Precisely controlling the atom number challenges the single-photon generation based on an individual neutral atom in experiments. A straightforward solution is to directly produce single photons via a number of atoms. We theoretically investigate such a single-photon emitter in which a bunch of Sr atoms are tightly confined in a blue-detuned optical lattice and weakly driven by an external field. The strong long-range interatomic interactions inducing large excitation-energy shifts results in a significant suppression of multiphoton emission. We consider two common approaches, which are widely used in experiments to generate single photons, in detail. The results indicate a potential to built up a single-photon source based on an ensemble of interacting atoms.

We study a waveguide QED system with a rectangular waveguide and a two-level system inside, where the transverse magnetic TM${}_{mn}$ modes define the quantum channels of guided photons. It is discovered that the loss of photons from the TM${}_{11}$ channel into the others can be overcome by replacing it with a certain coherent superposition of TM${}_{mn}$ channels, which is named the controllable channel (CC), as the photons in the CC can be perfectly reflected or transmitted by the two-level system and never lost into the other channels. A dark state emerges when the photon is incident from one of the scattering-free channels orthogonal to the CC. The underlying physics mechanism is the multichannel interference associated with Fano resonance.

The basic entanglement-swapping scheme can be seen as a process which allows one to redistribute the Bell states properties between different pairs of a four-qubit system. We analyze a similar scheme by performing a Bell–von Neumann measurement over two local qubits, each one initially correlated through an $X$ state with a spatially distant qubit. This process swaps the $X$ feature without conditions, whereas the input entanglement is partially distributed in the four possible outcome states under certain conditions. Specifically, we obtain two threshold values for the entanglement of formation of the input $X$ states in order for the outcome states to be nonseparable.

By counteracting gravity, the repulsive Casimir force enables stable levitation of a perfectly conducting particle near a liquid-air interface if the particle exists inside the liquid. In the present study, we examine the levitation of a gold particle near a bromobenzene-air interface and calculate the levitation height using the scattering-matrix formulation. In addition, we consider the Casimir force acting on a gold sphere near the interface between bromobenzene and water. At asymptotically large separations, the Casimir force is attractive because of the large static dielectric permittivity of water. However, the Casimir force changes from attractive to repulsive as the separation decreases. We also found that the gold particle can be levitated in bromobenzene above water.

The relaxation of trapped Cl${{}_2}^{-}$ ions and their resulting column density in a multipole radio-frequency (RF) ion trap have been investigated after loading the trap from an initial fast-moving beam exploiting a mechanism described recently [A. Svendsen et al., Phys. Rev. A 87, 043410 (2013)] where the injection is mediated through the exchange of energy between ions and the oscillating RF field. The temporal relaxation of the energy distribution of the trapped ion cloud was probed by observing the evolution of the resulting time-of-flight distribution of ions after extraction and fragment mass analysis in a quadrupole mass filter. The ion energy distribution was found to be essentially stationary after $\sim 20$ ms. The resulting column density of trapped ions after relaxation was probed by two-dimensional position-resolved photodissociation of the trapped Cl${{}_2}^{-}$ ions. A detailed statistical analysis of the ion column density in the ring-electrode trap is given, and by comparison to the experimental data, a value of the maximum adiabaticity parameter of ${\eta }_{\max }\simeq 0.28$ is inferred. It is further demonstrated how the present experimental system allows for time-resolved mass spectrometry by probing explicitly the populations of both parent (Cl${{}_2}^{-}$) and daughter (Cl${}^{-}$) ions as a function of time after closing the trap and after laser irradiation. Finally, it is discussed how the setup can be used to obtain absolute photodissociation cross sections via a tomographic method without assumptions on the decay law for the trapped ions.

Field-free molecular alignment has been used in order to track the collisional relaxation occurring in a molecular gas. CO${}_2$ molecules were initially irradiated by a short linearly polarized laser pulse resulting in the increase of their rotational energy. The evolution of the subsequent ultrafast relaxation process was optically probed after irradiating the sample with a second, weaker, short pulse leading to the alignment of the preheated molecules. Using classical molecular dynamic simulations, we were able to quantitatively reproduce the experimental shapes and amplitudes of the recorded revival transients for a time interval extending from 25 to 500 ps until thermalization of the gas sample is reached.

We propose a non-Hermitian generalization of stimulated Raman adiabatic passage (STIRAP), which allows one to increase speed and fidelity of the adiabatic passage. This is done by adding balanced imaginary (gain and loss) terms in the diagonal (bare energy) terms of the Hamiltonian and choosing them such that they cancel exactly the nonadiabatic couplings, providing in this way an effective shortcut to adiabaticity. Remarkably, for a STIRAP using delayed Gaussian-shaped pulses in the counterintuitive scheme the imaginary terms of the Hamiltonian turn out to be time independent. A possible physical realization of non-Hermitian STIRAP, based on light transfer in three evanescently coupled optical waveguides, is proposed.

With a neutron spin-echo reflectometer (OffSpec at ISIS, UK) it is possible to measure the gravitation-induced quantum phase difference between the two spin states of the neutron wave function in a magnetic field. In the small-angle approximation, this phase depends linearly on the inclination angle of the neutron beam with respect to the horizontal. This also holds for the Bonse-Hart interferometer used in the Colella-Overhauser-Werner experiments and should be taken into account. Neglecting this term could yield deviations up to 1% per degree inclination angle. The gravitation-induced quantum phase as measured with OffSpec with an accuracy of 0.1% agrees with the theoretically expected results.

We consider a potential scattering of a Bose-Hubbard dimer in a one-dimensional optical lattice. A numerical approach based on an effective non-Hermitian Hamiltonian has been developed for solving the scattering problem. It allows us to compute the tunneling and dissociation probabilities for an arbitrary shape of the potential barrier and an arbitrary kinetic energy of the dimer. The developed approach has been used to address the problem of two-particle decay out of a trap. In particular, it is shown that the presence of dissociation channels significantly decreases the nonescape probability due to single-particle escape to those channels.

We analyze the classical dynamics of a system composed of a one-dimensional cavity with a perfect, fixed mirror and a movable mirror with nonzero transparency interacting with a monochromatic laser. The movable mirror can deviate far from an equilibrium position, it is assumed to be thin so that it is modeled by a $\delta$ function, and we use the exact modes of the complete system. The transparency and the mirror-position-dependent cavity resonance frequencies are built into the modes and this allows us to deduce that the radiation pressure force comes from a periodic potential with period half the wavelength of the field. The exact modes and the radiation pressure potential allow us to give intuitive physical interpretations of the dynamics of the system and to obtain approximate analytic solutions for the motion of the mirror. Three regimes are identified depending on the intensity of the field and it is found that the dynamics can be qualitatively very different in each of them with some regimes being very sensitive to the values of the parameters. Moreover, we determine conditions when the Maxwell-Newton equations used to describe the system constitute accurate approximations of the exact equations governing the dynamics.

We consider theoretically how to extract mode-dependent single photons from a time and frequency multimode nonclassical beam. To achieve this task, we calculate the properties of sum frequency generation with a pulse shaped pump, taking into account both temporal and spatial degrees of freedom. We show that using a noncollinear configuration it is possible to achieve a mode-dependent weakly reflective beam splitter, with Schmidt number compatible with photon extraction for continuous variable regime tasks. We make explicit the possible application to the degaussification of highly multimode squeezed frequency combs.

We report the efficient creation and detection of hybrid entanglement between one photon's polarization and another photon's complex transverse polarization pattern including various polarization singularities. The polarization measurement of the first photon triggers a polarization-sensitive imaging of its partner photon, the vector photon, using a single-photon-sensitive camera. Thereby, the vector photon's complex polarization pattern is reconstructed tomographically dependent on the type of polarization measurement performed on its partner. To evaluate the quality of the observed patterns, we introduce a fidelity-like measure which compares the reconstructed polarization with the theoretically expected one at every transverse spatial position. We find that the measured polarization patterns and the theory overlap by more than 90%. Moreover, we visualize the varying strengths of polarization entanglement for different transverse regions and demonstrate an interesting phenomenon: Each vector photon can be both entangled and not entangled in polarization with its partner photon. We give an intuitive, information-theoretical explanation for our results. Our results pave the way to study quantum properties of the rich field of such complex light modes with the potential to improve various applications in quantum informational tasks.

Recently a new uncertainty relation was found as an alternative to a number-phase uncertainty relation for a harmonic oscillator. In this paper we determine numerically, via the discrete-variable-representation method known from quantum chemistry, the exact states that saturate this new uncertainty relation. We analyze the physical properties of the states and compare them to the intelligent states of the Pegg-Barnett uncertainty relation. We find that for a given set of expectation values of the physical parameters, which are the particle number and the two quadratures, the two kinds of intelligent states are equivalent. The intelligent states are the eigenstates corresponding to the lowest eigenvalue of a Hermitian operator, which, when interpreted as a Hamiltonian of a physical sytem, describes a nonlinear driven harmonic oscillator, for example, a Duffing oscillator for a certain parameter range. Hence, our results can be interpreted as the determination of the ground state of such physical systems in an explicit analytic form as well. As the Pegg-Barnett intelligent states we use are expressed in terms of a coherent-state superposition facilitating experimental synthesis, we relate the states determined here to experimentally feasible ones.

We solve the unextendible maximally entangled basis (UMEB) problem in ${\mathbb{C}}^d\otimes {\mathbb{C}}^{d^{\prime }}(d\ne d^{\prime }).$ The results show that a UMEB will always exist. In addition, there might be two sets of UMEBs with different numbers. The main difficulty is to prove the unextendibility of the set of states. We give an explicit construction of UMEBs by considering the Schmidt number of the complementary space of the states we construct.

We propose and analyze a multiphoton-state coherent transport protocol in a coupled-resonator quantum network. A multiphoton swap gate between two antipodes can be achieved with neither external modulation nor coupling strength engineering. Moreover, we extend this result to a coupled-resonator chain of arbitrary length with different coupling strengths. Effects of decoherence via quantum nondemolition interaction are studied with sources including vacuum quantum fluctuation and bath thermal excitations when the bath is in the thermal equilibrium state. These observations are helpful to understand the decoherence effects on quantum communication in quantum coupled-resonator systems.

The current limit on the electron's electric dipole moment, $|d_e|<8.7\times {10}^{-29}$ $e\mathrm{cm}$ (90% confidence), was set using the molecule thorium monoxide (ThO) in the $J=1$ rotational level of its $H{}^3{\Delta }_1$ electronic state [J. Baron et al., Science 343, 269 (2014)]. This state in ThO is very robust against systematic errors related to magnetic fields or geometric phases, due in part to its $\Omega$-doublet structure. These systematics can be further suppressed by operating the experiment under conditions where the $g$-factor difference between the $\Omega$ doublets is minimized. We consider the $g$ factors of the ThO $H{}^3{\Delta }_1$ state both experimentally and theoretically, including dependence on $\Omega$ doublets, the rotational level, and the external electric field. The calculated and measured values are in good agreement. We find that the $g$-factor difference between $\Omega$ doublets is smaller in $J=2$ than in $J=1$ and reaches zero at an experimentally accessible electric field. This means that the $H,J=2$ state should be even more robust against a number of systematic errors compared to $H,J=1$.

Dumbbell-shaped fullerene dimers $C_{118}^{+}$ and $C_{119}^{+}$ have recently been observed in mass spectra resulting from collisions between clusters of C${}_{60}$ molecules and keV He${}^{2+}$ or Ar${}^{2+}$ ions [H. Zettergren et al., Phys. Rev. Lett. 110, 185501 (2013) and F. Seitz et al., J. Chem. Phys. 139, 034309 (2013)]. To unveil the formation mechanisms of these fullerene dimers, systematic molecular dynamics (MD) simulations based on the self-consistent charge density functional tight-binding method have been performed for C${}_n^{+}$ + C${}_{60}$ ($n=58,59,60$) collisions following prompt atom knockouts by the fast ions. The statistics from the MD simulations indicate a much higher reactivity of $C_{59}^{+}$ and $C_{58}^{+}$ fragments compared to that of $C_{60}^{+}$. It is found that the covalently bonded dumbbell-shaped fullerene dimers $C_{118}^{+}$ and $C_{119}^{+}$ can be formed at very low-collision energies within 1 ps and are stable enough to survive on the microsecond time scale of the experiment. The thermodynamic and kinetic stabilities, as well as the bonding features, have been investigated for the most stable dumbbell dimers $C_{118}^{+}$, $C_{119}^{+}$, and $C_{120}^{+}$.

We study the phase diagram of strongly interacting fermions in one dimension at finite temperature, with mass and spin imbalance. By including the possibility of the existence of a spatially inhomogeneous ground state, we find regions where spatially varying superfluid phases are favored over homogeneous phases. We obtain estimates for critical values of the temperature, mass, and spin imbalance, above which these phases disappear. Finally, we show that an intriguing relation exists between the general structure of the phase diagram and the binding energies of the underlying two-body bound-state problem. Although our present study of the many-body aspects is based on a mean-field approximation, it may still yield phenomenologically important insights into the dynamics of strongly interacting fermions in one dimension. Most importantly, however, we expect that our present mean-field study can be considered worthwhile from a field-theoretical point of view as it may help to better understand the mechanisms underlying the emergence of inhomogeneous phases in higher dimensions as well as the general structure of the associated phase diagrams.

In this work, we study the BCS-BEC crossover and quantum phase transition in a Fermi gas under Rashba spin-orbit coupling close to a Feshbach resonance. By adopting a two-channel model, we take into account the closed-channel molecules, and show that combined with spin-orbit coupling, a finite background scattering in the open channel can lead to two branches of solution for both the two-body and the many-body ground states. The branching of the two-body bound-state solution originates from the avoided crossing between bound states in the open and the closed channels, respectively. For the many-body states, we identify a quantum phase transition in the upper branch regardless of the sign of the background scattering length, which is in clear contrast to the case without spin-orbit coupling. For systems with negative background scattering length in particular, we show that the bound state in the open channel, and hence the quantum phase transition in the upper branch, are induced by spin-orbit coupling. We then characterize the critical detuning of the quantum phase transition for both positive and negative background scattering lengths, and demonstrate the optimal parameters for the critical point to be probed experimentally.

We study theoretically a three-mode optomechanical system where two mechanical oscillators are independently coupled to a single cavity mode. By optimized two-tone or four-tone driving of the cavity, one can prepare the mechanical oscillators in an entangled two-mode squeezed state, even if they start in a thermal state. The highly pure, symmetric steady state achieved allows the optimal fidelity of standard continuous-variable teleportation protocols to be achieved. In contrast to other reservoir engineering approaches to generating mechanical entanglement, only a single reservoir is required to prepare the highly pure entangled steady state, greatly simplifying experimental implementation. The entanglement may be verified via a bound on the Duan inequality obtained from the cavity output spectrum. A similar technique may be used for the preparation of a highly pure two-mode squeezed state of two cavity modes, coupled to a common mechanical oscillator.