The operation of unambiguous state discrimination (USD) plays an important role in quantum information theory. Here, we study the generalization for this operation which can be applied where USD is not available, including the case of completely coinciding supports. We demonstrate the effect of overcoming the Holevo bound after postselective measurements, which works for a class of ensembles, including two arbitrary noncommuting quantum states. We also discuss the role of max-relative entropy in postselective measurements and give an alternative operational interpretation for this function.

The spontaneous radiative dissociation spectrum of the second bound state of positronium hydride (PsH) located just below the $\mathrm{H}\left(2p\right)+\mathrm{Ps}\left(2p\right)$ threshold energy is investigated by the variational and complex coordinate rotation methods. Convergence of the transition rates is examined in both length and velocity gauges. The spectrum indicates that the primary channel of radiative dissociation is the transition into the continuum of $\mathrm{H}\left(1s\right)+\mathrm{Ps}\left(2p\right)$, and the secondary one is that into the $\mathrm{H}\left(2p\right)+\mathrm{Ps}\left(1s\right)$ continuum. Radiative transition to the resonance state located close to the second bound state is found to exist. The total rate of the radiative dissociation is found to be $9.1\left(2\right)\times {10}^8{\mathrm{s}}^{-1}$.

In light of the work of Reed and Chen [Phys. Rev. A 48, 3644 (1993)], electron-impact excitation from the ground state $1s^2{}^{1}S_0$ to the excited energy level $1s2p{}^{1}P_1$ of heliumlike ions and subsequent electric dipole radiative decay $1s2p{}^{1}P_1\to 1s^2{}^{1}S_0$ are revisited by using the multiconfigurational Dirac-Fock method and relativistic distorted-wave theory. Special attention is paid to the effect of the Breit interaction on the angular and polarization behaviors of the characteristic electric dipole line radiated from heliumlike ions. It is found that the presently obtained linear polarization without inclusion of the Breit interaction agrees very well with the results of Reed and Chen for all the heliumlike ions and impact electron energies considered. In contrast to the results of Reed and Chen, the Breit interaction makes the electric dipole line much less linearly polarized and anisotropic, especially for high-$Z$ ions and high impact energies. This behavior quickly becomes more prominent with increasing atomic number $Z$ and impact energy.

We examine the dynamical role of the Pauli exclusion principle on resonant multielectron ionization. Using a simple essential-state model that permits us approximate but analytical solutions, we show that this principle can lead to several unexpected consequences. It has only a transient effect on the resonant multielectron dynamics and its dynamical impact decreases with increasing Rabi frequency. Due to mutually competing mechanisms, this principle can enhance the ionization probability, while at the same time it also decreases the probability for double ionization. Furthermore, it can also manifest itself in the energy distribution of the ionized electrons, but only if the energy accumulation range of the electronic detector is chosen to be sufficiently wide.

We report the experimental observation of the instability suppression of higher-order vortex solitons in cylindrical lead glass with thermal nonlocal nonlinearity. A scalar higher-order vortex soliton with a topological charge $l=10$, which is vulnerable to azimuthal breakup when it is perturbed by an initial noise, is stabilized by a coaxially propagating, mutually incoherent Gaussian beam, forming a Gauss-vortex vector soliton (GVVS). The vortical annular profile and topological charge in the GVVS, and the vortical annular profile in the Gauss-vortex vector breather (GVVB), can be preserved during the propagation. Numerical simulations for unperturbed and perturbed scalar vortex solitons, GVVS, and GVVB demonstrate close agreement with experimental results.

Cavity optomechanical systems have been widely studied for many of their interesting properties, which can be interpreted with their linearized dynamics. Previously, the genuine nonlinear dynamics of optomechanical systems was only considered for very few phenomena such as self-induced oscillation and optomechanical chaos. Beyond the scope of those previous studies, a phenomenon of frozen mechanical oscillation irrespective of the intensity variation on the driving fields was recently discovered for an optomechanical system under two pump fields having their frequencies properly matched [He et al., Phys. Rev. A 102, 011503(R) (2020)]. Here we significantly advance the study by showing that similar phenomena can manifest under more general conditions for the external driving fields. The mechanical oscillation in such doubly driven optomechanical systems can be locked in either its amplitude or both amplitude and phase. Simultaneous amplitude and phase locking is possible under a specific condition for the difference of the drive frequencies, whenever the drive intensity is sufficiently high. The variations of the phenomena also exist in two coupled cavities containing one mechanical breathing mode.

Non-Hermitian systems with parity-time reversal ($\mathcal{PT}$) or anti-$\mathcal{PT}$ symmetry have attracted a wide range of interest owing to their unique characteristics and counterintuitive phenomena. One of the most extraordinary features is the presence of an exceptional point, across which a phase transition with spontaneously broken $\mathcal{PT}$ symmetry takes place. We implement a Floquet Hamiltonian of a single qubit with anti-$\mathcal{PT}$ symmetry by periodically driving a dissipative quantum system of a single trapped ion. With stroboscopic emission and quantum state tomography, we obtain the time evolution of density matrix for an arbitrary initial state, and directly demonstrate information retrieval, eigenstate coalescence, and topological energy spectra as unique features of non-Hermitian systems.

We demonstrate real-time magnetometry by detecting fluorescence from a nitrogen-vacancy center in the setting of coherent population trapping and by estimating the magnetic field from the time series of the observed photon counts, which are correlated with the underlying field variation. The proof-of-principle experiment uses an external time-varying magnetic field that follows an Ornstein-Uhlenbeck (OU) process. A conventional estimator based on the use of average photon counts per relevant time interval is susceptible to photon number shot noise and performs poorly at relatively low average photon counts. In comparison, by taking into consideration the statistical properties of the OU process, a Bayesian inference-based estimator can effectively update dynamical information of the field in real time with the detection of just a single photon.

Temporal cavity solitons in ring microresonators provide broad and controllable generation of frequency combs with applications in frequency standards and precise atomic clocks. Three-level media in the $\mathrm{\Lambda }$ configuration inside microresonators displaying electromagnetically induced transparency can be used for the generation of temporal cavity solitons and frequency combs in the presence of anomalous dispersion and two external driving fields close to resonance. Here domain walls separating regions of two dark states due to quantum interference correspond to realizations of stimulated Raman adiabatic passage without input pulses. With no need of modulational instabilities, bright temporal cavity solitons and frequency combs are formed when these domain walls lock with each other. Wide stability ranges, close to resonance operation, and the optimal shape of the cavity solitons due to three-level quantum interference can make them preferable to those in two-level media.

We propose a two-frequency driving scheme in dynamic atomic force microscopy that maximizes the interaction time between tip and sample. Using a stochastic description of the cantilever dynamics, we predict large classical squeezing and a small amount of skewness of the tip's phase-space probability distribution. Strong position squeezing will require close contact between tip and surface, while momentum squeezing would also be possible in the van der Waals region of the tip-surface force. Employing a generalized Caldeira-Leggett model, we predict that surface-dependent dissipative forces may be the dominant source of quantum effects and propose a procedure to isolate quantum effects from thermal fluctuations.

Quantum thermometry exploits the high level of control in coherent devices to offer enhanced precision for temperature estimation. This highlights the need for constructing concrete estimation strategies. Of particular importance is collisional thermometry, where a series of ancillae are sent sequentially to probe the system's temperature. In this paper we put forth a complete framework for analyzing collisional thermometry using Bayesian inference. The approach is easily implementable and experimentally friendly. Moreover, it is guaranteed to always saturate the Cramér-Rao bound in the long-time limit. Subtleties concerning the prior information about the system's temperature are also discussed and analyzed in terms of a modified Cramér-Rao bound associated with Van Trees and Schützenberger.

Ion traps are promising architectures for implementing quantum computers, but they suffer from excessive “anomalous” ion motional heating that limit their overall coherence and practicality for scalable quantum computing. The exact microscopic origins of anomalous heating remain an open question, but experiments point to adsorbates on trap electrodes as one likely source. Many different models of anomalous heating have been proposed, but these models have yet to pinpoint the atomistic origin of the experimentally observed $1/\omega$ electric-field noise scaling seen in ion traps at frequencies between 0.1–10 MHz. In this work, we show that a model based on previously proposed surface-induced dipole fluctuations on adsorbates, but which also incorporates interparticle interaction dynamics through molecular dynamics simulations of up to multiple monolayers of adsorbates, gives rise to $1/\omega$ frequency scaling at the MHz frequencies typically employed in ion traps. These results demonstrate that moderate-to-high densities of adsorbates can give rise to a set of activated motions that produce the $1/\omega$ noise observed in ion traps and that collective adsorbate motions produce the observed noise spectra that a noninteracting model does not capture.

We report Doppler-sensitive laser-induced-fluorescence (LIF) imaging of an ultracold neutral plasma in a magnetic field. Local values of ion density, hydrodynamic fluid velocity, temperature, and spin polarization are obtained using a fluorescence model based on velocity-resolved rate equations (REs) including the transfer of ions between states due to laser coupling and spontaneous emission. The RE approach captures optical pumping of ions into states that are not driven by the LIF excitation laser, and this is validated with experimental data. Combined molecular-dynamics and quantum-trajectories simulations verify that velocity-changing collisions have a negligible impact on the state population evolution for typical experimental conditions. Relative intensities of Zeeman components of the LIF spectra provide clear evidence that the ions are electron-spin-polarized when created by photoionization of magnetically trapped ${}^{88}\mathrm{Sr}$ atoms. This probe opens many possibilities for studying thermal transport and the equilibration of neutral plasmas in overlapping regimes of strong coupling and magnetization.

Blackbody radiation, omnipresent at room temperature, couples nearby Rydberg states. The resulting state mixture features strong dipolar interactions, which may induce dephasing in a Rydberg many-body system. Here we report on a single atom resolved study of this state contamination and the emerging pairwise interactions in optical tweezers. For near-resonant laser detuning we observe characteristic correlations with a length scale set by the dipolar interaction. Our study reveals the microscopic origin of avalanche excitation observed in previous experiments.

We consider settings providing the existence of stable two-dimensional (2D) dissipative solitons with zero and nonzero vorticity in optical media with quadratic (${\chi }^{\left(2\right)}$) nonlinearity. To compensate the spatially uniform loss in both the fundamental-frequency (FF) and second-harmonic (SH) components of the system, a strongly localized amplifying region [“hot spot” (HS)] carrying the linear gain, is included, acting on either the FF component or the SH one. In both cases, the Gaussian radial gain profile supports stable fundamental dissipative solitons pinned to the HS. The structure of existence and stability domains for the 2D solitons is rather complex. They demonstrate noteworthy features, such as bistability and spontaneous symmetry breaking. A ring-shaped gain profile acting on the FF component supports stable vortex solitons, with the winding number up to 5, and multipoles. Nontrivial transformation of vortex-soliton profiles upon either growth of the gain value or stretching of the gain profile is observed.

Focusing on the transient regime, we explore atomic two-photon spectroscopy with self-aligned homodyne interferometry in a $\mathrm{\Lambda }$ system with large optical depth. The two light sources at the origin of the interference are the single-photon transient transmission of the probe and the slow light of the electromagnetically induced transparency. By abruptly switching off the probe laser (flash effect), the transient transmission signal is reinforced by cooperativity, showing enhanced sensitivity to the two-photon frequency detuning. If the probe laser is periodically switched on and off, the amplitude of the transmission signal varies and remains large even for high modulation frequency. This technique has potential applications in sensing, such as magnetometry and velocimetry, and in coherent population trapping clocks.

Understanding the behavior of a quantum system coupled to its environment is of fundamental interest in the general field of quantum technologies. It also has important repercussions on foundational problems in physics, such as the process of decoherence and the so-called quantum measurement problem. There have been many approaches to explore Markovian and non-Markovian dynamics within the framework of open quantum systems, but the richness of the ensuing dynamics is still not fully understood. In this paper we develop a non-Markovian extension of the standard Caldeira-Leggett model, based on expanding the dynamics of the reduced system at high temperature in inverse powers of the high-frequency cutoff of the Ohmic spectral density of the environment and derive a non-Markovian master equation for the reduced density matrix for the case of a general potential. We also obtain a fully analytical solution in the free particle case. While the short-time behavior of this solution does not diverge substantially from the Markovian behavior, at intermediate times we find a resurgence of coherence, which we name lateral coherence. We identify this with a corresponding transient negative entropy production rate, which is understood to be characteristic of non-Markovian dynamics. We also analyze the positivity of the reduced density matrix and derive the corresponding Fokker-Planck equation in the classical limit.

The complete geometry of quantum states in parameter space is characterized by the quantum geometric tensor, which contains the quantum metric and Berry curvature as the real and imaginary parts, respectively. When the quantum states are degenerate, the quantum metric and Berry curvature take non-Abelian forms. The non-Abelian (Abelian) Berry curvature and Abelian quantum metric have been experimentally measured. However, an experimentally feasible scheme to extract all the components of the non-Abelian quantum metric tensor is still lacking. Here we propose a generic protocol to directly extract the non-Abelian quantum metric tensor in arbitrary degenerate quantum states in any dimensional parameter space based on measuring the transition probabilities after parameter quenches. Furthermore, we show that the non-Abelian quantum metric can be measured to obtain the real Chern number of a generalized Dirac monopole and the second Chern number of a Yang monopole, which can be simulated in three- and five-dimensional parameter space of artificial quantum systems, respectively. We also demonstrate the feasibility of our quench scheme for these two applications with numerical simulations.

A single photon can be made to entangle simultaneously in its different internal degrees of freedom (DoF)—polarization, orbital angular momentum (OAM), and frequency—as well as in its external DoF—path. Such entanglement in multiple DoF is known as hyperentanglement and provides additional advantage for quantum information processing. We propose a passive optical setup using $q$ plates and polarization beam splitters to hyperentangle an incoming single photon in polarization, OAM, and path DoF. By mapping polarization DoF to a two-dimensional coin state, and path and OAM DoF to two spatial dimensions, $x$ and $y$, we present a scheme for realization of a two-dimensional discrete-time quantum walk using only polarization beam splitters and $q$ plates ensuing the generation of hyperentangled states. The amount of hyperentanglement generated is quantified by measuring the entanglement negativity between any two DoF. We further show that hyperentanglement generation can be controlled by using an additional coin operation or by replacing the $q$ plate with a $J$ plate.