Diatomic polar molecules have been the focus of research in the recent past as candidates for electron electric dipole moment (eEDM), $d_{\text{e}}$, measurements. In the present work, we focus on RaH molecule and have calculated three of its properties (in the $X{}^2{\mathrm{\Sigma }}^{+}$ state) the effective electric field ($E_{\text{eff}}$), the scalar-pseudoscalar (S-PS) interaction coefficient ($W_{\text{s,A}}$), and permanent electric dipole moment (PDM) that play crucial roles in probing fundamental symmetry violations. The calculations were based on the relativistic coupled-cluster singles and doubles (RCCSD) frame work. Our results for $\left|E_{\text{eff}}\right|$, $W_{\text{s,A}}$, and PDM are 80.31 GV/cm, 216.82 kHz, and 4.44 D respectively. In addition to possessing high magnitudes of $E_{\text{eff}}$ and $W_{\text{s,A}}$, RaH also has a large PDM, which is a highly desirable criterion for eEDM experiments. The analysis of our results clearly indicates the spectacular role of electron correlation in enhancing their magnitudes $\approx 16$% for PDM and $\approx 31$% for $E_{\text{eff}}$ and $W_{\text{s,A}}$. The ratio of $E_{\text{eff}}$ to $W_{\text{s,A}}$ for RaH is $89.56\times {10}^{18}{e}^{-1}$ $\mathrm{cm}{}^{-1}$. Based on our results and other experimental considerations, we propose RaH as a future candidate for experiments in this field.

The strong-field approximation (SFA) is widely used to theoretically describe the ionization of atoms and molecules in intense laser fields. We here propose an extension of the SFA to incorporate nondipole contributions in the interaction between the photoelectron and the driving laser field. To this end, we derive Volkov-type continuum wave functions of an electron propagating in a laser field of arbitrary spatial dependence. Based on previous work by L. Rosenberg and F. Zhou [Phys. Rev. A 47, 2146 (1993)], we show how to construct such Volkov-type solutions to the Schrödinger equation for an electron in a vector potential that can be written as an integral superposition of plane waves. These solutions are therefore not restricted to plane waves but are also appropriate to deal with more complex laser fields like twisted Bessel or Laguerre-Gaussian beams, where the magnetic field plays an important role. As an example, we compute photoelectron spectra in the above-threshold ionization of atoms with a single-mode plane-wave laser field of midinfrared wavelength. Especially, we demonstrate how peak offsets in the $p_z$ direction can be extracted that result from the nondipole nature of the interaction. Here, we find good agreement with previous theoretical and experimental studies for circular polarization and discuss differences for linear polarization.

We present a three-path photon interferometer, theoretically investigate the behavior of the photon passing through it, and revisit the surprising experimental outcomes introduced by Danan et al. [Phys. Rev. Lett. 111, 240402 (2013)]. Our study shows that the “discontinuous trajectory” behavior of the photon results from the interference of the three propagation paths of the photon, which originate from the light source and end in the detector. If two of the three paths are adjusted in a destructive interference condition, there will be a zero probability domain in the overlap region of these two paths. However, these two paths can still individually interfere with the third path and contribute to the final output. In this condition, the discontinuous trajectory phenomenon appears. Our theory has a clear physical image and agrees with the experimental outcomes very well.

Bell-state measurements (BSMs) play a vital role in quantum information. However, there is no definite solution of how to realize high-dimensional Bell-state measurements (HDBSMs). In this paper, we present a scheme for realizing arbitrary high-dimensional two-particle BSMs by auxiliary entanglement. In our scheme, the maximally entangled states with different degrees of freedom are used as auxiliary states, and then the HDBSM can be achieved by implementing a Bell measurement with different degrees of freedom. We present the detailed HDBSM scheme for three- and four-dimensional Bell states, and the general formula for arbitrary high-dimensional Bell states. Furthermore, we design an experimental scheme for three-dimensional BSM, and give a method of arbitrary dimension. This method can promote the high-dimensional quantum information task.

Deviations from classical physics when distant quantum systems become correlated are interesting both fundamentally and operationally. There exist situations where the correlations enable collaborative tasks that are impossible within the classical formalism. Here, we consider the efficiency of quantum computation protocols compared to classical ones as a benchmark for separating quantum and classical resources and argue that the computational advantage of collaborative quantum protocols in the discrete variable domain implies the nonclassicality of correlations. By analyzing a toy model, it turns out that this argument implies the existence of quantum correlations distinct from entanglement and discord. We characterize such quantum correlations in terms of the net global coherence resources inherent within quantum states and show that entanglement and discord can be understood as special cases of our general framework. Finally, we provide an operational interpretation of such correlations as those allowing two distant parties to increase their respective local quantum computational resources only using locally incoherent operations and classical communication.

While the circuit model of quantum computation defines its logical depth or “computational time” in terms of temporal gate sequences, the measurement-based model could allow totally different temporal ordering and parallelization of logical gates. By developing techniques to analyze Pauli measurements on multiqubit hypergraph states generated by the controlled-controlled-Z (ccz) gates, we introduce a deterministic scheme of universal measurement-based computation. In contrast to the cluster-state scheme where the Clifford gates are parallelizable, our scheme enjoys massive parallelization of ccz and swap gates, so that the computational depth grows with the number of global applications of Hadamard gates, or, in other words, with the number of changing computational bases. A logarithmic-depth implementation of an $N$-times controlled-Z gate illustrates a trade-off between space and time complexity.

An analytic description of high-order harmonic generation (HHG) is proposed in the adiabatic (low-frequency) limit for an initial $s$ state and a laser field having an arbitrary wave form. The approach is based on the two-state time-dependent effective range theory and is extended to the case of neutral atoms and positively charged ions by introducing ad hoc the Coulomb corrections for HHG. The resulting closed analytical form for the HHG amplitude is discussed in terms of real classical trajectories. The accuracy of the results of our analytic model is demonstrated by comparison with numerical solutions of the time-dependent Schrödinger equation for a strong bicircular field composed of two equally intense components with carrier frequencies $\omega$ and $2\omega$ and opposite helicities. In particular, we demonstrate the effect of ionization gating on HHG in a bicircular field, both for the case that the two field components are quasimonochromatic and for the case that the field components are time-delayed short pulses. We show how ionization in a strong laser field not only smooths the usual peak structures in HHG spectra but also changes the positions and polarization properties of the generated harmonics, seemingly violating the standard dipole selection rules. These effects appear for both short and long incident laser pulses. In the case of time-delayed short laser pulses, ionization gating provides an effective tool for control of both the HHG yield and the harmonic polarizations [Frolov et al., Phys. Rev. Lett. 120, 263203 (2018)]. For the case of short laser pulses, we introduce a simple two-dipole model that captures the physics underlying the harmonic emission process, describing both the oscillation patterns in HHG spectra and also the dependence of the harmonic polarizations on the harmonic energy.

The coherent emission from ionized nitrogen molecules is of interest for remote sensing and astronomical applications. To initiate the lasing process, we used an intense ultrashort near-infrared (NIR) pulse overlapped with a terahertz (THz) single-cycle pulse. We observed that coherent emission could be seeded and modulated by the amplitude of the THz field, which is the result of a combined effective second-order nonlinear polarization and the nonlinear effects induced by the NIR pump. Our results shed light on the role of intense transient fields in the coherent emission from photoexcited gas molecules.

Dynamical backaction has proven to be a versatile tool in cavity optomechanics, allowing for precise manipulation of a mechanical resonator's motion using confined optical photons. In this work, we present measurements of a silicon whispering-gallery-mode optomechanical cavity where backaction originates from opposing radiation-pressure and photothermal forces, with the former dictating the optomechanical spring effect and the latter governing the optomechanical damping. At high enough optical input powers, we show that the photothermal force drives the mechanical resonator into self-oscillations for a pump beam detuned to the low-frequency side of the optical resonance, contrary to what one would expect for a radiation-pressure-dominated optomechanical device. Using a fully nonlinear model, we fit the hysteretic response of the optomechanical cavity to extract its properties, demonstrating that this non-sideband-resolved device exists in a regime where photothermal damping could be used to cool its motion to the quantum ground state.

The existence of bound states in the continuum (BIC) manifests a general wave phenomenon first predicted in quantum mechanics by John von Neumann and Eugene Wigner [J. von Neumann and E. Wigner, Phys. Z. 30, 465 (1929)]. Today it is being actively explored in photonics, radiophysics, acoustics, and hydrodynamics. We report an experimental observation of an electromagnetic bound state in the radiation continuum in a one-dimensional array of dielectric particles. By measurement of the transmission spectra of the ceramic disk chain at GHz frequencies, we demonstrate how a resonant state in the vicinity of the center of the Brillouin zone turns into a symmetry-protected BIC with increase in the number of the disks. We estimate a number of the disks when the radiation losses become negligible in comparison to material absorption and, therefore, the chain could be considered practically as infinite. The presented analysis is supplemented by measurements of the near fields of the symmetry-protected BIC. All measurements are in a good agreement with the results of the numerical simulation and analytical model based on a tight-binding approximation. The obtained results provide useful guidelines for practical implementations of structures with bound states in the continuum that opens up horizons for the development of optical and radio-frequency metadevices.

We study the quantum Zeno effect of the circuit-QED system at finite temperature for both selective measurement and nonselective measurement proposals. For the selective measurement proposal, we explicitly show that the counter-rotating terms (CRTs) become as significant as rotating wave terms (RWTs) with increase of temperature. We also derive an analytic expression for the survival probability which correctly captures the leading contribution for both RWTs and CRTs in the frequent measurement limit. Beyond this limit, we observe a nonexponential decay of the survival probability by further considering the effect of the variation of initial states during repeated measurements. For the nonselective measurement proposal, we find that the survival probability demonstrates the same behavior as in the selective measurement case in the short-time regime while it deviates from an exponential form and approaches a nonvanishing value in the long-time scale due to the repopulation effect.

We consider a central fermion strongly interacting with a surrounding mesoscopic bath of fermions which is weakly coupled to a Markovian bath of fermions. The master equation of the system consisting of the central fermion and the mesoscopic bath is derived, and based on this master equation, the reduced dynamics and thermalization of the central fermion are investigated. As probes for studying the dynamics and thermalization, the mean number of the central fermion and the current density are calculated. The observed dynamics reveal that strong coupling strength induces non-Markovian behavior and that in the long time limit thermalization is attained leaving the central fermion in a thermal state that is described by the corresponding Gibbs distribution. Upon attaining the thermal state, the central fermion is found to be entangled with the collective mode of the surrounding mesoscopic bath in the high-temperature regime, whereas in the low-temperature regime, it is found to undergo sudden death of entanglement.

Recent experiments and simulations have shown that pulsed lasers can be used to push neutral Rydberg atoms forward or pull them back toward a light source. Referring to an earlier experiment [U. Eichmann et al., Nature 461, 1261 (2009)], we simulate both pushing and pulling effects on ${}^1\mathrm{H}$ and ${}^{12}\mathrm{C}$ in tightly focused laser fields with a high-order corrected optical description. Scaling laws of excited ${}^4\mathrm{He}$ acceleration by laser pulses are investigated via numerical simulations, which show that Rydberg atoms can be pushed or pulled to high speeds even up to hundreds of kilometers per second by tightly focused laser pulses. Moreover, we use the ponderomotive model and computer simulation to further analyze the validity of two formulas of the maximum radial and longitudinal velocity of outgoing accelerated Rydberg atoms. These studies have practical guiding significance to laser-based acceleration of neutral particle experiments in laboratories.

A many-order-of-magnitude difference in the harmonic yield from the two topological phases of finite, dimerizing linear chains in laser fields has recently been observed in all-electron time-dependent density-functional simulations [Bauer and Hansen, Phys. Rev. Lett. 120, 177401 (2018)]. In this work, we explore the robustness of the effect concerning the length of the chains, a continuous transition between the two topological phases, and disorder. A high robustness of both the degeneracy of the edge states in the topologically nontrivial phase as well as of the pronounced destructive interference, causing a dip in the harmonic spectra, in the topologically trivial phase is observed.

Quasi-one-dimensional lattice systems such as flux ladders with artificial gauge fields host rich quantum-phase diagrams that have attracted great interest. However, so far, most of the work on these systems has concentrated on zero-temperature phases while the corresponding finite-temperature regime remains largely unexplored. The question if and up to which temperature characteristic features of the zero-temperature phases persist is relevant in experimental realizations. We investigate a two-leg ladder lattice in a uniform magnetic field and concentrate our study on chiral edge currents and momentum-distribution functions, which are key observables in ultracold quantum-gas experiments. These quantities are computed for hard-core bosons as well as noninteracting bosons and spinless fermions at zero and finite temperatures. We employ a matrix-product-state based purification approach for the simulation of strongly interacting bosons at finite temperatures and analyze finite-size effects. Our main results concern the vortex-fluid-to-Meissner crossover of strongly interacting bosons. We demonstrate that signatures of the vortex-fluid phase can still be detected at elevated temperatures from characteristic finite-momentum maxima in the momentum-distribution functions, while the vortex-fluid phase leaves weaker fingerprints in the local rung currents and the chiral edge current. In order to determine the range of temperatures over which these signatures can be observed, we introduce a suitable measure for the contrast of these maxima. The results are condensed into a finite-temperature crossover diagram for hard-core bosons.

We analyze quantum effects occurring in optomechanical systems where the coupling between an optical mode and a mechanical mode is quadratic in displacement (membrane-in-the-middle geometry). We show that it is possible to observe quantum effects in these systems without achieving the single-photon strong-coupling regime. We find that zero-point energy causes a mechanical frequency shift, and we propose an experimental way to measure it. Further, we show that it is possible to determine the phonon statistics from the cavity transmission, and propose a way to infer the resonator temperature based on this feature. For completeness, we revisit the case of an isolated system and show that different types of mechanical quantum states can be created, depending on the initial cavity state. In this situation, mechanical motion undergoes collapse and revival, and we compute the collapse and revival times, as well as the degree of squeezing.

We experimentally test the recently predicted anisotropic invariance properties of pure three-qubit states, via generation and measurement of polarization-path entangled three-qubit states. These properties do not require aligned reference frames, and can be determined from measurements on any two of the qubits. They have several applications, such as a universal ordering of pairwise quantum correlations, strong monogamy relations for Bell inequalities and quantum steering, and a complementarity relation for Bell nonlocality versus 3-tangle, some of which we also test. The results indicate that anisotropic invariance, together with the three-qubit Bloch vector lengths, provide a robust and complete set of invariants for such states under local unitary transformations.

We investigate the role of retardation corrections to polarizability and to the refractive index. We found that the classical electromagnetic theory of dielectrics requires corresponding modifications in terms of the nonlocality of the dielectric constant. This nonlocality should be taken into account in the interpretation of accurate measurements of the optical refractivity.

We consider two non-Markovian models: random telegraph noise and non-Markovian dephasing. The memory in these models is studied from the perspective of quantum Fisher information flow. This is found to be consistent with the other well-known witnesses of non-Markovianity. The two noise channels are characterized quantum information theoretically by studying their gate and channel fidelities. Further, the quantum coherence and its balance with mixedness is studied. This helps to put in perspective the role that the two noise channels can play in various facets of quantum information processing and quantum communication.