Motivated by a recent experiment [Beattie, Moulder, Fletcher, and Hadzibabic, Phys. Rev. Lett. 110, 025301 (2013)], we study the superflow of atomic spinor Bose-Einstein condensates optically trapped in a ring-shaped geometry. Within a dissipative mean-field approach we simulate a two-component condensate in conditions adapted to the experiment. In qualitative agreement with the experimental findings, we observe stable superflow if the spin-population imbalance is above some well-defined threshold value. Below the stability threshold the persistent currents with higher-order circulation decay in quantized steps, and the vortex lines escape from the center of the ring through dynamically created “weak links” in the condensate annulus. These regions with reduced density of one spin component are filled by atoms of the other component. The vortices then leave the ring-shaped high-density region of the condensate and finally decay into elementary excitations.

Quantum theory can be viewed as a generalization of classical probability theory, but the analogy as it has been developed so far is not complete. Whereas the manner in which inferences are made in classical probability theory is independent of the causal relation that holds between the conditioned variable and the conditioning variable, in the conventional quantum formalism, there is a significant difference between how one treats experiments involving two systems at a single time and those involving a single system at two times. In this article, we develop the formalism of quantum conditional states, which provides a unified description of these two sorts of experiment. In addition, concepts that are distinct in the conventional formalism become unified: Channels, sets of states, and positive operator valued measures are all seen to be instances of conditional states; the action of a channel on a state, ensemble averaging, the Born rule, the composition of channels, and nonselective state-update rules are all seen to be instances of belief propagation. Using a quantum generalization of Bayes’ theorem and the associated notion of Bayesian conditioning, we also show that the remote steering of quantum states can be described within our formalism as a mere updating of beliefs about one system given new information about another, and retrodictive inferences can be expressed using the same belief propagation rule as is used for predictive inferences. Finally, we show that previous arguments for interpreting the projection postulate as a quantum generalization of Bayesian conditioning are based on a misleading analogy and that it is best understood as a combination of belief propagation (corresponding to the nonselective state-update map) and conditioning on the measurement outcome.

We derive the expression for the energy uncertainty of the final state of a decay of an unstable quantum state prepared at initial time $t=0$. This expression is a function of the time $t$ at which a measurement is performed to determine if the state has decayed and, if yes, in which one of the infinitely many possible final states. For large times the energy spread is, as expected, given by the decay width $\Gamma$ of the initial unstable state. However, if the measurement of the final state is performed at a time $t$ comparable to (or smaller than) the mean lifetime of the state $1/\Gamma$, then the uncertainty in the energy of the final state is much larger than the decay width $\Gamma$. Namely, for short times an uncertainty of type $1/t$ dominates, while at long times the usual spread $\Gamma$ is recovered. Then we turn to a generic two-body decay process and describe the energy uncertainty of each of the two outgoing particles. We apply these formulas to the two-body decays of neutral and charged pions and to the spontaneous emission process of an excited atom. As the last step, we study a case in which the nonexponential decay is realized and show that for short times eventual asymmetric terms are enhanced in the spectrum.

We present a method to apply the well-known matrix product state formalism to partially separable states in solid-state systems. The computational effort of our method is equal to the effort of the standard density matrix renormalization group (DMRG) algorithm. Consequently, it is applicable to all the usually considered condensed-matter systems where the DMRG algorithm is successful. We also show in exemplary cases that polymerization properties of ground states are closely connected to properties of partial separability, even if the ground state itself is not partially separable.

Entangled coherent states are useful for various applications in quantum information processing, but they are sensitive to loss. We propose a scheme to generate distributed entangled coherent states over a lossy environment in such a way that the fidelity is independent of the losses at detectors heralding the generation of the entanglement. We compare our scheme with a previous one for the same purpose [Ourjoumtsev et al., Nat. Phys. 5, 189 (2009)] and find parameters for which our scheme results in superior performance.

Spin chains are promising media for short-haul quantum communication. Their usefulness is manifested in all those situations where stationary information carriers are involved. In the majority of the communication schemes relying on quantum spin chains, the latter are assumed to be finite in length, with well-addressable end-chain spins. In this paper we propose that such a configuration could actually be achieved by a mechanism that is able to effectively cut a spin ring through the insertion of bond defects. We then show how suitable physical quantities can be identified as figures of merit for the effectiveness of the cut. We find that, even for modest strengths of the bond defect, a ring is effectively cut at the defect site. In turn, this has important effects on the amount of correlations shared by the spins across the resulting chain, which we study by means of a scattering-based mechanism of a clear physical interpretation.

The x-ray wavelengths and rates from double electron radiative transitions in Li isoelectronic sequences are evaluated for 17 ions with $14\le Z\le 54$ using fully relativistic multiconfiguration Dirac-Fock wave functions in the active space approximation with the inclusion of finite nuclear size, Breit interaction, self-energy, and vacuum polarization. A detailed discussion on the anomalous mixing coefficients contributions to the $1s2s{(}^3{S}_1)3p$ ${}^2{P}_{1/2}$ level for ions with $38\le Z\le 43$ is given. The sensitivity of transition rates to the Breit interaction is also analyzed. The present results are compared with other available theoretical data. The errors associated with the transitions are highlighted for some of the strongest lines, taking into account the uncertainties on the fine-structure energy levels and also on the line strengths.

Absolute double differential cross sections (DDCS) have been measured for the ionization of gas-phase uracil (C${}_4$H${}_4$N${}_2$O${}_2$) by 0.5, 1.0, and 2.0 MeV proton impacts. Measurements were performed by secondary electron spectroscopy using a 45${}^{\circ }$ parallel plate electrostatic spectrometer over an energy range of 1.0–1000 eV at emission angles from 15${}^{\circ }$ to 165${}^{\circ }$. Theoretical calculation of ionization cross sections has also been carried out by using the first-order Born approximation with correct boundary conditions. Single differential and total ionization cross sections deduced from DDCS revealed fairly good agreement between experimental and theoretical values at all the emission angles investigated.

We investigate coherent control of the two-photon transition pathways of a four-level atomic system in a diamond configuration. When an ultrashort laser pulse interacts with this system in the ground state $5S_{1/2}$ of rubidium, the two-photon transition probability amplitude of $5D_{3/2}$ is obtained by a summation of all possible resonant and nonresonant two-photon transition probability amplitudes via $5P_{1/2}$ and $5P_{3/2}$. Second-order perturbation theory predicts that the maximal constructive interference of the transition probability amplitudes occurs when the phases of eight different spectrum blocks satisfy four different phase relations. Experiments carried out with spectrally phase-coded laser pulses show good agreement with the theoretical prediction.

We investigate the effect of intramolecular interference in the high-order harmonic generation (HHG) from oriented asymmetric diatomic molecules interacting strongly with a laser field numerically. We focus on the two-center–interference-induced minimum in the two HHG channels of odd and even harmonics. As the interference usually results in a well-defined minimum for symmetric molecules, the odd or even HHG spectra of asymmetric molecules do not show a pronounced minimum in our simulations. We show that the interplay of different recombination routes, arising from the asymmetry of the molecule, significantly influences the minimum. A scheme is proposed to identify the interference effect in the odd versus even HHG spectra for diverse molecular parameters.

White-light seeded forward lasing action from a femtosecond laser filament in air was systematically investigated. Using the self-generated white-light seed from 800- and 400-nm laser filaments, 428- and 391-nm lasings, respectively, were observed. When an external white-light seed produced from a femtosecond laser supercontinuum in a water cell was used, simultaneous multiple-wavelength (388-, 391-, 428-, and 471-nm) lasing action was experimentally demonstrated.

We develop a strong-coupling ($t\ll U$) expansion technique for calculating the density profile for bosonic atoms trapped in an optical lattice with an overall harmonic trap at finite temperature and finite on-site interaction in the presence of superfluid regions. Our results match well with quantum Monte Carlo simulations at finite temperature. We also show that the superfluid order parameter never vanishes in the trap due to the proximity effect. Our calculations for the scaled density in the vacuum-to-superfluid transition agree well with the experimental data for appropriate temperatures. We present calculations for the entropy per particle as a function of temperature which can be used to calibrate the temperature in experiments. We also discuss issues connected with the demonstration of universal quantum critical scaling in the experiments.

We propose an experiment to demonstrate a strong coherent backward and forward microwave generation using forward-propagating optical fields only. This is achieved by applying laser fields to an ultradispersive medium with properly chosen detunings and/or Rabi frequencies to excite a spin coherence that generates a forward- or backward-propagating microwave. Applications to coherent scattering and remote sensing are discussed.

The violation of Bell inequalities where both detection and locality loopholes are closed is crucial for device-independent assessments of quantum information. While of a technological nature, the simultaneous closing of both loopholes still remains a challenge. In Teo et al. [Nat. Commun. 4, 2104 (2013)], a realistic setup to produce an atom-photon entangled state that could reach a loophole-free Bell inequality violation within current experimental technology is proposed. Here we improve the analysis of this proposal by giving an analytical treatment that shows that the state proposed in Teo et al. [Nat. Commun. 4, 2104 (2013)] can violate a Bell inequality for arbitrarily low photodectection efficiency, when all other losses are ignored. Moreover, it is also able to violate a Bell inequality considering only atomic and homodyne measurements eliminating the need to consider inefficient photocounting measurements. In this case, the maximum Clauser-Horne-Shimony-Holt inequality violation achievable is 2.29, and the minimum transmission required for violation is about 68%. Finally, we show that by postselecting on an atomic measurement, one can engineer superpositions of coherent states for various coherent state amplitudes.

We derive the analytical solution uncovering the origin of the ultrashort pulse formation from the resonant radiation in a hydrogenlike medium [Y. V. Radeonychev, V. A. Polovinkin, and O. Kocharovskaya, Phys. Rev. Lett. 105, 183902 (2010)], which is a quantum interference of the atomic transitions from the ground to the first excited energy level split by an intense far-off-resonant laser field due to the instantaneous Stark effect into the periodically oscillating sublevels and interference of the resonantly scattered radiation with the incident one. The analytical solution shows that the pulses are almost bandwidth limited and can be produced in a wide range of parameters in excellent agreement with the more general numerical simulation. The experimental schemes to form few-femtosecond pulses from 122-nm radiation in atomic hydrogen as well as few-hundred-attosecond pulses from 13.74-nm radiation in a Li${}^{2+}$ medium are discussed.

We propose to realize effective beam-splitter-like and two-mode-squeezing photon-photon interactions in an optomechanical interface by exploiting detuned driving lasers. In this interface, the transitions between the optical and the mechanical states are significantly suppressed by the large energy offsets, therefore protecting the photon-photon interactions from mechanical dissipations. Moreover, the destructive quantum interference between the eigenmodes of the interface is capable of reducing the effects of initial mechanical thermal occupations. This optomechanical interface can serve as a universal block for photon state engineering and hybrid quantum networks in high-temperature thermal bath and without the requirement of cooling the mechanical oscillator to the ground state.

We investigate the zero-temperature properties of a quasi-ideal Fermi gas with Rashba spin-orbit coupling. We find that the spin-orbit term strongly affects the speeds of zero sound and first sound in the Fermi gas, due to the presence of a third-order quantum phase transition. In addition, including a two-dimensional harmonic confinement, we show that also the shape of the density profile of the cloud crucially depends on the strength of the Rashba coupling.

We present an efficient method to simulate a quantum process subject to dissipation and noise. To describe the effect on any input state we evolve Monte Carlo wave functions for a principal and ancilla system, prepared initially in an entangled state. In analogy to experimental process tomography, the simulated propagator for the system density matrix is conveniently described by a process $\chi$ matrix - directly determined from the stochastic state vectors. Our method significantly reduces the computational complexity compared with standard theoretical characterization methods. It also delivers an upper bound on the trace distance between the ideal and simulated process based on the evolution of only a single wave function of the entangled system.

We study decoy-state measurement-device-independent quantum key distribution using heralded single-photon sources. This has the advantage that the observed error rate in the $X$ basis is at higher order and not so large. We calculate the key rate and transmission distance for two cases: one using only triggered events, and the other using both triggered and nontriggered events. We compare the key rates of various protocols and find that our scheme using triggered and nontriggered events can give a higher key rate and a longer secure distance. Moreover, we also show the different behavior of our scheme when using different heralded single-photon sources, i.e., in Poisson or thermal distributions. We demonstrate that the former can generate a higher secure key rate than the latter and can thus work more efficiently in practical quantum key distributions.