Consider a statistical model with an epistemic restriction such that, unlike in classical mechanics, the allowed distribution of positions is fundamentally restricted by the form of an underlying momentum field. Assume an agent (observer) who wishes to estimate the momentum field given information on the conjugate positions. We discuss a classically consistent, weakly unbiased, best estimation scheme for the momentum field (by minimizing the mean-squared error) based on which the abstract mathematical rules of quantum mechanics can be derived. The results suggest that a quantum wave function is not an objective agent-independent attribute of reality but represents the agent's best estimations of the momentum, given the positions, under epistemic restriction. Quantum uncertainty and complementarity between momentum and position find their epistemic origin from the trade-off between the mean-squared errors of simultaneous estimations of momentum field and mean position, with the Gaussian wave function representing the simultaneous efficient estimations, achieving the Cramér-Rao bounds of the associated mean-squared errors. We then argue that unitary time evolution and wave-function collapse in measurement are normative rules for an agent to update her estimations given information on the experimental settings.

In an ideal contextuality scenario it is assumed that, whenever two contexts overlap, the marginal distribution obtained for the intersection of these contexts must be the same. Although this nondisturbance idealization has been extremely useful in investigating the limitations of physical theories, in this paper, we will work out a more robust and possibly more experimentally friendly framework for contextuality. Simply put, we will restate the extended definition of noncontextuality provided by the noncontextuality by default and show that in this extended perspective it is possible to avoid weakening the nondisturbance hypothesis. By doing so, we also show how standard tools for characterizing and quantifying contextuality can also be applied to this extended scenario. In particular, we came up with a linear program to check whether or not a behavior is contextual in the extended sense. The connection with the usual definition of contextuality is also established because whenever nondisturbance is fully satisfied the extended conditions reduce to the original ones.

The measurement-based, or one-way, model of quantum computation for continuous variables uses a highly entangled state called a cluster state to accomplish the task of computing. Cluster states that are universal for computation are a subset of a class of states called graph states. These states are Gaussian states and therefore require that the homodyne detection (Gaussian measurement) scheme is supplemented with a non-Gaussian measurement for universal computation, a significant experimental challenge. Here we define a non-Gaussian class of states based on hypergraphs which we demonstrate satisfy the requirements of the Lloyd-Braunstein criteria while restricted to a Gaussian measurement strategy. Our main result is to show that, taking advantage of the intrinsic multimode nonlinearity, a hypergraph consisting of 3-edges can be used to apply a three-mode operation to an input three-mode state. As a special case, this technique can be used to apply the cubic phase gate to a single mode.

The task of sampling the output distribution of random quantum circuits will drive a multiqubit system into chaos, reaching the Porter-Thomas distribution where small multiqubit probabilities appear most often. Such a task is hard to be computed classically and has been proposed as an important measure for quantum supremacy. An equivalent route to reach chaos is the controlled dynamics uniformly exploring the large Hilbert space of coupled qubits, and a proof-of-concept experiment was recently demonstrated by Neill et al. [Science 360, 195 (2018)] in a chain of nine qubits featuring only neighboring couplings. Here, we show numerically that a faster convergence to chaos is possible when full connectivity, instead of partial connectivity, is present among the qubits, i.e., cycles or time required to reach chaotic regime increase only slightly with the number of qubits. We validate the numerical result by carrying out an experiment using up to 12 qubits in a fully connected superconducting quantum processor. Based on a machine learning model, we quantify the evolution error rate to be less than (0.5%/cycle)/qubit, which is mostly determined by qubit dephasing.

The family of codeword stabilized codes encompasses the stabilizer codes as well as many of the best known nonadditive codes. However, constructing optimal $n$-qubit codeword stabilized codes is made difficult by two main factors. The first of these is the exponential growth with $n$ of the number of graphs on which a code can be based. The second is the NP-hardness of the maximum clique search required to construct a code from a given graph. We address the second of these issues through the use of a heuristic clique finding algorithm. This approach has allowed us to find $\left(\right(9,97\le K\le 100,2\left)\right)$ and $\left(\right(11,387\le K\le 416,2\left)\right)$ codes, which are larger than any previously known codes. To address the exponential growth of the search space, we demonstrate that graphs that give large codes typically yield clique graphs with a large number of nodes. The number of such nodes can be determined relatively efficiently, and we demonstrate that $n$-node graphs yielding large clique graphs can be found using a genetic algorithm. This algorithm uses a crossover operation based on spectral bisection that we demonstrate to be superior to more standard crossover operations. Using this genetic algorithm approach, we have found ((13, 18, 4)) and ((13, 20, 4)) codes that are larger than any previously known code. We also consider codes for the amplitude damping channel. We demonstrate that for $n\le 9$, optimal codeword stabilized codes correcting a single amplitude damping error can be found by considering standard form codes that detect one of only three of the $3^n$ possible equivalent error sets. By combining this error-set selection with the genetic algorithm approach, we have found ((11, 68)) and ((11, 80)) codes capable of correcting a single amplitude damping error and ((11, 4)), ((12, 4)), ((13, 8)), and ((14, 16)) codes capable of correcting two amplitude damping errors.

To observe the dynamical Casimir effect (DCE) induced by a moving mirror is a long-standing challenge because the mirror velocity needs to approach the speed of light. Here, we present an experimentally feasible method for observing this mechanical DCE in an optomechanical system. It employs a detuned, parametric driving to squeeze a cavity mode, so that the mechanical mode, with a typical resonance frequency, can parametrically and resonantly couple to the squeezed cavity mode, thus leading to a resonantly amplified DCE in the squeezed frame. The DCE process can be interpreted as mechanically induced two-photon hyper-Raman scattering in the laboratory frame. Specifically, a photon pair of the parametric driving absorbs a single phonon and then is scattered into an anti-Stokes sideband. We also find that the squeezing, which additionally induces and amplifies the DCE, can be extremely small. Our method requires neither an ultrahigh mechanical-oscillation frequency (i.e., a mirror moving at nearly the speed of light) nor an ultrastrong single-photon optomechanical coupling and, thus, could be implemented in a wide range of physical systems.

We study optimization of fidelity for ultrafast transformation of a spin chain via external control of a local exchange coupling. We show that infidelity of such a process can be dramatically decreased by choosing a proper control profile in nonadiabatic time domain, predict main features of this profile analytically, corroborate them numerically with a gradient search algorithm, and discuss the corresponding quantum speed limit. For ultrafast transformations, the qualitative features of the obtained optimal control are system independent. Moreover, the main restrictions on its shape do not depend on the transformation time and remain valid up to the adiabatic limit. Our results can be applied to control a broad variety of quantum systems.

During the inaugural experiment at FLASH, the first vacuum ultraviolet (VUV) free-electron laser facility, Wabnitz et al. [Nature 420, 482 (2002)] irradiated xenon clusters and sparked a concerted theoretical and experimental effort to understand how dense, finite plasmas behave under intense irradiation. In this work, we revisit this experiment with a model that is based only on well-established atomic processes. We find that the experimental results can be explained by hybrid quantum-classical molecular-dynamics simulations if collisional excitation, recombination, and a sufficiently deep soft-core potential is used. Our recent theoretical model for inverse bremsstrahlung heating (IBH) is used to show that the measured energy absorbed by the cluster in the experiment is well predicted by our model.

We explore the x-ray second-harmonic generation process induced by resonant two-photon absorption in systems with inversion symmetry. We show that this process becomes allowed in the x-ray region due to nondipole contributions. It is found that, although a plane-wave pump field generates only a longitudinal second-harmonic field, a Gaussian pump beam creates also a radially polarized transverse second-harmonic field which is stronger than the longitudinal one. Contrary to the longitudinal component, the transverse second-harmonic field with zero intensity on the axis of the pump beam can run in free space. Our theory is applied to Ar and Ne atomic vapors and predicts an energy conversion efficiency of x-ray second-harmonic generation of $3.2\times {10}^{-11}$ and $1.3\times {10}^{-12}$, respectively.

Apart from maximizing the strength of optical electromagnetic fields achievable at high-intensity laser facilities, the collision of several phase-matched laser pulses has been identified theoretically as a trigger of and way to study various phenomena. These range from the basic processes of strong-field quantum electrodynamics to the extraordinary dynamics of the generated electron-positron plasmas. This has paved the way for several experimental proposals aimed at both fundamental studies of matter at extreme conditions and the creation of particle and radiation sources. Because of the unprecedented capabilities of such sources, they have the potential to open up new opportunities for experimental studies in nuclear and quark-gluon physics. We perform here a systematic analysis of different regimes and opportunities achievable with the concept of multiple colliding laser pulses, for both current and upcoming laser facilities. We reveal that several distinct regimes could be within reach of multi-petawatt laser facilities.

We develop a microscopic model to describe the Josephson tunneling between two superfluid reservoirs of ultracold fermionic atoms which accounts for the dependence of the critical current on both the barrier height and the interaction strength along the crossover from the BCS to the BEC regime. Building on a previous study [F. Meier and W. Zwerger, Phys. Rev. A 64, 033610 (2001)] of weakly interacting bosons, we derive analytic results for the Josephson critical current at zero temperature for homogeneous and trapped systems at arbitrary coupling. The critical current exhibits a maximum near the unitarity limit which arises from the competition between the increasing condensate fraction and a decrease in the chemical potential along the evolution from the BCS to the BEC limit. Our results agree quantitatively with numerical simulations and recent experimental data.

We probe the superconducting gap in the zero-temperature ground state of an attractively interacting spin-imbalanced two-dimensional Fermi gas with diffusion Monte Carlo simulations. A condensate fraction at nonzero pair momentum evinces a spatially nonuniform superconducting order parameter. Comparison with exact diagonalization studies confirms that the nonzero condensate fraction across a range of nonzero fermion pair momenta is consistent with nonexclusive pairing between majority and minority fermions, an extension beyond Fulde-Ferrell-Larkin-Ovchinnikov theory.

Atom interferometers typically use the total populations at the interferometer's output ports as the signal, but useful information can be encoded into spatial phase patterns. An interferometer translates the phase pattern into a measurable pattern in the atomic density that we use to perform a direct precision measurement of the ${}^7\mathrm{Li}$ tune-out wavelength near 671 nm. Expressed as a detuning from the $|2S_{1/2},F=2\rangle \to |2P_{1/2},F^{\prime }=2\rangle$ transition, we find 3329.5(1.4) MHz for the tensor-shifted tune out of the $|2S_{1/2},F=2,m_F=0\rangle$ state with ${\sigma }^{\pm }$ light polarization and 3310.6(4.9) MHz for the tune out of the scalar polarizability. This technique may be generalized for directly sensing spatially varying phase profiles.

Non-Hermitian systems provide new avenues to create topological defect states. An unresolved general question is how much the formation of these states depends on asymmetric backscattering, be it nonreciprocal as in the non-Hermitian skin effect or reciprocal as encountered between the internal states of asymmetric microresonators. Here, we demonstrate in a concrete, practically accessible setting of a lossy coupled-resonator optical waveguide that non-Hermitian defect states can exist in open optical systems due to lifetime differences, without the need for asymmetric backscattering within or between the individual resonators. We apply our findings to a finite system of coupled circular resonators perturbed by nanoparticles, following the concept of creating an interface by inverting the position of the nanoparticles in half of the chain. We compare a coupled-mode tight-binding approximation to full-wave numerical simulations, showing that spectrally isolated defect states can indeed be implemented in this simple non-Hermitian photonic device.

Electrically charged particles, moving faster than the speed of light in a medium, emit Cherenkov radiation. Theory predicts electric and magnetic dipoles to radiate as well, with a puzzling behavior for magnetic dipoles pointing in transversal direction [I. M. Frank, Izv. Akad. Nauk SSSR, Ser. Fiz. 6, 3 (1942)]. A discontinuous Cherenkov spectrum should appear at threshold, where the particle velocity matches the phase velocity of light. Here we deduce theoretically that light bullets [Y. Silberberg, Opt. Lett. 15, 1282 (1990)] emit an analogous radiation with exactly the same spectral discontinuity for point-like sources. For extended sources the discontinuity turns into a spectral peak at threshold. We argue that this Cherenkov radiation has been experimentally observed in the first attempt to measure Hawking radiation in optics [F. Belgiorno et al., Phys. Rev. Lett. 105, 203901 (2010)] thus giving experimental evidence for a puzzle in Cherenkov radiation instead.

Symmetry-protected bound states in the continuum (BICs) are nonradiative states with infinite lifetime and perfect confinement of energy even though lying in the radiation continuum due to the symmetry incompatibility. Herein, we study the symmetry-protected BIC supported by metasurfaces composed of silicon nanodisks. Through adding or removing parts of the nanodisks from the edge, a sharp Fano resonance emerges and demonstrates the excitation of quasi-BIC. Their $Q$ factors exhibit the same dependence on the asymmetry degree with these two opposite operations. Furthermore, from both qualitative and quantitative perspectives, analysis on far-field contributions from multipole moments along different directions combining with near-field distributions explains the evolution from BIC to quasi-BIC. The dominant contributor to the quasi-BIC is illustrated to be the electric quadrupole in the $x-y$ plane. Finally, the topological charge carried by the BIC is calculated to be $-1$, demonstrating the topological characteristics of our design. Such metasurfaces are robust in nanofabrication. Our results may provide a route for resonators with better performance applied in sensing, switching, nonlinear optics, and so on.

We advance a statistical theory of extreme event emergence in random nonlinear wave systems with self-similar intermediate asymptotics. We show, within the framework of a generic $(1+1)\mathrm{D}$ nonlinear Schrödinger equation with linear gain, that extreme events and even rogue waves in weakly nonlinear, statistical open systems emerge as parabolic-shape giant fluctuations in the self-similar asymptotic propagation regime. We demonstrate analytically the self-similar structure of the non-Gaussian statistics of emergent rogue waves, and we validate our results with numerical simulations. Our results shed new light on the generic statistical features of extreme events in nonlinear open systems with self-similar intermediate asymptotics.

We theoretically study the dynamics of the ground-state cooling and the quantum entanglement in a modulated optomechanical system, where the frequency of the mechanical oscillator and the optical field or the strength of the driving laser is time dependent. In this paper, we focus mainly on the fact that the system works in the regime of blue detuning. It is found that in the long-time limit the steady-state phonon number can be decreased significantly so that the mechanical oscillator is cooled to the ground state by appropriately selecting the frequency and the amplitude of the modulation. Further, with the help of the dynamical modulation in the system, the time evolution of the entanglement preevaluation value displays the death and rebirth of quantum entanglement between the mechanical oscillator and the optical field, which correspond, respectively, to the increase and decrease of the effective phonon number of the mechanical oscillator. In particular, when the steady-state dynamics of the system is quasiperiodic, the dynamics of the entanglement preevaluation value exhibits a quasiperiodic behavior, which means that the quantum phenomena can be represented in nonlinear classical dynamics.

We analyze the quantum Cheshire cat dilemma under an entangled scenario based on atom-field interferometric setup that can be readily executed experimentally. Once the accuracy and the faithful workability of the two-state vector formalism along with the weak measurement theme is guaranteed, our analysis suggests that the interpretation, i.e., separation of a certain specific feature from the quantum entity, furnished in the pioneering Cheshire cat paper [Y. Aharonov, S. Popescu, D. Rohrlich, and P. Skrzypczyk, New J. Phys. 15, 113015 (2013)], cannot be taken for granted as the sole reason for the phenomenon. Rather, our analysis of the considered entangled interferometric scenario suggests an alternative point of view where the inaccessibility of the defining features of a quantum system appears to be a more plausible reason than the disembodying of the feature from the system itself. Our proposal is based on the time-tested tools of the atomic Bragg diffraction, operating in off-resonant regime, and utilizes the entangled correlations engineered between two identical, two-level atoms in the momentum space.

A dissipation-based scheme is proposed to prepare the steady entangled state of two spatially separated nitrogen-vacancy (NV) centers fixed on the exterior surface of two microtoroidal resonators coupled by a whispering-gallery-mode (WGM) field. The effective operator method is used to get an effective master equation which simplifies the systematic dynamics into ground-state subspace. Based on the effective dynamics, the parameter condition to use the NV center's spontaneous emission and that to use the resonator's photon loss as resources to prepare entanglement are achieved. Interestingly, we find that these two conditions are almost the same. To some extent, these two dissipative factors are no longer detrimental, but can be used simultaneously as powerful resources to prepare the steady entangled state, which forms a sharp contrast with most of the currently schemes utilizing one dissipative factor. On the other hand, the different forms of the steady entangled state are achievable via modulating systematic parameters. The scheme is independent of initial states and does not require precise time control. Numerical results show that high fidelity and purity can be obtained and the scheme is robust against small parameter deviations.