable cost. These methods are somewhat like jpeg image compression, in that they get rid of less important information and keep only what's required to achieve accurate answers within the limits of the memory available.

Anand confirmed the accuracy of the quantum computer's results for the less complex calculations, but as the depth of the calculations grew, the results of the quantum computer diverged from those of the classical computer. For certain specific parameters, Anand was able to simplify the problem and calculate exact solutions that verified the quantum calculations over the classical computer calculations. At the largest depths considered, exact solutions were not available, yet the quantum and classical results disagreed.

The researchers caution that while they can't prove that the quantum computer's final answers for the hardest calculations were correct, Eagle's successes on the previous runs gave them confidence that they were.

"The success of the quantum computer wasn't like a fine-tuned accident. It actually worked for a whole family of circuits it was being applied to," Zaletel said.
IBM’s quantum computer is housed inside a cryogenic container (center) surrounded by a tangle of cables used to control and read out its qubits. Credit: IBM
Friendly competition

While Zaletel is cautious about predicting whether this error mitigation technique will work for more qubits or calculations of greater depth, the results were nonetheless inspiring, he said.

"It sort of spurred a feeling of friendly competition," he said. "I have a sense that we should be able to simulate on a classical computer what they're doing. But we need to think about it in a clever and better way—the quantum device is in a regime where it suggests we need a different approach."

One approach is to simulate the ZNE technique developed by IBM.

"Now, we're asking if we can take the same error mitigation concept and apply it to classical tensor network simulations to see if we can get better classical results," Anand said. "This work gives us the ability to maybe use a quantum computer as a verification tool for the classical computer, which is flipping the script on what's usually done."


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Virtual laboratory opens possibility for machine learning to understand promising class of quantum materials

 

Thomas Edison famously tried hundreds of materials and failed thousands of times before discovering that carbonized cotton thread burned long and bright in an incandescent light bulb. Experiments are often time-consuming (Edison's team spent 14 months) and expensive (the winning combination cost about $850,000 in today's money).

Expenses and time increase exponentially when developing the quantum materials that will revolutionize modern electronics and computing.

To make quantum material discovery possible, researchers turn to detailed databases as their virtual laboratory. A new database of understudied quantum materials that has been created by researchers at Pacific Northwest National Laboratory (PNNL) provides an avenue to discover new materials that could power gadgets far more powerful than Edison's lightbulb.
Beyond Edisonian trial and error

"We wanted to understand a general class of materials that have the same crystal structure, but different properties depending on how you combine and grow them," said materials scientist Tim Pope. This class of materials, known as transition metal dichalcogenides (TMDs), contains thousands of potential combinations, each of which requires a weeks'-long reaction to grow flakes of material the size of glitter.

Making the material is only the first step in understanding what it can do. As PNNL computational scientist Micah Prange said, each flake is "really small, really delicate," and quantum features will only emerge when studied at super-low temperatures. Essentially, "a whole research program could go into each flake."

Despite the difficulty of creating and measuring them, each combination holds promise to dramatically improve electronics, batteries, pollution remediation, and quantum computing devices.

]Prange said you can think of the flakes as "fancier graphene with a richer phenomenology and more practical possibilities." Tough, light, and flexible, graphene has been known as the material of the future, with uses in everything from aerospace to wearable electronics.

"The varied properties across this class of materials mean that as we better understand them, one of the combinations could be selected for a desired property and exactly paired to the ideal use," said Pope, "or even a brand new application."
Close up of a flake next to an image of its crystal structure overlaid with the model’s predicted crystal structure. Credit: Tim Pope | Pacific Northwest National Laboratory
Quantum material development of the future

Building the database began with PNNL's Chemical Dynamics Initiative, an effort to use PNNL's strength in data science to fill in knowledge gaps left by measurement challenges and experimental limitations.

These particular quantum materials are made by varying proportions of the 38 transition metals, like tungsten or vanadium, in combination with three elements in the sulfur family. They can also be grown in three different crystal structures, meaning there are thousands of potential combinations, all with distinct properties.

Using a type of modeling called density functional theory, the researchers computed the properties of 672 unique structures with a total of 50,337 individual atomic configurations. Before this research, there were fewer than 40 studied configurations, with only a rudimentary understanding of their properties.

"Models can work out the quantum mechanics of how atoms are arranged," said Prange. "From this, you can say if the material will conduct electricity or be transparent or how hard the material will be to compress or bend."

Using the database, PNNL's researchers revealed striking differences in the electrical and magnetic behaviors between different combinations. Importantly, the researchers also found other trends as they varied the transition metal, including a new understanding of transition metal chemistry at the quantum level.
Quantum combinations for machine learning

"When the crystal structure was overlaid with the database, it matched perfectly," said Pope, speaking of the PNNL-grown flakes that are beginning to validate the modeling results.

"The idea was really to develop a big data set of theoretical simulations so we could use data analytics to understand these materials," said Prange. "The immediate value of the project is that we did enough different cases to efficiently use machine learning."

The open-source dataset, published in Scientific Data, offers researchers a strong starting point for exploring relationships between initial structures and corresponding properties. With this information, they can downselect to specific materials for study.

"This project is one example of how we can use a large computational dataset to guide the experimental research," said CDI Chief Scientist Peter Sushko, "Projects like this provide critical data to the machine learning community and could streamline materials development. It is exciting to think about what needs to be understood next to enable synthesis of these materials with atomic precision."

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Scientists discover quantum oscillations in correlated insulators

                    


Quantum oscillations (QOs) of conductance in the magnetic field are widely observed in mesoscopic devices thanks to the Landau quantization, and thus QOs are commonly used as a powerful tool to measure the Fermi surface of metals. In contrast, QOs are usually absent for insulators due to the zero density of state in the gap.

A team led by Dr. Yang Wei and Dr. Zhang Guangyu from the Institute of Physics of the Chinese Academy of Sciences (CAS) reported the observation of anomalous quantum oscillations of correlated insulators in twisted double-bilayer graphene. Their paper, entitled "Quantum oscillations in field-induced correlated insulators of a moiré superlattice," was published in Science Bulletin.


Graphene-based moiré superlattices, which consist of two pieces of single or multilayer graphene stacked at a twisted angle, are known to host moiré flat bands and correlated states. A typical example is twisted double-bilayer graphene (2+2), whose band structure can be further tuned by electric field in addition to the twist angle, thus allowing tuning of flat bands and correlation strength in situ.

In this study, the researchers observed the spin-polarized and valley-polarized correlated insulators when the moiré bands were half filled in 2+2. With its highly tunable nature, 2+2 provides a new platform for discovering new exotic phases in the correlated insulating states.
The team has long been devoted to exploring the quantum transport behavior in moiré superlattices. Previously, they found that new correlated insulators with valley polarizations appear at half fillings of energy bands in twisted double bilayer graphene, thanks to the orbital Zeeman effect in perpendicular magnetic field.

To their surprise, they found that the resistance of correlated insulators in 2+2 oscillates periodically with the inverse of the magnetic field, similar to the Shubnikov de Haas oscillations in metal. The bulk insulating evidence of the QOs is revealed in the high oscillation amplitude of ~150 kΩ and its temperature dependence, as well as the antiphase behavior.

Moreover, the insulating QOs are strongly tunable by electric field. The carrier density extracted from the 1/B periodicity decreases almost linearly with D from -0.7 to -1.1V/nm, suggesting a reduced Fermi surface; the effective mass from the Lifshitz-Kosevich analysis depends nonlinearly on perpendicular electrical displacement field (D), reaching a minimum value of 0.1me at D = ~ -1.0V/nm.
Quantum oscillations in a hybridized gap with band inversion. Credit: YANG Wei
Electrical field tunable quantum oscillations. Credit: YANG Wei
Quantum oscillations in a hybridized gap with band inversion. Credit: YANG Wei
Electrical field tunable quantum oscillations. Credit: YANG Wei
                                                                    

To account for these anomalous phenomena, the researchers built a phenomenological inverted band model. With parameters extracted from experiments, density of states calculations from the model qualitatively reproduce the electric field tunable QOs of correlated insulators.

The observation of QOs of insulators in this study establishes a close connection to other strongly correlated systems such as Kondo insulators, topological insulators, and excitonic insulators, and it strongly suggests that more exotic phases are to be discovered in this system.

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