Quantum many-body systems with kinetic constraints exhibit intriguing relaxation dynamics. Recent experimental progress in the field of cold atomic gases offers a handle for probing the collective behavior of such systems, in particular, for understanding the interplay between constraints and disorder. Here, we explore a spin chain with facilitation constraints—a feature which is often used to model classical glass formers—together with disorder that originates from spin-spin interactions. The specific model we study, which is realized in a natural fashion in Rydberg quantum simulators, maps onto an $XX$ chain with nonlocal disorder. Our study shows that the combination of constraints and seemingly unconventional disorder may lead to interesting nonequilibrium behavior in experimentally relevant setups.

We propose and demonstrate a standard form of Einstein-Podolsky-Rosen (EPR) steering for an arbitrary bipartite Gaussian state under which a simple EPR steering criterion for two-photon correlation can be achieved. In terms of a beam splitter preparing three different steerable states, we further test that the simplified criterion is reliable and efficient when it is used to determine whether an arbitrary bipartite Gaussian state is steerable or not. Based on the same theoretical framework, the standard form and criterion of entanglement are reconstructed and discussed. Furthermore, it is found that EPR steering requires stronger two-photon correlation than entanglement, while its direction is strictly dependent on the mean photon number of two light modes. Moreover, the inherent asymmetric feature can be intuitively embodied in the corresponding criteria, and the essential difference and inherent relation between two standard forms is revealed. We hope the present results could be helpful for potential applications in quantum information processing.

In this work, we show that the quantum coherence among nondegenerate energy subspaces (CANES) is essential for the energy flow in any quantum system. CANES satisfies almost all of the requirements as a coherence measure, except that the coherence within degenerate subspaces is explicitly eliminated. We show that the energy of a system becomes frozen if and only if the corresponding CANES vanishes, which is true regardless of the form of interaction with the environment. However, CANES can remain zero even if the entanglement changes over time. Furthermore, we show how the power of energy flow is bounded by the value of CANES. An explicit relation connecting the variation of energy and CANES is also presented. These results allow us to bound the generation of system-environment correlation through the local measurement of the system's energy flow.

Calculating the energy spectrum of a quantum system is an important task, for example to analyze reaction rates in drug discovery and catalysis. There has been significant progress in developing algorithms to calculate the ground state energy of molecules on near-term quantum computers. However, calculating excited state energies has attracted comparatively less attention, and it is currently unclear what the optimal method is. We introduce a low depth, variational quantum algorithm to sequentially calculate the excited states of general Hamiltonians. Incorporating a recently proposed technique [O. Higgott, D. Wang, and S. Brierley, arXiv:1805.08138], we employ the low depth swap test to energetically penalize the ground state, and transform excited states into ground states of modified Hamiltonians. We use variational imaginary time evolution as a subroutine, which deterministically propagates toward the target eigenstate. We discuss how symmetry measurements can mitigate errors in the swap test step. We numerically test our algorithm on Hamiltonians which encode 3SAT optimization problems of up to 18 qubits, and the electronic structure of the lithium hydride molecule. As our algorithm uses only low depth circuits and variational algorithms, it is suitable for use on near-term quantum hardware.

We devise a method based on the tensor-network formalism to calculate genuine multisite entanglement in ground states of infinite spin chains containing spin-1/2 or spin-1 quantum particles. The ground state is obtained by employing an infinite time-evolving block decimation method acting upon an initial matrix product state for the infinite spin system. We explicitly show how such infinite matrix product states with translational invariance provide a natural framework to derive the generalized geometric measure, a computable measure of genuine multisite entanglement, in the thermodynamic limit of quantum many-body systems with both spin-1/2 and higher-spin particles.

We investigated the fragmentation dynamics of highly excited states of molecular oxygen using femtosecond transient photoelectron spectroscopy. An extreme ultraviolet (XUV) pulse populates the autoionizing Rydberg series converging to ${\mathrm{O}}_2^{+}c{}^4\mathrm{\Sigma }_u^{-}$, and a femtosecond near-infrared (IR) pulse was used to photoionize these states as they dissociate. Monitoring the differential photoelectron spectra as a function of time delay allowed us to obtain the relaxation lifetimes of these Rydberg states. We observed a photoelectron signal corresponding to the formation of a $4p$ excited atomic oxygen fragment, which is not an expected dissociation product of the $\left({\mathrm{O}}_2^{+}c{}^4\mathrm{\Sigma }_u^{-}\right)nl{\sigma }_g$ Rydberg series. Analysis of the time-dependent photoelectron spectra and photoionization calculations indicate that this fragment results from a previously unexplored $\left({\mathrm{O}}_2^{+}{}^4\mathrm{\Pi }_g\right)4p$ repulsive state and that, contrary to expectations, this multielectron excitation pathway presents a substantial cross section. Our study demonstrates that two-color time-resolved differential photoelectron spectroscopy is an excellent tool to study the fragmentation dynamics of such multielectron excited states, which are not easily probed by other means.

Multiparticle entangled states, essential ingredients for modern quantum technologies, are routinely generated in experiments of atomic Bose-Einstein condensates (BECs). However, the entanglement in ultracold interacting Fermi gases has not been yet exploited. In this work, by using an ansatz of composite bosons, we show that many-particle entanglement between two fermionic ensembles localized in spatially separated modes can be generated by splitting an ultracold interacting Fermi gas in the (molecular) BEC regime. This entanglement relies on the fundamental fermion exchange symmetry of molecular constituents and might be used for implementing quantum applications in oncoming experiments. We show that the generated fermionic ensembles can be highly entangled and exhibit nonlocal quantum correlations. Entanglement-induced suppression of fluctuations in the single-fermion spectral density of the ultracold fermionic gas is also observed.

Thermal fluctuation usually damages observations in physical systems. But here we demonstrate a unique contribution made by thermal fluctuation, that is, our observation of linear-zigzag phase transitions induced solely by temperature of the ${}^{40}\mathrm{Ca}^{+}$ ion crystals in a surface-electrode trap. In contrast to the previously observed counterparts based on change in the mechanical equilibrium conditions of the ions, our presented structural phase transition occurs due to controllable influence of thermal fluctuation. The ions' temperature is well controlled by tuning the cooling laser and the experimental observation could be fully understood by classical Langevin equation in addition to the effects from thermal fluctuation. Our experimental investigation indicates the possibility of the positive role thermal fluctuation played in the thermodynamic process at the atomic level, which might bridge the thermodynamics from the macroscopic domain to the quantum regime.

The ability to transmit light through an array of closely packed waveguides while minimizing interwaveguide coupling has important implications for fields such as discrete imaging and telecommunications. Proposals for achieving these effects have involved phenomena ranging from flat bands to Anderson localization. An approach based on Anderson localization is beneficial since every eigenstate localizes and this localization is applicable to arbitrarily large propagation distances, even in the presence of perturbations such as higher-neighbor couplings. However, the localization lengths can be large so that sites within a given finite region can exhibit significant crosstalk. Here, we pose an optimization problem in which we seek to maximally confine the eigenstates. We demonstrate that, for strongly detuned waveguides arranged in an equally spaced lattice in one dimension (1D) and a square lattice in two dimensions (2D), optimal eigenstate localization can be achieved by fractal potentials. We further show that these structures possess a localization-delocalization phase transition in both 1D and 2D. The structures are related to a more general family of self-similar potentials that can be constructed on the class of $k$-partite lattices satisfying the property that each sublattice forms a rescaled copy of the original lattice. The structures may also be approximated by a family of periodic structures, and we characterize the performance of these approximations in comparison to the full aperiodic structures.

We reveal the relevance between the nonlocality and the focusing and defocusing states in a nonlocally nonlinear system with a sine-oscillation response function, and predict a phenomenon that the self-focusing–self-defocusing property of the optical beam in the system depends on its degree of nonlocality. The transition from the focusing nonlinearity to the defocusing nonlinearity of the nonlinear refractive index will happen when the degree of nonlocality of the system goes across a critical value, and vice versa. We also discuss the bright soliton in the focusing state and the dark soliton in the defocusing state, respectively, for such a system.

Interaction-free measurement (IFM) has been proposed as a means of high-resolution, low-damage imaging of radiation-sensitive samples, such as biomolecules and proteins. The basic setup for IFM is a Mach-Zehnder interferometer, and recent progress in nanofabricated electron-diffraction gratings has made it possible to incorporate a Mach-Zehnder interferometer in a transmission electron microscope (TEM). Therefore, the limits of performance of IFM with such an interferometer and a shot-noise limited electron source (such as that in a TEM) are of interest. In this paper, we compare the error probability and sample damage for ideal IFM and classical imaging schemes, through theoretical analysis and numerical simulation. We consider a sample that is either completely transparent or completely opaque at each pixel. In our analysis, we also evaluate the impact of an additional detector for scattered electrons. Inclusion of the scattering detector results in reduction of error by up to an order of magnitude, for both IFM and classical schemes. We also investigate a sample reillumination scheme based on updating priors after each round of illumination and find that this scheme further reduces error. Implementation of these methods is likely achievable with existing instrumentation and would result in improved resolution in low-dose electron microscopy.

Second-order sideband generation (SSG) is investigated in an optomechanical system with atom-cavity-resonator coupling. We focus mainly on the effect of the atomic ensemble trapped in the cavity on the SSG. The numerical results show that the second-order sideband is induced not only for the blue probe-pump detuning resonant case but also for the red one while coupling the atoms with the cavity. Moreover, it is found that the efficiency of the SSG can be significantly enhanced while increasing the atom-cavity coupling strength. More interestingly, the SSG is also sensitive to the atom-pump detuning. It is demonstrated that when optimizing this detuning and the atom-cavity coupling strength together, a robust second-order sideband can be induced and the efficiency of the SSG even surpasses the one of the first-order sideband generation. In addition, the efficiency of the SSG can also be tuned effectively by the decay rate of the atom.

We demonstrate how heating of an environment can invert the line shape of a driven cavity. We consider a superconducting coplanar cavity coupled to multiple artificial atoms. The measured cavity transmission is characterized by Fano-type resonances with a shape that is continuously tunable by bias current through nearby (magnetic flux) control lines. In particular the same dispersive shift of the microwave cavity can be observed as a peak or a dip. We find that this Fano-peak inversion is possible due to a tunable interference between a microwave transmission through a background with reactive and dissipative properties and through the cavity affected by bias-current induced heating. The background transmission occurs due to crosstalk between the control and transmission lines. We show how such background can be accounted for by Jaynes-Cummings type models via modified boundary conditions between the cavity and transmission lines. We find generally that whereas resonance positions determine system energy levels resonance shapes give information on system fluctuations and dissipation.

We present simple yet extremely accurate coupled-wave models describing the formation of bound states in the continuum (BICs) in one-dimensional periodic guided-mode resonant gratings consisting of a slab waveguide layer with a binary grating attached to one or both of its interfaces. Using these models, we obtain simple closed-form expressions predicting the locations of the BICs and quasi-BICs in the $\omega \text{−}{k}_x$ parameter space. We study two mechanisms of the BIC formation: coupling between two counterpropagating guided modes and coupling between a guided mode and a Fabry-Pérot mode. The BIC conditions corresponding to the considered mechanisms are formulated in terms of the scattering coefficients of the binary grating. The predictions of the presented models are in excellent agreement with the results of full-wave simulations obtained using the Fourier modal method.

We identify a general connection between the physics of exceptional points in non-Hermitian systems and the few-photon bound states in waveguide quantum electrodynamics (QED) systems. We show that, in waveguide QED systems where the local quantum system exhibits an exceptional point, the tightest-bound few-photon bound state occurs at the exceptional point. We illustrate this connection with an explicit computation on a waveguide QED system in which a waveguide is coupled to a Jaynes-Cummings system. Our result provides a quantum signature of the exceptional point physics and indicates that the physics of exceptional point can be used to understand and control the photon-photon interaction.

We present two hybrid linear-optical architectures that simultaneously exploit spatial and temporal degrees of freedom of light to effect arbitrary discrete unitary transformations. Our architectures combine the benefits of spatial implementations of linear optics, namely, low loss and parallel operation, with those of temporal implementations, namely, modest resource requirements and access to transformations of potentially unbounded size. We arrive at our architectures by devising and employing decompositions of large discrete unitary transformations into smaller ones, decompositions we expect to have broad utility beyond spatiotemporal linear optics. We show that hybrid architectures promise important advantages over both spatial-only and temporal-only architectures.

In this work, we consider optimal discrimination among quantum states that are sent through a quantum channel. We show the conditions on a quantum channel and an ensemble of states to preserve a measurement for optimal state discrimination over the channel. In particular, we show that for an ensemble of states where the states are given with equal a priori probabilities, an optimal measurement can be preserved over any quantum channel by applying local operations and classical communication, that is, by manipulating the quantum states before and after the channel application. Examples are provided for illustration in the case of quantum communication protocols. Our results can be readily applied to optimal state discrimination over various types of noise.

Photoabsorption spectra of gaseous hydrides of $3p$ ($\mathrm{P}{\mathrm{H}}_3, {\mathrm{H}}_2\mathrm{S}$, HCl) and $4p$ elements ($\mathrm{Ge}{\mathrm{H}}_4, \mathrm{As}{\mathrm{H}}_3, {\mathrm{H}}_2\mathrm{Se}$, HBr) are measured in the energy region within 50 eV above the $K$ edge, to study coexcitations of valence electrons by photoeffect in the $K$ shell. The analysis of the valence coexcitations is extended to Ar, Kr, and $\mathrm{Si}{\mathrm{H}}_4$. Relative probabilities and energies of states in the individual coexcitation channels are recovered by modeling the spectral features with a minimal ansatz based on the features in the contiguous noble gas. The extracted parameters are compared to the results of theoretical calculations for molecules (orca code) and free atoms (Hartree-Fock code). The experimental results confirm that the valence coexcitations in the $3p$ and $4p$ hydride molecules can be satisfactorily described by a two-step process, with the shake of the outer electron following the excitation of the core electron. The total probability—relative to the $K$-edge jump—of the shake-up processes shows a steady decrease from 19% in Si to 14% in Cl, and from 15% in Ge to 12% in Br. The experimental values for Ar (12%) and Kr (10%) are in accord with the trend. The dominant contribution is the transition to quasiatomic orbitals, in contrast with the deeper coexcitation channels in hydride molecules where transition to molecular orbitals prevails.

The prospect of laser cooling of polyatomic molecules has opened a new avenue in the search for the electric dipole moment of the electron (eEDM). An upper bound on the eEDM would probe new physics arising from beyond the standard model of elementary particles. In this work, we report theoretical results for the effective electric field experienced by the electron in YbOH and its molecular electric dipole moment using a relativistic coupled cluster theory. We compare these two properties of YbOH with YbF, which also has a singly unoccupied orbital on the Yb ion. We also present the results of the effective electric field for different bond angles, which sheds light on the sensitivity that can be expected from an eEDM experiment with YbOH.