We find the spectral bandwidth scaling laws of the THz wave packets, produced from two-color laser filaments, as a function of the input laser-pulse duration and demonstrate how one can fully recover the original broadband THz wave packets even using narrow-band detection techniques such as the widespread electro-optic sampling.

We theoretically demonstrate the ground-state cooling of a mechanical oscillator in an optomechanical cavity in the presence of a quantum well, in the unresolved-sideband regime. Due to the presence of the quantum well, the cavity response gets modified and leads to asymmetric heating and cooling processes. The cooling rate of the mechanical resonator can potentially be enhanced by tuning the cavity-field detuning. It is demonstrated that, even when the cavity is in the unresolved-sideband regime, the effective interaction of the exciton and mechanical modes can bring the system back to an effective resolved-sideband regime. The time evolution of the mean phonon number in the mechanical resonator is studied using the quantum master equation. The average phonon occupancy in the mechanical resonator tends to 0 with time, exhibiting dynamic controllability of cavity dissipation.

In conventional quantum key distribution (QKD) protocols, security is guaranteed by estimating the amount of leaked information. Such estimation tends to overrate, leading to a fundamental threshold of the bit error rate, which becomes a bottleneck of practical QKD development. This bottleneck is broken through by the recent work of round-robin differential phase-shift (RRDPS) protocol, which eliminates the fundamental threshold of the bit error rate. The key challenge for the implementation of the RRDPS scheme lies in the realization of a variable-delay Mach–Zehnder interferometer, which requires active and random choice of many delays. By designing an optical system with multiple switches and employing an active phase stabilization technology, we successfully construct a variable-delay interferometer with 127 actively selectable delays. With this measurement, we experimentally demonstrate the RRDPS protocol and obtain a final key rate of 15.54 bps with a total loss of 18 dB and an error rate of 8.9%.

Random access codes are important for a wide range of applications in quantum information. However, their implementation with quantum theory can be made in two very different ways: (i) by distributing data with strong spatial correlations violating a Bell inequality or (ii) using quantum communication channels to create stronger-than-classical sequential correlations between state preparation and measurement outcome. Here we study this duality of the quantum realization. We present a family of Bell inequalities tailored to the task at hand and study their quantum violations. Remarkably, we show that the use of spatial and sequential quantum correlations imposes different limitations on the performance of quantum random access codes: Sequential correlations can outperform spatial correlations. We discuss the physics behind the observed discrepancy between spatial and sequential quantum correlations.

The refractive index $n$ of gaseous helium can be measured by optical interferometry so accurately that it can be used to establish a pressure standard which is expected to be superior to the current standard based on the height of a mercury column. The new standard requires knowledge of the dynamic polarizability of helium atom with accuracy significantly higher than obtainable in the best experiments, but possible to achieve computationally. Calculations of this quantity are presented at relativistic and quantum electrodynamics levels of theory including relativistic nuclear recoil effects. The uncertainties of the results are carefully estimated. Our recommended value of the dynamic polarizability at the He-Ne laser wavelength of 6329.908 Å, equal 1.39181197(14) a.u., has uncertainty at least two orders of magnitude smaller than that of recent measurements and is sufficiently accurate to establish a new pressure standard. Purely ab initio values of the refraction coefficient $n$ are computed using our polarizability, literature values of magnetic susceptibility, and dielectric virial coefficients. It is shown that $n-1$ can be predicted by theory as a function of density and temperature with uncertainty of 1 ppm for pressures up to 3 MPa.

The spectra of Rydberg atoms in the domain of three-level avoided crossings formed by the Rydberg $49S_{1/2}$ state and high-$l$ states in the $n=45$ manifold are observed in the standard magneto-optical trap of cesium. We develop a highly accurate method which uses the spectroscopic results in order to refine the quantum defect of the $49S_{1/2}$ state from the previous value $4.0495\pm 0.0001$ extrapolated from $n=6–30S$ states [K. H. Weber and C. J. Sansonetti, Phys. Rev. A 35, 4650 (1987)] to the new value $4.04978\pm 0.00003$. For this purpose, we determine two characteristic properties of the avoided crossings: (i) the minimum gap between the upper and the lower levels, and (ii) the value of equal gaps between the upper and the middle levels and between the middle and the lower levels. The experimental results for the avoided crossing near 4 V/cm are (i) $\left(111.7\pm 1.3\right)h\mathrm{MHz}$, and (ii) $\left(57.0\pm 2.1\right)h\mathrm{MHz}$. For comparison, quantum mechanical simulations based on the published quantum defect yield (i) $109.52h\mathrm{MHz}$, and (ii) $56.04h\mathrm{MHz}$. The refined quantum defect is adjusted such that it yields (i) $111.50h\mathrm{MHz}$, and (ii) $56.98h\mathrm{MHz}$, in even better agreement with the experimental results. The theoretical values of the electric fields for (i) and (ii) differ by 0.03 V/cm, in perfect agreement with the experimental results. The refinement also improves the agreement of their absolute values. The remaining ∼1.5% difference is attributed to effects of stray fields.

Optimal control of quantum systems with complex constrained external fields is one of the longstanding theoretical and numerical challenges at the frontier of quantum control research. Here, we present a theoretical method that can be utilized to optimize the control fields subject to multiple constraints while guaranteeing monotonic convergence towards desired physical objectives. This optimization method is formulated in the frequency domain in line with the current ultrafast pulse shaping technique, providing the possibility for performing quantum optimal control simulations and experiments in a unified fashion. For illustrations, this method is successfully employed to perform multiple constraint spectral-phase-only optimization for maximizing resonant multiphoton transitions with desired pulses.

Pulsed excitation on the two-photon Cs $6{}^2{S}_{1/2}\to 7{}^2{D}_{3/2,5/2}$ transition results in time-resolved fluorescence at 697 and 672 nm. The rates for fine-structure mixing between the $7{}^2{D}_{3/2,5/2}$ states have been measured for helium and argon rare-gas collision partners. The mixing rates are very fast, $1.26\pm 0.05\times {10}^{-9} {\mathrm{cm}}^3$/atom s for He and $1.52\pm 0.05\times {10}^{-10} {\mathrm{cm}}^3$/atom s for Ar, driven by the small energy splitting and large radial distribution for the valence electron. The quenching rates are considerably slower, $6.84\pm 0.09\times {10}^{-11}$ and $2.65\pm 0.04\times {10}^{-11} {\mathrm{cm}}^3$/atom s for He and Ar, respectively. The current results are placed in context with similar rates for other alkali-metal–rare-gas collision pairs using adiabaticity arguments.

In this work we present a coherent picture for the evolution of Higgs mode in neutral $s$-wave fermion superfluids. As the strength of attractive interaction between fermions increases from the BCS to the Bose-Einstein condensate (BEC) regime we find that the Higgs mode in the fermionic superfluids is pushed to higher energy while at the same time, gradually loses its spectral weight, which is because the system is tuned away from Lorentz invariance. Furthermore, when damping is taken into account, the spectral weight of the Higgs mode is significantly broadened due to coupling to phase mode in the whole BEC-BCS crossover.

Interacting Bose-Fermi mixtures possess a fermionic (super)symmetry when bosons and fermions in the mixture have equal masses, and when the interaction strengths are appropriately tuned. This symmetry is spontaneously broken in the ground state of the mixture, leading to a novel Goldstone mode with fermionic statistics and quadratic dispersion. Here we examine the effect of explicit symmetry-breaking perturbations on the Goldstone mode. When the symmetry is not exact and the system is allowed to deviate from the symmetric point, we find that the Goldstone mode acquires an energy gap. We show that the excitations manifest themselves as a nonanalyticity of the thermodynamic pressure.

We propose a symmetry-based method of using noncentrosymmetric optical lattices to systematically control topological nontrivial orbital hybridization. A crucial difference from the previous studies is the role of inversion symmetry breaking, which is applied to induce an exotic orbital-changing hopping perpendicular to the direction without inversion symmetry and opens a band gap, instead of reducing the codimension and producing gapless points. The orbital mixing here is reminiscent of the spin-orbit physics based on hyperfine states but differs in symmetry and origin. This nontrivial orbital hybridization produces a topological band structure. Attractively interacting fermionic atoms loaded in such a lattice are found to show an orbital topological Fulde-Ferrell superfluid state in the presence of onsite rotation. This state supports Majorana fermions on its edges. Our mechanism should pave an alternative way to achieve orbital topological phases in optical lattices of nonstandard geometry.

We obtain the phase diagram for a harmonically trapped dilute dipolar condensate with a short-ranged conservative three-body interaction. We show that this system supports two distinct fluid states: a usual condensate state and a self-cohering droplet state. We develop a simple model to quantify the energetics of these states, which we verify with full numerical calculations. Based on our simple model we develop a phase diagram showing that there is a first-order phase transition between the states. Using dynamical simulations we explore the phase transition dynamics, revealing that the droplet crystal observed in previous work is an excited state that arises from heating as the system crosses the phase transition. Utilizing our phase diagram we show that it is feasible to produce a single droplet by dynamically adjusting the confining potential.

Entangled light beams are important resources for quantum information processing. For some applications like teleportation, only the entanglement between two wave packets (two harmonic oscillators) is needed. So the calculation of output-photon entanglement involves projecting continuous output modes onto wave-packet modes by filter functions, thus resulting in a strong dependence of entanglement on the filter functions. In this paper, we aim at optimizing the filter functions to obtain a large entanglement in a relatively short time, which is important for utilizing the entangled light beams more efficiently in real experiments. We outline the general optimization procedures based on our previous schemes of generating entangled beams in a multimode optomechanical system. Moreover, we give analytic insights into as well as physical explanations of the wave-packet optimization, which are helpful for experimental estimations.

We study the $\mathcal{PT}$-symmetry breaking for the scattering problem in a one-dimensional non-Hermitian tight-binding lattice model with balanced gain and loss distributed on two adjacent sites. In the scattering process the system undergoes a transition from the exact $\mathcal{PT}$-symmetric phase to the phase with spontaneously breaking $\mathcal{PT}$ symmetry as the amplitude of complex potentials increases. Using the S-matrix method, we derive an exact discriminant, which can be used to distinguish different symmetry phases, and determine the exceptional point for the symmetry breaking analytically. In the $\mathcal{PT}$-symmetry-breaking region, we also confirm the appearance of the unique feature, i.e., the coherent perfect absorption laser, in this simple non-Hermitian lattice model. The study of the scattering problem of such a simple model provides an additional way to unveil the physical effect of non-Hermitian $\mathcal{PT}$-symmetric potentials.

Currently available separability criteria for continuous-variable states are generally based on the covariance matrix of quadrature operators. The well-known separability criterion of Duan et al. [L. M. Duan et al., Phys. Rev. Lett. 84, 2722 (2000)] and Simon [R. Simon, Phys. Rev. Lett. 84, 2726 (2000)], for example, gives a necessary and sufficient condition for a two-mode Gaussian state to be separable, but leaves many entangled non-Gaussian states undetected. Here we introduce an improvement of this criterion that enables a stronger entanglement detection. The improved condition is based on the knowledge of an additional parameter, namely, the degree of Gaussianity, and exploits a connection with Gaussianity-bounded uncertainty relations [A. Mandilara and N. J. Cerf, Phys. Rev. A 86, 030102 (2012)]. We exhibit families of non-Gaussian entangled states whose entanglement remains undetected by the Duan-Simon criterion.

We provide evidence for the validity of strong monogamy inequality of multiparty quantum entanglement using the square of convex-roof extended negativity (SCREN). We first consider a large class of multiqudit mixed states that are in a partially coherent superposition of a generalized $W$-class state and the vacuum, and provide some useful properties about this class of states. We show that monogamy inequality of multiqudit entanglement in terms of SCREN holds for this class of states. We further show that SCREN strong monogamy inequality of multiqudit entanglement also holds for this class of states. Thus SCREN is a good alternative for characterizing the monogamous and strongly monogamous properties of multiqudit entanglement.

How to generate genuine quantum randomness from untrusted devices is an important problem in quantum information processing. Inspired by previous work on a self-testing quantum random number generator [T. Lunghi et al., Phys. Rev. Lett. 114, 150501 (2015)], we present a method to generate quantum randomness from a prepare-and-measure scenario with independent devices. In existing protocols, the quantum randomness depends only on a witness value (e.g., Clauser-Horne-Shimony-Holt value), which is calculated with the observed probabilities. Differently, here all the observed probabilities are directly used to calculate the min-entropy in our method. Through numerical simulation, we find that the min-entropy of our proposed scheme is higher than that in the previous work when a typical untrusted Bennett-Brassard 1984 (BB84) setup is used. Consequently, thanks to the proposed method, more genuine quantum random numbers may be obtained than before.

We study electric quantum walks in two dimensions considering Grover, alternate, Hadamard, and discrete Fourier transform (DFT) quantum walks. In the Grover walk, the behavior under an electric field is easy to summarize: when the field direction coincides with the $x$ or $y$ axes, it produces a transient trapping of the probability distribution along the direction of the field, while when it is directed along the diagonals, a perfect two-dimensional (2D) trapping is frustrated. The analysis of the alternate walk helps one to understand the behavior of the Grover walk, as both walks are partially equivalent; in particular, it helps one to understand the role played by the existence of conical intersections in the dispersion relations, as we show that when these are removed, a perfect 2D trapping can occur for suitable directions of the field. We complete our study with the electric DFT and Hadamard walks in 2D, showing that the latter can exhibit perfect 2D trapping.

In this paper, we explicitly evaluate the one-shot quantum nonsignaling-assisted zero-error classical capacities ${\mathcal{M}}_0^{\mathrm{QNS}}$ for qubit channels. In particular, we show that for nonunital qubit channels ${\mathcal{M}}_0^{\mathrm{QNS}}=0$, which implies that in the one-shot setting nonunital qubit channels cannot transmit any information with zero probability of error even when assisted by quantum nonsignaling correlations. Furthermore, we show that for qubit channels ${\mathcal{M}}_0^{\mathrm{QNS}}$ equals the one-shot entanglement-assisted zero-error classical capacity. This means that for a single use of a qubit channel quantum nonsignaling correlations are not more powerful than shared entanglement.