We theoretically study trapped one-dimensional Fermi gases in the presence of spin-orbit coupling induced by Raman lasers. The gas changes from a conventional (nontopological) superfluid to a topological superfluid as one increases the intensity of the Raman lasers above a critical chemical-potential-dependent value. Solving the Bogoliubov-de Gennes equations self-consistently, we calculate the density of states in real and momentum space at finite temperatures. We study Majorana fermions (MFs) which appear at the boundaries between topologically trivial and topologically nontrivial regions. We linearize the trap near the location of a MF, finding an analytic expression for the localized MF wave function and the gap between the MF state and other edge states.

We investigate a quantum-dot-based cavity system via the master-equation approach. The dynamics of the system is greatly affected by dissipation and dephasing processes. We include these phenomena to the theory through the master equation. The dissipation effects such as cavity loss, spontaneous recombination of excitons, and incoherent pumping are considered. The dephasing process is included as an electron–acoustic-phonon interaction. An intrinsic feature of solid-state cavity systems is the presence of electron-phonon interaction, which distinguishes this system from atomic cavity quantum electrodynamics. Due to the temperature dependence of phonons we use the complete form of the master equation (temperature dependence of dissipation rates) in this paper. We study the emission spectrum and photon statistics of the system. We show that cavity mode emission depends on temperature, and temperature strongly affects the photon statistics.

Quantum optics dictates that amplification of a pure state by any linear deterministic amplifier always introduces noise in the signal and results in a mixed output state. However, it has recently been shown that noiseless amplification becomes possible if the requirement of a deterministic operation is relaxed. Here we propose and analyze a noiseless amplification scheme where the energy required to amplify the signal originates from the stochastic fluctuations in the field itself. In contrast to previous amplification setups, our setup shows that a signal can be amplified even if no energy is added to the signal from external sources. We investigate the relation between the amplification and its success rate as well as the statistics of the output states after successful and failed amplification processes. Furthermore, we also optimize the setup to find the maximum success rates in terms of the reflectivities of the beam splitters used in the setup and discuss the relation of our setup with the previous setups.

Entanglement between macroscopically populated states can easily be created by combining a single photon and a bright coherent state on a beamsplitter. Motivated by the simplicity of this technique, we report on a method using displacement operations in the phase space and basic photon detections to reveal such an entanglement. We show that this eminently feasible approach provides an attractive way for exploring entanglement at various scales, ranging from 1 to 1000 photons. This offers an instructive viewpoint to gain insight into why it is hard to observe quantum features in our macroscopic world.

We study quantum teleportation between two different types of optical qubits, one of which is “particlelike” and the other “fieldlike,” via hybrid entangled states under the effects of decoherence. We find that teleportation from particlelike to fieldlike qubits can be achieved with a higher fidelity than that in the opposite direction. However, teleportation from fieldlike to particlelike qubits is found to be more efficient in terms of the success probabilities. Our study shows that the direction of teleportation should be considered an important factor in developing optical hybrid architectures for quantum information processing.

We present an experimental and theoretical study of the ${}^2{F}_{5/2}\to {}^2{F}_{7/2}M1$ transition in Ag-like W (W${}^{27+}$). The experiments employed the Shanghai permanent magnet electron beam ion trap, which has been developed especially for assisting spectroscopic diagnostics of edge plasmas for magnetic fusion devices. The theoretical value was obtained using the grasp2k set of computer codes and included a comprehensive correlation study. The experimental $M$1 wavelength was measured as 3377.43±0.26 Å (3378.43 Å vacuum wavelength), whereas the calculated wavelength is in good agreement at 3381.80 Å. This good agreement shows the importance of fully understanding the electron correlation effects to predict the energy of the fine structure even in this, for tungsten, relatively simple case.

We rapidly change the scattering length $a_s$ of a ${}^{87}$Rb Bose-Einstein condensate by means of a Feshbach resonance, simultaneously releasing the condensate from its harmonic trapping potential. When $a_s$ is changed from positive to negative, the subsequent collapse of the condensate is stabilized by the kinetic energy imparted during the release, resulting in a deceleration of the loss rate near the resonance. We also observe an increase in the Thomas-Fermi radius, near the resonance, that cannot be understood in terms of a simple scaling model. Instead, we describe this behavior using the Gross-Pitaevskii equation, including three-body recombination, and hypothesize that the increase in cloud radius is due to self-interference of the condensate resulting in the formation of concentric shells.

We propose a mechanism for producing $N$-photon states on demand leaking from a single-mode optical cavity interacting with a single atom and a laser pulse. The number of photons can be chosen because it is determined by the Zeeman substructure of the ground state of the atom and its initial state. The deterministic generation of traveling light of $1\le n\le 2F$ photons is achieved, when a circularly polarized laser pulse completely transfers the atomic population between Zeeman sublevels of the ground hyperfine state $F$ through far-detuned Raman scattering, thus producing linearly polarized cavity photons. We describe analytically the evolution of optical field taking into account the spontaneous losses and the cavity damping. We analyze the photon statistics showing that it is close to Poissonian light. We show also that this technique provides a deterministic source of a train of identical multiphoton states with a definite number of photons if a sequence of left- and right-circularly polarized laser pulses is applied. The scheme expands the possibilities for using complex internal states of light to transmit data.

The purpose of a phase-preserving linear amplifier is to make a small signal larger, regardless of its phase, so that it can be perceived by instruments incapable of resolving the original signal, while sacrificing as little as possible in signal-to-noise ratio. Quantum mechanics limits how well this can be done: A high-gain linear amplifier must degrade the signal-to-noise ratio; the noise added by the amplifier, when referred to the input, must be at least half a quantum at the operating frequency. This well-known quantum limit only constrains the second moments of the added noise. Here we derive the quantum constraints on the entire distribution of added noise: We show that any phase-preserving linear amplifier is equivalent to a parametric amplifier with a physical state for the ancillary mode; the noise added to the amplified field mode is distributed according to the Wigner function of the ancilla state.

The dispersion coefficients for the long-range interaction of the first four excited states of He, i.e., He(2 ${}^{1,3}$$S$) and He(2 ${}^{1,3}$$P$), with the low-lying states of the alkali-metal atoms Li, Na, K, and Rb are calculated by summing over the reduced matrix elements of the multipole transition operators. For the interaction between He and Li the uncertainty of the calculations is 0.1–0.5$\%$. For interactions with other alkali-metal atoms the uncertainty is 1–3$\%$ in the coefficient $C_5$, 1–5$\%$ in the coefficient $C_6$, and 1–10$\%$ in the coefficients $C_8$ and $C_{10}$. The dispersion coefficients $C_n$ for the interaction of He(2 ${}^{1,3}$$S$) and He(2 ${}^{1,3}$$P$) with the ground-state alkali-metal atoms and for the interaction of He(2 ${}^{1,3}$$S$) with the alkali-metal atoms in their first ${}^2$$P$ states are presented in this Brief Report. The coefficients for other pairs of atomic states are listed in the Supplemental Material.

We report an Einstein-Podolsky-Rosen-Bohm experiment with a pulsed source of entangled pairs of photons, recording the time of arrival of the pulses and of the detection of each single photon. This allows varying the parameters of the analysis (as the size of the time coincidence window) at will after the experiment has ended. Among other results, we present the measurement of the time variation of the Clauser-Horne-Shimony-Holt parameter during the pulse. The obtained results close (or at least impose new and tight restrictions to) the last loophole that remains open in the tests of quantum mechanics vs local realism (the so-called coincidence loophole).

We present a method for solving few-body problems for trapped particles and apply it to three equal-mass particles in a one-dimensional harmonic trap, interacting via a contact potential. By expressing the relative Hamiltonian in Jacobi cylindrical coordinates, i.e., the two-dimensional version of three-body hyperspherical coordinates, we discover an underlying $C_{6v}$ symmetry. This symmetry simplifies the calculation of energy eigenstates of the full Hamiltonian in a truncated Hilbert space constructed from the trap Hamiltonian eigenstates. Particle superselection rules are implemented by choosing the relevant representations of $C_{6v}$. We find that the one-dimensional system shows nearly the full richness of the three-dimensional system, and can be used to understand separability and reducibility in this system and in standard few-body approximation techniques.

We justify that homodyne tomography turns out to be informationally complete when the number of independent quadrature measurements is equal to the dimension of the density matrix in the Fock representation. Using this as our thread, we examine the completeness of other schemes when continuous-variable observations are truncated to discrete finite-dimensional subspaces.

We explore the effect of Shor-state construction methods on logical state encoding and quantum error-correction for the [7,1,3] Calderbank-Shor-Steane quantum error-correction code in a nonequiprobable-error environment. We determine the optimum number of verification steps to be used in Shor-state construction and whether Shor states without verification are usable for practical quantum computation. These results are compared to the same processes of encoding and error correction where Shor states are not used. We demonstrate that the construction of logical zero states with no first-order error terms may not require the complete edifice of quantum fault tolerance. With respect to error correction, we show for a particular initial state that error correction using a single qubit for the syndrome measurement yields a similar output-state accuracy to error correction using Shor states as syndrome qubits. In addition, we demonstrate that error correction with Shor states has an inherent sensitivity to bit-flip errors. Finally, we suggest that in this type of error-correction scenario one should always repeat a syndrome measurement until attaining an all-zero readout (twice in row).

There are two important paradigms for defining quantum correlations in quantum information theory, viz. the information-theoretic and the entanglement-separability ones. We find an analytical relation between two measures of quantum correlations, one in each paradigm, and show that only a certain cone-like region on the two-dimensional space spanned by these measures is accessible to pure three-qubit states. The information-theoretic multiparty quantum correlation measure is related to the monogamy considerations of a bipartite information-theoretic quantum correlation measure, while the entanglement-separability multiparty measure is the generalized geometric measure, a genuine multiparty entanglement measure. We also find an analytical relation between two multiparty entanglement measures, and again obtain a cone-like accessible region in this case. One of the multisite measures in this case is related to the monogamy of a bipartite entanglement measure, while the other is again the generalized geometric measure.

We present a proposal for a qubit charge meter (detector) based on a linear wire of quantum dots placed between two electron reservoirs. A qubit formed by an excess electron in a double quantum dot is coupled electrostatically with a single wire site and the dynamics of this system is studied using the equation of motion for appropriate correlation functions and the evolution operator method. For the qubit-wire system, depending on the qubit position on the wire the readout current oscillates over a long time or the current oscillations decrease very rapidly. This effect cannot be explained in terms of different charges of the wire sites (the same occupancies of all sites are considered). We have found that the qubit's decoherence strongly depends on the structure of the local density of states of the site which is coupled with the qubit. Additionally, the period of the readout current oscillations changes with the wire-qubit electrostatic coupling.

We study entanglement concentration of continuous-variable Gaussian states by local photon subtractions enhanced by coherent displacements. Instead of the previously considered symmetric two-mode squeezed vacuum states, we investigate the protocol for input states in the form of split single-mode squeezed vacuum (i.e., states obtained by mixing a single-mode squeezed vacuum with a vacuum state on a beam splitter), which is an experimentally highly relevant configuration. We analyze two scenarios in which the displacement-enhanced photon subtraction is performed either only on one or on both of the modes and show that local displacements can lead to improved performance of the concentration protocol.

We study the two-dimensional toric-code Hamiltonian with effective long-range interactions between its anyonic excitations induced by coupling the toric code to external fields. It has been shown that such interactions allow an arbitrary increase in the lifetime of the stored quantum information by making $L$, the linear size of the memory, larger [Chesi et al., Phys. Rev. A 82, 022305 (2010)]. We show that for these systems the choice of boundary conditions (open boundaries as opposed to periodic boundary conditions) is not a mere technicality; the influence of anyons produced at the boundaries becomes in fact dominant for large enough $L$. This influence can be either beneficial or detrimental. In particular, we study an effective Hamiltonian proposed by Pedrocchi et al. [Phys. Rev. B 83, 115415 (2011)] that describes repulsion between anyons and anyon holes. For this system, we find a lifetime of the stored quantum information that grows exponentially in $L^2$ for both periodic and open boundary conditions, although the exponent in the latter case is found to be less favorable. However, $L$ is upper bounded through the breakdown of the perturbative treatment of the underlying Hamiltonian.

An analytical approximation of the asymptotic exchange energy of two interacting hydrogen atoms is obtained. This approximation depends only on functions of the internuclear distance $R$, which remain bounded as $R\to \infty$ and is derived using the Herring-Holstein surface-integral technique. It is found that, for large $R$, the exchange energy is $O\left(R^3\exp (-2R)\right)$ in contrast to earlier approximations of $O\left(R^{2.5}\exp (-2R)\right)$. Our result is similar to the classic Heitler-London expression without the unphysical term $O\left(R^3\ln \left(R\right)\exp (-2R)\right)$.

An anthracene molecule substituted at the central ring by two aniline moieties able to rotate about the C-N bonds can be considered to be a prototype molecular rotor. Electron attachment into the lowest empty orbitals of this molecule leads to formation of long-lived (microseconds) negative ions. Elimination of a hydrogen molecule from these anions was observed in the gas phase by means of dissociative electron attachment spectroscopy. The experimental findings were interpreted using density functional theory calculations. It was shown that the decay process must be accompanied by formation of a new covalent bond in which one aniline moiety is fixed to the adjacent carbon atom of the anthracene ring. The observed irreversible interruption of the rotational motion could occur in other artificial electron-driven molecular machines provided that suitable atoms approach each other under the rotation.