We present solutions for the Dirac equation in the form of Bessel or Laguerre-Gaussian beams for, respectively, a free electron or an electron subject to a longitudinal constant magnetic field. We calculate the probability and the current distributions and find a strong dependence on the spin angular momentum and the orbital angular momentum of the relativistic electron beam. The role of the intrinsic spin-orbital coupling is investigated.

We investigate the imaging of electron wave-packet dynamics by measuring the pattern of inelastically scattered photons. For this purpose, we develop a theory of time-resolved Compton scattering. We demonstrate that, by using a sufficiently short x-ray pulse, the scattering cross section directly reflects the instantaneous momentum density of the electron. Therefore, we propose time-resolved Compton scattering as a tool to image electron wave-packet dynamics in momentum space. To illustrate this, we simulate electron wave packets in argon by using the time-dependent configuration-interaction singles method. Specifically, we consider coherent particle-hole wave packets, where the hole is in either the $3p$ or the $3s$ shell, and the particle (the excited electron) is either in a Rydberg state or in the continuum. Our calculations confirm that the dynamics of electron wave packets can indeed be imaged by measuring the doubly differential Compton scattering cross section. When the x-ray detector has no energy resolution at all, the contribution of Compton scattering to the differential scattering cross section becomes stationary. In that case, the motion of the particle and the hole can no longer be inferred from the scattering pattern.

By developing the preceding work on the fast forward of transient phenomena of quantum tunneling by Khujakulov and Nakamura [Phys. Rev. A 93, 022101 (2016)], we propose a scheme of the exact fast forward of adiabatic control of stationary tunneling states with use of the electromagnetic field. The idea allows the acceleration of both the amplitude and phase of wave functions throughout the fast-forward time range. The scheme realizes the fast-forward observation of the transport coefficients under the adiabatically changing barrier with the fixed energy of an incoming particle. As typical examples, we choose systems with (1) Eckart's potential with tunable asymmetry and (2) double $\delta$-function barriers under tunable relative height. We elucidate the driving electric field to guarantee the stationary tunneling state during a rapid change of the barrier and evaluate both the electric-field-induced temporary deviation of transport coefficients from their stationary values and the modulation of the phase of complex scattering coefficients.

We study the quantum Zeno effect (QZE) and quantum anti-Zeno effect (QAZE) of a two-level system interacting with an environment of harmonic oscillators, the spin-boson model. By applying a numerically exact method based on matrix product states, the previously obtained picture on the influence of counter-rotating terms has to be modified: For physical bath initial states, the transition from QZE to QAZE with increasing measurement interval is only absent at weak coupling, while present at strong coupling. Furthermore, we find that the transition occurs always for the widely used bare bath initial state. Within a more realistic measurement scheme where only the qubit is projectively measured, the above scenario for the bare bath initial state remains qualitatively unchanged, apart from accelerated decay for intermediate measurement intervals.

Nonclassical correlations arising in complex quantum networks are attracting growing interest, both from a fundamental perspective and for potential applications in information processing. In particular, in an entanglement swapping scenario a new kind of correlations arise, the so-called nonbilocal correlations that are incompatible with local realism augmented with the assumption that the sources of states used in the experiment are independent. In practice, however, bilocality tests impose strict constraints on the experimental setup and in particular to the presence of shared reference frames between the parties. Here, we experimentally address this point showing that false positive nonbilocal quantum correlations can be observed even though the sources of states are independent. To overcome this problem, we propose and demonstrate a scheme for the violation of bilocality that does not require shared reference frames and thus constitutes an important building block for future investigations of quantum correlations in complex networks.

It was recently shown that transmission-line waveguides can mediate long-range fluctuation forces between neutral objects, potentially leading to novel Casimir forces in electric circuits. Here we present two approaches for the general description of these forces between electric components embedded in transmission-line circuits. The first, following ordinary quantum electrodynamics (QED), consists of the quantization and scattering theory of voltage and current waves inside transmission lines. The second approach relies on a simple circuit analysis with additional noisy current sources due to resistors in the circuit, as per the fluctuation-dissipation theorem (FDT). We apply the latter approach to derive a general formula for the Casimir force induced by circuit fluctuations between any two impedances. The application of this formula, considering the sign of the resulting force, is discussed. While both QED and FDT approaches are equivalent, we conclude that the latter is simpler to generalize and solve.

Contact has been well established as an important quantity to govern dilute quantum systems, in which the pairwise correlation at short distance traces a broad range of thermodynamic properties. So far, most studies have focused on contact in individual angular momentum channels. Here we point out that, to have a complete description of the pairwise correlation in a general dilute quantum systems, contact should be defined as a matrix. Whereas the diagonal terms of such a matrix include contact of all partial wave scatterings, the off-diagonal terms characterize the coherence of the asymptotic pairwise wave function in the angular momentum space and determine important thermodynamic quantities including the momentum distribution. The contact matrix allows physicists to access unexplored connections between short-range correlations and macroscopic quantum phenomena. As an example, we show the direct connection between the contact matrix and order parameters of a superfluid with mixed partial waves.

We study the transport of a single photon in two coupled one-dimensional semi-infinite coupled-resonator waveguides (CRWs), in which both end sides are coupled to a dissipative cavity. We demonstrate that a single photon can transfer from one semi-infinite CRW to the other nonreciprocally. Based on such nonreciprocity, we further construct a three-port single-photon circulator by a T-shaped waveguide, in which three semi-infinite CRWs are pairwise mutually coupled to each other. The single-photon nonreciprocal transport is induced by the breaking of the time-reversal symmetry and the optimal conditions for these phenomena are obtained analytically. The CRWs with broken time-reversal symmetry will open up a kind of quantum device with versatile applications in quantum networks.

We show that the form of the cluster decomposition principle, commonly used in quantum field theory, needs to be significantly generalized. As an illustration, we consider the general structure of two-photon $S$ matrix for a waveguide coupled to a local quantum system that supports multiple ground states. The presence of the multiple ground states results in a noncommutative aspect of the system with respect to the exchange of the orders of photons. Consequently, the two-photon $S$ matrix significantly differs from the standard form in quantum field theory.

We calculate the radiative, nonradiative, and plasmonic decay rates of an individual optical emitter near a metallic nanowire with a nonlocal dielectric response. The potential inside the cylindrical wire is obtained within the random-phase approximation, which allows the nonlocal treatment of the electronic response. The induced field at the site of the emitter is also found. Calculations are carried out using a hydrodynamic dielectric function and the effects introduced by spatial dispersion through surface plasmons and reflection amplitudes are discussed. Nonlocal corrections to the emission rates become more relevant for nanowires with radii in the range of a few nanometers, at exciting frequencies close to the resonances of surface modes, and when the distance between the source dipole and the wire edge is comparable to the wire radius or lies in the subnanometer scale.

We investigate the dynamics of dicationic metal-oxide molecules under large electric-field conditions, on the basis of ab initio calculations coupled to molecular dynamics. Applied to the case of ${\mathrm{ZnO}}^{2+}$ in the field of atom probe tomography (APT), our simulation reveals the dissociation into three distinct exit channels. The proportions of these channels depend critically on the field strength and on the initial molecular orientation with respect to the field. For typical field strength used in APT experiments, an efficient dissociation channel leads to emission of neutral oxygen atoms, which escape detection. The calculated composition biases and their dependence on the field strength show remarkable consistency with recent APT experiments on ZnO crystals. Our work shows that bond breaking in strong static fields may lead to significant neutral atom production, and therefore to severe elemental composition biases in measurements.

In projective measurements of energy, a target system is projected to an eigenstate of the system Hamiltonian and the measurement outcomes provide the information of corresponding eigenenergies. Recently, it has been shown that such a measurement can be, in principle, realized without detailed knowledge of the Hamiltonian by using probe qubits. However, in this approach, the size of the dimension for the probe increases as we increase the dimension of the target system; also, individual addressability of every qubit is required, which may not be possible for many experimental settings involving large systems. Here, we show that a single probe qubit is sufficient to perform such a projective measurement of energy if the target system is composed of noninteracting qubits whose resonant frequencies are unknown. Moreover, our scheme requires only global manipulations where every qubit is subjected to the same control fields. These results indicate the feasibility of our energy-projection protocols.

One of the main goals of any resource theory such as entanglement, quantum thermodynamics, quantum coherence, and asymmetry, is to find necessary and sufficient conditions that determine whether one resource can be converted to another by the set of free operations. Here we find such conditions for a large class of quantum resource theories which we call affine resource theories. Affine resource theories include the resource theories of athermality, asymmetry, and coherence, but not entanglement. Remarkably, the necessary and sufficient conditions can be expressed as a family of inequalities between resource monotones (quantifiers) that are given in terms of the conditional min-entropy. The set of free operations is taken to be (1) the maximal set (i.e., consists of all resource nongenerating quantum channels) or (2) the self-dual set of free operations (i.e., consists of all resource nongenerating maps for which the dual map is also resource nongenerating). As an example, we apply our results to quantum thermodynamics with Gibbs preserving operations, and several other affine resource theories. Finally, we discuss the applications of these results to resource theories that are not affine and, along the way, provide the necessary and sufficient conditions that a quantum resource theory consists of a resource destroying map.

We investigate high-order above-threshold ionization of model helium in the long-wavelength regime up to 2400 nm by solving the two-electron time-dependent Schrödinger equation in one dimension. To bypass the difficulty of solving the multielectron time-dependent Schrödinger equation with the long-wavelength laser interaction, we revisit and examine two typically used theoretical methods: the single-active-electron approximation and the strong-field approximation. For the description of the high-energy rescattered electrons in the ground-state ionic channel, the single-active-electron approximation performs better with increasing ponderomotive energy. Single ionization in the excited-state ionic channels, in general, has much weaker spectral intensity than that in the ground-state ionic channel. The above-threshold-ionization cutoffs in the excited-state ionic channels are clear signatures of two-electron dynamics, which cannot be explained within the single-active-electron approximation. By applying the two-electron strong-field approximation including rescattering and a saddle-point method analysis, we explain the channel-resolved cutoffs, and relate them to elastic and inelastic rescattering processes.

We study the robustness of quantum key distribution protocols using discrete or continuous variables to the channel noise. We introduce the model of such noise based on coupling of the signal to a thermal reservoir, typical for continuous-variable quantum key distribution, to the discrete-variable case. Then we perform a comparison of the bounds on the tolerable channel noise between these two kinds of protocols using the same noise parametrization, in the case of implementation which is perfect otherwise. Obtained results show that continuous-variable protocols can exhibit similar robustness to the channel noise when the transmittance of the channel is relatively high. However, for strong loss discrete-variable protocols are superior and can overcome even the infinite-squeezing continuous-variable protocol while using limited nonclassical resources. The requirement on the probability of a single-photon production which would have to be fulfilled by a practical source of photons in order to demonstrate such superiority is feasible thanks to the recent rapid development in this field.

We propose an improved bound for the difference between phase and bit error rate in measurement-device-independent quantum key distribution with uncharacterized qubits. We show by simulations that the bound considerably increases the final key rates.

Bloch oscillations of atoms in an optical lattice are a technique commonly used to accelerate atoms using photon recoils. The initial atomic state is crucial for optimizing the efficiency of the acceleration. Usually, a Bloch state is prepared by adiabatically loading atoms into the lattice. In this paper, using numerical simulations, we show that the optimal state (the Wannier-Bloch state) can significantly differ from the Bloch state. Furthermore, two experimental methods are presented in order to get closer to the optimal efficiency.

We study spinless fermions with repulsive nearest-neighbor interactions perturbed by an impurity particle or a local potential quench. Using the numerical time-evolving block decimation method and a simplified analytic model, we show that the perturbations create a soliton-antisoliton pair. If solitons are already present in the bath, the two excitations have a drastically different dynamics: The antisoliton does not annihilate with the solitons and is therefore confined close to its origin while the soliton excitation propagates. We discuss the consequences for experiments with ultracold gases.