In a recent experiment, the out-of-plane surface susceptibility of a single-layer two-dimensional atom crystal in the visible spectrum has been measured. This susceptibility gives a measurable contribution to the reflectivity of two-dimensional materials. Here we provide a complete theoretical description of the reflective properties, considering incoming $s$- and $p$-polarized plane waves at any angle of incidence on the crystal, computing local, reflected, and transmitted electromagnetic fields. We finally connect the microscopic polarizability to both the in-plane and the out-of-plane macroscopic surface susceptibilities.

Realizing and manipulating parity-time ($\mathcal{PT}$) symmetry in multidimensional systems are highly desirable for the exploration of nontrivial physics and the discovery of exotic phenomena in non-Hermitian systems. Finding non-Hermitian systems that still have all-real spectra even if their Hamiltonians possess only partial $\mathcal{PT}$ symmetry has also attracted tremendous attention in recent years. Here, we report the experimental observation of partial $\mathcal{PT}$ symmetry in a cesium atomic gas coupled with laser fields, where a two-dimensional partially $\mathcal{PT}$-symmetric optical potential for a probe laser beam is created. A transition of the partial $\mathcal{PT}$ symmetry from an unbroken phase to a broken one is observed through changing the beam-waist ratio of the control and probe laser beams, and the domains of unbroken, broken, and nonpartial $\mathcal{PT}$ phases are also discriminated unambiguously. Moreover, we develop a technique to precisely determine the exceptional point location of the partial $\mathcal{PT}$ symmetry breaking by measuring the asymmetry degree of the probe-beam intensity distribution. The findings reported here pave the way for controlling multidimensional laser beams in non-Hermitian systems via laser-induced atomic coherence, and have potential applications for designing light amplifiers and attenuators in different parts of laser beams.

We propose a method for the detection of the handedness of otherwise identical chiral molecules using the coupling to a high-$Q$ cavity at an exceptional point. In particular, we show that the rates of spontaneous emission for left- or right-handed enantiomers interacting with the cavity mode can be strongly enhanced or suppressed. This provides a strong spectroscopic signature to distinguish enantiomers. The enantioselectivity is shown to be robust against perturbations that move the cavity mode away from the exceptional point.

We consider tests of Leggett-Garg's macrorealism and of macroscopic local realism, where for spacelike separated measurements the assumption of macroscopic noninvasive measurability is justified by that of macroscopic Bell locality. We give a mapping between the Bell and Leggett-Garg experiments for microscopic qubits based on spin-$1/2$ eigenstates and gedanken experiments for macroscopic qubits based on two macroscopically distinct coherent states $|\alpha \rangle$ and $|-\alpha \rangle$ (as $\alpha \to \infty$). In this mapping, the unitary rotation $U_{\theta }$ of the Stern-Gerlach analyzer or polarizing beam splitter is realized by a local interaction $H=\mathrm{\Omega }{\widehat{n}}^4$ where $\widehat{n}$ is the number of quanta. By adjusting the time of interaction, one alters the measurement setting $\theta$. We thus predict violations of Leggett-Garg and Bell inequalities in a macroscopic regime where coarse-grained measurements $\widehat{M}$ need only discriminate between two macroscopically distinct coherent states. To interpret the violations, we distinguish between different definitions of macroscopic realism. Deterministic macroscopic local realism (dMR) assumes the system is in a state with a definite outcome ${\lambda }_{\theta }$ for the measurement $\widehat{M}$ prior to the unitary rotation $U_{\theta }$, and is negated by the violations. Weak macroscopic realism (wMR) assumes a definite outcome for systems prepared in a superposition ${\psi }_{\text{pointer}}$ of two macroscopically distinct eigenstates of $\widehat{M}$, after the unitary rotation $U_{\theta }$. We find that wMR can be viewed consistently with the violations and with the predictions of quantum mechanics for two tests of wMR. A model is presented in which wMR holds and the macroscopic violations emerge over the course of the unitary dynamics occurring at both sites. Implications for the realism of the micro-macro state generated during a quantum measurement are discussed. Finally, we point out an EPR-type paradox, which demonstrates inconsistency between wMR and the completeness of quantum mechanics at a microscopic level.

In this paper we demonstrate competitive Zeno physics in the dissipative Jaynes-Cummings model. The mechanism relies on Zeno coexistence between the single-mode field and the environment. We show that the Zeno suppression of coherent evolution predicted by the standard Jaynes-Cummings model can dramatically fail in the presence of the dissipative environment. For hydrogen atom, such a Zeno effect is not usually observable because the environment ignored previously would, in fact, work in a dominant manner. Moreover, beyond the extremely short-time measurement limit in atomic decay, the Zeno time for hydrogen reaches ${10}^{-10}$ s in the dressed-state picture, which presents an immediately realizable Zeno scheme for the decay of a real atom in the current technology.

The ensemble average of noninteracting particles in a nonlinear oscillator system is investigated. Depending on the initial phase-space distribution, the nonlinearity-induced dephasing mechanism can lead to temporal decays of the average particle position that can be quite different from the standard exponential decay. In fact, the approach to the equilibrium can be Gaussian or even nonmonotonic in time. In the long-time limit, it is possible to construct a single differential equation for the time evolution of the average position. Unlike the infinite set of coupled nonlinear differential equations derived from the standard approach based on the Liouville equation, this equation can be even linear. We also show that the predicted dephasing mechanisms have their direct counterpart in the corresponding dynamics of quantum mechanical wave packets.

We investigate the static and the dynamical behavior of localizable entanglement and its lower bounds on nontrivial loops of topological quantum codes with parallel magnetic field. Exploiting the connection between the stabilizer states and graph states in the absence of the parallel field and external noise, we identify a specific measurement basis, referred to as the canonical measurement basis, that optimizes localizable entanglement when measurement is restricted to single-qubit Pauli measurements only, thereby providing a lower bound. In situations where computing even the lower bound is difficult, we propose an approximation of the lower bound that can be computed for larger systems according to the computational resource in hand. Additionally, we compute a lower bound of the localizable entanglement that can be computed by determining the expectation value of an appropriately designed witness operator. We investigate the behavior of these lower bounds in the vicinity of the topological to nontopological quantum phase transition of the system, and perform a finite-size scaling analysis. We also investigate the dynamical features of these lower bounds when the system is subjected to Markovian or non-Markovian single-qubit dephasing noise. We find that in the case of the non-Markovian dephasing noise, at large time, the canonical measurement-based lower bound oscillates with a larger amplitude when the initial state of the system undergoing dephasing dynamics is chosen from the nontopological phase, compared to the same for an initial state from the topological phase. On the other hand, repetitive collapses followed by revivals to high value with time are observed for the proposed witness-based lower bound in the nontopological phase, which is absent in the topological phase. These features can be utilized to distinguish the topological phase of the system from the nontopological phase in the presence of dephasing noise.

We present a couple of counterdiabatic (CD) schemes for a rapid and high-fidelity transfer of quantum state across a one-dimensional spin chain, between two weakly coupled external qubits at each end. Employing the effective low-energy Hamiltonian of the system, we first construct the optimal CD terms based on the variational principle and then put forward two experimentally feasible shortcuts, which only need to manipulate couplings between the external qubits and chain. Compared to traditional adiabatic protocol, the resulting schemes allow a drastic increase in state-transfer fidelity in a short time. Furthermore, numerical simulation demonstrates that our speed-up protocols hold robustness against the imperfections of control fields and evolution time. The proposed schemes may be applicable to fast quantum-information transport in various spin-based physical systems.

Two-ion crystals constitute a platform for investigations of quantum nature that can be extended to any ion species or charged particle provided one of the ions in the crystal can be directly laser cooled and manipulated with laser radiation. This paper presents the formation of two-ion crystals for quantum metrology in a 7-tesla open-ring Penning trap. ${}^{40}\mathrm{Ca}^{+}$ ions are produced either internally by photoionization or externally in a (Paul-trap) source, transported through the strong magnetic field gradient of the superconducting solenoid, and captured in-flight with a mean kinetic energy of a few electronvolts with respect to the minimum of the Penning-trap potential well. Laser cooling of the two-ion crystal in a strong magnetic field towards reaching the quantum regime is also presented, with particular emphasis on the cooling of the radial modes.

We present the unambiguous determination of quantum phases in the multiphoton ionization (MPI) of potassium atoms by using a multichannel photoelectron interferometry scheme based on trichromatic pulse shaping. The colors of the laser field are chosen to produce three energetically separated photoelectron interferograms in the continuum. While the red pulse is two-photon resonant with the $3d$ state resulting in a $(2+1)$ resonance-enhanced MPI (REMPI), a $(1+2)$ REMPI occurs via the nonresonant intermediate $4p$ state with an initial green or blue pulse. We show that ionization via a nonresonant intermediate state lifts the degeneracy of photoelectron interferograms from pathways consisting of permutations of the colors. The analysis of the interferograms reveals a phase shift of $\pm \pi /2$ depending on the sign of the detunings in the $(1+2)$ REMPI pathways. In addition, we demonstrate that the photoionization time delay in the resonant $(2+1)$ REMPI pathway gives rise to a linear spectral phase in the photoelectron spectra. Insights into the underlying MPI processes are gained through an analytic perturbative description and numerical simulations of a trichromatic driven three-level system coupled to the continuum.

Trapping and self-assembly of particles by a single photonic chiral surface wave (PCSW) in a waveguide is investigated in this work. Through rigorous calculations, stability analysis, and physical interpretation using response theory of optical force, we show that particles of arbitrary sizes, shapes, and dielectric constants can be stably trapped and assembled in the waveguide which is counterintuitive since a propagating surface wave usually exerts a pushing or pulling force on the particle and transports it. The ability of confinement is due to that the PCSW can navigate the particle and recover to its original state. Because of the unique propagating and scattering properties, there is no interaction among the particles when the particles are far away enough. Thus, the particles are independently trapped during the self-assembly process. This work provides a different approach to manipulate small particles by using the PCSW.

Photonic orbital angular momentum (OAM) carrying orthogonal modes exhibits fascinating nature for information encoding. Here we focus on the propagation of OAM states in random media to investigate the turbulence-induced OAM degradation. We utilize the quadratic approximation of the phase structure function to derive a global continuous expression of OAM probability at the single-photon level. The expression provides a general insight into the energy transition between signal and crosstalk modes in random media. Especially for each crosstalk mode, our expression is accurate in predicting the second-order energy transition. The effectiveness of our model is validated via experimental data and simulations in various random media. We also reveal the universality of entangled optical vortex beams and obtain an expression of concurrence. It is demonstrated that utilizing high-order optical vortex beams can effectively improve the antijamming ability and enhance the propagation distance of entanglement. Our results provide a universal framework for OAM propagation through different random media.

We experimentally generate optical tornado waves using spatial multiplexing on a single-phase modulation device. In their focal region, the intensity pattern outlines a spiral of decreasing radius and pitch. We examine the propagation dynamics of such novel waves and reveal the key factors that lead to angular acceleration. Moreover, we propose a two-color scheme that makes it possible to generate dynamically twisting light, an optical analog of a drill, that can rotate at THz frequencies.

Cavity magnomechanics provides a readily controllable hybrid system, consisting of a cavity mode, magnon mode, and phonon mode, for quantum state manipulation and transfer. To implement a fast-and-robust state transfer between the hybrid photon-magnon mode and the phonon mode, we apply two accelerated-adiabatic-passage protocols based on the counterdiabatic Hamiltonian for transitionless quantum driving and the Levis-Riesenfeld invariant for inverse engineering. We construct the counterdiabatic Hamiltonian in a state-resolved way and express it through the creation and annihilation operators rather than the system eigenstates and their time derivatives. We can optimize the invariant-based inverse-engineering protocol with respect to the stability against the systematic errors of the coupling strength and frequency detuning. Both protocols apply for continuous-variable systems with arbitrary target states. We also discuss the decoherence effects from the thermal environment and the counterrotating interactions on both protocols. Our work contributes to the quantum memory for photonic and magnonic quantum information.

We propose a methodology to emulate quantum phenomena arising from any nonclassical quantum state using only a finite set of mixtures of coherent states. This allows us to successfully reproduce well-known quantum effects using resources that can be much more feasibly generated in the laboratory. We present a simple procedure to experimentally carry out quantum-state emulation with coherent states, illustrate it emulating multiphoton NOON states with few phase-averaged coherent states, and demonstrate its capabilities in observing fundamental quantum mechanical effects, such as the Hong-Ou-Mandel effect, violating Bell inequalities and witnessing quantum nonclassicality.

The quantum mean value (QMV) problem is a classically difficult problem that is the central part of many quantum algorithms. We show that using an approximation instead of the exact expectation results in a quadratic improvement in the efficiency of QMV and thus the underlying quantum algorithm. Together with efficient classical sampling algorithms, a quantum algorithm with minimal gate count can thus improve the efficiency of algorithms to solve general integer-value problems, such as the shortest vector problem investigated in this work.

Graph states are a large class of multipartite entangled quantum states that form the basis of schemes for quantum computation, communication, error correction, metrology, and more. In this work, we consider verification of graph states generated by an untrusted source and shared between a network of possibly dishonest parties. This has implications in certifying the application of graph states for various distributed tasks. We present a protocol which is globally efficient for a large family of useful graph states, including cluster states, GHZ states, cycle graph states, and more. For general graph states, efficiency with respect to the security parameter is maintained, though there is a cost increase with the size of the graph state. The protocols are practical, requiring only multiple copies of the graph state, local measurements, and classical communication.

This work aims to study the alignment properties of electron vacancies produced by proton bombardment in the low-energy region from 100 to 250 keV. The alignment parameter ${\mathcal{A}}_{20}$ of ${}_{42}\mathrm{Mo}, {}_{48}\mathrm{Cd}$, and ${}_{49}\mathrm{In}$ ions after $L_3$ subshell ionization has been investigated experimentally. The typical $L$ x-ray spectra are measured for each target at emission angles from ${115}^{\circ }$ to ${155}^{\circ }$. The angular dependence of differential intensity ratios $L_{\alpha }/L_{\beta 1}$ and $L_{\beta 2}/L_{\beta 1}$ is studied as a function of the second-order Legendre polynomial $P_2(\cos \theta$). Our result demonstrates that $L_{\alpha }$ and $L_{\beta 2}$ lines exhibit anisotropic emission spatially. The anisotropy parameter is converted to the alignment parameter by considering the Coster-Kronig (CK) correction coefficient and anisotropy coefficient. The results are compared with theoretical prediction within the framework of the semiclassical theory of inner-shell ionization, and good agreement is found in general. A small discrepancy around the inflection point of the alignment parameter is attributed to the atomic parameters employed only for single ionization in the CK correction coefficient.

When light diffuses in random media, the intensity decays and at the same time the polarization state is scrambled. As explanation for this apparent inseparability, theoretical work has identified a profound relation between length scales of optical transport and depolarization. Here, we experimentally confirm and quantify their proportionality with thickness-dependent depolarization measurements in colloidal suspensions of microscopic constituents. The observed equivalences accurately predict the nonlinear evolution rate of depolarization over a large range of penetration depths. Our results provide a simple relation, that connects light diffusion in strongly scattering media with measurable polarization signatures over wide spectral ranges and scatterer concentrations.