Tests of local realism and their applications aim for very high confidence in their results even in the presence of potentially adversarial effects. For this purpose, one can measure a quantity that reflects the amount of violation of local realism and determine a bound on the probability, according to local realism, of obtaining a violation at least that observed. In general, it is difficult to obtain sufficiently robust and small bounds. Here we describe an efficient protocol for computing such bounds from any set of Bell inequalities for any number of parties, measurement settings, or outcomes. The protocol can be applied to tests of other properties (such as entanglement or dimensionality) that are witnessed by linear inequalities.

We develop an SU(2)-invariant approach to the depolarization of quantum systems as the effect of random unitary SU(2) transformations. From it we derive an SU(2)-invariant Markovian master equation. This is applied to several quantum states examining whether nonclassical states are more sensible to depolarization than the classical ones. Furthermore, we show that this depolarization model provides a nontrivial generalization of depolarization channels to states of arbitrary dimension.

The stationary continuous-variable entanglement between an optical cavity and a nanomechanical resonator beam is generated by their common interaction with a quantum dot. To deal with the quantum dot which is modeled as a two-level system, we do not use the low excitation limit approximation to bosonize the spin operators, but we keep them in the quantum Langevin equations. We linearize the quantum Langevin equations reasonably and investigate the stationary continuous-variable entanglement in detail, and finally we show that a high degree of entanglement can be achieved for experimentally feasible parameters.

We want to certify in a black-box scenario that two parties simulating the teleportation of a qubit are really using quantum resources. If active compensation is part of the simulation, perfect teleportation can be faked by purely classical means. If active compensation is not implemented, a classical simulation is necessarily imperfect: In this case, we provide bounds for the certification of quantumness using only the observed statistics. In particular, if a uniform shrinking of the Bloch vector is observed on the teleported side, an average teleportation fidelity of 85% guarantees the use of quantum resources. In general, the criterion is not simply related to the fidelity; in an extreme case, the assessment of quantumness can be positive even for an average fidelity as low as 50%.

We show that entanglement can be utilized to extract the thermodynamic work beyond classical correlation via feedback control based on measurement on part of a composite system. The net work gain due to entanglement is determined by the change in the mutual information content between the subsystems that is accessible to the memory.

We propose a scheme to realize entanglement swapping via superradiance, entangling two distant cavities without a direct interaction. The successful Bell-state-measurement outcomes are performed naturally by the electromagnetic reservoir, and we show how, using a quantum trajectory method, the nonlocal properties of the state obtained after the swapping procedure can be verified by the steering inequality. Furthermore, we discuss how the unsuccessful measurement outcomes can be used in an experiment of delayed-choice entanglement swapping. An extension of testing the quantum steering inequality with the observers at three different times is also considered.

The subsidiary integral $W_4(k,l,m,n;\theta ,\alpha ,\beta ,\gamma )$ plays an essential role in the variational calculation of four-electron atomic systems using Hylleraas coordinates. With respect to the case where the ratio $\theta /(\theta +\alpha +\beta +\gamma )\sim 1$, an important special situation that may occur in the evaluation of the Bethe logarithm, existing approaches for evaluating the $W_4$ integral become impractical due to the problem of slow convergence. Based on our recent work for the three-electron subsidiary integral $W_3(l,m,n;\alpha ,\beta ,\gamma )$, we present a computationally efficient and numerically stable method, in which the $W_4$ integral can be expressed in terms of either a finite series or a finite recursion relation. Numerical experimentation is presented to validate our method.

The dynamics of photoelectrons ionized by XUV pulses in the presence of a strong infrared (IR) field is theoretically investigated by solving the time-dependent Schrödinger equation (TDSE). We study the ionization dynamics of He by an XUV pulse in the presence of a relatively strong IR laser field, which is different from the conventional attosecond streaking experiments where a rather weak IR field is applied. Comparing with the photoelectron spectra produced by the IR field only, we find the spectra ionized by the combined fields construct interesting interference structures for different XUV photon energies and pulse intensities. These features can be analyzed and explained by a quantum-trajectory theory based on the strong-field approximation (SFA). The SFA theory shows excellent agreement with the TDSE results, revealing that these structures originate from interferences among photoelectrons ionized by XUV pluses (streaked by the strong IR field) at different moments, and in some energy regions, together with the photoelectrons ionized by the intense IR pulse itself.

We demonstrate phase sensitivity in a horizontally guided, acceleration-sensitive atom interferometer with a momentum separation of $80\hslash k$ between its arms. A fringe visibility of 7% is observed. Our coherent pulse sequence accelerates the cold cloud in an optical waveguide, an inherently scalable route to large momentum separation and high sensitivity. We maintain coherence at high momentum separation due to both the transverse confinement provided by the guide and our use of optical $\delta$-kick cooling on our cold-atom cloud. We also construct a horizontal interferometric gradiometer to measure the longitudinal curvature of our optical waveguide.

We investigate single-particle properties of a mass-imbalanced Fermi gas in the BCS (Bardeen-Cooper-Schrieffer)–BEC (Bose-Einstein condensation) crossover region. In the presence of mass imbalance, we point out that the ordinary $T$-matrix approximation, which has been extensively used to clarify various BCS-BEC crossover physics in the mass-balanced case, unphysically gives a double-valued solution in terms of the superfluid phase transition temperature $T_{\mathrm{c}}$ in the crossover region. To overcome this serious problem, we include higher order strong-coupling corrections beyond the $T$-matrix level. Using this extended $T$-matrix theory, we calculate single-particle excitations in the normal state above $T_{\mathrm{c}}$. The so-called pseudogap phenomena originating from pairing fluctuations are shown to be different between the light-mass component and heavy-mass component, which becomes more remarkable at higher temperatures. Since Fermi condensates with hetero-Cooper pairs have recently been discussed in various fields, such as exciton (polariton) condensates as well as color superconductivity, our results would be useful for the further development of Fermi superfluid physics, beyond the conventional superfluid state with homo-Cooper pairs.

We develop a quantum Monte Carlo method to estimate the ground-state energy of a fermionic many-particle system in the configuration-interaction shell model approach. The fermionic sign problem is circumvented by using a guiding wave function in Fock space. The method provides an upper bound on the ground-state energy whose tightness depends on the choice of the guiding wave function. We argue that the antisymmetric geminal product class of wave functions is a good choice for guiding wave functions. We demonstrate our method for the trapped two-species fermionic cold atom system in the unitary regime of infinite scattering length using the particle-number projected Hartree-Fock-Bogoliubov wave function as the guiding wave function. We estimate the ground-state energy and energy-staggering pairing gap as a function of the number of particles. We compare our results with exact numerical diagonalization results and with previous fixed-node coordinate-space Monte Carlo calculations.

Scattering quantum bright solitons off barriers has been predicted to lead to nonlocal quantum superpositions, in particular the NOON state. The focus of this paper lies on signatures of both high- and low-fidelity quantum superposition states. We numerically demonstrate that a one-dimensional geometry with the barrier potential situated in the middle of an additional—experimentally typical—harmonic confinement gives rise to particularly well-observable signatures. In the elastic-scattering regime we investigate signatures of NOON states on the $N$-particle level within an effective potential approach. We show that removing the barrier potential and subsequently recombining both parts of the quantum superposition leads to a high-contrast interference pattern in the center-of-mass coordinate for narrow and broad potential barriers. We demonstrate that the presented signatures can be used to clearly distinguish quantum superposition states from statistical mixtures and are sufficiently robust against experimentally relevant excitations of the center-of-mass wave function to higher-lying oscillator states. For two-particle solitons we extend these considerations to low-fidelity superposition states: even for strong deviations from the two-particle NOON state we find interference patterns with high contrast.

We introduce an intuitive and accessible time-domain pulse-compression method. The mechanism is based on the coherent superposition of pulse replicas with different delays directly in the time domain. Simulations show that the delays can be chosen such that constructive interference occurs at the pulse peak, whereas destructive interference occurs in the wings of the pulse. Chirped and complex pulses are compressed to demonstrate the capability and versatility of the method. The pulse durations obtained after compression are close to the duration of the corresponding transform-limited pulse.

We demonstrate efficient generation of multicolor vacuum ultraviolet pulses with excellent mode quality through ${\chi }^{\left(3\right)}$-based four-wave sum-frequency mixing and third-order harmonic generation of 400- and 267-nm femtosecond laser pulses in argon gas. The ${\chi }^{\left(3\right)}$-based nonlinear optical processes were optimized with appropriate control of gas pressure and group velocity delay between the driving pulses. Furthermore, the pulse breakup effects were observed for tightly focused ultraviolet pulses.

We investigate the dynamics of the spaser-based nanolaser in the strong incoherent pumping regime in the quantum limit when the photon number is the order of unity. We consider the situation where the newly irradiated photon finds itself in the cloud of earlier irradiated photons that are not thermalized. As the result the entanglement of nanoparticle with quantum dot degrees of freedom in the nanolaser and the lasing intensity increases several times. In fact, the nonthermal bath effectively makes the nanolaser “more quantum” and the master equation for the nanolaser density matrix nonlinear and self-consistent.

Two physical principles, macroscopic locality (ML) and information causality (IC), so far have been most successful in distinguishing quantum correlations from post-quantum correlations. However, there are also some post-quantum probability distributions which cannot be distinguished with the help of these principles. Thus, it is interesting to see whether consideration of these two principles, separately, along with some additional physically plausible constraints, can explain some interesting quantum features which are otherwise hard to reproduce. In this paper we show that in a Bell-Clauser-Horne-Shimony-Holt scenario, ML along with the constraint of equal bias for the concerned observables, almost reproduces the quantum joint probability distribution corresponding to a maximal quantum Bell violation, which is unique up to relabeling. From this example and earlier work of Cavalcanti, Salles, and Scarani, we conclude that IC and ML are inequivalent physical principles; satisfying one does not imply that the other is satisfied.

The nonlocal correlations exhibited when measuring entangled particles can be used to certify the presence of genuine randomness in Bell experiments. While nonlocality is necessary for randomness certification, it is unclear when and why nonlocality certifies maximal randomness. We provide a simple argument to certify the presence of maximal local and global randomness based on symmetries of a Bell inequality and the existence of a unique quantum probability distribution that maximally violates it. We prove the existence of $N$-party Bell tests attaining maximal global randomness by identifying those combinations of two-outcome measurements by each party providing $N$ perfect random bits.

This paper describes an approach to construct temporally shaped control pulses that drive a quantum system toward desired properties. A parametrization in terms of periodic functions with predefined frequencies permits us to realize a smooth, typically simple shape of the pulses; their optimization can be performed based on a variational analysis with Floquet theory. As we show with selected specific examples, this approach permits us to control the dynamics of interacting spins, such that gate operations and entanglement dynamics can be implemented with very high accuracy.

Photon-echo-based quantum memories use inhomogeneously broadened, optically thick ensembles of absorbers to store a weak optical signal and employ various protocols to rephase the atomic coherences for information retrieval. We study the application of two consecutive, frequency-chirped control pulses for coherence rephasing in an ensemble with a “natural” inhomogeneous broadening. Although propagation effects distort the two control pulses differently, chirped pulses that drive adiabatic passage can rephase atomic coherences in an optically thick storage medium. Combined with spatial phase-mismatching techniques to prevent primary echo emission, coherences can be rephased around the ground state to achieve secondary echo emission with close to unit efficiency. Potential advantages over similar schemes working with $\pi$ pulses include greater potential signal fidelity, reduced noise due to spontaneous emission, and better capability for the storage of multiple memory channels.

We have developed a one-electron detachment theory from a closed-shell atomic configuration in the relativistic Fock-space coupled-cluster ansatz. Using this method, we determine sensitivity coefficients to the variation of the fine-structure constant in the first three important low-lying transitions of the astrophysically interesting highly charged Mn ix, Fe x, Co xi, and Ni xii ions. The potential of this method has been assessed by evaluating the detachment energies of the removed electrons and determining the lifetimes of the atomic states in the above ions. To account for the sensitivity of the higher-order relativistic effects, we have used the four-component wave functions of the Dirac-Coulomb-Breit Hamiltonian with the leading-order quantum electrodynamics (QED) corrections. A systematic study has been carried out to highlight the importance of the Breit and QED interactions in the considered properties of the above ions.