The simultaneous quantum and classical communication (SQCC) protocol is an appealing protocol since it allows continuous-variable quantum key distribution (CV-QKD) and classical communication to be implemented simultaneously by using the same communication infrastructure and on a single wavelength. However, the main challenge of the SQCC protocol is its low tolerance of phase noise, limiting the transmission distance under the real local oscillator (LO) design. To eliminate the phase drift without the aid of the reference signal, the classical carrier phase estimation (CPE) algorithm in the field of coherent optical communication is employed for phase recovery and data extraction. Besides, the power of the residual phase noise in classical communication and the power of the excess phase noise in CV-QKD are derived, including the effects of the employed laser linewidth, the transversal filter length, and the classical signal power. On this basis, the performances of the SQCC protocol under the trusted and the untrusted phase noise model are discussed, respectively. Our work confirms the feasibility of the CPE algorithm in the SQCC protocol and demonstrates the conditions required for this protocol to operate on a coherent optical communication system.

We report a measurement of the valence orbital-momentum profiles of mercury (Hg) using a high-sensitivity binary $(e,2e)$ electron-momentum spectrometer at incident energies of 600 and 1200 eV plus the binding energy. The $6s_{1/2}$ orbital and the spin-orbit components of $5d_{5/2}$ and $5d_{3/2}$ orbitals are well separated in the measured binding-energy spectra. The experimental momentum distributions for the individual orbitals and the branching ratio of $5d_{5/2}$ to $5d_{3/2}$ are obtained and compared with predictions from a plane-wave impulse approximation (PWIA) in which the orbital wave functions are calculated using the nonrelativistic (NR) and spin-orbital (SO) relativistic theories. The SO relativistic calculations are in better agreement with experiment than the NR, indicating clearly the importance of relativistic effects in the electronic structure of Hg. We also observe some discrepancies between experiment and PWIA calculations in the high-momentum region of $6s_{1/2}$ and the low-momentum region of $5d$ orbitals which display a dynamic dependence on the impact energy. These discrepancies become smaller at a higher energy of about 1200 eV and thus can be qualitatively assigned to the distorted-wave effect in the $(e,2e)$ reaction of Hg.

We have examined fine-structure mixing between the rubidium $5{}^{2}P_{3/2}$ and $5{}^{2}P_{1/2}$ states along with quenching of these states due to collisions with methane gas. Measurements are carried out using ultrafast laser-pulse excitation to populate one of the Rb $5{}^{2}P$ states, with the fluorescence produced through collisional excitation transfer observed using time-correlated single-photon counting. Fine-structure mixing rates and quenching rates are determined by the time dependence of this fluorescence. As $\mathrm{Rb}(5{}^{2}P$) collisional excitation transfer is relatively fast in methane gas, measurements were performed at methane pressures of $2.5–25$ Torr, resulting in a collisional transfer cross section ($5{}^{2}P_{3/2}\to 5{}^{2}P_{1/2}$) of $(4.23\pm 0.13)\times {10}^{-15}{\mathrm{cm}}^2$. Quenching rates were found to be much slower and were performed over methane pressures of $50–4000$ Torr, resulting in a quenching cross section of $(7.52\pm 0.10)\times {10}^{-19}{\mathrm{cm}}^2$. These results represent a significant increase in precision compared to previous work, and also resolve a discrepancy in previous quenching measurements.

Rescattering mediated by intense laser fields is one of the dominant processes in strong-field physics. It was usually described classically or semiclassically. Recently, laser-induced recollisions with the tunneling barriers have attracted great theoretical interest. However, fully distinguishing the “quantum” effect of rescattering with the tunneling barrier on photoelectron energy and momentum distributions is very difficult if using the linearly polarized light field. Here we perform the measurement of photoelectron momentum distributions by strong-field ionization of Ar atoms in orthogonally polarized two-color laser fields with comparable intensities. We observe the strong ionization enhancement due to rescattering on two-dimensional momentum distribution, which is in striking disagreement with the predictions by the plain strong-field approximation and the classical-trajectory Monte Carlo simulation. With the help of the analysis on the electron complex trajectories in both sub-barrier and quasicontinuum regions, the enhancement is illustrated to originate from the quantum effect of forward scattering between the tunneling electron and its parent ion. The study shows that the electron probability will be significantly modified when the rescattered electrons excurse in the continuum and the “classical” rescattering process needs the quantum description.

Recently, studies of high-order harmonic generation (HHG) from atoms driven by bichromatic counter-rotating circularly polarized laser fields have received considerable attention for this process could be a potential source of coherent circularly polarized extreme ultraviolet (XUV) and soft-x-ray beams in a tabletop-scale setup. In this paper, we address the problem with molecular targets and perform a detailed quantum study of the ${\mathrm{H}}_2{}^{+}$ molecule in bichromatic (${\omega }_0, 2{\omega }_0$) counter-rotating circular polarized laser fields where we adopt wavelengths (790 and 395 nm) and intensities ($2\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$) reported in a recent experiment [K. M. Dorney et al., Phys. Rev. Lett. 119, 063201 (2017)]. Here, we demonstrate appropriate conditions to produce perfectly circular polarized harmonics. The calculated radiation spectrum contains doublets of left and right circularly polarized harmonics which display perfect circular polarization with use of the trapezoidal pulse shape, and substantial deviations from perfect circular polarization with use of the sine-squared pulse shape. We also study in detail short- and long-cycle counter-rotating circularly polarized driving pulses with a time delay between the two driving fields, ${\omega }_0$ and $2{\omega }_0$. These time delayed circularly polarized driving pulses are applied to H atoms and ${\mathrm{H}}_2{}^{+}$ molecules, and in both atomic and molecular cases we conclude a zero time delay corresponds to the highest HHG intensity for short pulses. For longer pulses there are no distinct differences in HHG intensities between the zero and nonzero time delays if the latter are within a few optical cycles of the fundamental frequency.

Precision optical spectroscopy of exotic ions reveals accurate information about nuclear properties such as charge radii and magnetic and quadrupole moments. Thorium ions exhibit unique nuclear properties with high relevance for testing symmetries of nature. We report loading and trapping of single ${}^{232}\mathrm{Th}^{+}$ ions in a linear Paul trap, embedded into and sympathetically cooled by small crystals of trapped ${}^{40}\mathrm{Ca}^{+}$ ions. Trapped thorium ions are identified in a nondestructive manner from the voids in the laser-induced calcium fluorescence pattern emitted by the crystal, and alternatively, by means of a time-of-flight signal when extracting ions from the Paul trap and steering them into an external detector. We have loaded and handled a total of 231 individual thorium ions. We reach a time-of-flight detection efficiency of $\gtrsim 95\%$, consistent with the quantum efficiency of the detector. The sympathetic cooling technique is expected to be applicable for other isotopes and various charge states of thorium, e.g., for future studies of ${}^{229\mathrm{m}}\mathrm{Th}$.

The universal forms of quantum density and phase correlations after an interaction quench are found for dilute one-dimensional (1D), 2D, and 3D condensates. A Bogoliubov approach in a local density approximation is used. We obtain compact expressions for the most visible effects. Our results show how loss of phase coherence and antibunching are built up after the quench by quantum fluctuations. We demonstrate further that the density correlations can be observed even with imaging resolution much worse than healing length. This indicates that the direct measurement of counterpropagating atom pairs in situ in a continuum system is realistic. The conditions in contemporary 1D experiments are especially favorable for the correlation wave observations.

Multimode optomechanical systems are an emerging platform for studying fundamental aspects of matter near the quantum ground state and are useful in sensitive sensing and measurement applications. We study optomechanical cooling in a system where two nearly degenerate mechanical oscillators are coupled to a single microwave cavity. Due to an optically mediated coupling the two oscillators hybridize into a bright mode with a strong optomechanical cooling rate and a dark mode nearly decoupled from the system. We find that at high coupling, sideband cooling of the dark mode is strongly suppressed. Our results are relevant to novel optomechanical systems where multiple closely spaced modes are intrinsically present.

Plasmon resonance, with strong coupling of light to electrons at a metal-dielectric interface, allows light confinement and control at subwavelength scale. It is fundamentally limited by the inherent mobility of the free electrons, leading to the corresponding nonlocality of the electromagnetic response [C. Ciraci et al., Science 337, 1072 (2012); G. Toscano et al., Nat. Commun. 6, 7132 (2015)]. We report that this nonlocality also results in the formation of a hyperbolic layer near the metal-dielectric interface, with a strong anisotropy of its electromagnetic response. While the resulting hyperbolic blockade leads to the suppression of the conventional plasmon resonance, the hyperbolic layer also supports a different class of surface waves, which offer longer propagation distance and stronger field confinement, simultaneously. Furthermore, these hyperplasmons are not limited to the proximity of the plasmon resonance, which extends the operational bandwidth of plasmonic devices.

The exchange of spin-0 or spin-1 bosons between fermions or spin-polarized macroscopic objects gives rise to various spin-dependent potentials. We derive the coordinate-space nonrelativistic potentials induced by the exchange of such bosons, including contact terms that can play an important role in atomic-scale phenomena, and correct for errors and omissions in the literature. We summarize the properties of the potentials and their relevance for various types of experiments. These potentials underpin the interpretation of experiments that search for new bosons, including spectroscopy, torsion-pendulum measurements, magnetometry, parity nonconservation, and electric dipole moment experiments.

Designing high-precision and efficient protocols is of crucial importance for quantum parameter estimation in practice. Estimation based on continuous quantum measurement is one possible type of this, which also appears to be the most natural choice for continuous dynamical processes. In this work we consider the state-of-the-art superconducting circuit quantum-electrodynamics (QED) systems, where high-quality continuous measurements have been extensively performed in the past decade. Within the framework of Bayesian estimation and particularly using the quantum Bayesian rule in circuit QED, we numerically simulate the likelihood function as an estimator for the Rabi frequency of qubit oscillations. We find that, by proper design of the interaction strength of measurement, the estimate precision can scale with the measurement time beyond the standard quantum limit, which is usually assumed for this type of continuous measurement. This unexpected result is supported by the simulated Fisher information and can be understood as a consequence of the quantum correlation between the output signals by simulating the effect of quantum efficiency of measurement.

We report loading of laser-cooled cesium atoms into a hollow-core photonic-band gap fiber and confining the atoms in the fiber's $7\text{−}\mu \mathrm{m}$-diameter core with a magic-wavelength dipole trap at $\sim 935$ nm. The use of the magic wavelength removes the ac Stark shift of the 852 nm optical transition in cesium caused by the dipole trap in the fiber core and suppresses the inhomogeneous broadening of the atomic ensemble that arises from the radial distribution of the atoms. This opens the possibility to continuously probe the atoms over timescales of a millisecond—approximately 1000 times longer than what was reported in previous works, because the dipole trap does not have to be modulated. We describe our atom-loading setup and its unique features and present spectroscopy measurements of the cesium's $D_2$ line in the continuous-wave dipole trap with up to $1.7\times {10}^4$ loaded inside the hollow-core fiber.

The presumed connection in attoclock setups between the electron tunneling time and its asymptotic momentum has triggered vigorous debates. In neutral atomic systems investigated so far, the action of the long-range Coulomb potential on the electron momentum hinders extracting the effect of the tunneling process on the offset angle of the asymptotic electron momentum. We propose and investigate an attoclock experiment using ${\mathrm{F}}^{-}$ or ${\mathrm{Cl}}^{-}$ to circumvent this difficulty. Our calculations, performed with realistic laser parameters in the tunneling regime, could be checked directly against experiment and predict essentially a “zero” offset angle with no detectable effect of polarization.

Quantum theory allows direct measurement of the average of a non-Hermitian operator using the weak value of the positive semidefinite part of the non-Hermitian operator. Here, we experimentally demonstrate the measurement of the weak value and the average value of non-Hermitian operator by an interferometric technique. Our scheme is unique as we can directly obtain the weak value from the interference visibility and the phase shift in a Mach-Zehnder interferometer without using any weak measurement or postselection. Both the experiments discussed here were performed with laser sources, but the results would be the same with average statistics of single-photon experiments. Thus, the present experiment opens up the possibility of measuring the weak value and the average value of a non-Hermitian operator without weak interaction and postselection, which can have several technological applications.

The data of four recent experiments—conducted in Delft, Vienna, Boulder, and Munich with the aim of refuting nonquantum hidden-variables alternatives to the quantum-mechanical description—are evaluated from a Bayesian perspective of what constitutes evidence in statistical data. We find that each of the experiments provides strong, or very strong, evidence in favor of quantum mechanics and against the nonquantum alternatives. This Bayesian analysis supplements the previous non-Bayesian ones, which refuted the alternatives on the basis of small $p$ values but could not support quantum mechanics.

We develop a general framework for parameter estimation that allows only trusted parties to access the result and achieves optimal precision. The protocols are designed such that adversaries can access some information indeterministically, but only at the risk of getting caught (cheat sensitivity); under the assumption that the adversary can access the channel only once, then the protocol is unconditionally secure. By combining techniques from quantum cryptography and quantum metrology, we devise cryptographic procedures for single-parameter estimation when an arbitrary number of parties are involved.

In this work, we exhaustively investigate $1\to 2$ local and nonlocal broadcasting of entanglement as well as correlations beyond entanglement (geometric discord) using asymmetric Pauli cloners with the most general two-qubit state as the resource. We exemplify asymmetric broadcasting of entanglement using “maximally entangled mixed states.” We demonstrate the variation of broadcasting range with the amount of entanglement present in the resource state as well as with the asymmetry in the cloner. We show that it is impossible to optimally broadcast geometric discord with the help of these asymmetric Pauli cloning machines. We also study the problem of $1\to 3$ broadcasting of entanglement using nonmaximally entangled state as the resource. For this task, we introduce a method we call successive broadcasting which involves application of $1\to 2$ optimal cloning machines multiple times. We compare and contrast the performance of this method with the application of direct $1\to 3$ optimal cloning machines. We show that $1\to 3$ optimal cloner does a better job at broadcasting than the successive application of $1\to 2$ cloners and the successive method can be beneficial in the absence of $1\to 3$ cloners. We also bring out the fundamental difference between the tasks of cloning and broadcasting in the final part of the paper. We create examples to show that there exist local unitaries which can be employed to give a better range for broadcasting. Such unitary operations are not only economical, but also surpass the best possible range obtained using existing cloning machines enabling broadcasting of lesser entangled states. This result opens up a direction in exploration of methods to facilitate broadcasting which may outperform the standard strategies implemented through cloning transformations.

We investigate unambiguous discrimination between given quantum states with a sequential measurement, which is restricted to local measurements and one-way classical communication. If the given states are binary or each of their individual state spaces is two dimensional, then it is in some cases known whether a sequential measurement achieves a globally optimal unambiguous measurement. In contrast, for more than two states each of whose individual systems is more than two dimensional, the problem becomes extremely complicated. This paper focuses on symmetric ternary separable pure states each of whose individual systems is three dimensional, which include phase shift keyed (PSK) optical coherent states and a lifted version of “double trine” states. We provide a necessary and sufficient condition for an optimal sequential measurement to be globally optimal for the bipartite case. A sufficient condition of global optimality for multipartite states is also presented. One can easily judge whether these conditions hold for given states. Some examples are given, which demonstrate that, despite the restriction to local measurements and one-way classical communication, a sequential measurement can be globally optimal in quite a few cases.

We employed the method of continued fractions in conjunction with the positron correlation polarization potential model with different levels of polarization to calculate the elastic integral and differential cross sections for positron scattering by ${\mathrm{O}}_2$ molecules. We show how the addition of higher-order terms in the polarization interaction is necessary in order to achieve convergence of the scattering potential. Also, we present the effect that such terms have over the integral cross sections, comparing with results of previous studies. When convergence of the scattering potential is attained, the resulting cross sections fairly agree with the most recent total cross section measurements up to the positronium formation threshold and moreover exhibit good agreement with the available experimental quasielastic differential cross sections.