The quantum open-system simulation is an important category of quantum simulation. By simulating the thermalization process at zero temperature, we can solve the ground-state problem of quantum systems. To realize the open-system evolution on the quantum computer, we need to encode the environment using qubits. However, usually the environment is much larger than the system, i.e., numerous qubits are required if the environment is directly encoded. In this paper, we propose a way to simulate open-system dynamics by reproducing reservoir correlation functions using a minimized Hilbert space. In this way, we only need a small number of qubits to represent the environment. To simulate the $n\mathrm{th}-\mathrm{order}$ expansion of the time-convolutionless master equation by reproducing up to $n$-time correlation functions, the number of qubits representing the environment is $\sim \lfloor \frac{n}{2}\rfloor {log}_2\left(N_{\omega }{N}_{\beta }\right)$. Here, $N_{\omega }$ is the number of frequencies in the discretized environment spectrum, and $N_{\beta }$ is the number of terms in the system-environment interaction. By reproducing two-time correlation functions, i.e., taking $n=2$, we can simulate the Markovian quantum master equation. In our algorithm, the environment on the quantum computer could be even smaller than the system.

Entanglement can be considered as a special quantum correlation, but not the only kind. Even for a separable quantum system, nonclassical correlations are allowed to exist. Here we propose two dissipative schemes for generating a maximally correlated state of two qubits in the absence of quantum entanglement, which was proposed by Galve et al. [F. Galve, G. L. Giorgi, and R. Zambrini, Phys. Rev. A 83, 012102 (2011)]. These protocols take full advantage of the interaction between four-level atoms and strongly lossy optical cavities. In the first scenario, we alternatively change the phases of two classical driving fields, while the second proposal introduces a strongly lossy coupled-cavity system. Both schemes can realize all Lindblad terms required by the dissipative dynamics, guaranteeing the maximally quantum dissonant state to be the unique steady state for a certain subspace of the system. Moreover, since the target state is a mixed state, the performance of our method is evaluated by the definition of superfidelity $G({\rho }_1,{\rho }_2)$, and the strictly numerical simulations indicate that fidelity outstripping $99\%$ of the quantum dissonant state is achievable with the current cavity quantum electrodynamics parameters.

We propose a scheme for the construction of the one-particle Green's function (GF) of an interacting electronic system via statistical sampling on a quantum computer. Although the nonunitarity of creation and annihilation operators for the electronic spin orbitals prevents us from preparing specific states selectively, probabilistic state preparation is demonstrated to be possible for the qubits. We provide quantum circuits equipped with at most two ancillary qubits for obtaining all the components of the GF. We perform simulations of such construction of GFs for LiH and ${\mathrm{H}}_2\mathrm{O}$ molecules based on the unitary coupled-cluster (UCC) method to demonstrate the validity of our scheme by comparing the quasiparticle and satellite spectra exact within the UCC method and those from full configuration-interaction calculations. We also examine the accuracy of the sampling method by exploiting the Galitskii-Migdal formula, which gives the total energy only from the GF.

Experimental validation and control of quantum traits for an open quantum system are important for any quantum information purpose. We consider a traveling atom qubit as a quantum memory with adjustable velocity inside a leaky cavity, adopting a quantum witness as a figure of merit for quantumness assessment. We show that this model constitutes an inherent physical instance where the quantum witness does not work properly if not suitably optimized. We then supply the optimal intermediate blind measurements which make the quantum witness a faithful tester of quantum coherence. We thus find that larger velocities protect quantumness against noise, leading to a lifetime extension of hybrid qubit-photon entanglement and to higher phase estimation precision. Control of qubit motion thus reveals itself as a quantum enhancer.

This paper presents rotational-state-selective field-ionization spectra of highly excited triplet $nd{\text{H}}_2$ Rydberg states. A fast $6\mathrm{keV}$ beam of metastable $c{}^3{\mathrm{\Pi }}_u^{-}2p\pi$ molecules is excited to $v=0, N^{+}=1$ Rydberg states ($n=17$ to 27) by a frequency-doubled tunable dye laser and ionized by static electric fields. The resulting measured total ionization yields are analyzed using a model that considers a diabatic ionization pathway through the Stark map. A field of up to $\sim 4\mathrm{kV}{\mathrm{cm}}^{-1}$ mixes Stark manifolds, while a field of up to $\sim 36\mathrm{kV}{\mathrm{cm}}^{-1}$ ionizes the resulting mixture. Rotational-state population transfer of up to 50% is observed, populating neighboring $N^{+}=3$ manifolds ($n=12$ to $n=16$). Control over the rotational quantum state composition of the field-ionized molecules is demonstrated.

We propose a technique for the accurate, flexible, and robust generation of arbitrary preselected coherent superpositions of two quantum states. It uses a sequence of two adiabatic pulses split by a phase jump serving as a control parameter. Each pulse has a chirped detuning, which induces a half crossing, and acts approximately as a half-$\pi$ pulse in the adiabatic regime. The phase jump is imprinted onto the population ratio of the created superposition state. Of the various possible relations between the two pulses, we select the case when the Rabi frequency and the detuning of the second pulse are mirror images of those of the first pulse, and the two detunings have opposite signs. Then the mixing angle of the created superposition state depends on the phase jump only. For other arrangements, the superposition mixing angle is shifted by the dynamic phases of the propagators, which makes these cases suitable for state tomography. This sandwich setup comes along with the advantage that it reduces the error $\epsilon$ of each individual pulse down to $4{\epsilon }^2$ overall. Therefore, the proposed technique combines the benefits of robustness stemming from adiabatic evolution with accuracy generated by the twin-pulse error suppression, and flexibility of the created superposition state controlled by the value of the phase jump $\varphi$. In addition to the general analysis, we present a simple exactly soluble trigonometric model in order to illustrate the proposed technique. In this model, when the pulse area $A$ increases, the nonadiabatic oscillations are damped as $A^{-1}$ for a single pulse and $A^{-2}$ for the two-pulse sequence. Finally, the proposed technique is iteratively extended to sequences of $N=2^n$ pulses by concatenating half-$\pi$ sequences and splitting them by a phase jump, thereby further reducing the nonadiabatic error $\epsilon$ to ${\left(2\epsilon \right)}^N$. This makes the proposed technique suitable for generating high-fidelity quantum rotation gates even when starting with errant pulses.

This article presents a computational study of the x-ray fluorescence induced by the irradiation of thin layered media by intense, short x-ray pulses. The treatment is based on a numerical solution of the Helmholtz wave equation both for the pump and for the fluorescence signal. Consistently with a possible heating of the medium during the x-ray pulse, complex refractive indices are calculated at each time step from the results of an underlying treatment of atomic physics. In the context of an important core-hole production as a result of photoionization, we discuss the peculiarities of the resulting amplified fluorescence grazing emission and of the Bragg diffraction which can be realized at some angles inside a multilayer material or even in a perfect crystal.

Highly stable laser sources based on narrow atomic transitions provide a promising platform for direct generation of stable and accurate optical frequencies. Here we investigate a simple system operating in the high-temperature regime of cold atoms. The interaction between a thermal ensemble of ${}^{88}\mathrm{Sr}$ at mK temperatures and a medium-finesse cavity produces strong collective coupling and facilitates high atomic coherence, which causes lasing on the dipole forbidden ${}^{1}S_0\leftrightarrow {}^{3}P_1$ transition. We experimentally and theoretically characterize the lasing threshold and evolution of such a system and investigate decoherence effects in an unconfined ensemble. We model the system using a Tavis-Cummings model and characterize the velocity-dependent dynamics of the atoms as well as the dependency on the cavity detuning.

Here we present the results of a classical ghost imaging experiment accomplished at an XUV free-electron laser (FEL). To perform such experiment at an FEL source each x-ray pulse was transmitted through a moving diffuser, which created a noncorrelated speckled beam. This beam was then split in two identical branches by introducing a beam splitter in the form of a transmission grating. In one of these branches the sample was positioned. We demonstrate the possibility of image formation, a double bar in our case, in the beam that has never interacted with the sample. With this experiment we extend the quantum optics methodology to the FEL community.

A quantum system interacting with a dilute gas experiences irreversible dynamics. The corresponding master equation can be derived within two different approaches: The fully quantum description in the low-density limit and the semiclassical collision model, where the motion of gas particles is classical whereas their internal degrees of freedom are quantum. The two approaches have been extensively studied in the literature, but their predictions have not been compared. This is mainly due to the fact that the low-density limit was extensively studied for mathematical physics purposes, whereas the collision models have been essentially developed for quantum information tasks such as a tractable description of the open quantum dynamics. Here we develop and compare both approaches for a spin system interacting with a gas of spin particles. Using some approximations, we explicitly find the corresponding master equations including the Lamb shifts and the dissipators. The low-density limit in the Born approximation for fast particles is shown to be equivalent to the semiclassical collision model in the stroboscopic approximation. We reveal that both approaches give exactly the same master equation if the gas temperature is high enough. This allows to interchangeably use complicated calculations in the low-density limit and rather simple calculations in the collision model.

Thermodynamics is one of the fascinating branches of traditional physics which certifies the occurrence of many natural processes. On the other hand, quantum theory is the most acceptable description of the microscopic world. In the present paper, we have studied how the structure of quantum theory prohibits cloning or masking of several thermodynamic quantities, viz., work and energy stored in a quantum state. Our results have important consequences in quantum partial cloning, quantum masking, and on the action of quantum channels.

In this work we investigate the probability of violation of local realism under random measurements in parallel with the strength of these violations as described by resistance to white noise admixture. We address multisetting Bell scenarios involving up to seven qubits. As a result, in the first part of this manuscript, we report statistical distributions of a quantity reciprocal to the critical visibility for various multipartite quantum states subjected to random measurements. The statistical relevance of different classes of multipartite tight Bell inequalities violated with random measurements is investigated. We also introduce the concept of typicality of quantum correlations for pure states as the probability to generate a nonlocal behavior with both random state and measurement. Although this typicality is slightly above 5.3% for two qubits, for a modest increase in the number of involved particles it quickly surpasses 99.99%.

To quantitatively analyze the control power of arbitrary $d$-dimensional controlled teleportation (CT), this paper investigates channels of entangled high-dimensional three-qudit standard Greenberger-Horne-Zeilinger (GHZ) and GHZ-type states. The upper bound on the controller's power is established in the $d$-dimensional CT scheme. We identify a trade-off between the control power and the capacity of the quantum channels. The control power can be enhanced by reducing the dimension of the teleported information in a suitably high-dimensional quantum channel, or by increasing the dimension of the qudit held by the controller. Under controlled teleporting of a single qubit in a perfect CT scheme, the control power is maximized in the three-dimensional GHZ-type state channel. This maximum control power exceeds the highest predicted value in previous works and the upper bounds of control power in a two-dimensional quantum channel. Our studies provide a feasible method for achieving higher control power, and it may provide approaches to define high-dimensional entanglement of multibody systems.

We introduce two independent quantifications for three-mode and four-mode entanglement. We investigate the conversion of one type of nonclassicality, i.e., single-mode nonclassicality, into another type of nonclassicality, i.e., multimode entanglement, in beam splitters. We observe parallel behavior of the two quantifications. The methods can be generalized to the quantification of any multimode entanglement.

Overcoming repeaterless bounds for the secret key rate capacity of quantum key distribution protocols is still a challenge with current technology. Luong et al. [Appl. Phys. B 122, 96 (2016).] proposed a protocol to beat a repeaterless bound using one pair of quantum memories. However, the required experimental parameters for the memories are quite demanding. We extend the protocol with multiple pairs of memories, operated in a parallel manner to relax these conditions. We quantify the amount of relaxation in terms of the most crucial memory parameters, given the number of applied memory pairs. In the case of high-loss channels, we found that adding only a few pairs of memories can make the crossover possible.

We analyze the molecular electric dipole moments (PDMs) and static electric dipole polarizabilities of heteronuclear alkali dimers in their ground states by employing coupled-cluster theory, both in the nonrelativistic and four-component relativistic frameworks. The roles of electron correlations as well as relativistic effects are demonstrated by studying them at different levels of theory, followed by a comprehensive treatment of error estimates. We compare our obtained values with the previous nonrelativistic calculations, some of which include lower-order relativistic corrections, as well as with the experimental values, wherever available. We find that the PDMs are sensitive to relativistic effects as compared to polarizabilities. We show that consideration of relativistic values of PDMs improves significantly the isotropic van der Waals $C_6$ coefficients of the investigated alkali dimers over the previously reported nonrelativistic calculations. The dependence of dipole polarizabilites on molecular volume is also illustrated.

We present a simple method for determination of absolute vibrational numbering of isolated near dissociation levels in diatomic molecules, usually observed in cold-physics experiments. The method is based on the isotope shift and works even when energies of only two levels from one isotopologue and one level from another isotopologue have been measured. It is demonstrated with data from recently reported precise measurements of binding energies of levels lying close to the dissociation limits in ultracold ${\mathrm{Yb}}_2$, CsYb, RbSr, and RbYb molecules. Its predictions agree with those of much more elaborate multi-isotope potential curve fitting.

Atomic clocks based on an ${\mathrm{Al}}^{+}$ ion sympathetically cooled by a laser-cooled alkaline-earth-metal ion have achieved unprecedented accuracy. Here, we investigate theoretically interactions and charge-transfer dynamics of an ${\mathrm{Al}}^{+}$ ion immersed in an ultracold gas of Rb and Sr atoms. We calculate potential energy curves and transition electric dipole moments for the ${(\mathrm{Al}+\mathrm{Rb})}^{+}$ and ${(\mathrm{Al}+\mathrm{Sr})}^{+}$ ion-atom systems using coupled cluster and multireference configuration interaction methods with scalar relativistic effects included within the small-core energy-consistent pseudopotentials in Rb and Sr atoms. The long-range interaction coefficients are also reported. We use the electronic structure data to investigate cold collisions and charge-transfer dynamics. Scattering of an ${\mathrm{Al}}^{+}$ ion with alkali-metal or alkaline-earth-metal atom is governed by one potential energy curve whereas charge transfer can lead to several electronic states mixed by the relativistic spin-orbit coupling. We examine the branching ratios resulting from the interplay of the short- and long-range effects, as well as the prospects for the laser-field control and formation of molecular ions. We propose to employ the atomic clock transition in an ${\mathrm{Al}}^{+}$ ion to monitor ion-atom scattering dynamics via quantum logic spectroscopy. The presented results pave the way for the application of atomic ions other than alkali-metal and alkaline-earth-metal ones in the field of cold hybrid ion-atom experiments.

We present a theoretical study of rotationally elastic and inelastic collisions between molecular nitrogen ions and Rb atoms in the sub-Kelvin temperature regime prevalent in ion-atom hybrid trapping experiments. The cross sections for rotational excitation and de-excitation collisions were calculated using quantum-scattering methods on ab initio potential energy surfaces for the energetically lowest singlet electronic channel of the system. We find that the rotationally inelastic collision rates are at least an order of magnitude smaller than the charge-exchange rates found in this system, rendering inelastic processes a minor channel under the conditions of typical hybrid trapping experiments.

We study spin-dipole oscillations of a binary fermionic mixture at nonzero temperatures. We apply the atomic-orbital method combined with a Monte Carlo technique based sampling to probe finite temperatures. Our results agree quantitatively with a recent experiment [G. Valtolina et al., Nat. Phys. 13, 704 (2017)] showing the appearance of the ferromagnetic phase at stronger repulsion between components when the temperature is increased.