An ultralow-temperature binary mixture of Bose-Einstein condensates adsorbed at an optical wall can undergo a wetting phase transition in which one of the species excludes the other from contact with the wall. Interestingly, while hard-wall boundary conditions entail the wetting transition to be of first order, using Gross-Pitaevskii theory we show that first-order wetting as well as critical wetting can occur when a realistic exponential optical wall potential (evanescent wave) with a finite turn-on length $\lambda$ is assumed. The relevant surface excess energies are computed in an expansion in $\lambda /{\xi }_i$, where ${\xi }_i$ is the healing length of condensate $i$. Experimentally, the wetting transition may best be approached by varying the interspecies scattering length $a_{12}$ using Feshbach resonances. In the hard-wall limit, $\lambda \to 0$, exact results are derived for the prewetting and first-order wetting phase boundaries.

Quantum squeezing in mechanical systems is not only a key signature of macroscopic quantum effects, but can also be utilized to advance the metrology of weak forces. Here we show that strong mechanical squeezing in the steady state can be generated in an optomechanical system with mechanical nonlinearity and red-detuned monochromatic driving on the cavity mode. The squeezing is achieved as the joint effect of nonlinearity-induced parametric amplification and cavity cooling and is robust against thermal fluctuations of the mechanical mode. We also show that the mechanical squeezing can be detected via an ancilla cavity mode.

A fermionic analog of a parametric amplifier is used to describe the joint quantum state of the two interacting fermionic modes. Based on a two-mode generalization of the time-dependent density operator, time evolution of the fermionic density operator is determined in terms of its two-mode Wigner and $P$ function. It is shown that the equation of motion of the Wigner function corresponds to a fermionic analog of Liouville's equation. The equilibrium density operator for fermionic fields developed by Cahill and Glauber is thus extended to a dynamical context to show that the mathematical structures of both the correlation functions and the weight factors closely resemble their bosonic counterpart. It has been shown that the fermionic correlation functions are marked by a characteristic upper bound due to Fermi statistics, which can be verified in the matter wave counterpart of photon down-conversion experiments.

Kerr-lens mode-locked Ti:sapphire lasers are known to display three coexistent modes of operation: continuous wave, transform limited pulses (P1), and positive chirped pulses (P2). Extreme events (sometimes also called optical rogue waves), in the form of pulses of high energy appearing much often than in a Gaussian distribution, are observed in the chaotic regime of the P2 mode, but not of P1. The extreme events in P2 appear unpredictably, but their separation (measured in number of round trips) is a simple combination of the numbers 11 and 12 (which were named “magic numbers”). The existence of extreme events in P2 and not in P1, and also of the magic numbers, have been successfully reproduced by numerical simulations based on a five-variables iterative map, but the intuitive insight on the physical causes has been limited. In this paper, we present evidence that the extreme events in this laser appear if the amount of self-phase modulation on the pulses is above a certain threshold, and also that the P1 mode becomes unstable before crossing that threshold. This explains why the extreme events are observed in P2, and not in P1. Remarkably, even though the values of self-phase modulation on all the pulses (in the chaotic regime) are widely spread, the values inside the set of extreme events are relatively well defined. Finally, the magic numbers are found to be the residuals of the periodical orbits of the “cold” laser cavity when they are perturbed by the opposite effects of a dissipative term, due to the presence of transversal apertures, and an expansive term, due to the self-focusing.

There has been a surge of research activity recently on the role of joint measurability of unsharp observables in nonlocal features, viz., violation of Bell inequality and EPR steering. Here, we investigate the entropic uncertainty relation for a pair of noncommuting observables (of Alice's system) when an entangled quantum memory of Bob is restricted to record outcomes of jointly measurable positive operator valued measures. We show that with this imposed constraint of joint measurability at Bob's end, the entropic uncertainties associated with Alice's measurement outcomes—conditioned by the results registered at Bob's end—obey an entropic steering inequality. Thus, Bob's nonsteerability is intrinsically linked to his inability to predict the outcomes of Alice's pair of noncommuting observables with better precision, even when they share an entangled state. As a further consequence, we prove that in the joint measurability regime, the quantum advantage envisaged for the construction of security proofs in quantum key distribution is lost.

Universal quantum computation requires the implementation of arbitrary control operations on the quantum register. In most cases, this is achieved by external control fields acting selectively on each qubit to drive single-qubit operations. In combination with a drift Hamiltonian containing interactions between the qubits, this allows the implementation of arbitrary gate operations. Here, we demonstrate an alternative scheme that does not require local control for all qubits: we implement one- and two-qubit gate operations on a set of target qubits indirectly, through a combination of gates on directly controlled actuator qubits with a drift Hamiltonian that couples actuator and target qubits. Experiments are performed on nuclear spins, using radio-frequency pulses as gate operations and magnetic-dipole couplings for the drift Hamiltonian.

Distinguishing hot from cold is the most primitive form of thermometry. Here we consider how well this task can be performed using a single qubit to distinguish between two different temperatures of a bosonic bath. In this simple setting, we find that letting the qubit equilibrate with the bath is not optimal, and depending on the interaction time it may be advantageous for the qubit to start in a state with some quantum coherence. We also briefly consider the case that the qubit is initially entangled with a second qubit that is not put into contact with the bath and show that entanglement allows for even better thermometry.

We study tunneling ionization in a one-dimensional three-body model of a molecule, treating both electronic and nuclear degrees of freedom exactly. In a recent paper [Phys. Rev. Lett. 111, 153003 (2013)] it was demonstrated using a finite-range potential that the Born-Oppenheimer approximation breaks down for fields weaker than a critical field $F_{\mathrm{BO}}$ when describing tunneling ionization of molecules, but works for stronger fields. It was also demonstrated that the weak-field asymptotic theory allows for an accurate description in this weak-field limit. In the present paper, we consider a potential with a Coulomb tail and nonzero dipole moment, modeling polar molecules. Our study shows that the conclusions of the aforementioned paper also hold for this potential.

We present a formalism to calculate phase-space acceptance in a Zeeman decelerator. Using parameters closely mimicking previous Zeeman deceleration experiments, this approach reveals a velocity dependence of the phase stability which we ascribe to the finite rise and fall times of the current pulses that generate the magnetic fields inside the deceleration coils. It is shown that changing the current switch-off times (characterized by the reduced position of the synchronous particle ${\kappa }_0$) as the sequence progresses, so as to maintain a constant mean acceleration per pulse, can lead to a constant phase stability and hence a beam with well-defined characteristics. We also find that the time overlap between fields of adjacent coils has an influence on the phase-space acceptance. Previous theoretical and experimental results [A. W. Wiederkehr et al., Phys. Rev. A 82, 043428 (2010); J. Chem. Phys. 135, 214202 (2011).] suggested unfilled regions in phase space that influence particle transmission through the decelerator. Our model provides a means to directly identify the origin of these effects due to coupling between longitudinal and transverse dynamics. Since optimum phase stability is restricted to a rather small parameter range in terms of the reduced position of the synchronous particle ${\kappa }_0$, only a limited range of final velocities can be attained using a given number of coils. We evaluate phase stability for different Zeeman deceleration sequences, and by comparison with numerical three-dimensional particle-trajectory simulations, we demonstrate that our model provides a valuable tool to find optimum parameter sets for improved Zeeman deceleration schemes. An acceleration-deceleration scheme is shown to be a useful approach to generating beams with well-defined properties for variable-energy collision experiments. More generally, the model provides significant physical insights that are applicable to other types of particle decelerators with finite rise and fall time fields.

We present an efficient tool capable of measuring the spectral correlations between photons emerging from a Hong-Ou-Mandel interferometer. We show that for our spectrally factorizable spontaneous down-conversion source, the Hong-Ou-Mandel interference visibility decreases as the photons' frequency spread is increased to a maximum of 165 nm. Unfiltered, we obtained a visibility of $92.0\%\pm 0.2\%.$ The maximum visibility was $97\%\pm 0.2\%$ after applying filtering. We show that the tool can be useful for the study of spectral correlations that impair high-visibility and high-fidelity multisource interference applications. The nature of this tool also allows for arbitrary post-selective spectral filtering and high-rate multiphoton spectral correlation measurements.

We show that both the power and repetition rate of a frequency comb generated in a nonlinear ring resonator, pumped with continuous-wave (cw) coherent light, are modulated. The modulation is brought about by the interaction of the cw background with optical pulses excited in the resonator, and occurs in resonators with nonzero high-order chromatic dispersion and wavelength-dependent quality factor. The modulation frequency corresponds to the detuning of the pump frequency from the eigenfrequency of the pumped mode in the resonator.

The effects of small relativistic corrections to the off-resonant polarizability, hyperpolarizability, and second hyperpolarizability are investigated. Corrections to linear- and nonlinear-optical coefficients are demonstrated in the three-level ansatz, which includes corrections to the Kuzyk limits when scaled to semirelativistic energies. It is also shown that the maximum value of the hyperpolarizability is more sensitive than the maximum polarizability or second hyperpolarizability to lowest-order relativistic corrections. These corrections illustrate how the intrinsic nonlinear-optical response is affected at semirelativistic energies.

Optomechanics is concerned with the coupling between optical cavities and mechanical resonators. Most early works are concentrated in the physics of optomechanics in the small-displacement regime and consider one single optical cavity mode participating in the optomechanical coupling. In this article, we focus on optomechanics in the extremely-large-amplitude regime in which a mechanical resonator is coupled with multiple optical cavity modes during the oscillation. We explicitly show that the mechanical resonator can present self-sustained oscillations with limit cycles in the shape of sawtooth-edged ellipses and can exhibit dynamical multistability. By analyzing the mechanical oscillation process and the accompanying variation of the optical cavity occupation, we develop an energy-balanced condition to ensure the stability of self-sustained oscillation. The effect of the mechanical nonlinearities on the dynamics of the mechanical resonator is also investigated.

We numerically investigate entropic Bell inequalities for a pair of entangled qutrits using information-theoretic distances. We show that for this class of inequalities the Tsallis entropy is more suitable than the Shannon entropy, as it reveals nonclassicality for a larger set of quantum states. Finally, we find that, like probability-based inequalities, entropic ones are maximally violated by the non–maximally entangled qutrit state.

We study, in the paradigm of open quantum systems, the entanglement dynamics of two uniformly accelerated atoms with the same acceleration perpendicular to the separation. The two-atom system is treated as an open system coupled with a bath of fluctuating massless scalar fields in the Minkowski vacuum, and the master equation that governs its evolution is derived. It has been found that, for accelerated atoms with a nonvanishing separation, entanglement sudden death is a general feature when the initial state is entangled, while for those in a separable initial state, entanglement sudden birth only happens for atoms with an appropriate interatomic separation and sufficiently small acceleration. Remarkably, accelerated atoms can get entangled in certain circumstances while the inertial ones in the Minkowski vacuum cannot. A comparison between the results of accelerated atoms and those of static ones in a thermal bath shows that uniformly accelerated atoms exhibit features distinct from those immersed in a thermal bath at the Unruh temperature in terms of entanglement dynamics.

Exploiting multidimensional quantum walks as feasible platforms for quantum computation and quantum simulation attracts constantly growing attention from a broad experimental physics community. Here, we propose a two-dimensional quantum walk scheme with a single-qubit coin that presents, in the considered regimes, a strong localizationlike effect on the walker. The result could provide new possible directions for the implementation of quantum algorithms or from the point of view of quantum simulation. We characterize the localizationlike effect in terms of the parameters of a step-dependent qubit operation that acts on the coin space after any standard coin operation, showing that a proper choice can guarantee a nonnegligible probability of finding the walker in the origin even for large times. We finally discuss the robustness to imperfections, a qualitative relation with coherences behavior, and possible experimental realizations of this model with the current state-of-the-art settings.

In this paper, we mainly study the local indistinguishability of mutually orthogonal bipartite maximally entangled states. We construct sets of fewer than $d$ orthogonal maximally entangled states which are not distinguished by one-way local operations and classical communication (LOCC) in the Hilbert space of $d\otimes d$. The proof, based on the Fourier transform of an additive group, is very simple but quite effective. Simultaneously, our results give a general unified upper bound for the minimum number of one-way LOCC indistinguishable maximally entangled states. This improves previous results which only showed sets of $N\ge d-2$ such states. Finally, our results also show that previous conjectures in Zhang et al. [Z.-C. Zhang, Q.-Y. Wen, F. Gao, G.-J. Tian, and T.-Q. Cao, Quant. Info. Proc. 13, 795 (2014)] are indeed correct.

We study the $2e^{-}2e^{+}$ equal-mass charge-neutral four-body system in the adiabatic hyperspherical framework. The lowest few adiabatic potentials are calculated for zero orbital angular momentum, positive parity, and charge conjugation symmetries. Propagating the $R$ matrix, the low-energy $s$-wave scattering lengths of the singlet-singlet and triplet-triplet spin configurations are calculated. Last, we calculate the $S$ matrix for energies above the ionic threshold to estimate the transition rates between the single ionic fragmentation channel and the lowest few dimer-dimer fragmentation channels.

The local theory of relaxation of ${}^3\mathrm{He}$ by Brownian motion in an inhomogeneous magnetic field, originally proposed by Gamblin and Carver [Phys. Rev. 138, A946 (1965)] and by Schearer and Walters [Phys. Rev. 139, A1398 (1965)], is reconsidered in detail to clarify (or not) the approximations involved. Notably, a discussion is given of frames rotating at time-dependent angular velocities (both in magnitude and in direction), and attention is drawn to the often overlooked long tail that confinement adds to velocity correlation functions. A stepwise application of the Redfield theory of relaxation eventually provides quantitative predictions for the rate of depolarization and the linear-in-electric-field frequency shift, in terms of the components of the local inhomogeneity tensor and the usual parameters (nuclear magnetic resonance frequency, diffusion coefficient, mean time between collisions, etc.), valid in the common situation of mean free path much smaller than the size of the container. These predictions agree with almost all results of previous publications, most of which are based on the global approach which evaluates correlation functions by reference to the diffusion equation in spite of its limitations for short delays.