Physics – Page 44 – Chemistry and Physics

October 13, 2022October 13, 2022

Physicists predict the novel entangled states on programmable quantum simulators

Quantum science has not only deepened human understanding of the structure of matter and its microscopic interactions, but also introduced a new paradigm of computing and information science—quantum computing and quantum simulation. Quantum informatics research has won the 2022 Nobel Prize in Physics.

Among many quantum computing and simulation platforms, Rydberg Atom Arrays is considered the most promising system to show quantum superiority among many programmable quantum simulator platforms in recent years due to its largest number of qubits and highest experimental accuracy.

Such optical lattices consist of individual neutral alkaline-earth atoms with significant dipole moments trapped in arrays of microscopic dipole traps, which can be optically moved at will to make desired lattice geometry. Each atom can be excited to its Rydberg state, and a pair of excited states interact through their dipole moments via a long-range interaction.

Such Rydberg atom arrays are believed by many to be the system with the highest level of experimental precision and number of qubits in recent years within all platforms for programmable quantum simulators. The observations of quantum phase transitions and the signature of topological orders from Rydberg atom arrays have been reported with tremendous speed.

However, the long-range interaction and the Rydberg blockade mechanism in these optical lattices can have both pros and cons. On the one hand, they give rise to the high precision of experimental quantum control, as mentioned above. Still, on the other hand, they enforce the constraints on modeling the system.

Such quantum-constrained many-body systems are among the hardest to study from theoretical and numerical perspectives. Without a precise theoretical understanding of the complete phase diagrams and novel quantum phases, future experiments will have no guidance to continue.

Such an apparent conundrum, faced by all scientists in the field, is partially and substantially solved by a joint effort of Research Assistant Professor Zheng Yan and Associate Professor Zi Yang Meng from the Department of Physics, the University of Hong Kong (HKU) and the renowned physicist Professor Subir Sachdev (member of American Academy of Arts and Sciences) from Harvard University and his then student Dr. Rhine Samajdar now Postdoctoral fellow at Princeton University and Dr. Yan-Cheng Wang (Researcher) from Beihang Hangzhou Innovation Institute Yuhang at Hangzhou. Their research work has been published in the recent issue of Nature Communications.

They designed a new triangular lattice quantum dimer model with soft constraints to be as close as possible to experiment conditions and developed the sweeping cluster algorithm for quantum Monte Carlo simulations that could solve such soft-constrained quantum many-body systems efficiently.

Their simulations and theoretical analysis successfully map out the expected phase diagram of Rydberg arrays on the Kagome lattice. They found not only the expected and conventional nematic and stagger types of the solid phases but also the exotic highly entangled Z2 quantum spin liquids (QSL) with large parameter regimes in the phase diagram.

They identify these novel phases by designing non-local measurements of string operators and other physical observables in the quantum Monte Carlo simulations. The difference between QSL and trivial paramagnetic phase is distinguished successfully. The most exciting thing is that a path connecting odd Z2 QSL, trivial paramagnetic (PM) phase, and even QSL and solid phases has been revealed, which is very useful in guiding the Rydberg arrays experiment.

In addition, they have also studied the dynamics and interactions of the fractional quasiparticles (visons) in the Z2 QSL to give more experimentally probable evidence.

These results highlight the richness of their constrained models derived for Rydberg array systems and exploit various new phases induced by the long-range interactions and Rydberg blockade mechanism.

October 13, 2022October 13, 2022

Physicists reach qubit computing breakthrough

Researchers from Arizona State University and Zhejiang University in China, along with two theorists from the United Kingdom, have been able to demonstrate for the first time that large numbers of quantum bits, or qubits, can be tuned to interact with each other while maintaining coherence for an unprecedentedly long time, in a programmable, solid state superconducting processor.

Previously, this was only possible in Rydberg atom systems.

In a paper to be published on Thursday, Oct. 13, in Nature Physics, ASU Regents Professor Ying-Cheng Lai, his former ASU doctoral student Lei Ying and experimentalist Haohua Wang, both professors at Zhejiang University in China, have demonstrated a “first look” at the emergence of quantum many-body scarring (QMBS) states as a robust mechanism for maintaining coherence among interacting qubits. Such exotic quantum states offer the appealing possibility of realizing extensive multipartite entanglement for a variety of applications in quantum information science and technology to achieve high processing speed and low power consumption.

“QMBS states possess the intrinsic and generic capability of multipartite entanglement, making them extremely appealing to applications such as quantum sensing and metrology,” explained Ying.

Classical, or binary computing relies on transistors—which can represent only the “1” or the “0” at a single time. In quantum computing, qubits can represent both 0 and 1 simultaneously, which can exponentially accelerate computing processes.

“In quantum information science and technology, it is often necessary to assemble a large number of fundamental information-processing units—qubits—together,” explained Lai. “For applications such as quantum computing, maintaining a high degree of coherence or quantum entanglement among the qubits is essential.

“However, the inevitable interactions among the qubits and environmental noise can ruin the coherence in a very short time—within about ten nanoseconds. This is because many interacting qubits constitute a many-body system,” said Lai.

Key to the research is insight about delaying thermalization to maintain coherence, considered a critical research goal in quantum computing.

“From basic physics, we know that in a system of many interacting particles, for example, molecules in a closed volume, the process of thermalization will arise. The scrambling among many qubits will invariably result in quantum thermalization—the process described by the so-called Eigenstate Thermalization Hypothesis, which will destroy the coherence among the qubits,” said Lai.

According to Lai, the findings moving quantum computing forward will have applications in cryptology, secure communications and cybersecurity, among other technologies.

October 13, 2022October 13, 2022

Changing direction: Research team discovers switchable electronic chirality in an achiral Kagome superconductor

An international research team led by the Department of Microstructured Quantum Matter at the MPSD reports the first observation of switchable chiral transport in a structurally achiral crystal, the Kagome superconductor CsV3Sb5. Their work has been published in Nature.

Whether or not an object is indistinguishable from its mirror image has important consequences for its physical behavior. Say you watch a basketball player in a mirror. The ball, the player and their surroundings are, at first glance, just the same in the mirror as in real life. But if observed closely, some details are different. The ball in the player’s right hand now appears in their left hand in the mirror. While the mirror image still shows the same hand, it has clearly changed from a left to a right hand or vice versa. Many other physical objects also have mirror images that differ in a key aspect, just like hands, which is why scientists call them handed or chiral (from Greek χϵρι = hand). Others, like the ball, cannot be distinguished from their mirror image, which makes them achiral.

Chirality is one of the most fundamental geometric properties and plays a special role in biology, chemistry and physics. It can cause surprising effects: One version of the carvone molecule, for example, produces a spearmint smell but its chiral—mirrored—equivalent smells of caraway.

In material science, one distinguishes between crystals in which the periodic arrangement of the atoms is chiral or not. If it is, the electrons and electric currents flowing within it must also differ somehow from their mirror image, a property that can lead to exotic responses and novel applications. One example is a diode-like effect where the electric currents flowing from left to right are different from those flowing right to left, a property called electronic magneto-chiral anisotropy (eMChA). So far, this phenomenon has only been found in structurally chiral crystals.

Now, however, an international research team has reported the first observation of this chiral transport in a structurally achiral crystal, the Kagome superconductor CsV3Sb5. The team involved scientists from the MPSD and the Max Planck Institute for Chemical Physics of Solids (Germany), EPFL and the University of Zurich in Switzerland, the University of the Basque Country (Spain) and Qingdao University in China.

The quantum puzzle is as simple as it is deep: If the atomic positions in the crystal are just the same as in their mirror image, how is it possible that its electrons are not? Clearly a novel mechanism beyond a simple shape effect as in our hands must be at play. Unlike structural chirality, which is as firmly imprinted in a crystal as it is in a human hand, this new electronic chirality can be switched using magnetic fields. Switchable chirality has never before been observed and may well find its application in future technology.

It is clear that this unusual behavior is directly linked to the strong electronic interactions. The team proposes a model in which the electrons arrange themselves in patterns that violate mirror symmetry, even though the atoms are arranged symmetrically.

CsV3Sb5 is already known for many such interacting electronic structures, such as the formation of an unconventional chiral charge order featuring a tantalizing charge modulation. These chiral electronic structures may rotate spontaneously, causing an orbital magnetic response associated to a behavior known as “loop currents,” as described by C. Mielke et al., Nature 2022.

CsV3Sb5 appears to be a fantastic sandbox to probe correlated quantum phenomena, including the first example of switchable electronic chirality. The next steps include increasing the operational range from cryogenic to room temperature and enhancing the magnitude of this response. Clearly, much more is to come from interacting systems on geometrically frustrated lattices.

October 13, 2022October 13, 2022

Dissipative soliton generation and real-time dynamics in microresonator-filtered fiber lasers

Kerr microcombs are sets of equidistant spectral lines generated by pumping a high-Q microresonator with a resonant continuous-wave single-mode laser. They have arguably created a new field in cavity nonlinear photonics, with a strong cross-fertilization between theoretical, experimental, and technological research.

Despite the great success of Kerr microcombs, issues such as the self-starting mechanism, low mode efficiency, and low output power cannot be fully addressed by conventional Kerr microcomb designs. On the other hand, by nesting a passive nonlinear microresonator in an active fiber laser cavity, the recently-demonstrated laser cavity soliton (LCS) microcomb shows great potential to solve the problems of conventional Kerr microcombs.

In a new paper published in Light: Science & Applications, a team of scientists led by Professor Shu-Wei Huang from University of Colorado Boulder and co-workers theoretically and experimentally study the dissipative soliton generation and real-time dynamics of this new member to the microcomb family.

“We bring theoretical insight into the mode-locking principle of this new LCS microcomb architecture. We studied the dependence of soliton properties on microresonator parameters and predicted the existence of chirped bright dissipative soliton with flat-top spectral shape, a long-sought-after feature for high-capacity optical communication but not attainable in the conventional microcomb architecture,” the scientists say.

“Based on our proposed experimental guidelines, we demonstrated a self-starting LCS microcomb with low noise, high mode efficiency, and high output power. The LCS output power is as high as 12 mW with the mode efficiency up to 90.7%, both representing orders-of-magnitude enhancement to conventional Kerr microcombs. Equally important, we achieved the record low fundamental comb linewidth of 32 mHz and repetition rate phase noise of -137 dBc/Hz at 1 MHz offset frequency,” they write.

“Moreover, we utilized a time magnifier to ‘slow down’ the ultrafast nonlinear cavity dynamics by 70 times so soliton formation and interaction dynamics can be studied with state-of-the-art optoelectronics. We were particularly excited to observe the intriguing soliton Newton’s cradle phenomenon with the time magnifier for the first time,” they added.

“The study opens new avenues to compact, high-efficiency and self-starting microcombs for real-world applications and highlights the importance of time magnifier for real-time ultrafast dynamics,” the scientists said.

October 13, 2022October 13, 2022

New measurements quantifying qudits provide glimpse of quantum future

Using existing experimental and computational resources, a multi-institutional team has developed an effective method for measuring high-dimensional qudits encoded in quantum frequency combs, which are a type of photon source, on a single optical chip.

Although the word “qudit” might look like a typo, this lesser-known cousin of the qubit, or quantum bit, can carry more information and is more resistant to noise—both of which are key qualities needed to improve the performance of quantum networks, quantum key distribution systems and, eventually, the quantum internet.

Classical computer bits categorize data as ones or zeroes, whereas qubits can hold values of one, zero or both—simultaneously—owing to superposition, which is a phenomenon that allows multiple quantum states to exist at the same time. The “d” in qudit stands for the number of different levels or values that can be encoded on a photon. Traditional qubits have two levels, but adding more levels transforms them into qudits.

Recently, researchers from the U.S. Department of Energy’s Oak Ridge National Laboratory, Purdue University and the Swiss Federal Institute of Technology Lausanne, or EPFL, fully characterized an entangled pair of eight-level qudits, which formed a 64-dimensional quantum space—quadrupling the previous record for discrete frequency modes. These results were published in Nature Communications.

“We’ve always known that it’s possible to encode 10- or 20-level qudits or even higher using the colors of photons, or optical frequencies, but the problem is that measuring these particles is very difficult,” said Hsuan-Hao Lu, a postdoctoral research associate at ORNL. “That’s the value of this paper—we found an efficient and novel technique that is relatively easy to do on the experimental side.”

Qudits are even more difficult to measure when they are entangled, meaning they share nonclassical correlations regardless of the physical distance between them. Despite these challenges, frequency-bin pairs—two qudits in the form of photons that are entangled in their frequencies—are well suited to carrying quantum information because they can follow a prescribed path through optical fiber without being significantly modified by their environment.

“We combined state-of-the-art frequency-bin production with state-of-the-art light sources, and then used our technique to characterize high-dimensional qudit entanglement with a level of precision that hasn’t been shown before,” said Joseph Lukens, a Wigner Fellow and research scientist at ORNL.

The researchers began their experiments by shining a laser into a micro-ring resonator—a circular, on-chip device fabricated by EPFL and designed to generate nonclassical light. This powerful photon source takes up 1 square millimeter of space—comparable in size to the point of a sharpened pencil—and allowed the team to generate frequency-bin pairs in the form of quantum frequency combs.

Typically, qudit experiments require researchers to construct a type of quantum circuit called a quantum gate. But in this case, the team used an electro-optic phase modulator to mix different frequencies of light and a pulse shaper to modify the phase of these frequencies. These techniques are studied extensively at the Ultrafast Optics and Optical Fiber Communications Laboratory led by Andrew Weiner at Purdue, where Lu studied before joining ORNL.

These optical devices are commonplace in the telecommunications industry, and the researchers performed these operations at random to capture many different frequency correlations. According to Lu, this process is like rolling a pair of six-sided dice and recording how many times each combination of numbers appears—but now the dice are entangled with each other.

“This technique, which involves phase modulators and pulse shapers, is heavily pursued in the classical context for ultrafast and broadband photonic signal processing and has been extended to the quantum avenue of frequency qudits,” Weiner said.

To work backward and infer which quantum states produced frequency correlations ideal for qudit applications, the researchers developed a data analysis tool based on a statistical method called Bayesian inference and ran computer simulations at ORNL. This accomplishment builds on the team’s previous work focused on performing Bayesian analyses and reconstructing quantum states.

The researchers are now fine-tuning their measurement method to prepare for a series of experiments. By sending signals through optical fiber, they aim to test quantum communication protocols such as teleportation, which is a method of transporting quantum information, and entanglement swapping, which is the process of entangling two previously unrelated particles.

Karthik Myilswamy, a graduate student at Purdue, plans to bring the micro-ring resonator to ORNL, which will enable the team to test these capabilities on the laboratory’s quantum local area network.

“Now that we have a method to efficiently characterize entangled frequency qudits, we can perform other application-oriented experiments,” Myilswamy said.

October 12, 2022October 13, 2022

Measuring Zak phase in room-temperature atoms

In topological insulators, global geometric phases of energy bands determine the existence of unidirectionally propagating edge states, which are immune to local defects, bringing a new stage for designing noise-resilient devices of electrons, photons and atoms. The geometric phase is measured by adiabatically transporting a quantum state through a Brillouin zone, which requires sophisticated quantum control technique. In atoms, such measurement can only be carried out at an extremely low temperature.

The modern theory of polarization in solids sheds new light on the measurement of geometric phases. Based on that theory, the Wannier centers (the expected positions of the Wannier functions of energy bands) are determined by the geometric phases. In a static electric field, the Wannier centers determine the energies of the Wannier-Stark ladders, from which the geometric phases can be extracted.

In a new paper published in Light: Science & Applications, a team of scientists led by Da-Wei Wang and Han Cai from Zhejiang University developed a new technique to measure geometric phases in thermal atoms. The work is implemented in superradiance lattices, i.e., momentum-space lattices of collectively excited states of atoms, the so-called timed Dicke states. The authors summarize the essence of their technique as follows.

“In momentum-space lattices, the motion of atoms plays the role of an electric field in real-space lattices. Atoms moving through the real-space Brillouin zone of superradiance lattices follow the same dynamics of lattice electrons traveling through a momentum-space Brillouin zone in a DC electric field,” the scientists explained.

“In particular, we find that the velocity of atoms is proportional to the strengths of the effective electric field. In thermal atoms, we obtain the energy spectra of superradiance lattices by integrating Wannier-Stark spectra of atoms with different velocities. When two ladders from different energy bands have the same energy for a certain velocity, an anti-crossing happens, which is characterized by a dip in the energy spectra.

“By measuring the energies of those dips, we manage to reconstruct the geometric phases of the energy bands. The technique is applied to various one-dimensional lattices, including the Semenoff insulators, the Su-Schriefer-Heeger models, and the generalized Rice-Mele model.

“Our experiment demonstrates a conceptually novel technique in detecting band geometry of laser-dressed atoms and can be generalized to measure other topological invariants in higher-dimensional systems. Meanwhile, it also paves a way for the application of topological physics in optical devices at ambient temperature,” the scientists said.

Provided by Chinese Academy of Sciences