Category: Physics

Researchers at Penn Engineering have created a chip that outstrips the security and robustness of existing quantum communications hardware. Their technology communicates in “qudits,” doubling the quantum information space of any previous on-chip laser.

Liang Feng, Professor in the Departments of Materials Science and Engineering (MSE) and Electrical Systems and Engineering (ESE), along with MSE postdoctoral fellow Zhifeng Zhang and ESE Ph.D. student Haoqi Zhao, debuted the technology in a recent study published in Nature. The group worked in collaboration with scientists from the Polytechnic University of Milan, the Institute for Cross-Disciplinary Physics and Complex Systems, Duke University and the City University of New York (CUNY).

Bits, qubits and qudits

While non-quantum chips store, transmit and compute data using bits, state-of-the-art quantum devices use qubits. Bits can be 1s or 0s, while qubits are units of digital information capable of being both 1 and 0 at the same time. In quantum mechanics, this state of simultaneity is called “superposition.”

A quantum bit in a state of superposition greater than two levels is called a qudit to signal these additional dimensions.

“In classical communications,” says Feng, “a laser can emit a pulse coded as either 1 or 0. These pulses can easily be cloned by an interceptor looking to steal information and are therefore not very secure. In quantum communications with qubits, the pulse can have any superposition state between 1 and 0. Superposition makes it so a quantum pulse cannot be copied. Unlike algorithmic encryption, which blocks hackers using complex math, quantum cryptography is a physical system that keeps information secure.”

Qubits, however, aren’t perfect. With only two levels of superposition, qubits have limited storage space and low tolerance for interference.

The Feng Lab device’s four-level qudits enable significant advances in quantum cryptography, raising the maximum secrete key rate for information exchange from 1 bit per pulse to 2 bits per pulse. The device offers four levels of superposition and opens the door to further increases in dimension.

“The biggest challenge,” says Zhang, “was the complexity and non-scalability of the standard setup. We already knew how to generate these four-level systems, but it required a lab and many different optical tools to control all the parameters associated with the increase in dimension. Our goal was to achieve this on a single chip. And that’s exactly what we did.”

The physics of cybersecurity

Quantum communication uses photons in tightly controlled states of superposition. Properties such as location, momentum, polarization and spin exist as multiplicities at the quantum level, each of which is governed by probabilities. These probabilities describe the likelihood of a quantum system—an atom, a particle, a wave—taking on a single attribute when measured.

In other words, quantum systems are neither here nor there. They are both here and there. It is only the act of observation—detecting, looking, measuring—that causes a quantum system to take on a fixed property. Like a subatomic game of Statues, quantum superpositions take on a single state as soon as they are observed, making it impossible to intercept them without detection or copy them.

The hyperdimensional spin-orbit microlaser builds on the team’s earlier work with vortex microlasers, which sensitively tune the orbital angular momentum (OAM) of photons. The most recent device upgrades the capabilities of the previous laser by adding another level of command over photonic spin.

This additional level of control—being able to manipulate and couple OAM and spin—is the breakthrough that allowed them to achieve a four-level system.

The difficulty of controlling all these parameters at once is what had been hindering qudit generation in integrated photonics and represents the major experimental accomplishment of the team’s work.

“Think of the quantum states of our photon as two planets stacked on top of each other,” says Zhao. “Before, we only had information about these planets’ latitude. With that, we could create a maximum of two levels of superposition. We didn’t have enough information to stack them into four. Now, we have longitude as well. This is the information we need to manipulate photons in a coupled way and achieve dimensional increase. We are coordinating each planet’s rotation and spin and holding the two planets in strategic relation to each other.”

Quantum cryptography with Alice, Bob and Eve

Quantum cryptography relies on superposition as a tamper-evident seal. In a popular cryptography protocol known as Quantum Key Distribution (QKD), randomly generated quantum states are sent back and forth between sender and receiver to test the security of a communications channel.

If sender and receiver (always Alice and Bob in the storyworld of cryptography) discover a certain amount of discrepancy between their messages, they know that someone has attempted to intercept their message. But, if the transmission remains mostly intact, Alice and Bob understand the channel to be safe and use the quantum transmission as a key for encrypted messages.

How does this improve on non-quantum communication security? If we imagine the photon as a sphere rotating upwards, we can get a rough idea of how a photon might classically encode the binary digit 1. If we imagine it rotating downwards, we understand 0.

When Alice sends classical photons coded in bits, Eve the eavesdropper can steal, copy and replace them without Alice or Bob realizing. Even if Eve cannot decrypt the data she has stolen, she may be squirreling it away for a near future when advances in computing technology might allow her to break through.

Quantum communication adds a stronger layer of security. If we imagine the photon as a sphere rotating upwards and downwards at the same time, coding 1 and 0 simultaneously, we get an idea of how a qubit maintains dimension in its quantum state.

When Eve tries to steal, copy and replace the qubit, her ability to capture the information will be compromised and her tampering will be apparent in the loss of superposition. Alice and Bob will know the channel is not secure and will not use a security key until they can prove that Eve has not intercepted it. Only then will they send the intended encrypted data using an algorithm enabled by the qubit key.

However, while the laws of quantum physics may prevent Eve from copying the intercepted qubit, she may be able to disturb the quantum channel. Alice and Bob will need to continue generating keys and sending them back and forth until she stops interfering. Accidental disturbances that collapse superposition as the photon travels through space also contribute to interference patterns.

A qubit’s information space, limited to two levels, has a low tolerance for these errors.

To solve these problems, quantum communication requires additional dimensions. If we imagine a photon rotating (the way the earth rotates around the sun) and spinning (the way the earth spins on its own axis) in two different directions at once, we get a sense of how the Feng Lab qudits work.

If Eve tries to steal, copy and replace the qudit, she will not be able to extract any information and her tampering will be clear. The message sent will have a much greater tolerance for error—not only for Eve’s interference, but also for accidental flaws introduced as the message travels through space. Alice and Bob will be able to efficiently and securely exchange information.

“There is a lot of concern,” says Feng, “that mathematical encryption, no matter how complex, will become less and less effective because we are advancing so quickly in computing technologies. Quantum communication’s reliance on physical rather than mathematical barriers make it immune to these future threats. It’s more important than ever that we continue to develop and refine quantum communication technologies.”

More information: Zhifeng Zhang et al, Spin–orbit microlaser emitting in a four-dimensional Hilbert space, Nature (2022). DOI: 10.1038/s41586-022-05339-z

Journal information: Nature 

Provided by University of Pennsylvania 

Scientists and government representatives meeting at a conference in France voted on Friday to scrap leap seconds by 2035, the organization responsible for global timekeeping said.

Similar to leap years, leap seconds have been periodically added to clocks over the last half century to make up for the difference between exact atomic time and the Earth’s slower rotation.

While leap seconds pass by unnoticed for most people, they can cause problems for a range of systems that require an exact, uninterrupted flow of time, such as satellite navigation, software, telecommunication, trade and even space travel.

It has caused a headache for the International Bureau of Weights and Measures (BIPM), which is responsible for Coordinated Universal Time (UTC)—the internationally agreed standard by which the world sets its clocks.

A resolution to stop adding leap seconds by 2035 was passed by the BIPM’s 59 member states and other parties at the General Conference on Weights and Measures, which is held roughly every four years at the Versailles Palace west of Paris.

The head of BIPM’s time department, Patrizia Tavella, told AFP that the “historic decision” would allow “a continuous flow of seconds without the discontinuities currently caused by irregular leap seconds”.

“The change will be effective by or before 2035,” she said via email.

She said that Russia voted against the resolution, “not on principle”, but because Moscow wanted to push the date it comes into force until 2040.

Other countries had called for a quicker timeframe such as 2025 or 2030, so the “best compromise” was 2035, she said.

The United States and France were among the countries leading the way for the change.

Tavella emphasized that “the connection between UTC and the rotation of the Earth is not lost”.

“Nothing will change” for the public, she added.

A leap minute?

Seconds were long measured by astronomers analyzing the Earth’s rotation, however the advent of atomic clocks—which use the frequency of atoms as their tick-tock mechanism—ushered in a far more precise era of timekeeping.

But Earth’s slightly slower rotation means the two times are out of sync.

To bridge the gap, leap seconds were introduced in 1972, and 27 have been added at irregular intervals since—the last in 2016.

Under the proposal, leap seconds will continue to be added as normal for the time being.

But by 2035, the difference between atomic and astronomical time will be allowed to grow to a value larger than one second, Judah Levine, a physicist at the US National Institute of Standards and Technology, told AFP.

“The larger value is yet to be determined,” said Levine, who spent years helping draft the resolution alongside Tavella.

Negotiations will be held to find a proposal by 2035 to determine that value and how it will be handled, according to the resolution.

Levine said it was important to protect UTC time because it is run by “a worldwide community effort” in the BIPM.

GPS time, a potential UTC rival governed by atomic clocks, is run by the US military “without worldwide oversight”, Levine said.

A possible solution to the problem could be letting the discrepancy between the Earth’s rotation and atomic time build up to a minute.

It is difficult to say exactly how long that might take, but Levine estimated anywhere between 50 to 100 years.

Instead of then adding on a leap minute to clocks, Levine proposed a “kind of smear”, in which the last minute of the day takes two minutes.

“The advance of a clock slows, but never stops,” he said.

© 2022 AFP

Researchers studying the magnetic behavior of a cuprate superconductor may have explained some of the unusual properties of their conduction electrons.

Cuprate superconductors are used in levitating trains, quantum computing and power transmission. They are of a family of materials made of layers of copper oxides alternating with layers of other metal oxides, which act as charge reservoirs.

The largest use of superconductors is currently for manufacturing superconducting magnets used for medical MRI machines and for scientific applications such as particle accelerators.

For the potential applications of superconducting materials to be fully realized, developing superconductors that maintain their properties at higher temperatures is crucial for scientists. The cuprate superconductors currently exhibit relatively high transition point temperatures and therefore give scientists an opportunity to study what makes higher temperature superconductivity possible.

In this study, published in Nature Physics, a collaboration involving the University of Bristol and the ISIS Pulsed Neutron and Muon Source, they focused on the cuprate superconductor La_2-xSr_xCuO₄ (LSCO). Superconductivity in this system is very sensitive to the exact ratio of Lanthanum (La) to Strontium (Sr) offering the ability to understand which properties are correlated with superconductivity. LSCO is also close to being magnetically ordered and one possibility is that the magnetic fluctuations are what enables its superconductivity.

Inelastic neutron scattering offers an excellent method to study these magnetic fluctuations. The researchers were able to measure over a wide range of reciprocal space and energy scales. This enabled them to build a full picture of the spin fluctuations and phonons, allowing very low energy spin fluctuations to be isolated.

Although cuprate superconductors are metals above the temperature where they become superconducting, the electrons that carry current behave very strangely. As the temperature is increased, their ability to carry current is dramatically reduced. The low-energy spin fluctuations could scatter the conduction electrons and explain this strange metal behavior.

Furthermore, when the superconductor was cooled and the superconductivity suppressed with a magnetic field, the spin fluctuations became stronger and slow down suggesting the material is close to magnetic order. This could help to explain the unusual electronic properties of the cuprates.

Prof Stephen Hayden of Bristol’s School of Physics said, “This study has demonstrated the potential importance of spin fluctuations in understanding cuprates. A deeper understanding of their properties and their relation to superconductivity is another step towards designing materials with higher superconducting temperatures.

“In the future they should be used for quantum computing, transport including levitating trains and compact motor as well as power transmission. There are already demonstration projects for the latter.

“The work relies on the unique instrumentation and sample environment available at ISIS.”

More information: M. Zhu et al, Spin fluctuations associated with the collapse of the pseudogap in a cuprate superconductor, Nature Physics (2022). DOI: 10.1038/s41567-022-01825-3

Journal information: Nature Physics 

Provided by University of Bristol 

New research conducted by Princeton University physicists is delving with high resolution into the complex and fascinating world of topological quantum matter—a branch of physics that studies the inherent quantum properties of materials that can be deformed but not intrinsically changed. By repeating an experiment first conducted by researchers at Kyoto University, the Princeton team has clarified key aspects of the original experiment, and importantly, reached novel and divergent conclusions—conclusions that advance our understanding of topological matter.

As chronicled in a paper published in the journal Nature Materials, the Princeton researchers used a special type of magnetic insulator realized in ruthenium chloride (α-RuCl₃) to demonstrate the first example of a magnetic insulator that exhibits the thermal Hall effect arising from quantum edge modes of bosons in the presence of a novel force field called the Berry curvature.

Background to the experiment

The experiment has its origins in the work of Princeton physicist and 1977 Nobel Prize-winner Phil Anderson, who theorized a novel state of matter called spin liquids. These are classes of magnetic materials that—even under extremely low temperatures—do not undergo what physicists call a magnetic phase transition. This describes an abrupt transition to a state in which the spin at each lattice site either aligns in a perfectly parallel pattern, called ferromagnetic order, or alternates in an orderly fashion between up and down, called antiferromagnetic order. Over ninety-nine percent of magnetic materials experience this phase transition when cooled to sufficiently low temperatures. Anderson suggested the term “geometric frustration” to describe how spin liquids are prevented from undergoing such phase transitions.

“To illustrate this concept, imagine trying to seat couples around a dinner table under the rule that every woman is to be seated between two men and vice versa,” said N. Phuan Ong, the Eugene Higgins Professor of Physics at Princeton University and the senior author of the paper. “If we have a guest who arrives alone, this arrangement is geometrically impossible.”

In 2006, Russian physicist Alexei Kitaev at the California Institute of Technology (Caltech) proposed that Anderson’s spin liquid state could be achieved without invoking Anderson’s concept of geometric frustration. He outlined this in a series of elegant equations, and importantly, predicted the existence of new particles called Majoranas and visons. The Majorana particle is an especially strange and elusive subatomic particle that was first theorized in 1937 by Italian physicist Ettore Majorana. It is a type of fermion; in fact, it is the only fermion recognized as identical to its own antiparticle.

Kitaev’s work sparked a flurry of research to find materials that could realize his model calculations in the laboratory. Two years later, two physicists, George Jackeli and Giniyat Khailyulin of the Max Planck Institute in Stuttgart, Germany, predicted ruthenium chloride (α-RuCl₃) to be the closest proximate. This material, which crystallizes in a honeycomb lattice, is an excellent insulator.

Consequently, in the past decade, α-RuCl₃ has become one of the most intensively investigated candidates for quantum spin liquids. The research received a considerable boost in 2018 when physicist Yuji Matsuda and his colleagues at Kyoto University reported the observation of the “half-quantized” thermal Hall effect predicted in Kitaev’s calculations.

The thermal Hall effect, which is analogous to the more familiar electrical Hall effect, describes how an intense magnetic field deflects sideways an applied heat current. The sideways deflection engenders a weak temperature difference between two edges of the sample, which reverses sign if the direction of the magnetic field is reversed. While the thermal Hall effect is well established in metals such as copper and gallium, it is very rarely observed in insulators. This is because, in insulators, a heat current is conveyed by lattice vibrations called phonons that are indifferent to the magnetic field, Ong noted.

Matsuda reported that their measurements of the thermal Hall conductivity revealed that it was “half-quantized.” The magnitude depends only on the Planck constant and the Boltzmann constant, and nothing else, as predicted by Kitaev. “This experiment, implying the observation of a current of Majorana particles, attracted enormous interest in the community.”

But Ong and his research team, long familiar with thermal Hall experiments, felt that there was something amiss with Matsuda’s conclusion. “I couldn’t quite put my finger on it,” Ong said.

The experiment

Ong and his colleagues decided to repeat the experiment. But this time, they aimed to conduct the experiment at a higher resolution and over a much larger temperature interval—from one-half degrees Kelvin to ten degrees Kelvin.

The high level of resolution was critical to the success of the experiment, explained Peter Czajka, the lead author of the paper and a graduate student in physics. “Our experiment is a great example of something that is conceptually quite simple, but very difficult in practice. It’s relatively easy to measure the electrical resistance of something but measuring the thermal conductivity of a sample is much harder.”

The first part of the experiment required the researchers to select a sample of ruthenium chloride that had several specific characteristics, including a very thin crystal structure with a distinct hexagonal shape. They then attached sensitive thermometers to measure the temperature gradients.

“All we’re really doing is measuring very small temperature gradients on a crystal,” said Czajka. “But to do this we need a resolution of a thousandth to a millionth of a degree—something in between that scale.”

The researchers cooled the material down to temperatures of one Kelvin or lower, and subjected the sample to a strong magnetic field, which was applied parallel to the heat current. They then used an electrical heater to warm up one edge of the crystal and measured the temperature gradients. The experiment—measurements of temperature gradients—required, amazingly, a period of several months.

“The sample was cold for about six months,” said Czajka, “and during that time we thoroughly mapped out the temperature and field dependence. This was unprecedented because most researchers aren’t willing to put six months into a single experiment.”

The first thing the researchers noticed, in a finding parallel with Matsuda’s, was the presence of the thermal Hall effect. The researchers recognized this when the thermometers detected that the flow of the heat current was deflected to one side or the other depending on the magnetic field.

To explain this, Ong used the analogy of a raft going downstream, with the river current symbolizing the heat current and the raft symbolizing a packet of heat entropy. “Although you’re going with the flow of the river, you find that your raft is being pushed to one side of the river, say the left bank. And all the rafts following you are similarly being pushed to the left bank,” he said. This leads to a slight increase in the left bank’s temperature.

The signal is also sensitive to the direction of the magnetic field, said Ong. “If you repeat the experiment with the magnetic field reversed in direction, you will find all the rafts, which are still going downstream, accumulating on the right bank.”

In the vast majority of insulators, this effect does not occur. “The rafts will not accumulate on either the left or right side; they will just flow down the river,” said Ong.

But in these new topological materials the effect is startling. And the reason for this is because of a phenomenon known as the Berry curvature.

In principle, all crystalline materials display an internal force field called the Berry curvature, named after Michael Berry, a mathematical physicist at the University of Bristol. The Berry Curvature describes how wave functions twist and turn throughout the space spanned by momentum. In magnetic and topological materials, the Berry curvature is finite. It acts on charged particles, such as electrons, as well as neutral ones, such as phonons and spins, much like an intense magnetic field.

“The Berry curvature is a concept that was missing for the last sixty years, but has now come to the fore in the last five years or so,” said Ong. “It’s the Berry Curvature that we proved in this paper that is actually the cause of Matsuda’s experimental observation.”

Equally important, the Princeton researchers were not able to confirm the presence of the Majorana fermion, as originally predicted in Matsuda’s experiment. Rather, the researchers traced the thermal Hall effect to another kind of particle, a boson.

All particles in nature are either fermions or bosons. Electrons are fermions, while particles such as photons, phonons and gluons are bosons. Bosons originate from the wave-like collective excitations of the magnetic moments at high magnetic field. Both types of particles can give rise to the thermal Hall effect if the materials used are topological in nature.

“In our study, we demonstrate rather convincingly that the observed particles are bosons rather than fermions,” said Ong. “If the Kyoto group had been correct—if the particles were identified as fermions—the signal would be independent of temperature. But the signal is, in fact, strongly temperature dependent, and its temperature dependence very precisely corresponds to a quantitative model for topological boson excitations.”

“Our experiment is the first example of what is called a bosonic material displaying quantum edge transport,” Ong added.

Implications and future research

Ong and his team believe their research has robust implications for fundamental physics research.

“What our experiment accomplished—by clarifying the presence of bosons rather than fermions—is to open the door to using the thermal Hall effect in the same way that the quantum Hall Effect has been used to uncover many novel quantum states,” said Ong.

Ong also said that the particles discovered in experiments like this one might have practical applications for such things as topological quantum computing or quantum devices, though achieving such breakthroughs are likely twenty or more years down the road. Ong and the members of his research laboratory intend to continue their research by searching for similar bosonic Hall effects in related materials, and study the quantum possibilities of ruthenium chloride in even greater detail. The experiments were performed in collaboration with scientists at Oak Ridge National Laboratories, the University of Tennessee, Tokyo University and Purdue University.

More information: Peter Czajka et al, Planar thermal Hall effect of topological bosons in the Kitaev magnet α-RuCl3, Nature Materials (2022). DOI: 10.1038/s41563-022-01397-w

Provided by Princeton University 

Femtosecond pulsed lasers—which emit light in ultrafast bursts lasting a millionth of a billionth of a second—are powerful tools used in a range of applications from medicine and manufacturing, to sensing and precision measurements of space and time. Today, these lasers are typically expensive table-top systems, which limits their use in applications that have size and power consumption restrictions.

An on-chip femtosecond pulse source would unlock new applications in quantum and optical computing, astronomy, optical communications and beyond. However, it’s been a challenge to integrate tunable and highly efficient pulsed lasers onto chips.

Now, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a high-performance, on-chip femtosecond pulse source using a tool that seems straight out of science fiction: a time lens.

The research is published in Nature.

“Pulsed lasers that produce high-intensity, short pulses consisting of many colors of light have remained large,” said Marko Lončar, the Tiantsai Lin Professor of Electrical Engineering at SEAS and senior author of the study.

“To make these sources more practical, we decided to shrink a well-known approach, used to realize conventional—and large—femtosecond sources, leveraging a state of the art integrated photonics platform that we have developed. Importantly, our chips are made using microfabrication techniques like those used to make computer chips, which ensures not only reduced cost and size, but also improved performance and reliability of our femtosecond sources.”

Traditional lenses, like contact lenses or those found in magnifying glasses and microscopes, bend rays of light coming from different directions by altering their phase so that they hit the same location in space—the focal point.

Time lenses, on the other hand, “bend” light beams in similar ways—but they alter the phase of light beams in time rather than space. In this way, different colors of light, which travel at different speeds, are re-timed so that they each hit the focal plane at the same time.

Imagine a car race, in which each color of light is a different car. First, the time lens staggers the leave time of each car, then sets their speed so they arrive at the finish line at the same time.

To generate femtosecond pulses, the team’s device uses a series of optical waveguides, couplers, modulators and optical grating on the lithium niobate platform pioneered by Lončar’s lab.

The team starts by passing a continuous-wave, single-color laser beam through an amplitude modulator that controls the amount of light going through the time-lens, a function similar to an aperture in a conventional lens. The light then propagates through the “bendy” part of the lens, a phase modulator in this case, where a frequency comb of different colors is generated. Going back to the car analogy, the phase modulator creates and then releases the cars of different colors at different starting times.

Then the final component of the laser comes in—a fishbone grating along the waveguide. The grating changes the speed of the different colors of light to bring them all in line with each other, neck and neck in the race, so that they hit the finish line (or focal plane) at the same time

Because the device controls how fast different wavelengths travel and when they hit the focal plane, it effectively transforms the continuous, single color laser beam into a broadband, high-intensity pulse source that can produce ultra-fast, 520 femtosecond bursts.

The device is highly tunable, integrated onto a 2cm by 4mm chip and, because of lithium niobate’s electro-optical properties, requires significantly reduced power than table-top products.

“We’ve shown that integrated photonics offers simultaneous improvements in energy consumption and size,” said Mengjie Yu, a former postdoctoral fellow at SEAS and first author of the study.

“There’s no tradeoff here; you save energy at the same time you save space. You just get better performance as the device gets smaller and more integrated. Just imagine—in the future we can carry around femtosecond pulse lasers in our pockets to sense how fresh fruit is or track our well-being in real time, or in our cars to do distance measurement.”

Next, the team aims to explore some of the applications for both the laser itself and the time lens technology, including in lensing systems like telescopes as well as in ultrafast signal processing and quantum networking.

More information: Mengjie Yu et al, Integrated femtosecond pulse generator on thin-film lithium niobate, Nature (2022). DOI: 10.1038/s41586-022-05345-1

Journal information: Nature 

Provided by Harvard John A. Paulson School of Engineering and Applied Sciences 

A group of scientists from Aalto University, IQM Quantum Computers, and VTT Technical Research Center have discovered a new superconducting qubit, the unimon, to increase the accuracy of quantum computations. The team has achieved the first quantum logic gates with unimons at 99.9% fidelity—a major milestone on the quest to build commercially useful quantum computers. This research was just published in the journal Nature Communications.

Of all the different approaches to build useful quantum computers, superconducting qubits are in the lead. However, the qubit designs and techniques currently used do not yet provide high enough performance for practical applications. In this noisy intermediate-scale quantum (NISQ) era, the complexity of the implementable quantum computations is mostly limited by errors in single- and two-qubit quantum gates. The quantum computations need to become more accurate to be useful.

“Our aim is to build quantum computers which deliver an advantage in solving real-world problems. Our announcement today is an important milestone for IQM, and a significant achievement to build better superconducting quantum computers,” said Professor Mikko Möttönen, joint Professor of Quantum Technology at Aalto University and VTT, and also a Co-Founder and Chief Scientist at IQM Quantum Computers, who was leading the research.

Today, Aalto, IQM and VTT have introduced a new superconducting-qubit type, the unimon, which unites in a single circuit the desired properties of increased anharmonicity, full insensitivity to dc charge noise, reduced sensitivity to magnetic noise, and a simple structure consisting only of a single Josephson junction in a resonator. The team achieved fidelities from 99.8% to 99.9% for 13-nanoseconds-long single-qubit gates on three different unimon qubits.

“Because of the higher anharmonicity, or non-linearity, than in transmons, we can operate the unimons faster, leading to fewer errors per operation,” said Eric Hyyppä who is working on his Ph.D. at IQM.

To experimentally demonstrate the unimon, the scientists designed and fabricated chips, each of which consisted of three unimon qubits. They used niobium as the superconducting material apart from the Josephson junctions, in which the superconducting leads were fabricated using aluminum.

The team measured the unimon qubit to have a relatively high anharmonicity while requiring only a single Josephson junction without any superinductors, and bearing protection against noise. The geometric inductance of the unimon has the potential for higher predictability and yield than the junction-array-based superinductors in conventional fluxonium or quarton qubits.

“Unimons are so simple and yet have many advantages over transmons. The fact that the very first unimon ever made worked this well, gives plenty of room for optimization and major breakthroughs. As next steps, we should optimize the design for even higher noise protection and demonstrate two-qubit gates,” added Prof. Möttönen.

“We aim for further improvements in the design, materials, and gate time of the unimon to break the 99.99% fidelity target for useful quantum advantage with noisy systems and efficient quantum error correction. This is a very exciting day for quantum computing,” concluded Prof. Möttönen.

More information: Eric Hyyppä et al, Unimon qubit, Nature Communications (2022). DOI: 10.1038/s41467-022-34614-w

Journal information: Nature Communications 

Provided by Aalto University 

Teams of astrophysicists worldwide are trying to observe different possible types of dark matter (DM), hypothetical matter in the universe that does not emit, absorb or reflect light and would thus be very difficult to detect. Fermionic DM, however, which would be made of fermions, has so far been primarily explored theoretically.

The PandaX Collaboration, a large consortium of researchers in China involved in the PandaX-4T experiment, has recently carried out a study aimed at extending the sensitive mass window for experiments aimed at directly detecting fermionic DM from above GeV to MeV or even keV ranges.

The team recently published two papers in Physical Review Letters outlining the results of the two searches for the absorption of fermionic DM using data gathered as part of the Panda X-4T experiment, a large-scale research effort aimed at detecting DM using a dual-phase time projection chamber (TPC) in China.

“With a massive DM converted to a massless neutrino, the DM mass is absorbed and converted to the kinetic energy of the neutrino and most importantly the recoiled electron or nuclear targets,” Prof. Shao-Feng Ge, one of the researchers who carried out the study, told Phys.org.

“With efficient mass conversion to energy, according to the Einstein relation E = mc², even keV (MeV) DM can deposit a large enough recoil energy in the recoil electron (nuclei).”

The idea of observing light fermionic DM by detecting the recoil energy resulting from the absorption of its mass first emerged a few years ago and has since been explored by different groups of theoretical physicists. While these studies offered valuable theoretical predictions, these predictions had so far never been tested experimentally.

“Past phenomenological papers established the basic features of this unique channel for fermionic DM r searches,” Prof. Ge explained. “The PandaX collaboration worked hard to first search for the predicted signals using real data.”

Theoretical studies predict that in nuclear absorption reactions, the mass of DM is converted into kinetic energy that charges the outgoing neutrino and nucleus. This energy, known as “nuclear recoil energy,” should be approximately proportional to the square of the DM mass, resulting in a unique mono-energetic spectrum. In their first study, the PandaX-4T collaboration tried to detect the energy resulting from the absorption of fermionic DM by nuclei.

“This mono-energetic spectrum is dramatically different from the traditional elastic scattering spectrum and has not been searched dedicatedly before in the DM direct detection experiment,” Dr. Yi Tao, another researcher involved in the study, told Phys.org. “As part of this PandaX-4T search, we performed dedicated studies on the nuclear recoil energy reconstruction and then compared simulation and neutron calibration data.”

The researchers found that there was a good consistency between the data collected by their dual-phase time projection chamber (TPC) and their detector response theoretical model. More specifically, the signal region they scanned corresponded to nuclear recoil energy up to 100 keV, which covers the DM mass parameter from 30 MeV/c² to 125 MeV/c².

In a similar way to nuclear absorption processes, electronic absorption processes are also predicted to be sensitive to light DM, but in a different mass range. In fact, electronic absorption processes imply the conversion of a hypothetical fermionic DM particle’s static mass into the kinetic energy of electrons, creating a free electron.

Theoretically, fermionic DM should thus induce electronic recoiling signals in liquid xenon detectors that could be experimentally detected. In their second study, the PandaX-4T collaboration searched for this other potential trace of fermionic DM.

Electrons are much lighter than nuclei and thus easier to be ejected during absorption processes. Therefore, electronic absorption searches can be sensitive to the sub-MeV mass range.

“In addition, unlike nuclear recoiling signals where quite a bit of energy is quenched into heat and cannot be detected in a liquid xenon detector, most of the electronic recoiling energy is detectable,” Dr. Dan Zhang, another researcher who carried out the study, told Phys.org.

“For more detailed theoretical models, different hypothetical six-dimensional operators in the four-fermion process (fermionic DM + electron -> electron + neutrino) have been studied with an effective field theory approach. It turns out electronic absorption signals will be similar regardless of operators in the direct detection experiments, but the interpretations on the couplings are quite different, and the comparison with other cosmological and astrophysical observations are also different.”

The search for sub-MeV fermionic DM absorbed by electrons carried out by Dr. Zhang and the rest of the PandaX-4T collaboration did not lead to the detection of any significant signals over the expected background. Nonetheless, the team was able to set the strongest limits on the axial-vector and vector interactions for DMs with a mass of several tens keV/c², which surpass the existing astronomy and cosmology constraints for such light fermion DMs.

“About two years ago, XENON1T reported a low-energy excess, which could be interpreted as an electronic absorption of 60 keV/c² fermionic DM according to phenomenological studies,” Dr. Zhang said. “This possibility is now challenged by our data.”

The recent searches performed by the PandaX-4T collaboration highlight the potential of nuclear absorption and electronic absorption processes as channels to search for light mass DM. In the future, they could inspire other astrophysics collaborations worldwide to perform similar searches.

“Once any excess is observed, the energy of the excess would indicate the mass of DM,” Prof. Ning Zhou, another researcher involved in the study, told Phys.org. “For this channel, we obtained model-independent constraints on the sub-GeV DM-nucleon scattering cross section and probe down to the 10^-50 cm² region for 35 MeV/c² DM mass, for the first time. In addition, we study a UV-complete model with Z’ mediator, which brings together the cosmology constraint, the collider constraint, and our limit from direct detection.”

So far, the Panda X-4T collaboration successfully set new limits for experiments aimed at directly detecting fermionic DM. As their experiment is ongoing and is thus still collecting data, the team will soon be conducting additional searches for elusive, light DM.

“The data we reported is equivalent to exposing a 600-kg liquid xenon target for one year to the illumination of this hypothetical DM,” Prof. Jianglai Liu, Spokesperson for the PandaX Collaboration, told Phys.org. “When PandaX-4T concludes in 2025, we anticipate a cumulative exposure of 10 times greater. We also expect to obtain a more precise understanding of our detector to the nuclear recoil and electronic recoil signals via thorough calibrations and we are excited to see how the story unfolds in the future.”

More information: Linhui Gu et al, First Search for the Absorption of Fermionic Dark Matter with the PandaX-4T Experiment, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.161803

Dan Zhang et al, Search for Light Fermionic Dark Matter Absorption on Electrons in PandaX-4T, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.161804

Jeff A. Dror et al, Directly Detecting Signals from Absorption of Fermionic Dark Matter, Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.124.181301

Jeff A. Dror et al, Absorption of sub-MeV fermionic dark matter by electron targets, Physical Review D (2021). DOI: 10.1103/PhysRevD.103.035001

Jeff A. Dror et al, Erratum: Absorption of sub-MeV fermionic dark matter by electron targets [Phys. Rev. D 103 , 035001 (2021)], Physical Review D (2022). DOI: 10.1103/PhysRevD.105.119903

Jeff A. Dror et al, Absorption of fermionic dark matter by nuclear targets, Journal of High Energy Physics (2020). DOI: 10.1007/JHEP02(2020)134

Shao-Feng Ge et al, Revisiting the fermionic dark matter absorption on electron target, Journal of High Energy Physics (2022). DOI: 10.1007/JHEP05(2022)191

Journal information: Physical Review Letters  , Physical Review D 

Spin glasses are alloys formed by noble metals in which a small amount of iron is dissolved. Although they do not exist in nature and have few applications, they have nevertheless been the focus of interest of statistical physicists for some 50 years. Studies of spin glasses were crucial for Giorgio Parisi’s 2021 Nobel Prize in Physics.

The scientific interest of spin glasses lies in the fact that they are an example of a complex system whose elements interact with each other in a way that is sometimes cooperative and sometimes adversarial. The mathematics developed to understand their behavior can be applied to problems arising in a variety of disciplines, from ecology to machine learning, not to mention economics.

Spin glasses are magnetic systems, that is, systems in which individual elements, the spins, behave like small magnets. Their peculiarity is the co-presence of ferromagnetic-type bonds, which tend to align the spins, with antiferromagnetic-type bonds, which tend to orient them in opposite directions.

This causes lower-energy configurations to exhibit residual frustration: it is not possible to find an arrangement of spins that satisfies all bonds. The frustrated configurations are also clustered in a huge (exponential!) number of possible equilibria. This is in stark contrast to what happens in purely ferromagnetic systems, where at low temperature only two states are admissible (spin aligned “up” or spin aligned “down”).

To make an analogy with an ecosystem, having a high number of equilibria indicates a resilient ecosystem, able to cope, for example, with the disappearance of a species, through a limited number of rearrangements. A low equilibrium number describes a fragile system, which requires numerous and complicated rearrangements to return to equilibrium and can, therefore, be seriously damaged, if not destroyed, by relatively small perturbations.

This phenomenology has been well elucidated and mathematically described in systems living in infinite dimension, so-called mean-field systems, the solution to which was provided by Parisi in 1979 and then better understood in subsequent years with the help of Marc Mézard (now a full professor at Bocconi) and Michelangelo Virasoro.

“One of the most debated issues,” as Carlo Lucibello, Assistant Professor in the Department of Computing Sciences and co-author, with Parisi and others, of a paper just published in Physical Review Letters explains, “is to what extent mean-field phenomenology applies in low dimensionality.”

For we know that in dimension 1, that is, on one spin chain, the system is always in a paramagnetic phase, so by lowering the temperature there are no transitions either to a spin glass phase with its many equilibria or to a simple ferromagnetic phase.

“There is a so-called critical upper dimension,” Lucibello says, “above which the mean-field theory applies, allowing us to predict the exponents governing the transition. At the moment, however, no one can say for sure what this dimension is (5, 6, or a non-integer number?) and what happens below it.”

The paper just published by Lucibello and co-authors introduces a new mathematical technique for analyzing finite-dimensional systems. The new theory predicts a critical higher dimension of 8, so we can reasonably conclude that spin glasses in our three-dimensional world are unlikely to be described by a mean-field theory and that there is still a lot of work to do in this branch of theoretical physics.

More information: Maria Chiara Angelini et al, Unexpected Upper Critical Dimension for Spin Glass Models in a Field Predicted by the Loop Expansion around the Bethe Solution at Zero Temperature, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.128.075702

Journal information: Physical Review Letters 

There’s an awkward, irksome problem with our understanding of nature’s laws which physicists have been trying to explain for decades. It’s about electromagnetism, the law of how atoms and light interact, which explains everything from why you don’t fall through the floor to why the sky is blue.

Our theory of electromagnetism is arguably the best physical theory humans have ever made—but it has no answer for why electromagnetism is as strong as it is. Only experiments can tell you electromagnetism’s strength, which is measured by a number called α (aka alpha, or the fine-structure constant).

The American physicist Richard Feynman, who helped come up with the theory, called this “one of the greatest damn mysteries of physics” and urged physicists to “put this number up on their wall and worry about it.”

In research just published in Science, we decided to test whether α is the same in different places within our galaxy by studying stars that are almost identical twins of our sun. If α is different in different places, it might help us find the ultimate theory, not just of electromagnetism, but of all nature’s laws together—the “theory of everything.”

We want to break our favorite theory

Physicists really want one thing: a situation where our current understanding of physics breaks down. New physics. A signal that cannot be explained by current theories. A sign-post for the theory of everything.

To find it, they might wait deep underground in a gold mine for particles of dark matter to collide with a special crystal. Or they might carefully tend the world’s best atomic clocks for years to see if they tell slightly different time. Or smash protons together at (nearly) the speed of light in the 27-km ring of the Large Hadron Collider.

The trouble is, it’s hard to know where to look. Our current theories can’t guide us.

Of course, we look in laboratories on Earth, where it’s easiest to search thoroughly and most precisely. But that’s a bit like the drunk only searching for his lost keys under a lamp-post when, actually, he might have lost them on the other side of the road, somewhere in a dark corner.

Stars are terrible, but sometimes terribly similar

We decided to look beyond Earth, beyond our solar system, to see if stars which are nearly identical twins of our sun produce the same rainbow of colors. Atoms in the atmospheres of stars absorb some of the light struggling outwards from the nuclear furnaces in their cores.

Only certain colors are absorbed, leaving dark lines in the rainbow. Those absorbed colors are determined by α—so measuring the dark lines very carefully also lets us measure α.

The problem is, the atmospheres of stars are moving—boiling, spinning, looping, burping—and this shifts the lines. The shifts spoil any comparison with the same lines in laboratories on Earth, and hence any chance of measuring α. Stars, it seems, are terrible places to test electromagnetism.

But we wondered: if you find stars that are very similar—twins of each other—maybe their dark, absorbed colors are similar as well. So instead of comparing stars to laboratories on Earth, we compared twins of our sun to each other.

A new test with solar twins

Our team of student, postdoctoral and senior researchers, at Swinburne University of Technology and the University of New South Wales, measured the spacing between pairs of absorption lines in our sun and 16 “solar twins”—stars almost indistinguishable from our sun.

The rainbows from these stars were observed on the 3.6-meter European Southern Observatory (ESO) telescope in Chile. While not the largest telescope in the world, the light it collects is fed into probably the best-controlled, best-understood spectrograph: HARPS. This separates the light into its colors, revealing the detailed pattern of dark lines.

HARPS spends much of its time observing sun-like stars to search for planets. Handily, this provided a treasure trove of exactly the data we needed.

From these exquisite spectra, we have shown that α was the same in the 17 solar twins to an astonishing precision: just 50 parts per billion. That’s like comparing your height to the circumference of Earth. It’s the most precise astronomical test of α ever performed.

Unfortunately, our new measurements didn’t break our favorite theory. But the stars we’ve studied are all relatively nearby, only up to 160 light years away.

What’s next?

We’ve recently identified new solar twins much further away, about half way to the center of our Milky Way galaxy.

In this region, there should be a much higher concentration of dark matter—an elusive substance astronomers believe lurks throughout the galaxy and beyond. Like α, we know precious little about dark matter, and some theoretical physicists suggest the inner parts of our galaxy might be just the dark corner we should search for connections between these two “damn mysteries of physics.”

If we can observe these much more distant suns with the largest optical telescopes, maybe we’ll find the keys to the universe.

More information: Michael T. Murphy et al, A limit on variations in the fine-structure constant from spectra of nearby Sun-like stars, Science (2022). DOI: 10.1126/science.abi9232

Journal information: Science 

This article is republished from The Conversation under a Creative Commons license. Read the original article.

When 2D layered materials are made thinner (i.e., at the atomic scale), their properties can dramatically change, sometimes resulting in the emergence of entirely new features and in the loss of others. While new or emerging properties can be very advantageous for the development of new technologies, retaining some of the material’s original properties is often equally important.

Researchers at Tsinghua University, the Chinese Academy of Sciences and the Frontier Science Center for Quantum Information have recently been able to realize tailored Ising superconductivity in a sample of intercalated bulk niobium diselenide (NbSe₂), a characteristic of bulk NbSe₂ that is typically compromised in atomically thin layers. The methods they used, outlined in a paper published in Nature Physics, could pave the way towards the fabrication of 2D thin-layered superconducting materials.

“Atomically thin 2D materials exhibit interesting properties that are often distinct from their bulk materials, which consist of hundreds and thousands of layers,” Shuyun Zhou, one of the researchers who carried out the study, told Phys.org. “However, atomically thin films/flakes are difficult to fabricate, and the emerging new properties are sometimes achieved by sacrificing some other important properties.”

Zhou and his colleagues have been trying to identify experimental methods to achieve novel properties comparable to atomically thin samples without losing any vital material properties for some years now. In their recent study, they specifically evaluated the effectiveness of electrochemical intercalation, a valuable strategy for tuning the electronic properties of layered solid materials.

“The bulk material is immersed in the ionic liquid, which consists of cations and anions,” Zhou explained. “Such ionic liquids have been widely used for injecting electrons into few-layer samples, while the ions remain in the liquid. We have found out that by applying a larger negative voltage, the large-size organic cations can be driven into the van der Waals gap (the empty space between the active layers, NbSe₂ layers in this case), forming hybrid materials.”

In their experiments, Zhou and his colleagues found that intercalation is an effective strategy for controlling both the dimensionality and carrier concentration of their NbSe₂ layered sample. Using this strategy, they were able to attain a tailored Ising superconductivity that exceeded both that observed in bulk NbSe₂ crystals and monolayer NbSe₂ samples, but in an intercalated bulk NbSe₂ sample.

Essentially, intercalation strategies consist in the immersion of a bulk material in an ionic liquid and the subsequent application of electrical voltage. This process prompts an increase in the spacing between a bulk layered material’s active layers, reducing interactions between them.

“Although the intercalated NbSe₂ material still consists of many layers, its properties behave quite similarly to those of monolayer NbSe₂ samples,” Zhou said. “Specifically, the intercalated material’s superconductivity can survive under a large in-plane magnetic field, but the superconducting transition temperature is higher than monolayer NbSe₂. In addition, the cations can transfer charges to the active layers and act as protecting layers, making the hybrid material stable in the air.”

While Zhou and his colleagues specifically used their intercalation-based strategy to broaden the properties of a layered 2D NbSe₂ sample, the exact same strategy could also be applied to a wide range of layered materials to achieve properties comparable to those of monolayer versions of these materials, or even better. So far, this method has enabled tailored Ising superconductivity in NbSe₂, enhanced superconductivity in Weyl semimetal MoTe₂ and semiconducting-to-superconducting transition in SnSe₂.

“Our intercalation method is quite generic and can be readily extended to a large variety of layered materials and a large selection of ionic liquids with different cations,” Zhou added. “Therefore, our work provides an important pathway for creating hybrid materials with tunable functionalities possibly exceeding the bulk crystals and monolayer samples. Besides superconductors, we would like to apply this strategy to many other layered materials to obtain more intriguing properties. We expect that thanks to intercalation, intriguing properties exceeding both bulk crystals and monolayer samples will soon be enabled in a growing number of layered materials.”

More information: Haoxiong Zhang et al, Tailored Ising superconductivity in intercalated bulk NbSe2, Nature Physics (2022). DOI: 10.1038/s41567-022-01778-7

Journal information: Nature Physics 

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