We explore the interplay between acceleration radiation and the dynamical Casimir effect in the field of superconducting quantum technologies, analyzing the generation of entanglement between two qubits by means of the dynamical Casimir effect in several states of qubit motion. We show that the correlated absorption and emission of photons are crucial for entanglement, which in some cases can be linked to the notion of simultaneity in special relativity.

Covert and secret quantum key distribution aims at generating information-theoretically secret bits between distant legitimate parties in a manner that remains provably undetectable by an adversary. We propose a framework in which to precisely define and analyze such an operation, and we show that covert and secret key expansion is possible. For fixed and known classical-quantum wiretap channels, we develop and analyze protocols based on forward and reverse reconciliation. The crux of our approach is the use of information reconciliation and privacy amplification techniques that are able to process the sparse signals required for covert operation and the Shannon entropy of which scales as the square root of their length. In particular, our results show that the coordination required between legitimate parties to achieve covert communication can be achieved with a negligible number of secret key bits.

A theoretical study of the relatively unexplored area of low-lying Rydberg states of van der Waals molecules (vdW) in the particular case of the CdAr system is presented. The fully ab initio calculation of the potential energy curves (PECs) of the excited vdW complex CdAr above the $\left(5s6s\right){}^{3}S_1,{}^{1}S_0$ atomic asymptotes, reaching the $\left(5s6p\right){}^{3}P_{0,1,2}, \left(5s5d\right){}^{1}D_2{,}^3{D}_{1,2,3}, \left(5s6p\right){}^{1}P_1$, and $\left(5s7s\right){}^{3}S_1,{}^{1}S_0$, is performed. All the calculated PECs of the Rydberg states of $\mathrm{\Sigma }$ symmetry exhibit undulations resulting in the double-well character that was already observed experimentally in the case of the lowest Rydberg states of various 12 group metal–rare gas vdW complexes. The outer wells obtained within the present work are shallow with depths not exceeding 20 ${\mathrm{cm}}^{-1}$ below the atomic asymptote and reaching equilibrium distance of roughly 27 bohr in the case of states correlating with the $\left(5s7s\right){}^{3}S_1,{}^{1}S_0$ asymptotes. The largest calculated values of permanent electric dipoles at equilibrium distances exceed 10 D in the case of ${}^1\mathrm{\Sigma }$ Rydberg states correlating with $\left(5s6p\right){}^{1}P_1$ and $\left(5s5d\right){}^{1}D_2$ atomic asymptotes. The calculations were performed within the second-order multireference perturbation theory. The performance of this particular theoretical ab initio approach is discussed in the context of the considered type of systems and compared, wherever it was possible, with the benchmark results of the coupled-cluster method obtained in this work, with other ab initio calculations based on multireference perturbation theory and experimental data.

We present ultrafast magnetic-field pulse generation in aligned molecules from numerical solutions of time-dependent Schrödinger equations. The one-electron molecular ion ${{\mathrm{H}}_2}^{+}$ as a benchmark model is used to describe the ultrafast photophysics process. Schemes with bichromatic high-frequency corotating and counterrotating $(\omega ,2\omega )$ and ($\omega ,3\omega )$ circularly polarized UV laser pulses are presented to produce the spatial and temporal evolution of the generated magnetic field. We discuss how interference effects between multiple resonant excitations modulate the evolution of the generated magnetic field. It is found that the modulation of generated magnetic fields is dependent on the pulse frequency and helicity combination and the molecular alignment, which is attributed to the different resonant excitation processes with various molecular orbitals that vary the intramolecular coherent electron currents. The spatiotemporal evolution of generated magnetic fields is shown to be related and strongly different for molecular around-axis and in-plane electron currents. The scheme allows one to control the induced magnetic-field-pulse generation as a tool for ultrafast optical magnetism.

We study the interactions between oscillating nonresonant rf fields and atoms with strong spin-exchange collisions in the presence of a weak dc magnetic field. We find that the atomic Larmor precession frequency shows a new functional form to the rf field parameters when the spin-exchange collision rate is tuned. In the weak rf field amplitude regime, a strong modification of atomic Larmor frequency appears when the spin-exchange rate is comparable to the rf field frequency. This effect has been neglected before due to its narrow observation window. We compare the experimental results with density matrix calculations, and explain the data by an underdamped oscillator model. When the rf field amplitude is large, there is a minimum atomic gyromagnetic ratio point due to the rf photon dressing, and we find that strong spin-exchange interactions modify the position of such a point.

The absorption spectrum of a bound state with a finite lifetime gives rise to the well-known Lorentzian line shape. This fundamental mechanism breaks down at high intensities. Here we demonstrate how even a single intense multicycle excitation pulse can result in Fano absorption profiles in the frequency domain. The intensity-dependent line shapes indicate the dipole responses are phase modulated by the single excitation pulse itself. Further, as the atomic density increases, we observe broader Fano-like shapes and the appearance of novel substructures in absorption lines. These are clear signatures of propagation effects in which collective interactions need to be taken into account. An analytical model is applied, which describes both effects observed in the experiment. Understanding the transition from optically dilute to dense medium is crucial to distinguish between single-atom and macroscopic effects.

Young's two-beam interference and the Pancharatnam-Berry geometric phase constitute two landmark results in optical physics. Employing the spectral interference law of electromagnetic waves, we establish a general expression for the Pancharatnam-Berry phase in Young's two-pinhole setup. We show that the phase is elegantly specified by the electric field Stokes parameters at the openings. The geometric phase can also be deduced from the interference pattern using Pancharatnam's definition of the generalized phase, as we demonstrate both numerically and experimentally. Our results imply a way to control the phase differences in two-beam electromagnetic interference by adjusting the input-field intensities and polarization states.

The reflection and transmission of a paraxial light beam carrying the spin and intrinsic orbital angular momenta (IOAMs) at a plane interface between two isotropic transparent media is considered. The surface transverse linear momenta (STLMs), i.e., the momenta that are localized near the interface and whose direction is perpendicular to the plane of incidence, are investigated. The IOAM-dependent and spin-dependent STLMs of the beams of homogeneous and inhomogeneous plane waves as well as the interference STLMs in the first medium are calculated. A detailed comparison of these STLMs is made. The way of experimental investigation of STLMs based on the relation between the global STLM and the transverse shift of the center of gravity of the global electromagnetic field is discussed.

We demonstrate experimentally how semiconductor lasers subjected to double optical feedback change the statistics of their chaotic spiking dynamics from Gaussian to long-tail power-law distributions associated to the emergency of bursting. These chaotic regimes, which are features of excitable complex systems, are quantified by the tail exponent $\alpha$ and appear by changing the ratio between the feedback times. Transitions to bursting occur in the neighborhood of low-order Farey fractions. The physics behind these transitions is related to the variation of threshold pump current in the compound system as obtained from a deterministic set of rate equations. Numerical integration also verifies the observed chaos transitions indicating the possibility of controlling the bursting chaotic statistics.

Dark states are eigenstates or steady states of a system that are decoupled from the radiation. Their use, along with associated techniques such as stimulated Raman adiabatic passage, has extended from atomic physics, where it is an essential cooling mechanism, to more recent versions in the condensed phase where it can increase the coherence times of qubits. These states are often discussed in the context of unitary evolution and found with elegant methods exploiting symmetries, or via the Morris-Shore transformation. However, the link with dissipative systems is not always transparent, and distinctions between classes of coherent population trapping are not always clear. We present a detailed overview of the arguments to find stationary dark states in dissipative systems, and examine their dependence on the Hamiltonian parameters, their multiplicity, and purity. We evidence the class of dark states that depends not only on the detunings of the lasers but also on their relative intensities and phases. We illustrate the criteria with the more complex physical system of the hyperfine transitions of ${}^{87}\mathrm{Rb}$ and show how a knowledge of the dark-state manifold can guide the preparation of pure states.

We study information scrambling, as diagnosed by the out-of-time order correlations (OTOCs), in a system of large spins collectively interacting via spatially inhomogeneous and incommensurate exchange couplings. The model is realizable in a cavity QED system in the dispersive regime. Fast scrambling, signaled by an exponential growth of the OTOCs, is observed when the couplings do not factoriZe into the product of a pair of local interaction terms, and at the same time the state of the spins points initially coplanar to the equator of the Bloch sphere. When one of these conditions is not realized, OTOCs grow algebraically with an exponent sensitive to the orientation of the spins in the initial state. The impact of initial conditions on the scrambling dynamics is attributed to the presence of a global conserved quantity, which critically slows down the evolution for initial states close to the poles of the Bloch sphere.

A quantum channel (operation) is commutativity-preserving if it maps commutating states to commutating states. We show that commutativity-preserving channels cannot increase quantumness of ensembles, as quantified in terms of the commutator of square roots of constituent states in a quantum ensemble recently introduced [Phys. Rev. A 96, 022132 (2017)].

We design a quasi-one-dimensional spin chain with engineered coupling strengths such that the natural dynamics of the spin chain evolves a single excitation localized at the left-hand site to any specified single-particle state on the whole chain. Our treatment is an exact solution to a problem which has already been addressed in approximate ways. As two important examples, we study the $W$ states and Gaussian states of arbitrary width.

To date, only active-decoy-state methods have been adopted in current measurement-device-independent (MDI) quantum key distribution (QKD) protocols, causing security loopholes when modulating multiple light intensities with practical devices. To solve this problem, the passive-decoy-state method will be a good choice. Until today practical passive-decoy-state MDI QKD is still missing. In this paper, based on a recently discussed structure of heralded single-photon sources, we propose a passive-decoy-state MDI QKD protocol. In present work, one does not need to modulate light sources into different intensities, but utilizes the built-in local detection events to passively generate different decoy states. Furthermore, through carrying out an appropriate mathematical method, we are able to give very precise estimations for single-photon-pair contributions. As a result, our protocol can exhibit distinct advantages compared with state-of-the-art MDI QKD schemes even when the finite-key-size effect is taken into account. Therefore, our protocol seems a promising candidate to enhance both the security and performance of MDI QKD in practical implementation.

There have been several upper bounds on the quantum capacity of the single-mode Gaussian channels with thermal noise, such as thermal attenuator and amplifier. We consider a class of attenuator and amplifier with more general noises, including squeezing or even non-Gaussian noises. We derive upper bounds the energy-constrained quantum capacity of those channels by using the quantum conditional entropy power inequality. Also, we obtain lower bounds for the same channels by means of Gaussian optimizer with fixed input entropy. They give narrow bounds when the transmissivity is near unity and the energy of input state is low.

For incident x-ray energies below the core level ionization threshold, one-photon absorption (OPA) leads to off-resonant x-ray excitations that are mediated by a virtual intermediate electronic state. In the linear regime of x-ray interaction, this intermediate state may decay radiatively and the maximum energy of the emitted photon is given by the initial and final atomic state as well as the energy of the incident photon. In the nonlinear excitation regime, however, this intermediate state may be depleted by a second x-ray absorption process as described in the sequential two-photon absorption (TPA) mechanism. Since the same intermediate state is involved in the OPA and TPA processes, the cross-section values for OPA and TPA are interconnected. We report on the determination of cross-section values for off-resonant excitation conditions, and show how the data can be used for estimating the TPA cross section in the sequential absorption mechanism. The determined off-resonant cross sections for cobalt exhibit a Lorentz-like energy-dependent behavior allowing for an energy-dependent description of the OPA and TPA cross sections with semiempirical models. The dependence of TPA cross sections on atomic number is discussed and different scaling laws are investigated. The effect of the lifetimes of a virtual intermediate state is also studied in relation to the incidence x-ray energy.

The implementation of quantum networks involving quantum memories and photonic channels without the need for cryogenics would be a major technological breakthrough. Nitrogen-vacancy centers have excellent spin properties even at room temperature, but phonon-induced broadening makes it challenging to coherently interface these spins with photons at noncryogenic temperatures. Inspired by recent progress in achieving high mechanical quality factors, we propose that this challenge can be overcome using spin-optomechanical transduction. We quantify the coherence of the interface by calculating the indistinguishability and single-photon purity of photons emitted from such a device and describe promising paths towards experimental implementation. Our results show that for ultrahigh mechanical quality factor frequency products, as have recently been achieved, our proposed interface could generate single photons with high indistinguishability and efficiency without cryogenic cooling, an important step towards room-temperature quantum networks.

We demonstrate feedback cooling of the center-of-mass motion of single charged nanoparticles to millikelvin temperatures in three dimensions via applying oscillating electric fields synchronized to their optically observed motion. The observed motional temperatures at weak feedback agree with a simple model and allow us to estimate the charge number of trapped nanoparticles. The agreement between our model and experiments is confirmed by independent measurements of the charge numbers based on a shift in the oscillation frequency induced by a constant electric field. The demonstrated temperature of less than $10\mathrm{mK}$ at $4\times {10}^{-3}\mathrm{Pa}$ is lower than that with the conventional optical cooling approach at this pressure by one to two orders of magnitude. Our results form the basis of manipulating cold charged nanoparticles and pave the way to quantum mechanical studies with trapped nanoparticles near their ground state.

We address the quantum dynamics of a system composed of a qubit globally coupled to a many-body system characterized by short-range interactions. We employ a dynamic finite-size scaling framework to investigate the out-of-equilibrium dynamics arising from the sudden variation (turning on) of the interaction between the qubit and the many-body system, in particular when the latter is in proximity to a quantum first-order or continuous-phase transition. Although the approach is quite general, we consider $d$-dimensional quantum Ising spin models in the presence of transverse and longitudinal fields as paradigmatic quantum many-body systems. To characterize the out-of-equilibrium dynamics, we focus on a number of quantum-information-oriented properties of the model. Namely, we concentrate on the decoherence features of the qubit, the energy interchange between the qubit and the many-body system during the out-of-equilibrium dynamics, and the work distribution associated with the quench. The scaling behaviors predicted by the dynamic finite-size scaling theory are verified through extensive numerical computations for the one-dimensional Ising model, which reveal a fast convergence to the expected asymptotic behavior with increasing system size.