We introduce a method to determine whether a given generalized quantum measurement is isolated or if it belongs to a family of measurements having the same prescribed symmetry. The technique proposed reduces to solving a linear system of equations in some relevant cases. As a consequence, we provide a simple derivation of the maximal family of symmetric informationally complete positive operator-valued measure SIC-POVM in dimension 3. Furthermore, we show that the following remarkable geometrical structures are isolated, so that free parameters cannot be introduced: (a) maximal sets of mutually unbiased bases in prime power dimensions from 4 to 16, (b) SIC-POVM in dimensions from 4 to 16, and (c) contextual Kochen-Specker sets in dimension 3, 4, and 6, composed of 13, 18, and 21 vectors, respectively.

Broadcasting quantum and classical information is a basic task in quantum information processing, and is also a useful model in the study of quantum correlations including quantum discord. We establish a full operational characterization of two-sided quantum discord in terms of bilocal broadcasting of quantum correlations. Moreover, we show that both the optimal fidelity of unilocal broadcasting of the correlations in an arbitrary bipartite quantum state and that of broadcasting an arbitrary set of quantum states can be formulized as semidefinite programs (SDPs), which are efficiently computable. We also analyze some properties of these SDPs and evaluate the broadcasting fidelities for some cases of interest.

Local distinguishability of bipartite unitary operations has recently received much attention. A nontrivial and interesting question concerning this subject is whether there is a sequential scheme for locally discriminating between two bipartite unitary operations, because a sequential scheme usually represents the most economic strategy for discrimination. An affirmative answer to this question was given in the literature, however with two limitations: (i) the unitary operations to be discriminated were limited to act on $d\otimes d$, i.e., a two-qudit system, and (ii) the inverses of the unitary operations were assumed to be accessible, although this assumption may be unrealizable in experiment. In this paper, we improve the result by removing the two limitations. Specifically, we show that any two bipartite unitary operations acting on $d_A\otimes d_B$ can be locally discriminated by a sequential scheme, without using the inverses of the unitary operations. Therefore, this paper enhances the applicability and feasibility of the sequential scheme for locally discriminating unitary operations.

Due to recent experimental efforts on light muonic atoms, we consider the quantum electrodynamics (QED) theory of two-body muonic atoms, namely, of those with $A=3$. Here we present the results for the Lamb shift and fine structure up to the order ${\alpha }^{5}m$ for the $n=2$ levels, which in particular include (1) pure QED contributions, (2) the coefficient to the $r_N^2$ term with QED corrections (where $r_N$ is the rms nuclear charge radius), and (3) the general expressions for the nuclear-structure contributions consistent with the presented QED theory. We revisit theory for the muonic helium-3 ion by rechecking all the relevant theoretical contributions and develop a theory of muonic tritium. We also reestimate the uncertainty of the nuclear-structure contribution.

Quantum control of the pathway along which a Rydberg electron field ionizes is experimentally and computationally demonstrated. Selective field ionization is typically done with a slowly rising electric field pulse. The ${(1/n^{*})}^4$ scaling of the classical ionization threshold leads to a rough mapping between arrival time of the electron signal and principal quantum number of the Rydberg electron. This is complicated by the many avoided level crossings that the electron must traverse on the way to ionization, which in general leads to broadening of the time-resolved field ionization signal. In order to control the ionization pathway, thus directing the signal to the desired arrival time, a perturbing electric field produced by an arbitrary wave-form generator is added to a slowly rising electric field. A genetic algorithm evolves the perturbing field in an effort to achieve the target time-resolved field ionization signal.

We investigate work extraction from integrable quantum systems under unitary operations. As a model system, we consider noninteracting fermions in one dimension. Thanks to its integrability, this system does not thermalize after a perturbation, even though it does reach a steady state which can be described by a generalized Gibbs ensemble (GGE). Such a GGE has an excess free energy compared to a thermal state and we propose to extract this energy by applying Bragg pulses. We show how all the available work in the GGE can be extracted in the adiabatic limit while some excess energy is left at finite times. The unextracted work reaches the adiabatic limit as a power law with exponent $z=-2$ for small systems and with $z=-1$ in the thermodynamic limit. Two distinct protocols for combining the Bragg operations are compared, and in some systems an extensive difference in efficiency arises. From the unextracted work and the entropy production, a notion of temperature is defined and compared to the Boltzmann-Gibbs temperature of the system.

We present a technique to control the spatial state of a small cloud of interacting particles at low temperatures with almost perfect fidelity using spatial adiabatic passage. To achieve this, the resonant trap energies of the system are engineered in such a way that a single, well-defined eigenstate connects the initial and desired states and is isolated from the rest of the spectrum. We apply this procedure to the task of separating a small pre-defined number of particles (up to 10) from an initial cloud and show that it can be implemented in radio-frequency traps using experimentally realistic parameters.

The regime of strong light-matter coupling is typically associated with weak excitation. With current realizations of cavity-QED systems, strong coupling may persevere even at elevated excitation levels sufficient to cross the threshold to lasing. In the presence of stimulated emission, the vacuum-Rabi doublet in the emission spectrum is modified and the established criterion for strong coupling no longer applies. We provide a generalized criterion for strong coupling and the corresponding emission spectrum, which includes the influence of higher Jaynes-Cummings states. The applicability is demonstrated in a theory-experiment comparison of a few-emitter quantum-dot–micropillar laser as a particular realization of the driven dissipative Jaynes-Cummings model. Furthermore, we address the question if and for which parameters true single-emitter lasing can be achieved and provide evidence for the coexistence of strong coupling and lasing in our system in the presence of background emitter contributions.

We consider a wide-aperture surface-emitting laser with a saturable absorber section subjected to time-delayed feedback. We adopt the mean-field approach assuming a single longitudinal mode operation of the solitary vertical-cavity surface-emitting laser (VCSEL). We investigate cavity soliton dynamics under the effect of time-delayed feedback in a self-imaging configuration where diffraction in the external cavity is negligible. Using bifurcation analysis, direct numerical simulations, and numerical path-continuation methods, we identify the possible bifurcations and map them in a plane of feedback parameters. We show that for both the homogeneous and localized stationary lasing solutions in one spatial dimension, the time-delayed feedback induces complex spatiotemporal dynamics, in particular a period doubling route to chaos, quasiperiodic oscillations, and multistability of the stationary solutions.

Boson sampling is a quantum mechanical task involving Fock basis state preparation and detection and evolution using only linear interactions. A classical algorithm for producing samples from this quantum task cannot be efficient unless the polynomial hierarchy of complexity classes collapses, a situation believed to be highly implausible. We present a method for constructing a device which uses Fock state preparations, linear interactions, and Gaussian continuous-variable measurements for which one can show that exact sampling would be hard for a classical algorithm in the same way as boson sampling. The detection events used from this arrangement do not allow a similar conclusion to be drawn for the classical hardness of approximate sampling. We discuss the details of this result outlining some specific properties required by approximate sampling hardness.

We compute the entanglement between the electronic and vibrational motions in the simplest molecular system, the hydrogen molecular ion, considering the molecule as a bipartite system, electron and vibrational motion. For that purpose we compute an accurate total non-Born-Oppenheimer wave function in terms of a huge expansion using nonorthogonal B-spline basis sets that expand separately the electronic and nuclear wave functions. According to the Schmidt decomposition theorem for bipartite systems, widely used in quantum-information theory, it is possible to find a much shorter but equivalent expansion in terms of the natural orbitals or Schmidt bases for the electronic and nuclear half spaces. Here we extend the Schmidt decomposition theorem to the case in which nonorthogonal bases are used to span the partitioned Hilbert spaces. This extension is first illustrated with two simple coupled systems, the former without an exact solution and the latter exactly solvable. In these model systems of distinguishable coupled particles it is shown that the entanglement content does not increase monotonically with the excitation energy, but only within the manifold of states that belong to an existing excitation mode, if any. In the hydrogen molecular ion the entanglement content for each non-Born-Oppenheimer vibronic state is quantified through the von Neumann and linear entropies and we show that entanglement serves as a witness to distinguish vibronic states related to different Born-Oppenheimer molecular energy curves or electronic excitation modes.

We report on the experimental observation and quantitative determination of the laser-induced frequency shift (LIFS) of the ultracold polar molecules formed by photoassociation (PA). The experiments are performed by detecting a series of double PA spectra with a molecular hyperfine structure, which are induced by two PA lasers with a precise and adjustable frequency reference. We find that the LIFS of the molecular hyperfine levels shows a linear dependence on PA laser intensity.

We report a combined experimental and theoretical study to elucidate nonsequential double-ionization dynamics of argon atoms at laser intensities near and below the recollision-induced ionization threshold. Three-dimensional momentum measurements of two electrons arising from strong-field nonsequential double ionization are achieved with a custom-built electron-electron-ion coincidence apparatus, showing laser intensity-dependent Coulomb repulsion effect between the two outgoing electrons. Furthermore, a previously predicted feature of double ionization from doubly excited states is confirmed in the distributions of sum of two-electron momenta. A classical ensemble simulation suggests that Coulomb-repulsion-assisted double ionization from doubly excited states is at play at low laser intensity. This mechanism can explain the dependence of Coulomb repulsion effect on the laser intensity, as well as the transition from side-by-side to back-to-back dominant emission along the laser polarization direction.

When atoms are irradiated by two-color circularly polarized laser fields the resulting strong-field processes are dramatically different than when the same atoms are irradiated by a single-color ultrafast laser. For example, electrons can be driven in complex two-dimensional trajectories before rescattering or circularly polarized high harmonics can be generated, which was once thought impossible. Here, we show that two-color circularly polarized lasers also enable control over the ionization process itself and make a surprising finding: the ionization rate can be enhanced by up to $700\%$ simply by switching the relative helicity of the two-color circularly polarized laser field. This enhancement is experimentally observed in helium, argon, and krypton over a wide range of intensity ratios of the two-color field. We use a combination of advanced quantum and fully classical calculations to explain this ionization enhancement as resulting in part due to the increased density of excited states available for resonance-enhanced ionization in counter-rotating fields compared with co-rotating fields. In the future, this effect could be used to probe the excited state manifold of complex molecules.

We study strong-field molecular ionization, with a focus on indirect ionization to dissociative excited ionic states. Indirect ionization, also known as post-ionization excitation, refers to the excitation of the molecular cation following ionization to a lower-lying state. We propose two possible mechanisms underlying indirect ionization: resonant transitions facilitated by nuclear dynamics and nonadiabatic transitions driven by the laser field off resonance. We compare them by measuring the dependence of the indirect ionization yield on pulse duration for cations with different electronic structures. Both experiments and simulations confirm the importance of nuclear dynamics in indirect ionization and indicate the presence of off-resonant nonadiabatic transitions.

Inner-shell photodetachment from $\mathrm{N}{\mathrm{i}}^{-}\left(\left[\mathrm{Ar}\right]3d^{9}4s^2\right)$ leading to $\mathrm{N}{\mathrm{i}}^{+},\mathrm{N}{\mathrm{i}}^{2+}$, and $\mathrm{N}{\mathrm{i}}^{3+}$ ion production was studied near and above the $3p$ excitation region, in the 60–90 eV photon energy range, using a merged ion-photon beam technique. The absolute photodetachment cross section of $\mathrm{N}{\mathrm{i}}^{-}$ leading to $\mathrm{N}{\mathrm{i}}^{+}$ ion production was measured. The $3p\to 3d$ photoexcitation in $\mathrm{N}{\mathrm{i}}^{-}$ gives rise to a giant Feshbach resonance. In the near-threshold region, a Fano profile, modified by a Wigner $s$-wave ($l=0$) threshold law, accurately fits the $\mathrm{N}{\mathrm{i}}^{-}$ single-photodetachment cross section. A lower-order $R$-matrix calculation shows overall agreement with essential features of the experimental data, confirming the nature of the strong, asymmetric Fano profile of the giant $3p\to 3d$ photoexcitation-autodetachment resonance in $\mathrm{N}{\mathrm{i}}^{-}$.

The propagation time delay due to the finite speed of light (FSL) in atom gravimeters introduces a bias in the gravity measurement, as well as that in classical free-falling corner-cube gravimeters, which is usually termed the FSL effect. For a typical atom gravimeter, the FSL time delay is about several nanoseconds, resulting in the FSL effect, a non-negligible bias in the gravity-acceleration measurement. However, a time delay of about several microseconds, achieved by controlling the Raman-pulse timing directly, contributes a negligible effect. This interesting phenomenon motivates us to make clear two questions: first, what are the origins of the FSL effect in atom gravimeters, and second, what is the difference between the two time delays? Our analysis shows that the FSL effect in atom gravimeters is not just a matter of FSL time delay to a great extent but also the change in the effective wave vector; moreover, the FSL time delay can be quantitatively regarded as the same as the pulse time delay since both actually affect the gravity measurement by changing the two interferometer pulse separations.

X-ray echo spectroscopy, a space-domain counterpart of neutron spin echo, is a recently proposed inelastic x-ray scattering (IXS) technique. X-ray echo spectroscopy relies on imaging IXS spectra and does not require x-ray monochromatization. Due to this, the echo-type IXS spectrometers are broadband, and thus have a potential to simultaneously provide dramatically increased signal strength, reduced measurement times, and higher resolution compared to the traditional narrow-band scanning-type IXS spectrometers. The theory of x-ray echo spectrometers presented earlier [Yu. Shvyd'ko, Phys. Rev. Lett. 116, 080801 (2016)] is developed here further with a focus on questions of practical importance, which could facilitate optical design and assessment of the feasibility and performance of the echo spectrometers. Among others, the following questions are addressed: spectral resolution, refocusing condition, echo spectrometer tolerances, refocusing condition adjustment, effective beam size on the sample, spectral window of imaging and scanning range, impact of the secondary source size on the spectral resolution, angular dispersive optics, focusing and collimating optics, and detector's spatial resolution. Examples of optical designs and characteristics of echo spectrometers with 1-meV and 0.1-meV resolutions are presented.

Spontaneous synchronization is a significant collective behavior of weakly coupled systems. Due to their inherent nonlinear nature, optomechanical systems can exhibit self-sustained oscillations which can be exploited for synchronizing different mechanical resonators. In this paper, we explore the synchronization dynamics of two membranes coupled to a common optical field within a cavity, and pumped with a strong blue-detuned laser drive. We focus on the system quantum dynamics in the parameter regime corresponding to synchronization of the classical motion of the two membranes. With an appropriate definition of the phase difference operator for the resonators, we study synchronization in the quantum case through the covariance matrix formalism. We find that for sufficiently large driving, quantum synchronization is robust with respect to quantum fluctuations and to thermal noise up to not too large temperatures. Under synchronization, the two membranes are never entangled, while quantum discord behaves similarly to quantum synchronization, that is, it is larger when the variance of the phase difference is smaller.