Probability current fluctuations play an important role in nonequilibrium statistical mechanics, and are a key object of interest in both theoretical studies and in practical applications. So far, most of the studies were devoted to the fluctuations of the time-averaged probability current, the zero-frequency Fourier component of the time-dependent current. However, in many practical applications the fluctuations at other frequencies are of equal importance. Here we study the statistics of all the probability current's Fourier component in periodically driven stochastic systems. We restrict our study to “trapped” systems where the degrees of freedom of the system cannot achieve arbitrarily large values as time becomes large, in contrast to, e.g., diffusing systems. First, we discuss possible methods to calculate the current statistics, valid even when the current's Fourier frequency is incommensurate with the driving frequency, breaking the time periodicity of the system. Somewhat surprisingly, we find that the cumulant generating function (CGF), that encodes all the statistics of the current, is composed of a continuous background at any frequency accompanied by either positive or negative discontinuities at current's frequencies commensurate with the driving frequency. We show that cumulants of increasing orders display discontinuities at an increasing number of locations but with decreasing amplitudes that depend on the rational frequency ratio. All these discontinuities are then transcribed in the behavior of the CGF. As the measurement time increases, these discontinuities become sharper but keep the same amplitude and eventually lead to discontinuities of the CGF at all the frequencies that are commensurate with the driving frequency in the limit of infinitely long measurement. We demonstrate our formalism and its consequences on three types of models: an underdamped Brownian particle in a periodically driven harmonic potential; a periodically driven run-and-tumble particle; and a two-state system. Our results show a rich and interesting structure in experimentally accessible and important objects: the fluctuations of alternating currents as a function of their frequency.

Probing quantum mechanics and quantum aspects of general relativity, along with the sensing and constraining of classical gravity, can all be enabled by unprecedented spatial sizes of superpositions of massive objects. In this paper, we show that there is a feasible setup sourced by realizable magnetic field gradients $O\left(10\text{–}100\right)\mathrm{T}$ ${\mathrm{m}}^{-1}$ to construct a large spatial superposition of $O\left({10}^{-4}\text{–}{10}^{-8}\right)\mathrm{m}$ for $O\left({10}^{-17}\text{–}{10}^{-14}\right)\mathrm{kg}$ masses within a time of up to 0.1–10 seconds. The scale of superpositions is unrestricted as long as quantum coherence can be maintained for a required amount of time.

We construct a unified operator framework for quantum holonomies generated from bosonic systems. For a system whose Hamiltonian is bilinear in the creation and annihilation operators, we find a holonomy group determined only by a set of selected orthonormal modes obeying a stronger version of the adiabatic theorem. This photon-number independent description offers deeper insight as well as a computational advantage when compared to the standard formalism on geometric phases. In particular, a strong analogy between quantum holonomies and linear optical networks can be drawn. This relation provides an explicit recipe of how any linear optical quantum computation can be made geometrically robust in terms of adiabatic or nonadiabatic geometric phases.

An electron or electron-positron beam streaming through a plasma is notoriously prone to microinstabilities. For a dilute ultrarelativistic infinite beam, the dominant instability is a mixed mode between longitudinal two-stream and transverse filamentation modes, with a phase velocity oblique to the beam velocity. A spatiotemporal theory describing the linear growth of this oblique mixed instability is proposed which predicts that spatiotemporal effects generally prevail for finite-length beams, leading to a significantly slower instability evolution than in the usually assumed purely temporal regime. These results are accurately supported by particle-in-cell (PIC) simulations. Furthermore, we show that the self-focusing dynamics caused by the plasma wakefields driven by finite-width beams can compete with the oblique instability. Analyzed through PIC simulations, the interplay of these two processes in realistic systems bears important implications for upcoming accelerator experiments on ultrarelativistic beam-plasma interactions.

In quantum-electrodynamics scatterings the transfer of angular momentum between initial and final states can be resolved using the vortex scattering theory, where involved particles are described by quantum vortex states with featured orbital angular momentum (OAM). Here we employ the vortex scattering scenario in bremsstrahlung of a plane-wave electron by describing the final electron and photon in vortex states. We find that, while the total cross-section and angular distribution of the vortex scattering scenario are consistent with those from the ordinary plane-wave scattering, the final states in the former can gain non-negligible OAM values even though the incident electron does not contain any OAM. Moreover, introducing the OAM degree of freedom conserves the total angular momentum in polarized scattering. The dependence of the OAM on the opening angle of the vortex state and the energy ratio of photon to electron are obtained. It is shown that the emitted photon gains higher OAM at larger angles and energy ratios.

In this work, the generalization of the Friedel formula and Krein's theorem in complex potential scattering theory is presented. The consequences of various symmetry constraints on dynamical systems are discussed. In addition, the Muskhelishvili-Omnès representation of Krein's theorem is also given and discussed.

On-chip chiral quantum light-matter interfaces, which support directional interactions, provide a promising platform for efficient spin-photon coupling, nonreciprocal photonic elements, and quantum logic architectures. We present full-wave three-dimensional calculations to quantify the performance of conventional and topological photonic crystal waveguides as chiral emitter-photon interfaces. Specifically, the ability of these structures to support and enhance directional interactions while suppressing subsequent backscattering losses is quantified. Broken symmetry waveguides, such as the nontopological glide-plane waveguide and topological bearded interface waveguide are found to act as efficient chiral interfaces, with the topological waveguide modes allowing for operation at significantly higher Purcell enhancement factors. Finally, although all structures suffer from backscattering losses due to fabrication imperfections, these are found to be smaller at high enhancement factors for the topological waveguide. These reduced losses occur because the optical mode is pushed away from the air-dielectric interfaces where scattering occurs, and not because of any topological protection. These results are important to the understanding of light-matter interactions in topological photonic crystal and design of efficient, on-chip chiral quantum devices.

Ferropericlase ($fp$), $\left({\mathrm{Mg}}_{1-x}{\mathrm{Fe}}_x\right)\mathrm{O}$, the second most abundant mineral in the Earth's lower mantle, is expected to be an essential component of the mantles of super-Earths. Here, we present an ab initio investigation of the structure and magnetic ground state of $fp$ up to $\sim 3\mathrm{TPa}$ with iron concentrations ($x_{\mathrm{Fe}}$) varying from 0.03 to 0.12. Calculations were performed using LDA + $U_{\mathrm{sc}}$ and PBE exchange-correlation functionals to elucidate the pressure range for which the Hubbard $U$ ($U_{\mathrm{sc}}$) is required. Similar to the end members FeO and MgO, $fp$ also undergoes a $B1$-to-$B2$ phase transition that should be essential for modeling the structure and dynamics of the mantles of super-Earths. This structural transition involves a simultaneous change in magnetic state from a low-spin (LS) $B1$ phase with iron total spin $S=0$ to an intermediate-spin (IS) $B2$ phase with $S=1$. This is a rare form of pressure/strain-induced magnetism produced by local cation coordination changes. Phonon calculations confirm the dynamical stability of the iron $B2$-IS state. Free energy calculations are then carried out including vibrational effects and electronic and magnetic entropy contributions. The phase diagram is then obtained for low-concentration $fp$ using a quasi-ideal solid solution model. For $x_{\mathrm{Fe}}>0.12$, this approach is no longer valid. At ultrahigh pressures, there is an IS-to-LS spin state change in Fe in the $B2$ phase, but the transition pressure depends sensitively on thermal electronic excitations and on $x_{\mathrm{Fe}}$.

In the absence of time-reversal symmetry, the asymmetric parameter $q$ of the Fano resonance is extended into the complex space, where its trajectory can be utilized to investigate the decoherence process. By embedding the ensemble of M$\ddot{\mathrm{o}}$ssbauer nuclei in the thin-film planar cavity in this work, the trajectories of asymmetric parameter $q$ are studied via the collective effect of the nuclear ensemble, which is regulated by the nuclear abundance and angle offset. Due to the diverse controllable methods of the collective resonant strength, there are different straight lines and arc-shaped trajectories in the complex plane, in which the slopes and the radius can be adjusted by the angle offset and nuclear abundance, respectively. It is demonstrated that the dissipation process can be “suppressed” equivalently by the strong energy exchange between the cavity and the nuclear ensemble. The present results could enrich the behaviors of the asymmetric parameter $q$ in the complex plane and would provide a new opportunity for decoherence research through the thin-film planar cavity.

Lipid membranes form the barrier between the inside and outside of cells and many of their subcompartments. As such, they bind to a wide variety of nano- and micrometer sized objects and, in the presence of strong adhesive forces, strongly deform and envelop particles. This wrapping plays a key role in many healthy and disease-related processes. So far, little work has focused on the dynamics of wrapping. Here, using a model system of micron-sized colloidal particles and giant unilamellar lipid vesicles with tunable adhesive forces, we measure the velocity of the particle during wrapping as well as the forces exerted on it by the lipid membrane. Dissipation near the contact line appears to be the main factor determining the wrapping velocity and time to wrap an object.

Fixed-frequency superconducting quantum processors are one of the most mature quantum computing architectures with high-coherence qubits and simple controls. However, high-fidelity multiqubit gates pose tight requirements on individual qubit frequencies in these processors, and these constraints are difficult to satisfy when constructing larger processors due to the large dispersion in the fabrication of Josephson junctions. In this paper, we propose a mixed-integer-programming-based optimization approach that determines qubit frequencies to maximize the fabrication yield of quantum processors. We study traditional qubit and qutrit (three-level) architectures with cross-resonance interaction processors. We compare these architectures to a differential ac-Stark shift based on entanglement gates and show that our approach greatly improves the fabrication yield and also increases the scalability of these devices. Our approach is general and can be adapted to problems where one must avoid specific frequency collisions.

Designing unconventional interactions between single phonons and spins is fascinating for its applications in quantum phononics. Here we propose a reliable scheme for coupling spins and phonons in phononic dimer and trimer lattices, with the combination of solid-state defects and diamond phononic (optomechanical) crystals. The dimer and trimer lattices used are an array of coupled phononic cavities with spatially modulated hopping rates. We predict a series of unconventional sound-matter interaction phenomena in this hybrid quantum system. In the dimer lattice we show the formation of chiral spin-phonon bound states and topology-dependent phononic collective radiation. While in the trimer lattice, chiral bound states still exist and the spin relaxation is sublattice dependent. The chiral bound states existing in both types of lattices are robust to a large amount of disorder, which can mediate chiral and robust spin-spin interactions. This work provides a promising platform for phonon-based quantum information processing and quantum simulation.

Resting-state networks usually show time-limited self-sustaining oscillatory patterns (TLSOPs) with the characteristic features of multiscaled rhythms and frequent switching between different rhythms, but the underlying mechanisms remain unclear. To reveal the mechanisms of multiscaled rhythms, we present a simplified reaction-diffusion model of activation propagation to reproduce TLSOPs in real brain networks. We find that the reproduced TLSOPs do show multiscaled rhythms, depending on the activating threshold and initially chosen activating nodes. To understand the frequent switching between different rhythms, we present an approach of dominant activation paths and find that the multiscaled rhythms can be separated into individual rhythms denoted by different core networks, and the switching between them can be implemented by a time-dependent activating threshold. Further, based on the microstates of TLSOPs, we introduce the concept of a return loop to study the distribution of the return times of microstates in TLSOPs and find that it satisfies the Weibull distribution. Then, to check it for real data, we present a method of a shifting window to transform a continuous time series into a discrete two-state time series and interestingly find that the Weibull distribution also exists in resting-state EEG and fMRI data. Finally, we show that the TLSOP lifetime depends exponentially on the core network size and can be explained by a theory of the complete graphs.

It is commonly assumed that no accurate experimental information can be obtained on the path taken by a particle when quantum interference between the paths is observed. However, recent progress in the measurement and control of quantum systems may provide the missing information by circumventing the conventional uncertainty limits. Here, we experimentally investigate the possibility that an individual neutron moving through a two-path interferometer may actually be physically distributed between the two paths. For this purpose, it is important to distinguish between the probability of finding the complete particle in one of the paths and the distribution of an individual particle over both paths. We accomplish this distinction by applying a magnetic field in only one of the paths and observing the exact value of its effect on the neutron spin in the two output ports of the interferometer. The results show that individual particles experience a specific fraction of the magnetic field applied in one of the paths, indicating that a fraction or even a multiple of the particle was present in the path before the interference of the two paths was registered. The obtained path presence equals the weak value of the path projector and is not a statistical average but applies to every individual neutron, verified by the recently introduced method of feedback compensation.

The complex dynamical phases of quantum systems are dictated by atomic interactions that usually evoke an emergent periodic order. Here, we study a quantum many-body system with two competing and substantially different long-range interaction potentials where the dynamical instability towards density order can give way to a disordered amorphous solid, which exhibits local density modulations but no long-range periodic order. We consider a two-dimensional Bose-Einstein condensate in the Rydberg-dressing regime coupled to an optical standing wave resonator. The dynamic pattern formation in this system is governed by the competition between the two involved interaction potentials: repulsive soft-core interactions arising due to Rydberg dressing and infinite-range sign changing interactions induced by the cavity photons. The amorphous phase is found when the two interaction potentials introduce incommensurate length scales. The dynamic formation of this peculiar phase can be attributed to frustration induced by the two competing interaction energies and length scales.

Strong light-induced interactions between atoms are known to cause nonlinearities at a few-photon level, which are crucial for applications in quantum information processing. Compared to free space, the scattering and the light-induced dipolar interaction of atoms can be enhanced by a dielectric environment. For this Purcell effect, either a cavity or a waveguide can be used. Here, we combine the high densities achievable in thermal atomic vapors with an efficient coupling to a slot waveguide. In contrast to free-space interactions, atoms aligned within the slot exhibit repulsive interactions that are further enhanced by a factor of 8 due to the Purcell effect. The corresponding blueshift of the transition frequency of atoms arranged in the essentially one-dimensional geometry vanishes above the saturation, providing a controllable nonlinearity at the few-photon level. The experimental results are in good agreement with Monte Carlo simulations that include the dielectric environment, dipolar interactions, and motional effects. The results pave the way towards a robust scalable platform for quantum nonlinear optics and all-optical quantum information processing at room temperature.