We present a spin-adapted time-dependent coupled-cluster singles and doubles model for the molecular response to a sequence of ultrashort laser pulses. The implementation is used to calculate the electronic response to a valence-exciting pump pulse, and a subsequent core-exciting probe pulse. We assess the accuracy of the integration procedures used in solving the dynamic coupled-cluster equations, in order to find a compromise between computational cost and accuracy. The transient absorption spectrum of lithium fluoride is calculated for various delays of the probe pulse with respect to the pump pulse. We observe that the transient probe absorption oscillates with the pump-probe delay, an effect that is attributed to the interference of states in the pump-induced superposition.

The microscopic control available over cold atoms in optical lattices has opened new opportunities to study the properties of quantum spin models. While a lot of attention is focused on experimentally realizing ground or thermal states via adiabatic loading, it would often be more straightforward to prepare specific simple product states and to probe the properties of interacting spins by observing their dynamics. We explore this possibility for spin-1/2 and spin-1 models that can be realized with bosons in optical lattices, and which exhibit $\mathit{XY}$-ferromagnetic (or counterflow spin-superfluid) phases. We consider the dynamics of initial spin-rotated states corresponding to a mean-field version of the phases of interest. Using matrix product state methods in one dimension, we compute both nonequilibrium dynamics and ground and thermal states for these systems. We compare and contrast their behavior in terms of correlation functions and induced spin currents, which should be directly observable with current experimental techniques. We find that although spin correlations decay substantially at large distances and on long timescales, for induction of spin currents, the rotated states behave similarly to the ground states on experimentally observable timescales.

We develop a unified theoretical framework for the efficient description of multiphoton states generated and propagating in loop-based optical networks which contain nonlinear elements. These active optical components are modeled as nonlinear media, resembling a two-mode squeezer. First, such nonlinear components can be seeded to manipulate quantum states of light, as such enabling photon addition protocols. And, second, they can function as an amplifying medium for quantum light. To prove the practical importance of our approach, the impact of multiple round trips is analyzed for states propagating in experimentally relevant loop configurations of networks, such as time-multiplexed driven quantum walks and iterative photon-number state generation protocols. Our method not only enables us to model such complex systems but also allows us to propose alternative setups that overcome previous limitations. To characterize the systems under study, we provide exact expressions for fidelities with target states, success probabilities of heralding-type measurements, and correlations between optical modes, including many realistic imperfections. Moreover, we provide an easily implementable numerical approach by devising a vector-type representation of photonic states, measurement operators, and passive and active processes.

Through a quantitative analysis of an atomic transition driven strongly by quantized electromagnetic fields of various quantum states, we explore the role of quantum fluctuations on the behavior of the system. The emphasis is on fields with super-Poissonian statistics manifested in photon bunching, with the case of squeezed vacuum radiation serving as a prototype of superbunching. When combined with nonlinear coupling of the resonant states, bunching and superbunching lead to counterintuitive behavior. The connection to recent progress in squeezed vacuum sources and the opportunity for experimental investigation, as well as challenging open theoretical problems, are also outlined. In yet another context, the possibility of exploring related effects in strongly driven Auger resonances by FEL radiation is discussed.

An anomaly of the Earth's gravitational field could increase the second-order gravity-gradient tensor more than an order of magnitude and third-order gravity-gradient tensor more than four orders of magnitude. As a result an estimate of the systematic error in the atomic gravimetry considered in the article by Utrecht and Giese [Phys. Rev. A 101, 053615 (2020)] should be proportionally enlarged.

Recently, we introduced [C. Ufrecht and E. Giese, Phys. Rev. A 101, 053615 (2020)] a technique to calculate the phase of light-pulse atom interferometers caused by the presence of perturbation potentials and underlined its power by an illustrative example. In the preceding Comment [B. Dubetsky, Phys. Rev. A 102, 027301 (2020)], it was pointed out that other, less idealized situations could have been calculated as well. Our Reply emphasizes that our method is correct, the results from our example can be trivially generalized to other perturbations, and intricate effects of local environments can be even more prominent but also treated by our technique.

The definition of memory in operational approaches to quantum non-Markovianity depends on the statistical properties of different sets of outcomes related to successive measurement processes performed over the system of interest. Using projector techniques we develop a perturbation theory that enable to expressing both joint probabilities and outcome correlations in terms of the unperturbed system density-matrix propagator. This object defines the open system dynamics in the absence of measurement processes. Successive series terms, which are scaled by the system-environment interaction strength, consist in a convolution structure involving system propagators weighted by higher-order bath correlations. The formalism is corroborated by studying different dynamics that admit an exact description. Using the perturbative approach, unusual memory effects induced by the interplay between the system-environment interaction and measurement processes are found in finite temperature reservoirs.

The uncertainty relation reveals the intrinsic difference that distinguishes the quantum world from the classical world. In this paper, the quantum uncertainty relations of the sandwiched Rényi relative entropy of coherence (SRREOC) and unified $(\alpha ,\beta )$-relative entropy of coherence (UREOC) are studied. First, the uncertainty relation of the SRREOC for pure states, with extension to mixed states by convex-roof construction, is given in a $d$-dimensional quantum system. Second, the analytical formula of the UREOC is presented; it is shown that the UREOC is a good candidate for a quantum coherence measure. Furthermore, the quantum uncertainty relation of the UREOC for pure states is obtained in a $d$-dimensional quantum system, then it is generalized to mixed states. In addition, our results cover the quantum uncertainty relations of the other three coherence measures. Third, two methods are given to compare the lower bounds of the SRREOC and UREOC for single-qubit pure states. By comparison, we find that the results in this paper not only can be applied to single-qubit states but also can be extended to a $d$-dimensional quantum system.

Non-Hermiticity-induced quantum control is an unique phenomenon, which cannot be observed in Hermitian quantum systems. Here we study the quantum control of localization in non-Hermitian QW systems. When a phase defect is introduced to a specific site of non-Hermitian QW, the walker will not only be localized at the site of phase defect but also at its neighboring sites. Moreover, we find that as the system transforms from the $\mathcal{PT}$-symmetry-unbroken regime to $\mathcal{PT}$-symmetry-broken regime, most of probability distributions can be found at the positions far away from the phase defect in a long period of time. Our discoveries about the control of walker in non-Hermitian QW can provide valuable references for the study of quantum search in dissipative quantum systems.

We investigate a non-Hermitian extension of a Kitaev chain by considering imaginary $p$-wave pairing amplitudes. The exact solution shows that the phase diagram consists of two phases with real and complex Bogoliubov–de Gennes spectra, associated with the $\mathcal{PT}$-symmetry breaking, which is separated by a hyperbolic exceptional line. The exceptional points (EPs) correspond to a specific Cooper pair state $(1+c_k^{†}{c}_{-k}^{†})\left|0\right.〉$ with movable $k$ when the parameters vary along the exceptional line. The non-Hermiticity around EP supports resonant generation of such a pair state from the vacuum state $\left|0\right.〉$ of fermions via the critical dynamic process. In addition, we propose a scheme to generate a superconducting state through a dynamic method.

We introduce a quasiprobability phase space distribution with two pairs of azimuthal-angular coordinates. This representation is well adapted to describe quantum systems with discrete symmetry. Quantum error correction of states encoded in continuous variables using translationally invariant states is studied as an example of application. We also propose an experimental scheme for measuring such distribution.

Absolutely maximally entangled (AME) states are maximally entangled for every bipartition of the system. They are crucial resources for various quantum information protocols. We present techniques for verifying that two AME states are equivalent concerning stochastic local operations and classical communication (SLOCC). The conjecture that for a given multipartite quantum system all AME states are SLOCC equivalent is proven false. We also show that the existence of AME states with minimal support of six or more particles results in the existence of infinitely many such non-SLOCC-equivalent states. Moreover, we present AME states which are not SLOCC equivalent to the existing AME states with minimal support.

Weak coin flipping is among the fundamental cryptographic primitives which ensure the security of modern communication networks. It allows two mistrustful parties to remotely agree on a random bit when they favor opposite outcomes. Unlike other two-party computations, one can achieve information-theoretic security using quantum mechanics only: both parties are prevented from biasing the flip with probability higher than $1/2+\epsilon$, where $\epsilon$ is arbitrarily low. Classically, the dishonest party can always cheat with probability 1 unless computational assumptions are used. Despite its importance, no physical implementation has been proposed for quantum weak coin flipping. Here, we present a practical protocol that requires a single photon and linear optics only. We show that it is fair and balanced even when threshold single-photon detectors are used, and reaches a bias as low as $\epsilon =1/\sqrt{2}-1/2\approx 0.207$. We further show that the protocol may display a quantum advantage over a few-hundred meters with state-of-the-art technology.

Quantum techniques, developed in recent decades, provide new approaches to achieving high-precision measurements beyond the classical bounds. In this paper, we theoretically demonstrate a metrology method for improving the sensitivity of the interferometric optical gyroscope, robust against the loss, by using coherent-light stimulated two-mode squeezed beams as the light source. The detection protocol is based on a simple intensity measurement, and the quantum noise is far below the shot-noise limit. The enhancement factors for different coherent light fields are analyzed in detail. Additionally, the influence of loss during the propagation in the optical path is studied, and the conditions for achieving sub-shot-noise measurement sensitivity are obtained. We also find that the phase sensitivity of the proposed gyroscope scheme becomes closer to the quantum Cramér-Rao bound with increasing of the photon number of the coherent beams.

Quadrature correlations between subsystems of a Gaussian quantum state are fully characterized by its covariance matrix. For example, the covariance matrix determines the amount of entanglement or decoherence of the state. Here, we establish when it is possible to remove correlations between conjugate quadratures using only passive operations. Such correlations are usually undesired and arise due to experimental cross-quadrature contamination. Using the Autonne-Takagi factorization, we present necessary and sufficient conditions to determine when such removal is possible. Our proof is constructive, and whenever it is possible we obtain an explicit expression for the required passive operation.

We have applied a tomographic imaging method to recreate the full three-dimensional distribution of photoelectrons produced in a strong-field photodetachment process. The method is general and can be applied to any laser polarization. This stands in contrast to traditional imaging inversion methods, such as Abel inversion, which require prior knowledge of the symmetries of the electron distribution and are therefore limited to experiments using linear or circular polarization. Our method is also useful in a situation where linear polarization is used, since it compensates for detector inhomogeneities by spreading the information on a larger detector surface. In addition, it facilitates a method to detect polarization defects. Measurements were made for photodetachment of ${\mathrm{Ag}}^{-}$ at laser wavelengths of 1310 and 2055 nm, and were found to agree well with simulations in the strong field approximation. The data in the 1310 nm case revealed an unexpected asymmetry in the plane in which the laser polarization axis is rotated. Using a quasistationary quasienergy state model, a residual elliptical polarization of $\varepsilon =0.21\pm 0.01$ consistently explains the observed asymmetry. We conclude that the method described in this paper has the potential to be applied in experiments where a more complete characterization of electron emission distributions is required.

We use exact quantum Monte Carlo techniques to study the properties of quantum droplets in two-component bosonic mixtures with contact interactions in one spatial dimension. We systematically study the surface tension, the density profile and the breathing mode as a function of the number of particles in the droplet and of the ratio of coupling strengths between intraspecies repulsion and interspecies attraction. We find that deviations from the predictions of the generalized Gross-Pitaevskii equation are small in most cases of interest.

We investigate the dynamics of a three-dimensional Bose-Einstein condensate of ultracold atomic gases with a soft-core-shaped long-range interaction, which is induced by laser dressing the atoms to a highly excited Rydberg state. For a homogeneous condensate, the long-range interaction drastically alters the dispersion relation of the excitation, supporting both roton and maxon modes. Rotons are typically responsible for the creation of supersolids, while maxons are normally dynamically unstable in BECs with dipolar interactions. We show that maxon modes in the Rydberg-dressed condensate, on the contrary, are dynamically stable. We find that the maxon modes can be excited through an interaction quench, i.e., turning on the soft-core interaction instantaneously. The emergence of the maxon modes is accompanied by oscillations at high frequencies in the quantum depletion, while rotons lead to much slower oscillations. The dynamically stable excitation of the roton and maxon modes leads to persistent oscillations in the quantum depletion. Through a self-consistent Bogoliubov approach, we identify the dependence of the maxon mode on the soft-core interaction. Our study shows that maxon and roton modes can be excited dynamically and simultaneously by quenching Rydberg-dressed long-range interactions. This is relevant to current studies in creating and probing exotic states of matter with ultracold atomic gases.

We design dipolar quantum many-body Hamiltonians that will facilitate the realization of exotic quantum phases under current experimental conditions achieved for polar molecules. The main idea is to modulate both single-body potential barriers and two-body dipolar interactions on a spatial scale of tens of nanometers to strongly enhance energy scales and, therefore, relax temperature requirements for observing new quantum phases of engineered many-body systems. We consider and compare two approaches. In the first, nanoscale barriers are generated with standing-wave optical light fields exploiting optical nonlinearities. In the second, static electric-field gradients in combination with microwave dressing are used to write nanostructured spatial patterns on the induced electric dipole moments, and thus dipolar interactions. We study the formation of interlayer and interface bound states of molecules in these configurations, and provide detailed estimates for binding energies and expected losses for present experimental setups.