Elaborating versatile strategies for new lasing schemes is important for both fundamental research and realistic applications. By using atomic photonic crystals, two-color distributed feedback lasing can be shown to occur in a nonlinear regime of balanced optical gain. Within the context of a simple model based on coupled-wave theory and applied to alkali-metal-atom crystals, we find that the two-color lasing threshold in cold atoms is within state-of-the-art experimental reach.

We report on the preservation of transverse spatial coherence of an optical pulse stored in atomic vapor quantum memory. The high visibility Young-type spatial fringes formed by interference between the retrieved and the delayed optical pulses clearly demonstrate that the atomic vapor quantum memory based on electromagnetically induced transparency preserves transverse spatial coherence. This demonstration has important implications in quantum imaging and multimode quantum information processing.

The spatial optical solitons with ellipse-shaped spots have generally been considered to be a result of either linear or nonlinear anisotropy. We investigate a class of spiraling elliptic solitons in nonlocal nonlinear media without both linear and nonlinear anisotropy. The spiraling elliptic solitons carry the orbital angular momentum (OAM), which plays a key role in the formation of such solitons, and are stable for any degree of nonlocality except the local case if the response function is Gaussian. During the propagation of such solitons, the rate of decay of the OAM is extremely low. The formation of such solitons can be attributed to effective anisotropic diffraction (linear anisotropy) resulting from the OAM. Our variational analytical result is confirmed by direct numerical simulation of the nonlocal nonlinear Schrödinger equation.

We study the optical bistability of an optomechanical system in which the position of a mechanical oscillator modulates the cavity frequency. The steady-state mean-field equation of the optical mode is identical to the one for a Kerr medium, and thus we expect it to have the same characteristic behavior with a lower, a middle, and an upper branch. However, the presence of position fluctuations of the mechanical resonator leads to a new feature: the upper branch will become unstable at sufficiently strong driving in certain parameter regimes. We identify the appropriate parameter regime for the upper branch to be stable, and we confirm, by numerical investigation of the quantum steady state, that the mechanical mode indeed acts as a Kerr nonlinearity for the optical mode in the low-temperature limit. This equivalence of the optomechanical system and the Kerr medium will be important for future applications of cavity optomechanics in quantum nonlinear optics and quantum information science.

We study the tunneling and traversal of ultracold $\Lambda$-type three-level atoms through vacuum-induced potentials in a high-$Q$ mazer cavity. In particular, we discuss the effects of driving-induced atomic coherence on the passage of ultracold atoms through a high-$Q$ mazer cavity. We consider phase time to study quantum tunneling which exhibits interesting features due to atomic coherence. For example, negative phase time appears for transmission of the atoms in both excited and ground states due to the presence of atomic coherence. Further, for certain values of the driving field, it is found that the phase tunneling time shows an alternate subclassical and superclassical traversal behavior with the increase in atomic momentum.

The fundamental quantum interferometry bound limits the sensitivity of an interferometer for a given total rate of photons and for a given decoherence rate inside the measurement device. We theoretically show that the recently reported quantum-noise-limited sensitivity of the squeezed-light-enhanced German-British gravitational wave detector GEO 600 is exceedingly close to this bound, given the present amount of optical loss. Furthermore, our result proves that the employed combination of a bright coherent state and a squeezed vacuum state is generally the optimum practical approach for phase estimation with high precision on absolute scales. Based on our analysis we conclude that the application of neither Fock states nor NOON states nor any other sophisticated nonclassical quantum state would have yielded an appreciably higher quantum-noise-limited sensitivity.

We study the geometric phase for the ground state of a generalized one-dimensional non-Hermitian quantum $XY$ model, which has transverse-field-dependent intrinsic rotation-time reversal symmetry. Based on the exact solution, this model is shown to have a full real spectrum in multiple regions for the finite-size system. The result indicates that the phase diagram or exceptional boundary which separates the unbroken- and broken-symmetry regions corresponds to the divergence of the Berry curvature. The scaling behaviors of the ground-state energy and Berry curvature are obtained in an analytical manner for a concrete system.

We adopt the beam-splitter model for losses to analyze the performance of a recent compact continuous-variable entanglement distillation protocol [A. Datta et al., Phys. Rev. Lett. 108, 060502 (2012)] implemented using realistic quantum memories. We show that the decoherence undergone by a two-mode squeezed state while stored in a quantum memory can strongly modify the results of the preparatory step of the protocol. We find that the well-known method for locally increasing entanglement, phonon subtraction, may not result in entanglement gain when losses are taken into account. Thus, we investigate the critical number $m_c$ of phonon subtraction attempts from the matter modes of the quantum memory. If the initial state is not de-Gaussified within $m_c$ attempts, the protocol should be restarted to obtain any entanglement increase. Moreover, the condition $m_c>1$ implies an additional constraint on the subtraction beam-splitter interaction transmissivity, viz., it should be about $50\%$ for a wide range of protocol parameters. Additionally, we consider the average entanglement rate, which takes into account both the unavoidable probabilistic nature of the protocol and its possible failure as a result of a large number of unsuccessful subtraction attempts. We find that a higher value of the average entanglement can be achieved by increasing the subtraction beam-splitter interaction transmissivity. We conclude that the compact distillation protocol with the practical constraints coming from realistic quantum memories allows a feasible experimental realization within existing technologies.

We calculate the cross sections for elastic scattering and Zeeman relaxation in binary collisions of molecules in the rovibrational ground state of a ${}^2\Sigma$ electronic state and the Zeeman state with the electron spin projection $M_S=\frac{1}{2}$ on the magnetic field axis. This is the lowest-energy state of ${}^2\Sigma$ molecules confined in a magnetic trap. The results are averaged over calculations with multiple molecule-molecule interaction potentials, which yield the expectation intervals for the cross sections and the elastic-to-inelastic cross-section ratios. We find that the elastic-to-inelastic cross-section ratios under conditions corresponding to trapped molecular ensembles at $T\sim {10}^{-3}$ K exceed 100 for the majority of ${}^2\Sigma$ molecules. The range of ${}^2\Sigma$ molecules expected to be collisionally unstable in magnetic traps at $T<{10}^{-3}$ K is limited to molecules with the spin-rotation interaction constant ${\gamma }_{\mathrm{SR}}>0.5$ cm${}^{-1}$ and the rotational constant $B_e<4$ cm${}^{-1}$.

The impact of both exchange interaction and electron correlation, as well as their combined impact, on electron elastic scattering off a semifilled-shell Mn($\cdots 3d^5$$4s^2$, ${}^{6}S$) atom are theoretically studied in the electron energy range of $\epsilon =0$–25 eV. Corresponding elastic-scattering phase shifts ${\delta }_{\ell }\left(\epsilon \right)$ as well as partial ${\sigma }_{\ell }\left(\epsilon \right)$ and total $\sigma \left(\epsilon \right)$ cross sections are found to be subject to a strong correlation impact. The latter is shown to drastically differ for oppositely spin-polarized scattered electrons in some cases, thereby bringing significant differences in corresponding ${\delta }_{\ell }\left(\epsilon \right)$s, ${\sigma }_{\ell }\left(\epsilon \right)$s, and $\sigma \left(\epsilon \right)$s between these electrons. This is proven to be an inherent feature of electron scattering off a semifilled-shell atom in general. Electron correlation is accounted for in the framework of the self-energy part $\Sigma$ of the Green function of a scattered electron concept. The latter is calculated both in the second-order perturbation theory in the Coulomb interelectron interaction as well as beyond it by solving the Dyson equation for $\Sigma$. The significance of the “Dyson” correlation corrections in $e^{-}+$Mn scattering is unraveled. They are shown to noticeably increase the inherent differences between elastic-scattering phase shifts and cross sections of spin-up ($\uparrow$) and spin-down ($\downarrow$) polarized electrons scattered off a spin-polarized Mn atom, in some cases. In particular, the existence of a narrow resonant maximum in ${\sigma }^{\downarrow }\left(\epsilon \right)$ near $\epsilon \approx 8$ eV but the absence of such in ${\sigma }^{\uparrow }\left(\epsilon \right)$ in $e^{-}+$Mn scattering is predicted.

We investigate the contribution of interatomic Coulombic decay induced by ion impact in neon and argon dimers (Ne${}_2$ and Ar${}_2$) to the production of low-energy electrons. Our experiments cover a broad range of perturbation strengths and reaction channels. We use 11.37 MeV/u S${}^{14+}$, 0.125 MeV/u He${}^{1+}$, 0.1625 MeV/u He${}^{1+}$, and 0.150 MeV/u He${}^{2+}$ as projectiles and study ionization, single and double electron transfer to the projectile, as well as projectile electron loss processes. The application of a COLTRIMS reaction microscope enables us to retrieve the three-dimensional momentum vectors of the ion pairs of the fragmenting dimer into Ne${}^{q+}$-Ne${}^{1+}$ and Ar${}^{q+}$-Ar${}^{1+}$ ($q=$ 1, 2, 3) in coincidence with at least one emitted electron.

Recently it has been recognized that electronic conical intersections in molecular systems can be induced by laser light even in diatomics. As is known a direct consequence of these accidental degeneracies is the appearance of nonadiabatic effects which has a strong impact on the nuclear quantum dynamics. Studying the photodissociation process of the ${\mathrm{D}}_2$${}^{+}$ molecule, we report here some observable quantum interference phenomena that arise from the topological singularity induced by a strong laser field.

We report the realization of a matter-wave interferometer based on Raman transitions which simultaneously interrogates two different atomic species (${}^{87}$Rb and ${}^{85}$Rb). The simultaneous aspect of our experiment presents encouraging preliminary results for future dual-species atom interferometry projects and seems very promising by taking advantage of a differential acceleration measurement. Indeed, the resolution of our differential accelerometer remains lower than $3.9\times {10}^{-8}\text{g}$ even with vibration levels up to $1\times {10}^{-3}\text{g}$ thanks to common-mode vibration noise rejection. An atom-based test of the weak equivalence principle has also been carried out leading to a differential free fall measurement between both isotopes of $\Delta g/g=(1.2\pm 3.2)\times {10}^{-7}$.

Under the influence of an external magnetic field and spin-changing collisions, the band insulator state of one-dimensional $s$-wave repulsively interacting four-component fermions at half-filling transforms into Mott insulator states with a spontaneously doubled unit cell: a dimerized state for shallow lattices and a Néel state for deep lattices via an intermediate topological state. These Mott insulator phases could be of special interest for experiments as they can be reached starting from band insulator state and changing magnetic field adiabatically.

We investigate thermally excited, dipolar Bose-Einstein condensates (BECs). Quasiparticle excitations of the atomic cloud cause density fluctuations which can induce the collapse of the condensate if the interparticle interaction is attractive. Within a variational approach, we identify the collectively excited stationary states of the gas which form transition states on the way to the BEC's collapse. We analyze transition states with different $m$-fold rotational symmetry and identify the one which mediates the collapse. The latter's symmetry depends on the trap aspect ratio of the external trapping potential, which determines the shape of the BEC. Moreover, we present the collapse dynamics of the BEC and calculate the corresponding decay rate using transition-state theory. We observe that the thermally induced collapse mechanism is important near the critical scattering length, where the lifetime of the condensate can be significantly reduced. Our results are valid for an arbitrary strength of the dipole-dipole interaction. Specific applications are discussed for the elements ${}^{52}$Cr, ${}^{164}$Dy, and ${}^{168}$Er with which dipolar BECs have been experimentally realized.

Ultracold bosonic atoms are confined by an optical lattice inside an optical resonator and interact with a cavity mode whose wavelength is incommensurate with the spatial periodicity of the confining potential. We predict that the intracavity photon number can be significantly different from zero when the atoms are driven by a transverse laser whose intensity exceeds a threshold value and whose frequency is suitably detuned from the cavity and the atomic transition frequency. In this parameter regime the atoms form clusters in which they emit in phase into the cavity. The clusters are phase locked, thereby maximizing the intracavity photon number. These predictions are based on a Bose-Hubbard model, whose derivation is reported here in detail. The Bose-Hubbard Hamiltonian has coefficients which are due to the cavity field and depend on the atomic density at all lattice sites. The corresponding phase diagram is evaluated using quantum Monte Carlo simulations in one dimension and mean-field calculations in two dimensions. Where the intracavity photon number is large, the ground state of the atomic gas lacks superfluidity and possesses finite compressibility, typical of a Bose glass.

We study spin liquid phases of spin-5/2 alkaline-earth-metal atoms on a honeycomb lattice at finite temperatures. Our analysis is based on a Gutzwiller projection variational approach recast to a path-integral formalism. In the framework of a saddle-point approximation we determine spin liquid phases with lowest free energy and study their temperature dependence. We identify a critical temperature, where all the spin liquid phases melt and the system goes to the paramagnetic phase. We also study the stability of the saddle-point solutions and show that a time-reversal symmetry breaking state, a so-called chiral spin liquid phase, is realized even at finite temperatures. We also determine the spin structure factor, which, in principle, is an experimentally measurable quantity and is the basic tool to map the spectrum of elementary excitations of the system.

We develop quantum-optical input-output theory for resonators with arbitrary coupling strength, and for input fields whose spectrum can be wider than the cavity free-spectral range, while ensuring that the field-operator commutator relations in space-time variables are correct. The cavity-field commutator exhibits a series of space-time “echoes,” representing causal connections of certain space-time points by light propagation. We apply the theory to two-photon wave-packet shaping by cavity reflection, which displays a remarkable illustration of dispersion cancellation. We also show that the theory is amenable to inclusion of intracavity absorbing and emitting atoms, allowing, for example, dissipative losses within the cavity to be incorporated in a quantum mechanically correct way.

In this paper, we investigate the ground-state properties of a Bose-Einstein condensate in an optomechanical cavity, in which a nonlinear photon-phonon interaction is created through the radiation pressure. We find that this nonlinear photon-phonon interaction can enhance the macroscopic collective excitations, and more importantly, it can generate a phase, called the dynamical unstable phase, in a regime of strong interaction. Moreover, the transition from the superradiant phase to the dynamical unstable phase shows no breaking of symmetry but is an intrinsic transition governed only by the ground-state stability. The possible physical explanation is also illustrated. These predictions can be detected by measuring the atom population, the mean photon number, or the mean phonon number in experiments.