Colinear laser spectroscopy of fast atomic beams has been established as one of the main tools to perform precision experiments with atoms containing short-lived nuclei. Although highly sensitive, the spectral resolution of these techniques is typically limited to several MHz. Here, we study the use of transient interference phenomena to potentially improve the experimental resolution. Previous attempts to implement Ramsey-type measurements with fast beams were limited by the lack of understanding of the observed line shapes. By using a simple model, we provide a satisfactory description of the previously observed line shapes and propose alternative experimental schemes to improve the resolution in fast ion-beam experiments.

In this work, we use the quantum-jump approach to study light-matter interactions in a four-level scheme giving rise to frequency up-conversion amplification without inversion (AWI). The results obtained apply to the case of neutral Hg vapor where the AWI of a probe field in the UV regime was recently reported, opening the way for lasing without inversion in this range of frequencies. We show that, in this scheme, the key element to obtain maximum amplification is the fulfillment of the three-photon resonance condition, which favors coherent evolution periods associated with the probe field gain. We also investigate the parameter values to optimize amplification. The present study extends the theoretical understanding of the underlying mechanisms involved in AWI.

The quantum approximate optimization algorithm (QAOA) requires that circuit parameters are determined that allow one to sample from high-quality solutions to combinatorial optimization problems. Such parameters can be obtained using either costly outer-loop optimization procedures and repeated calls to a quantum computer or, alternatively, via analytical means. In this work, we consider a context in which one knows a probability density function describing how the objective function of a combinatorial optimization problem is distributed. We show that, if one knows this distribution, then the expected value of strings, sampled by measuring a Grover-driven, QAOA-prepared state, can be calculated independently of the size of the problem in question. By optimizing this quantity, optimal circuit parameters for average-case problems can be obtained on a classical computer. Such calculations can help deliver insights into the performance of and predictability of angles in QAOA in the limit of large problem sizes, in particular, for the number partitioning problem.

We study Hilbert-space localization of the many-body dynamics due to ergodicity breaking and analyze this effect in terms of the entanglement entropy and the entanglement spectrum. We find a transition from a regime driven by quantum tunneling to a regime that is dominated by boson-boson interaction, where the latter exhibits ergodicity breaking. Properties of this transition are captured by observation time averaging, which effectively suppresses the large dynamical entanglement fluctuations near the critical point. We employ this approach to the experimentally available bosonic Josephson junction. In this example, the transition from a tunneling regime to Hilbert-space localization reveals clear signatures in the entanglement entropy and entanglement spectrum. Interestingly, the transition point is reduced by quantum effects in comparison to the well-known result of the mean-field approximation in the form of self-trapping. This indicates that quantum fluctuations reduce the classical self-trapping. Different scaling with respect to the number of bosons, $N$, is found in the tunneling and the localization regime: While the entanglement entropy grows logarithmically with $N$ in the tunneling regime, it increases linearly in the localized regime. Our results indicate that entanglement provides a concept for a sensitive diagnosis for the transition from a quantum tunneling regime to Hilbert-space localization.

A neutral-atom system serves as a promising platform for realizing gate-based quantum computing because of its capability to trap and control several atomic qubits in different geometries and the ability to perform strong, long-range interactions between qubits; however, the two-qubit entangling gate fidelity lags behind competing platforms such as superconducting systems and trapped ions. The aim of our work is to design a fast, robust, high-fidelity controlled-$Z$ (cz) gate, based on the Rydberg-blockade mechanism, for neutral atoms. We propose a gate procedure that relies on simultaneous and transitionless quantum driving of a pair of atoms using broadband lasers. By simulating a system of two interacting cesium atoms, including spontaneous emission from excited levels and parameter fluctuations, we yield a Rydberg-blockade cz gate with fidelity 0.9985 over an operation time of $0.12\mathrm{\mu s}$. Our gate procedure delivers cz gates that are superior than the state-of-the-art experimental cz gate and the simulated cz gates based on adiabatic driving of atoms. Our results show that our gate procedure carries significant potential for achieving scalable quantum computing using neutral atoms.

We perform a study on Rayleigh scattering of highly linearly polarized hard x rays on a thin Au foil target. In the study the angular distribution of the scattered radiation is analyzed in a relativistic regime both within and out of the plane of polarization of the incident beam. Within this experiment we scatter a synchrotron beam with a photon energy of 175 keV on a high-$Z$ target foil, namely, gold. Our findings correlate well with state-of-the-art calculations of the scattering process performed in the framework of quantum electrodynamics and may have considerable impact on future experiments regarding a polarization-resolved analysis of Delbrück scattering. Furthermore, we show that the angular distribution of Rayleigh scattering can be used for a highly sensitive determination of the degree and orientation of the linear polarization of the incident hard-x-ray beam, if we rely on the theoretical framework.

The Schwinger multichannel method with pseudopotentials was employed to obtain elastic and electronically inelastic cross sections for the scattering of electrons by formamide up to impact energies of 50 eV. The scattering calculations were carried out according to the minimal orbital basis for the single-configuration-interaction strategy and considered up to 179 open target states in the composition of the space of coupled channels for the description of the multichannel coupling. To include polarization effects, we consider only the excitations related to the hole-particle pairs used in the active space of the minimal orbital basis for the single-configuration-interaction approach. The elastic and electronically inelastic cross sections for the excitation from the ground state to the first four low-lying electronically excited states of formamide are presented, and these results are compared to previous calculations available in the literature. The partial and total ionization cross sections calculated with the binary-encounter-Bethe model and the total cross section calculated as the sum of the elastic, total electronically inelastic, and total ionization cross sections are also reported.

We revisit the problem involving two constantly accelerating Unruh-DeWitt detectors using open effective field theory methods. We study the time evolution of the joint detector state using a Markovian approximation which differs from the standard one taken in the literature. We show that this Markovian limit already implies the complete positivity of the dynamical evolution map without invoking the rotating wave approximation (RWA), in contrast to standard derivations of open system master equations. By calculating explicitly the domain of validity of this Markovian approximation, we argue that the lack of complete positivity in the usual microscopic derivation stems from the (subtle) fact that the Redfield equation is used outside its domain of validity. We give two well-known cases studied in the literature that violate the validity of the Markovian approximation: (i) the “stacked trajectory” limit (when detector trajectories are taken to be on top of one another) and (ii) the large gap-to-acceleration ratio. Since Markovian dynamics with or without RWA can lead to different qualitative predictions for entanglement dynamics, our work emphasizes the need to properly track the regime of validity of all approximations.

We study properties of the work distribution of a many-body system driven through a quantum phase transition in finite time. We focus on the non-Gaussianity of the distribution, which we characterize through two quantitative metrics: skewness and negentropy. In particular, we focus on the quantum Ising model and show that a finite duration of the ramp enhances the non-Gaussianity of the distribution for a finite size system. By examining the characteristics of the full distribution, we observe that there is a clear intermediate regime between the sudden quench and adiabatic limits, where the distribution becomes increasingly skewed.

Quantum entanglement has been verified experimentally and applied in quantum computing, quantum sensing, and quantum networks. It is of great significance to find measures to characterize the quantum entanglement faithfully. In this work, by exploiting the Schmidt decomposition of bipartite states, we first establish a one-to-one correspondence between the characteristic polynomial of the reduced state of a bipartite pure state and the trace of the reduced state. We introduce a family of entanglement measures based on the eigenvalues of the reduced density matrices. Specific measures called informationally complete entanglement measures (ICEMs) are presented. It is demonstrated that such ICEMs can characterize better the entanglement than the existing well-known entanglement measures. The ICEMs also give rise to criteria of state transformations under local operation and classical communication. Moreover, it is shown that the ICEMs can be efficiently estimated on a quantum computer. The full separability, entanglement, and genuine multipartite entanglement can be detected faithfully on quantum devices.

Very recently, dissipative coupling was discovered, which develops and broadens methods for controlling and utilizing light-matter interactions. Here, we propose a scheme to generate the tripartite $W$ state in a dissipatively coupled system, where one qubit and two resonators simultaneously interact with a common reservoir. With appropriate parameters, we find the $W$ state is a dark state of the system. By driving the qubit, the dissipatively coupled system will evolve from the ground state to the tripartite $W$ state. Because the initial state is the ground state of the system and no measurement is required, our scheme is easy to implement in experiments. Moreover, the $W$ state decouples from the common reservoir and thus has a very long lifetime. This scheme is applicable to a wide class of dissipatively coupled systems and we specifically illustrate how to prepare the $W$ state in a hybrid qubit-photon-magnon system by using this scheme.

Communication games are one of the widely used tools that are designed to demonstrate quantum supremacy over classical resources in which two or more parties collaborate to perform an information processing task to achieve the highest success probability of winning the game. We propose a specific two-party communication game in the prepare-measure scenario that relies on an encoding-decoding task of specific information. We first demonstrate that quantum theory outperforms the classical preparation noncontextual theory and the optimal quantum success probability of such a communication game enables the semi-device-independent certification of qubit states and measurements. Further, we consider the sequential sharing of quantum preparation contextuality and show that, at most, two sequential observers can share the quantum advantage. The suboptimal quantum advantages for two sequential observers form an optimal pair which certifies a unique value of the unsharpness parameter of the first observer. Since the practical implementation inevitably introduces noise, we devised a scheme to demonstrate the robust certification of the states and unsharp measurement instruments of both the sequential observers.

Gas phase ${\mathrm{C}}_{70}$ molecules have been ionized with single photons of energies between 16 and 70 eV, and the electron spectra measured with velocity map imaging in coincidence with the ions. The doubly ionized and unfragmented species was present at photon energies of 22 eV and up, and triply charged ions were present from 55 eV. The low-kinetic-energy parts of the spectra are explained with thermal emission of transient hot electrons. We propose a generally applicable mechanism, named resonance ionization shadowing, for the creation of hot electrons by absorption of above-threshold energy photons.

We study an interacting one-dimensional gas of spin-1/2 fermions with two-body losses. The dynamical phase diagram that characterizes the approach to the stationary state displays a wide quantum Zeno region, identified by a peculiar behavior of the lowest eigenvalues of the associated non-Hermitian Hamiltonian. We characterize the universal dynamics of this Zeno regime using an approximation scheme based on an effective decoupling of charge and spin degrees of freedom, where the latter effectively evolve according to a non-Hermitian Heisenberg Hamiltonian. We present detailed results for the time evolution from initial states with one particle per site with either incoherent or antiferromagnetic spin order, showing how peculiar charge properties witnessed by the momentum distribution function build up in time.

Polarization-sensitive second-harmonic-generation microscopy (pSHGM) allows one to determine the molecular orientation of harmonophores. Conventional point scanning based pSHGM is time consuming and subject to the assumption of the cylinder symmetry of the sample. Here, we propose a wide-field pSHGM measurement scheme that is able to measure the second-order nonlinear polarization tensor. The measurement scheme is based on first-order Born approximation, from which the relation between incident fundamental wave, second harmonic wave, and nonlinear polarization tensor has been established. It suggests that the polarization tensor can be solved by measuring the vectorial second harmonic complex fields corresponding to three independent polarization states of the incident fundamental wave. An experiment on measuring the supramolecular orientations, in terms of their symmetric axis, of myosin in rat muscle tissue has been carried out to demonstrate the proposed measurement scheme. Benefiting from the ability of recording the second harmonic signal from different positions parallelly, the proposed method possesses a higher imaging frame rate compared to point scanning based pSHGM. With the present configuration, it takes 0.01 s to acquire a 128×128 pixels image, which is mainly limited by the excitation power density for wide-field illumination. For the same data throughput using pixel-by-pixel scanning, 0.16 s acquisition time is required for a pixel dwell time of 10 µs. Having the ability of wide-field imaging and polarization measurement, the present work also lays a foundation for high-resolution SHG microscopy using computed tomography.

The two-state vector formalism (TSVF) proposed by Aharonov, Bergmann, and Lebowitz [A. Aharonov, P. G. Bergmann, and J. L. Lebowitz, Phys. Rev. 134, B1410 (1964)] allows a counterfactual assignment of probabilities of outcomes of contemplated (but unperformed) measurements on quantum systems. The probabilities assigned by the Aharonov-Bergmann-Lebowitz (ABL) rule and the associated weak values have been used to provide insights into quantum situations and to unearth underlying quantum contextuality. We apply the principle of exclusivity on ABL probabilities which are assigned to mutually orthogonal projectors to define paradoxical and nonparadoxical scenarios. Given a pre- and a postselected pair of states, we consider the nonparadoxical sector with a view to explore the demonstration of quantum contextuality. For the Klyachko-Can-Binicioğlu-Shumovsky scenario, we numerically show that the ABL probabilities of the TSVF in the nonparadoxical sector do not offer any contextual advantage. Our approach can be easily generalized to other contextual scenarios as well. We thus argue that several previous proofs of the emergence of contextuality in pre- and postselected scenarios are only possible if the principle of exclusivity is violated and are therefore classified as paradoxical.

We investigate the maximal amount of quantum work in a driven atom as a quantum battery which is coupled to a massless scalar field in a Minkowski vacuum. The relativistic thermodynamics of the quantum battery is explored from the prospective of quantum channels. It is found that quantum work extraction can be enhanced in the battery whose trajectory is characterized by a combination of a linear accelerated motion and a component of the four velocity. In the nonrelativistic case, the accelerated motion results in the attenuation of energy extraction, while the velocity component can prevent the degradation of quantum work to a certain extent. The work extraction can increase in the relativistic regime. Moreover, for ultrarelativistic velocities, the relativistic thermodynamics of the quantum battery can behave as if it were a closed system owing to the relativity correction associated with the velocity. The extractable work can be shielded from the impacts of vacuum fluctuations and relativistic motion. It is demonstrated that the work extraction relies on quantum coherence of the relativistic battery and nonclassical nature in the Minkowski vacuum.