Along its lowest rotational band, the antiprotonic helium atom undergoes several changes of structure. It evolves from an hydrogenlike atom with broader and broader resonant states, to a quasistable molecularlike structure with narrow resonances, and then to a quasistable Rydberg pseudoatom. The antiprotonic helium atom is studied with high accuracy as a nonrelativistic three-body Coulomb system in the framework of the Lagrange-mesh method. Its metastable states are first determined from $L=0$ to $L=80$ by searching conditions of calculation for which energies are stationary. Mean values of distances between the particles are then easily deduced. They show the evolution of the structure of the system. Broad resonances are also analyzed with the complex scaling method. Physical interpretations are provided.

We study one-dimensional (1D) models of diatomic molecules where both the electrons and nuclei are treated as quantum particles, going beyond the usual Born-Oppenheimer approximation. The continuous system is approximated by a grid which computationally resembles a ladder, with the electrons living on one leg and the nuclei on the other. To simulate density-matrix renormalization group efficiently with this system, a three-site algorithm has been implemented. We also use a compression method to treat the long-range interactions between charged particles. We find that 1D diatomic molecules with spin-1/2 nuclei in the spin triplet state will unbind when the mass of the nuclei reduces to only a few times larger than the electron mass, while the molecule with nuclei in the singlet state always binds.

Many atoms and negative ions are able to bind a positron. Their structure consists of an atomic core with an orbiting positron or positronium (Ps). A few systems can also bind two positrons, but much less is known about their internal structure. We examine the family $p{e}_n^{-}{e}_m^{+}$ with a proton $p$, $n\le 4$ electrons, and $m\le 2$ positrons. Using quantum Monte Carlo techniques, we study the energetics and structure of $p{e}_4^{-}{e}_2^{+}$. We compute its ground-state energy [−1.05910(1) hartree] and its binding energy (0.0079 hartree). We show that to construct a bound variational wave function, despite having a single nucleus, we must adopt a molecular description of this system, using HPs, $\mathrm{P}{\mathrm{s}}^{\text{–}}$, ${\mathrm{H}}^{\text{–}}$, and $\mathrm{P}{\mathrm{s}}_2$ as interacting fragments. The analysis of the electronic and positronic probability distributions reveals that at zero order, the internal structure of this system consists of a $\mathrm{P}{\mathrm{s}}^{\text{–}}$ interacting with the HPs core. Based on these results, we speculate about the existence of similar systems containing three positrons.

Pyragas control allows us to stabilize unstable states in applied nonlinear science. We propose to apply a quantum version of the Pyragas protocol to control individual photon probabilities in an otherwise only globally accessible photon-probability distribution of a quantum light emitter. The versatility of quantum Pyragas control is demonstrated for the case of a two-level emitter in a pulsed laser-driven half cavity. We show that one- and two-photon events respond in a qualitatively different way to the half-cavity-induced feedback signal. One-photon events are either enhanced or suppressed, depending on the choice of parameters. In contrast, two-photon events undergo exclusively an enhancement up to $50\%$ for the chosen pulse areas. We hereby propose an implementation of quantum Pyragas control via a time-delayed feedback setup.

Motivated by a recent experiment [Fuchs et al., Nat. Phys. 11, 964 (2015)], we theoretically investigate the process of x-ray nonlinear Compton scattering (XNLC). Our approach is based on the time-dependent Schrödinger equation for an atomic system subject to an intense x-ray pulse, and explicitly accounts for the spontaneous scattering into a quantized photonic mode. We employ our framework to study multiple nonlinear scattering scenarios. Initially, we consider soft x rays at $500\text{eV}$ photon energy to scatter nonlinearly off a helium target. For this, we find that XNLC is dominated by certain third-order processes rather than the naïvely expected mechanisms pertaining to the lowest order of perturbation theory. Subsequently, we apply our model to XNLC in helium at $4.0\text{keV}$ photon energy and beryllium at $9.7\text{keV}$. Contrary to the conclusions drawn from the experimental observations, our results suggest a good agreement of the XNLC spectrum with simple, free-electron model predictions. Moreover, our studies reveal striking qualitative similarities of linear and nonlinear Compton scattering cross sections in this regime.

We discuss two different approaches for splitting the wave function of a single-particle box (SPB) into two equal parts. Adiabatic insertion of a barrier in the center of a SPB in order to make two compartments which each have probability 1/2 of finding the particle in it is one of the key steps for a Szilard engine. However, any asymmetry between the volume of the compartments due to an off-center insertion of the barrier results in a particle that is fully localized in the larger compartment, in the adiabatic limit. We show that rather than exactly splitting the eigenfunctions in half by a symmetric barrier, one can use a nonadiabatic insertion of an asymmetric barrier to induce excitations to only the first excited state of the full box. As the barrier strength goes to infinity the excited state of the full box becomes the ground state of one of the new boxes. Thus, we can achieve close to exact splitting of the probability between the two compartments using the more realistic nonadiabatic, not perfectly centered barrier, rather than the idealized adiabatic and central barrier normally assumed.

We introduce and discuss the concept of chiral relativistic qubit as an irreducible amount of quantum information related to a one-half spin relativistic chiral elementary system (carrier particle). We propose a Lorentz-covariant time evolution of the qubit which on the level of the density matrix is unitary. Next we investigate behavior of the Bloch vector as a function of time during the relativistic uniformly accelerated motion of the carrier particle. In particular, we select the same special evolutions which correspond to the hyperbolic, rotational, and structurally unstable motion. Finally, we consider two-qubit systems. We extend the proposed Lorentz-covariant and unitary evolution on this case in a way preserving tensor product structure of the two-particle space of states. We also discuss a correlation function in an Einstein-Podolsky-Rosen type experiment with uniformly accelerating particles; as an example we calculate correlations in the evolving Bell state.

The time evolution of an initially uncorrelated system is governed by a completely positive (CP) map. More generally, the system may contain initial (quantum) correlations with an environment, in which case the system evolves according to a not-completely positive (NCP) map. It is an interesting question what the relative measure is for these two types of maps within the set of positive maps. After indicating the scope of the full problem of computing the true volume for generic maps acting on a qubit, we study the case of Pauli channels in an abstract space whose elements represent an equivalence class of maps that are identical up to a non-Pauli unitary. In this space, we show that the volume of NCP maps is twice that of CP maps.

We propose a long-distance continuous-variable measurement-device-independent quantum key distribution (CV-MDI-QKD) protocol with discrete modulation. This kind of discrete-modulated scheme has good compatibility with efficient error correction code, which leads to higher reconciliation efficiency even at low signal-to-noise ratio. Security analysis shows that the proposed protocol is secure against arbitrary collective attacks in the asymptotic limit with the proper use of decoy states. With the use of discrete modulation, the proposed CV-MDI-QKD protocol has simpler implementation and outperforms previous protocols in terms of achievable maximal transmission distance, which precisely solved the bottleneck of the original Gaussian-modulated CV-MDI-QKD protocol.

We show that within the Krieger-Li-Iafrate and common-energy-denominator approximations, the linearized time-dependent Kohn-Sham equations for orbital functionals can be solved very efficiently using the frequency-dependent Sternheimer scheme. The Kohn-Sham response can be obtained without the need to explicitly evaluate the exchange-correlation kernel as a functional derivative with respect to the density. Instead, it suffices to compute functional derivatives with respect to the orbitals. The scheme allows for the computationally efficient use of orbital functional potential approximations in Kohn-Sham response theory.

We investigate electron correlation effects in internuclear-distance-dependent enhanced ionization of ${\mathrm{H}}_2$, LiH, and HF molecules by intense near-infrared laser pulses using a three-dimensional description of the systems with the time-dependent generalized-active-space configuration-interaction method. This method systematically incorporates electron-electron correlation of the quantum many-electron system under consideration. Our correlated description of diatomic molecules shows that enhanced ionization occurs at certain critical internuclear separations and electron correlation systematically improves the ionization probability in this process until convergence is reached. We demonstrate the failure of the single-active-electron and the configuration-interaction singles approximations to produce the correct internuclear position and probability of the strong-field enhanced-ionization process. We elucidate the role of low-lying electronic excited states in the enhanced-ionization process of diatomic molecules. There is clear evidence that an accurate description of low-lying electronically excited states is important to describe the nonperturbative enhanced-ionization phenomenon in the ultrashort intense near infrared laser pulses.

We analyze the time evolution of the Bose-Hubbard model after a sudden quantum quench to a weakly interacting regime. Specifically, motivated by a recent experiment at Kyoto University, we numerically simulate redistribution of the kinetic and on-site interaction energies at an early time, which was observed in nonequilibrium dynamics of ultracold Bose gases in a cubic optical lattice starting with a singly occupied Mott-insulator state. In order to compute the short-time dynamics corresponding to the experimental situation, we apply the truncated-Wigner approximation to the Bose-Hubbard model on a cubic lattice. We show that our semiclassical approach quantitatively reproduces the fast redistribution dynamics. We further analyze spatial spreading of density-density correlations at equal time in the Bose-Hubbard model on a square lattice with a large filling factor. When the system is initially prepared in a coherent state, we find that a propagation velocity of the correlation wave packet in the correlation function strongly depends on the final interaction strength, and it is bounded by twice the maximum group velocity of the elementary excitations. In contrast, when the system is initially in a Mott-insulator state, the propagation velocity of the wave packet is approximately independent of the final interaction strength.

We theoretically propose a method for on-demand generation of traveling Schrödinger cat states, namely, quantum superpositions of distinct coherent states of traveling fields. This method is based on deterministic generation of intracavity cat states using a Kerr-nonlinear parametric oscillator (KPO) via quantum adiabatic evolution. We show that the cat states generated inside a KPO can be released into an output mode by dynamically controlling the parametric pump amplitude. We further show that the quality of the traveling cat states can be improved by using a shortcut-to-adiabaticity technique.

The evolution of a quantum particle propagating in a nonlinear medium and under the action of electric and magnetic fields is studied. For this purpose, the integrable unification of the Dirac and Manakov equations is derived. The integrability conditions of the system and the corresponding representation of the zero curvature of the system are found. We have applied an approach based on the Riemann-Hilbert problem to derive soliton solutions of the unified model. The study of the solution shows that the action of the electric and magnetic fields leads to the Zitterbewegung in the nonlinear interaction mode. It was also found that the collision of two solitons in the presence of an electric field can lead to the rectification of the particle trajectory and the disappearance of its Zitterbewegung.

A recent paper [Yu and Prasad Phys. Rev. Lett. 121, 180504 (2018)] considered the problem of quantum-limited estimation of the separation vector for a pair of incoherent point sources in all three dimensions. Here we extend our analysis to treat the problem of simultaneously estimating the location of the centroid and the separation of the source pair, which is equivalent to localizing both sources simultaneously. We first calculate the quantum Fisher information for simultaneous pair centroid-separation estimation and then discuss the fundamental, estimation-theoretic tradeoffs between the two tasks, which we confirm using simulations.

Fluctuation theorems are relations constraining the out-of-equilibrium fluctuations of thermodynamic quantities like the entropy production that were initially introduced for classical or quantum systems in contact with a thermal bath. Here we show, in the absence of thermal bath, the dynamics of continuously measured quantum systems can also be described by a fluctuation theorem, expressed in terms of a recently introduced arrow of time measure. This theorem captures the emergence of irreversible behavior from microscopic reversibility in continuous quantum measurements. From this relation, we demonstrate that measurement-induced wave-function collapse exhibits absolute irreversibility, such that Jarzynski-like equalities are violated, and that this property is inherent to quantum information acquisition. We apply our results to different continuous measurement schemes on a qubit: dispersive measurement, homodyne, and heterodyne detection of qubit's fluorescence.

The Redfield and Lindblad forms of the master equations are obtained under not only the Born and Markov approximations but also the secular approximation. In this paper, we propose a computable form of the master equation for open multilevel quantum systems beyond the secular approximation. The proposed form can retain some dynamical effects that are lost due to use of the secular approximation. Two kinds of open multilevel quantum systems are investigated by using this method, and the results are compared to the ones obtained from the Redfield equation and an exactly numerical path integral approach. It is shown that the results obtained from this computable form match those from the exact numerical path integral approach better than results from the Redfield equation. The normal-diffusion coefficient, momentum-damping coefficient, anomalous-diffusion coefficient, etc. in the master equation are also reevaluated.

In the tests for macrorealism proposed by Leggett and Garg, the temporal correlation functions of a dichotomic variable $Q$ must be measured in a noninvasive way to rule out alternative classical explanations of Leggett-Garg inequality violations. Ideal negative measurements in which a null result is argued to be a noninvasive determination of the system's state are often used. From a quantum-mechanical perspective, such a measurement collapses the wave function and will therefore typically be found to be invasive under any experimental check. Here, a simple modified ideal negative measurement protocol is described for measuring the correlation functions, which is argued to be noninvasive from both classical and quantum perspectives and hence the noninvasiveness can then be checked experimentally, thereby permitting a quantitative measure of the degree of clumsiness of the measurement. It is also shown how this procedure may be extended to measure higher-order correlation functions, and a number of higher-order conditions for macrorealism are derived.