We propose a tunable phonon laser prototype with a large trapped ion array, where some of the ions are effectively pinned by optical tweezers, thus isolating a subset of ions that mimics an acoustic cavity used as a phonon lasing resonator. The cavity loss can then be controlled by the tweezer strength and the “wall thickness”, the number of pinned ions for isolation. We pump the resonator by applying blue-sideband lasers, and investigate the lasing dynamics of the “cavity” motional modes such as threshold behavior, population distribution, the second-order coherence, and linewidth narrowing. This scheme can be generalized to multimode resonators, for which we report the mode competition phenomenon and lasing mode multistability. Our work provides an excellent platform for acoustic quantum state manipulation, paving the way towards phonon-mediated quantum computing, communications, and metrology.

Successful feedback control of small systems allows for the rectification of thermal fluctuations, converting them into useful energy; however, control itself requires work. This paper emphasizes the fact that the controller is a physical entity interacting with the feedback-controlled system. For a specifically designed class of controllers, reciprocal interactions become nonreciprocal due to large timescale separation, which considerably simplifies the situation. We introduce a minimally dissipative controller model, illustrating the findings using a simple example. We find that the work required to run the controller must at least compensate for the decrease in entropy due to the control operation.

Models of meme propagation on social networks, in which memes compete for limited user attention, can successfully reproduce the heavy-tailed popularity distributions observed in online settings. While system-wide popularity distributions have been derived analytically, the dynamics of individual meme trajectories have thus far evaded description. To address this, we formulate the diffusion of a given meme as a one-dimensional stochastic process, whose fluctuations result from aggregating local network dynamics using classic and generalized central limit theorems, with the latter based on stable distribution theory. Ultimately, our approach decouples competing trajectories of meme popularities, allowing them to be simulated independently, and thus parallelized and expressed in terms of Fokker-Planck equations.

Quantum simulation is one of the key applications of quantum computing, which accelerates research and development in the fields such as chemistry and material science. The recent development of noisy intermediate-scale quantum (NISQ) devices urges the exploration of applications without the necessity of quantum error correction. In this paper, we propose an efficient method to simulate quantum dynamics driven by a static Hamiltonian on NISQ devices, named subspace variational quantum simulator (SVQS). SVQS employs the subspace-search variational quantum eigensolver (SSVQE) [Phys. Rev. Res. 1, 033062 (2019)] to find a low-lying eigensubspace and extends it to simulate dynamics within the subspace with lower overhead compared to the existing schemes. We experimentally simulate the time-evolution operator in a low-lying eigensubspace of a hydrogen molecule. We also define the subspace process fidelity as a measure between two quantum processes in a subspace. The subspace time evolution mimicked by SVQS shows the subspace process fidelity of $0.896\text{–}0.989$.

Quantum simulation of lattice gauge theories has been proposed and used as a method to overcome theoretical difficulties in dealing with the nonperturbative nature of such models. In this work we focus on two important bottlenecks that make developing such simulators hard: one is the difficulty of simulating fermionic degrees of freedom, and the other is the redundancy of the Hilbert space, which leads to a waste of experimental resources and the need to impose and monitor the local symmetry constraints of gauge theories. This has previously been tackled in one-dimensional settings, using nonlocal methods. Here we show an alternative procedure for dealing with these problems, which removes the matter and the Hilbert space redundancy, and is valid for higher space dimensions. We demonstrate it for a ${\mathbb{Z}}_2$ lattice gauge theory and implement it experimentally via the IBMQ cloud quantum computing platform.

Parafermions are ${\mathbb{Z}}_n$ generalizations of Majorana quasiparticles, with fractional non-Abelian statistics. They can be used to encode topological qudits and perform Clifford operations by their braiding. Here we investigate the generation of quantum gates by allowing ${\mathbb{Z}}_3$ parafermions to interact in order to achieve universality. In particular, we study the form of the nontopological gate that arises through direct short-range interaction of the parafermion edge modes in a ${\mathbb{Z}}_3$ parafermion chain. We show that such an interaction gives rise to a dynamical phase gate on the encoded ground space, generating a non-Clifford gate which can be tuned to belong to even levels of the Clifford hierarchy. We illustrate how to access highly noncontextual states using this dynamical gate. Finally, we propose an experiment that simulates the braiding and dynamical evolutions of the ${\mathbb{Z}}_3$ topological states with Rydberg atom technology.

Electron-electron interactions in graphene are sufficiently strong to induce a correlated and momentum-conserving flow such that charge carriers behave similarly to the Hagen-Poiseuille flow of a classical fluid. In the current work, we investigate the electronic signatures of such a viscous charge flow in high-mobility graphene field effect transistors (FETs). In two complementary measurement schemes, we monitor differential resistance of graphene for different channel widths and for different effective electron temperatures. By combining both approaches, the presence of viscous effects is verified in a temperature range starting from 178 K and extending up to room temperature. Our experimental findings are supported by finite element calculations of the graphene channel, which also provide design guidelines for device geometries that exhibit increased viscous effects. The presence of viscous effects near room temperature opens up avenues for functional hydrodynamic devices such as geometric rectifiers like a Tesla valve and charge amplifiers based on the electronic Venturi effect.

Quantum neural networks form one pillar of the emergent field of quantum machine learning. Here quantum generalizations of classical networks realizing associative memories—capable of retrieving patterns, or memories, from corrupted initial states—have been proposed. It is a challenging open problem to analyze quantum associative memories with an extensive number of patterns and to determine the maximal number of patterns the quantum networks can reliably store, i.e., their storage capacity. In this work, we propose and explore a general method for evaluating the maximal storage capacity of quantum neural network models. By generalizing what is known as Gardner's approach in the classical realm, we exploit the theory of classical spin glasses for deriving the optimal storage capacity of quantum networks with quenched pattern variables. As an example, we apply our method to an open-system quantum associative memory formed of interacting spin-1/2 particles realizing coupled artificial neurons. The system undergoes a Markovian time evolution resulting from a dissipative retrieval dynamics that competes with a coherent quantum dynamics. We map out the nonequilibrium phase diagram and study the effect of temperature and Hamiltonian dynamics on the storage capacity. Our method opens an avenue for a systematic characterization of the storage capacity of quantum associative memories.

Film materials made of cellulose nanofibers (CNFs), i.e., so-called nanopapers, are fabricated from aqueous dispersions of CNFs. The drying in the container causes the phase transition from sol to gel and final state of transparent film. The nonequilibrium and nonuniform nature of the drying calls for the least invasive in-process measurement to understand the process. In this paper, we focus on the transient state of nanopaper formation through the time-series image analysis from optical measurements on the multiple spatiotemporal scales and resolutions. The combination of single-particle tracking and particle image velocimetry reveals the intermittent flow with a low symmetry of space pattern in the middle stage of drying process. The small portion of CNF is sufficient for suppression of coffee-ring phenomena, and the flow is in combination with the emergence of partial structural order. Furthermore, we find that the flow pattern depends on the sidewall material of the sample container. We characterize the three stages of drying process through the flow patterns by the correlation length of velocity autocorrelation function as well as the mean and standard deviation of the flow speed in the containers.

Nonradiative recombination leads to losses in efficiency in optoelectronic devices such as photovoltaic cells and light-emitting diodes. Charges trapped at point defects or self-trapped as a small polaron may act as recombination centers. Using various phases of titanium dioxide as an example, we provide first-principles predictions that small hole polarons in the bulk of the crystal would exhibit significant rates of recombination with electrons in the conduction band. However, small hole polarons trapped at a model grain boundary are predicted to have much higher nonradiative recombination rates, which can be attributed to softer phonon modes in the vicinity of the boundary as well as greater electron-phonon coupling. These findings have ramifications in materials other than titanium dioxide, and we propose strategies to reduce the degree of recombination that would occur at grain boundaries.

Quantum computers are expected to accelerate solving combinatorial optimization problems, including algorithms such as Grover adaptive search and quantum approximate optimization algorithm (QAOA). However, many combinatorial optimization problems involve constraints which, when imposed as soft constraints in the cost function, can negatively impact the performance of the optimization algorithm. In this paper, we propose fermionic quantum approximate optimization algorithm (FQAOA) for solving combinatorial optimization problems with constraints. Specifically, FQAOA tackles the constrains issue by using fermion particle number preservation to intrinsically impose them throughout QAOA. We provide a systematic guideline for designing the driver Hamiltonian for a given problem Hamiltonian with constraints. The initial state can be chosen to be a superposition of states satisfying the constraint and the ground state of the driver Hamiltonian. This property is important since FQAOA reduced to quantum adiabatic computation in the large limit of circuit depth $p$ and improved performance, even for shallow circuits with optimizing the parameters starting from the fixed-angle determined by Trotterized quantum adiabatic evolution. We perform an extensive numerical simulation and demonstrate that proposed FQAOA provides substantial performance advantages against existing approaches in portfolio optimization problems. Furthermore, the Hamiltonian design guideline is useful not only for QAOA but also Grover adaptive search and quantum phase estimation to solve combinatorial optimization problems with constraints. Since software tools for fermionic systems have been developed in quantum computational chemistry both for noisy intermediate-scale quantum computers and fault-tolerant quantum computers, FQAOA allows us to apply these tools for constrained combinatorial optimization problems.

We study theoretically the zero-temperature intrinsic anomalous Hall effect in an experimentally realized two-dimensional spin-orbit-coupled Bose gas. For anisotropic atomic interactions and as the spin-orbit-coupling strength increases, the system undergoes a ground-state phase transition from states exhibiting a total in-plane magnetization to those with a perpendicular magnetization along the $z$ direction. We show that finite frequency, or ac, Hall responses exist in both phases in the absence of an artificial magnetic field, as a result of finite interband transitions. However, the characteristics of the anomalous Hall responses are drastically different in these two phases because of the different symmetries preserved by the corresponding ground states. In particular, we find a finite dc Hall conductivity in one phase, but not the other. The underlying physical reasons for this are analyzed further by exploring relations of the dc Hall conductivity to the system's chirality and Berry curvatures of the Bloch bands. Finally, we discuss an experimental method of probing the anomalous Hall effect in trapped systems.