We investigate the destiny of a gray soliton in a repulsive one-dimensional Bose-Einstein condensate undergoing a sudden quench of the nonlinearity parameter. The outcome of the quench is found to depend dramatically on the ratio $\eta$ of the final and initial values of the speed of sound. For integer $\eta$ the soliton splits into exactly $2\eta -1$ solitons. For noninteger $\eta$ the soliton decays into multiple solitons and Bogoliubov modes. The case of integer $\eta$ is analyzed in detail. The parameters of solitons in the out state are found explicitly. Our approach exploits the inverse scattering method and can be easily used for similar quenches in any classical integrable system.

We derive perturbation analytic expressions for the Casimir free energy and entropy between two dissimilar ferromagnetic plates which are applicable at arbitrarily low temperature. The dielectric properties of metals are described using either the nondissipative plasma model or the Drude model taking into account the dissipation of free charge carriers. Both cases of constant and frequency-dependent magnetic permeability are considered. It is shown that for ferromagnetic metals described by the plasma model the Casimir entropy goes to zero when the temperature vanishes, i.e., the Nernst heat theorem is satisfied. For ferromagnetic metals with perfect crystal lattices described by the Drude model the Casimir entropy goes to a nonzero constant depending on the parameters of a system with vanishing temperature, i.e., the Nernst heat theorem is violated. This constant can be positive which is quite different from the earlier investigated case of two nonmagnetic metals.

In this work, we design a quantum key distribution protocol, allowing two limited semiquantum or “classical” users to establish a shared secret key with the help of a fully quantum server. A semiquantum user can prepare and measure qubits only in the computational basis and so must rely on this quantum server to produce qubits in alternative bases and also to perform alternative measurements. However, we assume that the server is untrusted and we prove the unconditional security of our protocol even in the worst case: when this quantum server is an all-powerful adversary. We also compute a lower bound of the key rate of our protocol, in the asymptotic scenario, as a function of the observed error rate in the channel, allowing us to compute the maximally tolerated error of our protocol. Our results show that a semiquantum protocol may hold similar security to a fully quantum one.

We propose two compact, economic, and scalable schemes for implementing optical controlled-phase-flip and controlled-controlled-phase-flip gates by using the input-output process of a single-sided cavity strongly coupled to a single nitrogen-vacancy-center defect in diamond. Additional photonic qubits, necessary for procedures based on the parity-check measurement or controlled-path and merging gates, are not employed in our schemes. In the controlled-path gate, the paths of the target photon are conditionally controlled by the control photon, and these two paths can be merged back into one by using a merging gate. Only one half-wave plate is employed in our scheme for the controlled-phase-flip gate. Compared with the conventional synthesis procedures for constructing a controlled-controlled-phase-flip gate, the cost of which is two controlled-path gates and two merging gates, or six controlled-not gates, our scheme is more compact and simpler. Our schemes could be performed with a high fidelity and high efficiency with current achievable experimental techniques.

In many areas of physics, the Kramers-Kronig relations are used to extract information about the real part of the optical response of a medium from its imaginary counterpart. In this paper we discuss an alternative but mathematically equivalent approach based on the Hilbert transform. We apply the Hilbert transform to transmission spectra to find the group and refractive indices of a Cs vapor and thereby demonstrate how the Hilbert transform allows indirect measurement of the refractive index, group index, and group delay while avoiding the use of complicated experimental setups.

We revisit the discrepancy between experiments and theory for the x-ray transitions of heliumlike ions and report on a measurement of the $n=2\to n=1$ x-ray transitions of heliumlike ${\mathrm{Cu}}^{27+}$. These measurements were carried at the Livermore electron beam ion trap facility and achieved an accuracy of 18 ppm. The measured values show reasonable agreement with theory, but they do not follow the trend established by other measurements with similar uncertainties.

We theoretically analyze the low-energy electrons appearing in strong-field ionization of atomic and molecular targets by laser pulses with large ellipticity. We present the semiclassical model used to calculate the photoelectron momentum distributions and the derivations leading to a parameter introduced in a recent combined theoretical and experimental work on naphthalene [D. Dimitrovski et al., Phys. Rev. Lett. 113, 103005 (2014)]. The parameter quantifies both the possibility to observe such low-energy electrons and their relative impact in the photoelectron momentum distributions. The numerical value of this parameter is calculated for various cases of strong-field ionization by elliptically polarized pulses available in the literature; in addition, the possibility to detect low-energy electrons for large $\epsilon$ and different targets, including simple atoms, is discussed.

We show that additional features can emerge in the linear absorption spectra of homonuclear diatomic molecules when the ions are described quantum mechanically. In particular, the widths and energies of the peaks in the optical spectra change with the initial configuration, mass, and charge of the molecule. We introduce a model that can describe these features and we provide a quantitative analysis of the resulting peak energy shifts and width broadenings as a function of the mass.

Recently, a new three-level structure, called the $\Gamma$ system, which shows strong multiphoton coupling and harmonic generation, has been identified and studied. Under certain circumstances, the $\Gamma$ system appears in one-dimensional (1D) double-well potentials, suggesting that strong multiphoton excitation and harmonic generation may be seen in molecules at favorable internuclear separations, as various aspects of molecules in strong fields can be modeled by 1D potentials. However, even 1D potentials will have additional states, besides the $\Gamma$ system, which may interfere with these multiphoton processes. This paper presents numerical solutions to the time-dependent Schrödinger equation (TDSE) of two electrons in a double-well potential approximating a dication ($A_2{}^{2+}$) at various internuclear separations $R$. These calculations are compared to two- and three-level approximations of the full energy level structure. We show (1) the presence of the $\Gamma$ system in the 1D double-well potential; (2) that harmonic generation is enhanced at certain $R$; (3) that the enhancement is due to the $\Gamma$ system by showing that the three-level system shows the enhancement, while the two-level system does not. This goes beyond the work of Berent and Parzyński [Phys. Rev. A 82, 023804 (2010)] by demonstrating that the additional levels in the full TDSE calculation do not disrupt this response. In addition, we investigate a neutral double-well potential ($A_2$) with both TDSE and five-level calculations. While the level structure is not as clear as the dication, enhanced harmonic generation is still present, from which we conclude that the $\Gamma$ system inherent in the double-well potential still allows for a strong multiphoton coupling, even in the presence of other levels. From these calculations, we expect that neutral diatomic molecules at intermediate $R$ should be a strong source of harmonic generation when exposed to intense laser fields. Indeed, in both charge states, the harmonic radiation is highly nonperturbative, as the power in the harmonics grows with harmonic order from 3 to 17, under certain conditions.

We propose coupling two internal atomic states using a pair of Raman beams operated in Laguerre-Gaussian laser modes with unequal phase windings. This generates a coupling between the atom's pseudospin and its orbital angular momentum. We analyze the single-particle properties of the system using realistic parameters and provide detailed studies of the spin texture of the ground state. Finally, we consider a weakly interacting atomic condensate subject to this angular spin-orbit coupling and show how the interatomic interactions modify the single-particle physics.

We report on results of quantum Monte Carlo simulations for bosons in a two-dimensional quasiperiodic optical lattice. We study the ground state phase diagram at unity filling and confirm the existence of three phases: superfluid, Mott insulator, and Bose glass. At lower interaction strength, we find that sizable disorder strength is needed in order to destroy superfluidity in favor of the Bose glass. On the other hand, at large enough interaction superfluidity is completely destroyed in favor of the Mott insulator (at lower disorder strength) or the Bose glass (at larger disorder strength). At intermediate interactions, the system undergoes an insulator to superfluid transition upon increasing the disorder, while a further increase of disorder strength drives the superfluid to Bose glass phase transition. While we are not able to discern between the Mott insulator and the Bose glass at intermediate interactions, we study the transition between these two phases at larger interaction strength and find no evidence of a Mott-glass-like behavior.

The ability to accurately transfer quantum information through networks is an important primitive in distributed quantum systems. While perfect quantum state transfer (PST) can be effected by a single particle undergoing continuous-time quantum walks on a variety of graphs, it is not known if PST persists for many particles in the presence of interactions. We show that if single-particle PST occurs on one-dimensional weighted path graphs, then systems of hard-core bosons undergoing quantum walks on these paths also undergo PST. The analysis extends the Tonks-Girardeau ansatz to weighted graphs using techniques in algebraic graph theory. The results suggest that hard-core bosons do not generically undergo PST, even on graphs which exhibit single-particle PST.

The quantum version of a fundamental entropic data-processing inequality is presented. It establishes a lower bound for the entropy that can be generated in the output channels of a scattering process, which involves a collection of independent input bosonic modes (e.g., the modes of the electromagnetic field). The impact of this inequality in quantum information theory is potentially large and some relevant implications are considered in this work.

To achieve universal quantum computation using continuous variables, one needs to jump out of the set of Gaussian operations and have a non-Gaussian element, such as the cubic phase gate. However, such a gate is currently very difficult to implement in practice. Here we introduce an experimentally viable “repeat-until-success” approach to generating the cubic phase gate, which is achieved using sequential photon subtractions and Gaussian operations. We find that our scheme offers benefits in terms of the expected time until success, as well as the fact that we do not require any complex off-line resource state, although we require a primitive quantum memory.

We present a mass spectrometric technique for identifying the masses and relative abundances of Coulomb-crystallized ions held in a linear Paul trap. A digital radio-frequency wave form is employed to generate the trapping potential, as this can be cleanly switched off, and static dipolar fields are subsequently applied to the trap electrodes for ion ejection. Close to 100% detection efficiency is demonstrated for ${\mathrm{Ca}}^{+}$ and ${\mathrm{CaF}}^{+}$ ions from bicomponent ${\mathrm{Ca}}^{+}-{\mathrm{CaF}}^{+}$ Coulomb crystals prepared by the reaction of ${\mathrm{Ca}}^{+}$ with ${\mathrm{CH}}_3\mathrm{F}.$ A quantitative linear relationship is observed between ion number and the corresponding integrated time-of-flight (TOF) peak, independent of the ionic species. The technique is applicable to a diverse range of multicomponent Coulomb crystals—demonstrated here for ${\mathrm{Ca}}^{+}-\mathrm{NH}{}_3{}^{+}-\mathrm{NH}{}_4{}^{+}$ and ${\mathrm{Ca}}^{+}-\mathrm{CaOH}{}^{+}-\mathrm{CaOD}{}^{+}$ crystals—and will facilitate the measurement of ion-molecule reaction rates and branching ratios in complicated reaction systems.

We study a system of two bosonic fields subject to two-mode squeezing in the presence of dissipation. We find the Lie algebra governing the dynamics of the problem and use the Wei-Norman method to determine the solutions. Using this scheme, we arrive at a closed-form expression for an infinitely dimensional density operator which we use to calculate the degree of entanglement (quantified by Horodeckis' negativity) between the modes. We compare our result to the known continuous variable entanglement measures. We analyze the conditions for entanglement generation and the influence of thermal environments on the state formed. The problem is relevant, in particular, for understanding of quantum dynamics of coupled optical and/or mechanical modes in optomechanical and nanomechanical systems.

We study the coupled even number of microcavities with the balanced gain and loss between any pair of their neighboring components. The effective non-Hermitian Hamiltonian for such a structure has the cyclic permutation-time symmetry with respect to the cavity modes, and this symmetry determines the patterns of the dynamical evolutions of the cavity modes. The systems also have multiple exceptional points for the degeneracy of the existing supermodes, exhibiting the “phase transition” of system dynamics across these exceptional points. We illustrate the quantum dynamical properties of the systems with the evolutions of cavity photon numbers and correlation functions. Moreover, we demonstrate the effects of the quantum noises accompanying the amplification and dissipation of the cavity modes. The reciprocal light transportation predicted with the effective non-Hermitian models for the similar couplers is violated by the unavoidable quantum noises.

The Goos-Hänchen shift of the system composed by two cavity walls and an intracavity atomic sample is presented. The atomic sample is treated as a four-level double-$\Lambda$ system, driven by the two counterpropagating coupling fields. The probe field experiences the discontinuous refractive index variation and is reflected. Moreover, under the phase-matching condition, the four-wave mixing effect based on electromagnetically induced transparency can cause effective reflection. The Goos-Hänchen shifts appear in both situations and are carefully investigated in this article. We refer to the first one with the incident and reflected light having identical wavelength as the linear Goos-Hänchen shift, and the second one with the reflection wavelength determined by the phase-matching condition as the nonlinear Goos-Hänchen shift. The differences between the two kinds of shifts, such as the incident angle range, conditions for the shift peaks, and controllability, are discussed.