Strong-field above-threshold detachment (ATD) is analyzed in the low-frequency (or adiabatic) limit. The explicit form of ATD amplitude is obtained within a developed low-frequency approximation. We show that in the low-frequency limit, the atomic structure effects are introduced in the ATD amplitude through the $T$ matrix, which in the limiting case coincides with the amplitude of elastic electron scattering. The rescattering part of ATD amplitude is evaluated in terms of the closed real classical electron trajectories. We further discuss the behavior of ATD amplitude near the caustic rescattering energies (classical energies for which two trajectories merge into a single one) of photoelectrons. Comparison of developed analytic theory with the numerical solution of time-dependent Schrödinger equation is presented for linearly polarized and time-delayed bicircular fields. Based on the analytical description, we predict an enhancement of ATD yield for time-delayed bicircular fields, which is similar to previous findings for the high harmonic yield [M.V. Frolov et al., Phys. Rev. Lett. 120, 263203 (2018)].

Theoretical study is presented for a spinor Bose-Einstein condensate, whose two components are coupled by copropagating Raman beams with different orbital angular momenta. The investigation is focused on the behavior of the ground state of this condensate, depending on the atom-light coupling strength. By analyzing the ground state, we have identified a number of quantum phases, which reflect the symmetries of the effective Hamiltonian and are characterized by the specific structure of the wave function. In addition to the well-known stripe, polarized, and zero-momentum phases, our results show that the system can support phases whose wave functions contain a complex vortex molecule. Such a molecule plays an important role in the continuous phase transitions of the system. The predicted behavior of vortex-molecule phases can be examined in cold-atom experiments using currently existing techniques.

Excited-state quantum phase transitions (ESQPTs) generalize the concept of quantum phase transitions to excited levels. Here we investigate how such ESQPTs can emerge in Kerr nonlinear oscillators, where a bosonic mode with a Kerr nonlinearity is subject to both coherent (one-photon) and parametric (two-photon) driving. We determine the structure and properties of the complete phase diagram by defining proper dimensionless parameters. Rich phases and phase transitions exist in the phase diagram, including phases with two or four critical energies above the ground-state level and first-order quantum phase transitions. We characterize the ESQPTs by singularities in the density of states, which show universal logarithmic divergence or discontinuity at the critical energies. Furthermore, we find that observables such as the photon number also exhibit singular behaviors at the critical energies and obey scaling laws. These results may extend our understanding of the driven Kerr oscillator and ESQPTs in finite-component systems.

We calculate the quantum Fisher information (QFI) for estimating, using a circular imaging aperture, the length of a uniformly bright incoherent line source with a fixed midpoint and the radius of a uniformly bright incoherent disk shaped source with a fixed center. Prolate spheroidal wave functions (PSWFs) on a centered line segment and their generalized versions on a centered disk furnish the respective bases for computing the eigenstates and eigenvalues of the one-photon density operator, from which we calculate QFI with respect to the spatial parameters of the two sources. Zernike polynomials provide a good set into which to project the full source wavefront, and such classical wavefront projection data can realize quantum limited estimation error bound in each case. We subsequently generalize our approach to analyze sources of arbitrary brightness distributions and shapes using a certain class of Bessel Fourier functions that are closely related to the PSWFs. We illustrate the general approach by computing QFI for estimating the lengths of the principal axes of a uniformly bright, centered elliptical disk.

Quantum memories, devices that can store and retrieve photonic quantum states on demand, are essential components for scalable quantum technologies. It is desirable to push the memory towards the broadband regime in order to increase the data rate. Here, we present a theoretical and experimental study of broadband optical memory based on the electromagnetically induced transparency (EIT) protocol. We first provide a theoretical analysis of the issues and requirements needed to achieve broadband EIT memory. We then present our experimental efforts for the movement of EIT memory in cold atoms towards the broadband or short-pulse regime. A storage efficiency of $\sim 80\%$ with a pulse duration of 30 ns (corresponding to a bandwidth of 14.7 MHz) is realized. Due to limitations of the available intensity of the control beam, we could not achieve an optimal storage for the even shorter pulses but were still able to obtain an efficiency of greater than $50\%$ with a pulse duration of 14 ns (31.4 MHz). The time-bandwidth product achieved at an efficiency of $50\%$ is 1267(89).

We consider simultaneous and continuous measurement of two noncommutative observables of the system whose commutator is not necessarily a $c$-number. We revisit the Arthurs-Kelly model and generalize it to describe the simultaneous measurement of two arbitrary observables of the system. Using this generalized model, we continuously measure the system by following the scheme proposed by Scott and Milburn [Scott and Milburn, Phys. Rev. A 63, 042101 (2001)]. We find that the unconditioned master equation reduces to the Lindblad form in the continuous limit. In addition, we find that the master equation does not contain a cross term of these two measurements. Finally, we propose a scheme to prepare the state of a two-level system in an external field by feedback control based on the simultaneous, continuous measurement of the two observables.

We show that in a linear quantum machine, a driven quantum system that evolves while coupled with thermal reservoirs, entanglement between the reservoir modes is unavoidably generated. This phenomenon, which occurs at sufficiently low temperatures and is at the heart of the third law of thermodynamics, is a consequence of a simple process: the transformation of the energy of the driving field into pairs of excitations in the reservoirs. For a driving with frequency ${\omega }_d$ we show entanglement exists between environmental modes whose frequencies satisfy the condition ${\omega }_i+{\omega }_j={\omega }_d$. We show that this entanglement can persist for temperatures that can be significantly higher than the lowest achievable ones with sideband resolved cooling methods.

This article provides a framework to solve linear differential equations based on partial commutativity, which is introduced by means of the Fedorov theorem. The framework is applied to specific types of three-level and four-level quantum systems. The efficiency of the method is evaluated and discussed. The Fedorov theorem appears to answer the need for methods that allow us to study dynamical maps corresponding with time-dependent generators. By applying this method, one can investigate countless examples of dissipative systems such that the relaxation rates depend on time.

Recently, there is growing interest in studying quantum mechanics from the information geometry perspective, where a quantum state is depicted by a point in the projective Hilbert space (PHS). However, the absence of high-dimensional measures limits information geometry in the study of multiparameter systems. In this paper, we propose a measure of the intrinsic density of quantum states (IDQS) in the PHS with the volume element of quantum Fisher information (QFI). Theoretically, the IDQS is a measure to define the (over)completeness relation of a class of quantum states. As an application, the IDQS is used to study quantum measurement and multiparameter estimation. We find that the density of distinguishable states (DDS) for a set of efficient estimators is measured by the invariant volume element of the classical Fisher information, which is the classical counterpart of the QFI and serves as the metric of statistical manifolds. The ability to infer the IDQS via quantum measurement is studied with a determinant-form quantum Cramér-Rao inequality. As a result, we find a gap between the IDQS and the maximal DDS over the measurements. The gap has tight connections with the uncertainty relationship. Exemplified by the three-level system with two parameters, we find that the Berry curvature characterizes the square gap between the IDQS and the maximal attainable DDS. Specific to vertex measurements, the square gap is proportional to the square of the Berry curvature.

The energy interval $\left(3S\text{−}1S\right)$ in muonic hydrogen is calculated on the basis of quasipotential approach in quantum electrodynamics. We take into account different corrections of orders ${\alpha }^3–{\alpha }^6$, which are determined by relativistic effects, the effects of vacuum polarization, nuclear structure, and recoil, as well as combined corrections including the above. Nuclear structure effects are expressed in terms of the charge radius of the proton in the case of one-photon interaction and the proton electromagnetic form factors in the case of two-photon exchange interaction. The value of the energy interval $\left(3S\text{−}1S\right)$ can be used for a comparison with future experimental data and determining the proton charge radius with greater accuracy.

Grazing-incidence fast atom diffraction (GIFAD) is an exceptionally sensitive method for surface analysis, which can be applied not only at room temperature but also at higher temperatures. In this work we use the He-LiF(001) system as a benchmark to study the influence of temperature on GIFAD patterns from insulator surfaces. Our theoretical description is based on the phonon-surface initial value representation (P0-SIVR) approximation, which is a semiquantum approach that includes the phonon contribution to the elastic scattering. Within the P0-SIVR approach the main features introduced by thermal lattice vibrations on the angular distributions of scattered projectiles are investigated as a function of the crystal temperature. We found that azimuthal and polar spectra are strongly affected by thermal fluctuations, which modify the relative intensities and the polar spread of the interference structures. These findings are relevant for the use of GIFAD in surface research. Moreover, the present results are contrasted with available experimental data at room temperature.

We explore the validity of adiabatic elimination in the derivation of an essential-states representation of the time-dependent Schrödinger equation in the presence of a strong laser field. We consider the elimination of off-resonant states in generating an effective two-level description of the light-matter interaction, where the initial and final states are two-photon resonant. The treatment is nonperturbative and can be generalized to $N$-photon absorption.

We present an experimental and theoretical energy- and angle-resolved study on the photoionization dynamics of nonresonant one-color two-photon single-valence ionization of neutral ${\mathrm{N}}_2$ molecules. Using 9.3-eV photons produced via high-order harmonic generation and a three-dimensional momentum imaging spectrometer, we detect the photoelectrons and ions produced from one-color two-photon ionization in coincidence. Photoionization of ${\mathrm{N}}_2$ populates the $X$ ${}^2\mathrm{\Sigma }_g^{+}, A$ ${}^2\mathrm{\Pi }_u$, and $B$ ${}^2\mathrm{\Sigma }_u^{+}$ ionic states of ${\mathrm{N}}_2{}^{+}$, where the photoelectron angular distributions associated with the $X$ ${}^2\mathrm{\Sigma }_g^{+}$ and $A$ ${}^2\mathrm{\Pi }_u$ states both vary with changes in photoelectron kinetic energy of only a few hundred meV. We attribute the rapid evolution in the photoelectron angular distributions to the excitation and decay of dipole-forbidden autoionizing resonances that belong to series of different symmetries, all of which are members of the Hopfield series, and compete with the direct two-photon single ionization.

Laser plasma instabilities are problematic for inertial confinement fusion because they can spoil illumination uniformity, reduce laser-target coupling, and create unwanted fast electrons. Recent experiments and simulations have shown that self-seeded stimulated rotational Raman scattering (SRRS) in air might achieve enough spectral broadening to mitigate these instabilities with only moderate unwanted broadening of the focal spot. The theoretical model for the simulations included chaotic broadband and spatially multimode light, but was a scalar formulation suitable for only linear polarization, where the SRRS gains and spectral broadening are limited by Stokes–anti-Stokes coupling. This paper derives a tensor formulation of SRRS theory suitable for modeling spectral broadening of arbitrarily polarized spatially and temporally incoherent light; it then describes the algorithms used to simulate the theory and provides some preliminary results that compare linear and elliptical polarizations. It begins with a paraxial wave equation for an arbitrarily polarized optical field envelope, which is phase modulated by a term proportional to a Raman driven molecular polarizability tensor. Treating the air molecules as rigid rotators, it uses a quantum treatment to derive a driven harmonic oscillator equation for that polarizability, then expresses these vector and tensor equations in terms of the field's right- and left-handed circular polarization components to derive the final coupled equations for arbitrary polarization. The formulation includes possible ac Stark shift contributions, but shows that they are negligible for intensities below $10\mathrm{GW}/{\mathrm{cm}}^2$. It then describes the algorithms used in the simulation code and the numerical model of the chaotic light, whose initial spectral bandwidth is broad enough to self-seed the SRRS. In this algorithm, the SRRS process accurately conserves the total energy at each axial plane along the propagation path. Finally, it compares simulations of power spectra and far-field profiles for elliptical vs linear polarization, which show that elliptically polarized light produces significantly more broadening of both profiles than linear polarization. For linear polarization, the SRRS process reduces the incident coherence time from 0.54 to 0.27 ps; for elliptical polarization, it reduces to 0.19 ps. The theory and simulation algorithms presented here provide a framework for evaluating techniques that combine beams of alternating circular polarizations with different spectra and angular divergences to improve SRRS spectral broadening without excessive focal spot broadening.

We develop a theoretical description of a device for coherent conversion of microwave-to-optical photons. For the device, dopant ions in a crystal are used as three-level systems, and interact with the fields inside overlapping microwave and optical cavities. We develop a model for the cavity fields interacting with an ensemble of ions and model the ions using an open quantum systems approach, while accounting for the effect of inhomogeneous broadening. Numerical methods are developed to allow us to accurately simulate the device. We also further develop a simplified model, applicable in the case of small cavity fields, which is relevant to quantum information applications. This simplified model is used to predict the maximum conversion efficiency of the device. We investigate the effect of various parameters, and predict that conversion efficiency of above 80% should be possible with currently existing experimental setups inside a dilution refrigerator.

The quantum fluctuations of a physical property can be observed in the measurement statistics of any measurement that is at least partially sensitive to that physical property. Quantum theory indicates that the effective distribution of values taken by the physical property depends on the specific measurement context based on which these values are determined and weak values have been identified as the contextual values describing this dependence of quantum fluctuations on the measurement context. Here, the relation between classical statistics and quantum contextuality is explored by considering systems entangled with a quantum reference. The quantum fluctuations of the system can then be steered by precise projective measurements of the reference, resulting in different contextual values of the quantum fluctuations depending on the effective state preparation context determined by the measurement of the reference. The results show that mixed-state statistics are consistent with a wide range of potential contexts, indicating that the precise definition of a context requires maximal quantum coherence in both state preparation and measurement.

We present analytical and numerical simulation results for squeezing, entanglement, and quantum discord in a dissipatively coupled coherent Ising machine (CIM). Both analytical solutions and numerical simulation results, which are obtained with positive-$P$, truncated-Wigner, and truncated-Husimi representations for the density operator, predict the presence of entanglement and quantum discord, below and above the threshold of CIM. The entanglement criteria and quantum discord are evaluated as a function of the dissipative coupling strength relative to the background loss. For coupled two DOPOs, while entanglement disappears as the background loss exceeds the Ising coupling strength, the quantum discord remains finite even with a large linear loss. For one-dimensional lattice of DOPOs, while entanglement disappears for DOPO pair with large distance, quantum discord remains finite.

Quantum Darwinism describes objectivity of quantum systems via their correlations with their environment—information that hypothetical observers can recover by measuring the environments. However, observations are done with respect to a frame of reference. Here we take the formalism of [Giacomini et al., Nat. Commun. 10, 494 (2019)] and consider the repercussions on objectivity when changing quantum reference frames. We find that objectivity depends on nondegenerative relative separations, conditional state localization, and environment macrofractions. There is different objective information in different reference frames due to the interchangeability of entanglement and coherence, and of statistical mixing and classical correlations. As such, objectivity is subjective across quantum reference frames.

Quantum devices are preparing increasingly more complex entangled quantum states. How can one effectively study these states in light of their increasing dimensions? Phase spaces such as Wigner functions provide a suitable framework. We focus on spherical phase spaces for finite-dimensional quantum states of single qudits or permutationally symmetric states of multiple qubits. We present methods to efficiently compute the corresponding spherical phase-space functions which are at least an order of magnitude faster than traditional methods. Quantum many-body states in much larger dimensions can now be effectively studied by experimentalists and theorists using spherical phase-space techniques.