Counterfactual quantum key distribution (QKD) enables two parties to share a secret key using an interaction-free measurement. Here, we point out that the efficiency of counterfactual QKD protocols can be enhanced by including noncounterfactual bits. This inclusion potentially gives rise to the possibility of noiseless attacks, in which Eve can gain knowledge of the key bits without introducing any errors in the quantum channel. We show how this problem can be resolved in a simple way that naturally leads to the idea of “counterfactual security,” whereby the noncounterfactual key bits are indicated to be secure by counterfactual detections. This method of enhancing the key rate is shown to be applicable to various existing quantum counterfactual key distribution protocols, increasing their efficiency without weakening their security.

Anyonic states, which are topologically robust because of their peculiar structure of Hilbert space, have important applications in quantum computing and quantum communication. Here we investigate the capacity of the deterministic quantum one-time pad (DQOTP) that uses Fibonacci anyons as an information carrier. We find that the Fibonacci particle-antiparticle pair produced from vacuum can be used to asymptotically send $2{log}_2{d}_{\tau }$ bits of classical information ($d_{\tau }$ is the quantum dimension of a Fibonacci anyon $\tau$), which equals anyonic mutual information of the pair. Furthermore, by studying the DQOTP via a parameterized state of six Fibonacci anyons with trivial total charge, we give the analytical results of the maximum number of messages that can be sent for different parameters, which is a step function with every step corresponding to a regular simplex from the viewpoint of geometry. The results for the maximum number of messages sent by the DQOTP can be explained by anyonic accessible information.

Attosecond probing of core-level electronic transitions in molecules provides a sensitive tool for real-time observation of chemical dynamics. Here, we employ ultrafast extreme-ultraviolet (XUV) transient-absorption spectroscopy to investigate the excited state electronic and nuclear dynamics in a prototype molecule, ${\mathrm{I}}_2$. A few-femtosecond visible pump pulse is employed to excite the ${\mathrm{I}}_2$ molecule and an attosecond XUV pulse is used to probe the dynamics through iodine-$4d$ core-to-valence transitions. A highly extended vibrational wave packet $({\nu }^{\prime }=10–50,{\nu }_{max}^{\prime }=25)$ is prepared by one-photon absorption in the valence excited $B{}^3{\mathrm{\Pi }}_{{0^{+}}_{\mathrm{u}}}$ state of ${\mathrm{I}}_2$ and its motion is directly mapped due to the strong shift of the XUV core-level transition with internuclear separation. Through the imaging of this vibrational motion, we directly reconstruct the transition energy between the valence and the core-excited states as a function of internuclear distance. Besides single-photon dynamics, distinct direct dissociation pathways arising from two-photon pump absorption are also revealed.

Interatomic Coulombic electron capture (ICEC) is an environment-enabled electron capture process in which a free electron can efficiently attach to a quantum system by transferring the excess energy to a neighbor thus ionizing it. Using the ab initio $R$-matrix method, we investigate the electron attachment to a proton in the neighborhood of a water molecule. The corresponding ICEC cross sections exhibit clear Fano profiles, resulting from interferences between the ICEC final states and resonant states. These Fano interferences, observed in the total ICEC cross sections, were discussed in our recent work on large system-neighbor separations [A. Molle et al., Phys. Rev. A 103, 012808 (2021)]. In the present study, we report on the ICEC cross sections at shorter distances which are relevant in biological and biochemical contexts. Furthermore, we investigate the partial ICEC cross sections and demonstrate that the ionization of a water molecule via ICEC is substantially different from that due to direct photoionization. Finally, we show that the distortion of the equilibrium geometry of the water molecule due to the presence of the proton influences the ICEC process.

Spontaneous symmetry breaking is a ubiquitous concept and is well described in many textbooks of physics. However, direct observation of spontaneous symmetry breaking is lacking. Here we present the observation and analysis of a spontaneously broken discrete time-translation symmetry in our driven system. We experimentally find a 50-50 split between two stable and temporal-distinct solutions, indicative of the breaking of a ${\mathbb{Z}}_2$ (Ising-like) time-translation symmetry. The experiment allows for further exploration of the symmetry breaking in our discrete time crystal and for engineering excitations in space and time in the quantum domain.

Dielectric spheres of various sizes may sustain electromagnetic whispering-gallery modes resonating at optical frequencies with very narrow linewidths. Arbitrary small deviations from the spherical shape typically shift and broaden such resonances. Our goal is to determine these shifted and broadened resonances. A boundary-condition perturbation theory for the acoustic vibrations of nearly circular membranes was developed by Rayleigh more than a century ago. We extend this theory to describe the electromagnetic excitations of nearly spherical dielectric cavities. This approach permits us to avoid dealing with decaying quasinormal modes. We explicitly find the frequencies and the linewidths of the optical resonances for arbitrarily deformed nearly spherical dielectric cavities, as power series expansions by a small parameter, up to and including second-order terms. We thoroughly discuss the physical conditions for the applicability of perturbation theory.

We study the effect of strong single scattering on the $C_0$ correlation of intensity fluctuations in a disordered ensemble of discrete scatterers. Diagrammatic calculations of the $C_0$ contribution to the spatial correlation function are carried out for both scalar and electromagnetic waves beyond the standard Born approximation. For a system of Mie particles, the dependence of $C_0$ on the size and refractive index of the scatterers is examined. It is found that for highly refracting particles, the $C_0$ correlation acquires a pronounced resonant character. Close to the resonances, due to correlations between more than two wave fields, the magnitude of $C_0$ grows drastically, greatly exceeding the value for Born scatterers.

We theoretically and numerically demonstrate the generation of a relativistic modified Bessel-Gaussian beam (MBGB) via plasma-based beam braiding. It is realized by injecting several intense Gaussian pulses with well-designed offsets and angles into an underdense plasma channel which acts as a laser-pulse combiner via refractive coupling. The MBGB propagates stably in the plasma channel with a well-controlled orbital angular momentum of large value, exciting a twisted plasma wave. After leaving the plasma, it becomes unguided and survives in vacuum for at least hundreds of femtoseconds. This method is insensitive to the initial laser injection conditions and thus should be robust for experimental implementation. It provides an alternative approach in generating high-quality tunable intense optical vortex beams which are desired for various applications.

We propose an experimental architecture where an array of optical tweezers affords site-dependent control over the confining potential of a conventional radio-frequency ion trap. The site-dependent control enables programmable manipulation of phonon modes of ions, with many potential applications in quantum information processing (QIP) and thermodynamics. We describe protocols for programming the array of optical tweezers to attain a set of target phonon modes with high accuracy. We propose applications of such controls in simulating quantum thermodynamics of a particle of programmable effective mass via Jarzynski's equality and improving the efficiency of sympathetic cooling and quantum logic gates in a multispecies ion system of disparate masses. We discuss the required optical parameters in a realistic ion trap system and potential adverse effects of optical tweezers in QIP. Our scheme extends the utility of trapped ions as a platform for quantum computation and simulation.

Positron-impact cross sections of 1-1-$C_2{H}_2{F}_2$ are reported in the energy range from 0.1 to 4000 eV. The multicenter Gaussian-type orbitals were expanded within a single determinant Hartree-Fock self-consistent field scheme to obtain molecular wave function. The elastic cross sections were computed within fixed nuclei approximation by invoking the single center expansion approach. The target interactions were described by their local nature. The direct ionization and electron excitation cross sections were estimated using the binary-encounter-Bethe model for positron and scaled-Born approximation, respectively. The experimental scattering data were corrected to account for the angular discrimination effects. The corrections enhanced experimental measurements substantially in the low-energy range. The elastic cross sections are in an excellent agreement with the forward angle corrected total cross sections below positronium formation. The cross sections obtained by summing the elastic and inelastic cross sections incoherently were in good agreement with the experimental results. An in-depth analysis of the angular limitation of spectrometers employed in measuring the positron impact scattering cross sections and corrections arising out of the polar molecule are an integral part of the study. Some key aspects of electron and positron scattering are also discussed.

The lasing of ${\mathrm{N}}_2{}^{+}$ at 391 nm, originating from the population inversion between $X{}^2{\mathrm{\Sigma }}_g{}^{+}(v=0)$ and $B{}^2{\mathrm{\Sigma }}_u{}^{+}(v=0)$ achieved by the irradiation of a pair of few-cycle intense near-IR laser pulses in a pump-probe scheme, is investigated both experimentally and theoretically. The characteristic periodic intensity modulations recorded experimentally in both $P$- and $R$-branch emission lines are reproduced well by the theoretical simulation, in which the population transfer processes among the rovibrational levels of the three lowest electronic states, $X{}^2{\mathrm{\Sigma }}_g{}^{+}, A{}^2{\mathrm{\Pi }}_u$, and $B{}^2{\mathrm{\Sigma }}_u{}^{+}$, of ${\mathrm{N}}_2{}^{+}$ induced by pump and probe few-cycle intense laser pulses are calculated. Based on the results of the theoretical simulation, the periodic intensity modulations are ascribed to the difference between the rotational selection rules in the $A{}^2{\mathrm{\Pi }}_u-X{}^2{\mathrm{\Sigma }}_g{}^{+}$ coupling, which suppresses the rotational excitation in the $X{}^2{\mathrm{\Sigma }}_g{}^{+}\left(0\right)\text{state}$, and those in the $B{}^2{\mathrm{\Sigma }}_u{}^{+}-X{}^2{\mathrm{\Sigma }}_g{}^{+}$ coupling, which enhances the rotational excitation in the $B{}^2{\mathrm{\Sigma }}_u{}^{+}$(0) state.

We consider a hybrid system of matter and light as a sensing device and quantify the role of cooperative effects. The latter generically enhance the precision with which modifications of the effective light-matter coupling constant can be measured. In particular, considering a fundamental model of $N$ qubits coupled to a single electromagnetic mode, we demonstrate that the ultimate bound for the precision shows double-Heisenberg scaling: $\mathrm{\Delta }\theta \propto 1/\left(Nn\right)$, with $N$ and $n$ the number of qubits and photons, respectively. Moreover, even using classical states and measuring only one subsystem, a Heisenberg-times-shot-noise scaling, i.e., $1/\left(N\sqrt{n}\right)$ or $1/\left(n\sqrt{N}\right)$, is reached. As an application, we show that a Bose-Einstein condensate trapped in a double-well optical lattice within an optical cavity can in principle be used to detect the gravitational acceleration $g$ with the relative precision of $\mathrm{\Delta }g/g\sim {10}^{-4}{\mathrm{Hz}}^{-1/2}$. The analytical approach presented in this study takes into account the leakage of photons through the cavity mirrors, and allows one to determine the sensitivity when $g$ is inferred via measurements on atoms or photons.

We study the dynamics of one-dimensional bosons trapped in a box potential in the presence of a barrier creating a tunable weak link, thus realizing a one-dimensional Bose-Josephson junction. By varying the initial population imbalance and the barrier height, we evidence different dynamical regimes. In particular, we show that at large barriers a two-mode model captures accurately the dynamics, while for low barriers the dynamics involves dispersive shock waves and solitons. We study a quench protocol that can be readily implemented in experiments and show that self-trapping resonances can occur. This phenomenon can be understood qualitatively within the two-mode model.

We study the properties of a single impurity in a dilute Bose gas, a Bose polaron, using the functional renormalization group. We use an ansatz for the effective action motivated by a derivative expansion, and we compute the energies of the attractive and repulsive branches of excitations in both two and three spatial dimensions. Three-body correlations play an important role in the attractive branch, and we account for those by including three-body couplings between two bath bosons and the impurity. Our calculations compare very favorably with state-of-the-art experimental measurements and numerical simulations.

A semiclassical theory of incoherent diffractive imaging is given, based on the Hanbury Brown and Twiss effect when used to image inner-shell x-ray fluorescence from heavy atoms excited by the femtosecond pulses of an x-ray laser. Interference between emission from different atoms is expected when the pulse duration is shorter than the fluorescent lifetime. Simulations for atoms at the vertices of an icosahedral virus capsid are given, and reconstructions are presented based on phasing of the pair correlation function between photons emitted independently from many different atoms at two different detector pixels. The dependence of the pair-correlation function on the fluorescence lifetime relative to the pulse duration of the x-ray free-electron laser (XFEL) is computed, and a simple expression is obtained for the contrast of incoherent diffractive imaging speckles as a function of the XFEL's flux and lifetime. This indicates that compact XFELs, with reduced flux but sub-femtosecond pulses, should be ideally suited to atomic-resolution three-dimensional mapping of heavy atoms in materials science, chemistry, and biology.

Based on a general transport theory for nonreciprocal non-Hermitian systems and a topological model that encompasses a wide range of previously studied examples, we (i) provide conditions for effects such as reflectionless and transparent transport, lasing, and coherent perfect absorption, (ii) identify which effects are compatible and linked with each other, and (iii) determine by which levers they can be tuned independently. For instance, the directed amplification inherent in the non-Hermitian skin effect does not enter the spectral conditions for reflectionless transport, lasing, or coherent perfect absorption, but allows to adjust the transparency of the system. In addition, in the topological model the conditions for reflectionless transport depend on the topological phase, but those for coherent perfect absorption do not. This then allows us to establish a number of distinct transport signatures of non-Hermitian, nonreciprocal, and topological behavior, in particular (1) reflectionless transport in a direction that depends on the topological phase, (2) invisibility coinciding with the skin-effect phase transition of topological edge states, and (3) coherent perfect absorption in a system that is transparent when probed from one side.

High-energy photons can decay to electron-positron pairs via the nonlinear Breit-Wheeler process when colliding with an intense laser pulse. The energy spectrum of the produced particles is broadened because of the variation of their effective mass in the course of the laser pulse. Applying a suitable chirp to the laser pulse can narrow the energy distribution of the generated electrons and positrons. We present a scenario where a high-energy electron beam is collided with a chirped laser pulse to generate a beam of quasimonoenergetic $\gamma$ photons, which then decay in a second chirped, UV pulse to produce a quasimonoenergetic source of high-energy electrons and positrons.