A key ingredient of cavity QED is the coupling between the discrete energy levels of an atom and photons in a single-mode cavity. The addition of periodic ultrashort laser pulses allows one to use such a system as a source of single photons—a vital ingredient in quantum information and optical computing schemes. Here we analyze and time-adjust the photon-counting statistics of such a single-photon source and show that the photon statistics can be described by a simple transport-like nonequilibrium model. We then show that there is a one-to-one correspondence of this model to that of nonequilibrium transport of electrons through a double quantum dot nanostructure, unifying the fields of photon-counting statistics and electron-transport statistics. This correspondence empowers us to adapt several tools previously used for detecting quantum behavior in electron-transport systems (e.g., super-Poissonian shot noise and an extension of the Leggett-Garg inequality) to single-photon-source experiments.

We characterize the operation of semiconductor microring lasers in an excitable regime. Our experiments reveal a statistical distribution of the characteristics of noise-triggered optical pulses that is not observed in other excitable systems. In particular, an inverse correlation exists between the pulse amplitude and duration. Numerical simulations and an interpretation in an asymptotic phase space confirm and explain these experimentally observed pulse characteristics.

We calculate the first-order perturbation correction to the ground-state energy and chemical potential of a harmonically trapped boson gas with contact interactions about the infinite repulsion Tonks-Girardeau limit. With $c$ denoting the interaction strength, we find that, for a large number of particles $N$, the $1/c$ correction to the ground-state energy increases as $N^{5/2}$, in contrast to the unperturbed Tonks-Girardeau value that is proportional to $N^2$. We describe a thermodynamic scaling limit for the trapping frequency that yields an extensive ground-state energy and reproduces the zero temperature thermodynamics obtained by a local-density approximation.

We report a room-temperature measurement of nanomechanical motion with an imprecision below the standard quantum limit (SQL), using cavity-enhanced optical near fields. Fundamental thermodynamical cavity frequency fluctuation limits the total imprecision to $3\hspace{0.5em}\mathrm{dB}$ below the SQL at room temperature [corresponding to $(530\hspace{0.5em}\mathrm{am}/\sqrt{\mathrm{Hz}}){}^2$ in absolute units] and is expected to reduce to negligible values at moderate cryogenic temperatures. Coupling strengths exceeding those required to reach the SQL by more than two orders of magnitude are achieved, allowing a shot-noise limited imprecision more than $10\hspace{0.5em}\mathrm{dB}$ below the SQL. The transducer, thus, in principle, allows access to the quantum backaction dominated regime, a prerequisite for exploring quantum backaction, measurement-induced squeezing, and obtaining sub-SQL sensitivity using backaction evading techniques.

We investigate the feasibility for the preservation of coherence and entanglement of one- and two-spin qubits coupled to an interacting quantum spin-1/2 chain within the dynamical decoupling (DD) scheme. The performance is examined by counting the number of computing pulses that can be applied periodically with periods of $T$ before qubits become decoherent, while an identical decoupling pulse sequence is applied within each cycle. By considering pulses with mixed directions and finite width controlled by magnetic fields, it is shown that pulse-width accumulation degrades the performance of sequences with larger numbers of pulses and that feasible magnetic fields in practice restrict consideration to sequences with few than 10 decoupling pulses within each cycle. Furthermore, within each cycle $T$, exact nontrivial pulse sequences are found to suppress the qubit-bath coupling to $O(T^{N+1})$ progressively with the minimum number of pulses being 4, 7, and 12 for $N=1,2,3$. These sequences, when applied to all qubits, are shown to preserve both the entanglement and coherence. Based on time-dependent density matrix renormalization, our numerical results show that for modest magnetic fields (10–40 T) available in laboratories, the overall performance is optimized when the number of pulses in each cycle is 4 or 7 with pulse directions alternating between $x$ and $z$. Our results provide useful guides for the preservation of coherence and entanglement of spin qubits in the solid state.

We study the continuous-variable quantum teleportation of states, statistical moments of observables, and scale parameters such as squeezing. We investigate the problem both in ideal and imperfect Vaidman-Braunstein-Kimble protocol setups. We show how the teleportation fidelity is maximized and the difference between output and input variances is minimized by using suitably optimized entangled resources. Specifically, we consider the teleportation of coherent squeezed states, exploiting squeezed Bell states as entangled resources. This class of non-Gaussian states, introduced by Illuminati and co-workers [F. Dell’Anno, S. De Siena, L. Albano, and F. Illuminati, Phys. Rev. A 76, 022301 (2007); F. Dell’Anno, S. De Siena, and F. Illuminati, Phys. Rev. A 81, 012333 (2010)], includes photon-added and photon-subtracted squeezed states as special cases. At variance with the case of entangled Gaussian resources, the use of entangled non-Gaussian squeezed Bell resources allows one to choose different optimization procedures that lead to inequivalent results. Performing two independent optimization procedures, one can either maximize the state teleportation fidelity, or minimize the difference between input and output quadrature variances. The two different procedures are compared depending on the degrees of displacement and squeezing of the input states and on the working conditions in ideal and nonideal setups.

Recently, the quantum information processing power of closed timelike curves has been discussed. Because the most widely accepted model for quantum closed timelike curve interactions contains ambiguities, different authors have been able to reach radically different conclusions as to the power of such interactions. By tracing the information flow through such systems we are able to derive equivalent circuits with unique solutions, thus allowing an objective decision between the alternatives to be made. We conclude that closed timelike curves, if they exist and are well described by these simple models, would be a powerful resource for quantum information processing.

Previous observations of Cu- through Ge-like high-$Z$ ions have demonstrated that accurate measurements and theory agree well for ions with a single valence electron but that additional electrons in the valence shell cause progressively worsening computational problems. We have obtained highly resolved euv spectra of Pt ($Z=78$) ions in an electron-beam ion trap. The measured wavelengths are compared to the results of a number of recent large-scale calculations, including our own multireference Møller-Plesset computations. The latter calculations match the best for Cu- and Zn-like ions and represent an order-of-magnitude improvement in predictive accuracy for Ga- and Ge-like ions.

Intermolecular interaction potentials of the silane dimer in 12 orientations have been calculated by using the Hartree-Fock (HF) self-consistent theory and the second-order Møller-Plesset (MP2) perturbation theory. We employed basis sets from Pople's medium-size basis sets [up to 6-$311+$$+$$G$($3\mathit{df}$, $3\mathit{pd}$)] and Dunning's correlation consistent basis sets (up to the triply augmented correlation-consistent polarized valence quadruple-zeta basis set). We found that the minimum energy orientations were the $G$ and $H$ conformers. We have suggested that the Si-H attractions, the central silicon atom size, and electronegativity play essential roles in weakly binding of a silane dimer. The calculated MP2 potential data were employed to parametrize a five-site force field for molecular simulations. The Si-Si, Si-H, and H-H interaction parameters in a pairwise-additive, site-site potential model for silane molecules were regressed from the ab initio energies.

${}^4\mathrm{He}$${}_2$–alkali-metal triatomic molecular systems are studied using the adiabatic hyperspherical representation. By adopting the best pairwise He-He and He-$X$ interaction potentials, we search for weakly bound states of ${}^4\mathrm{He}$${}_2$$X$ systems with $X={}^6$Li, ${}^7\mathrm{Li}$, ${}^{23}\mathrm{Na}$, ${}^{39}\mathrm{K}$, ${}^{40}\mathrm{K}$, ${}^{41}\mathrm{K}$, ${}^{85}\mathrm{Rb}$, ${}^{87}\mathrm{Rb}$, and ${}^{133}\mathrm{Cs}$. We consider not only zero total angular momentum $J=0$ states, but also $J>0$ states. We find that the ${}^4\mathrm{He}$${}_2^6$Li and ${}^4\mathrm{He}$${}_2^7$Li systems each possess two bound states with $J^{\Pi }=0^{+}$ symmetry and none with $J>0$, while the other ${}^4\mathrm{He}$${}_2$–alkali-metal species are found to support one $0^{+}$ and one $1^{-}$ bound state. We calculate the bound-state energies of these molecular species and discuss the essential features of the wave functions associated with these bound states.

The Casimir-Polder potential between a pair of atoms or molecules, either one of which may be excited, in a dielectric medium out of thermal equilibrium is obtained using a field operator that accounts for the attenuation or amplification of the radiation. Two contributions to the intermolecular potential are identified. One term is that induced by the excitations and relaxations that the interacting molecules experience due to the thermal bath, obtained using the resonance energy transfer rate and the Wigner-Weisskopf model. The second contribution arises as a consequence of the polarization induced in the interacting molecules by vacuum and thermal radiation. A number of disagreements between the present theory and several previously reported formulations examining this intermolecular potential are revealed.

Fluorescence of single, trapped ${}^{171}\mathrm{Yb}$${}^{+}$ ions has been experimentally studied as a function of laser polarization, power, and detuning and as a function of magnetic field strength. The suppression of efficient fluorescence by coherent population trapping and the counteracting effect of the magnetic field are found to agree with theoretical predictions. For comparison, a fluorescence study has also been made of the isotope ${}^{174}\mathrm{Yb}$${}^{+}$ for which coherent population trapping is absent on the main fluorescence and laser cooling transition. Finally, state-sensitive fluorescence detection of the ${}^{171}\mathrm{Yb}$${}^{+}$ hyperfine qubit is studied, including the role of coherent population trapping in the optimization of detection parameters. A qubit detection fidelity of greater than $97\%$ is achieved.

Scalar diffraction theory is combined with beam-propagation techniques to investigate the projection of near-field diffraction patterns to spatial locations away from the aperture for use in optically trapping cold neutral alkali-metal atoms. Calculations show that intensity distributions with localized bright and dark spots usually found within a millimeter of the diffracting aperture can be projected to a region free from optical components such as a cloud of cold atoms within a vacuum chamber. Calculations also predict that the critical properties of the optical dipole atom traps are not only maintained for the projected intensity patterns but also can be manipulated and improved by adjustment of the optical components outside the vacuum chamber.

We investigate the disorder-induced localization transition in Bose-Einstein condensates for the Anderson and Aubry-André models in the noninteracting limit using exact diagonalization. We show that, in addition to the standard superfluid fraction, other tools such as the entanglement and fidelity can provide clear signatures of the transition. Interestingly, the fidelity exhibits good sensitivity even for small lattices. Effects of the system size on these quantities are analyzed in detail, including the determination of a finite-size-scaling law for the critical disorder strength in the case of the Anderson model.

We simulate the transition from amplified spontaneous emission (ASE) to lasing in random systems with varying degrees of mode overlap. This is accomplished by solving the stochastic Maxwell-Bloch equations with the finite-difference time-domain method. Below lasing threshold, the continuous emission spectra are narrowed by frequency-dependent amplification. Our simulations reproduce the stochastic emission spikes in the spectra. Well-defined peaks, corresponding to the system resonances, emerge at higher pumping and are narrowed by stimulated emission before lasing takes place. Noise tends to distribute pump energy over many modes, resulting in multimode operation. Well above the lasing threshold, the effects of noise lessen and results become similar to those without noise. By comparing systems of different scattering strength, we find that weaker scattering extends the transition region from ASE to lasing, where the effects of noise are most significant.

We show experimentally that optical phase singularities (PSs) can be written and erased, locally and in a controllable manner, into a light beam using the giant Kerr optical nonlinearities of liquid crystals. The method relies on the nonlinear optical spin-orbit coupling experienced by a collimated probe beam when a collinear focused pump beam imprints a radial birefringent pattern into a nematic film. In addition, experimental data are quantitatively described, accounting for the elastic anisotropy of the material and its nonlocal spatial response to the pump light field. Since we show that the optical intensity of a light beam (the “pump”) controls the phase of another beam (the “probe”) in a singular fashion (i.e., with the generation of a screw PS) via their interaction in a nonlinear medium that involves spin-orbit coupling, we dubbed such a nonlinear optical process as spin-orbit optical cross-phase-modulation.

We propose a scheme that is able to generate on demand a steady-state entanglement between two nondegenerate cavity modes. The scheme relies on the interaction of the cavity modes with driven two- or three-level atoms, which act as a coupler to build entanglement between the modes. We show that, in the limit of a strong driving, it is crucial for the generation of entanglement between the modes to imbalance populations of the dressed states of the driven atomic transition. In the case of a three-level $V$-type atom, we find that a stationary entanglement can be created on demand by tuning the Rabi frequency of the driving field to the difference between the atomic transition frequencies. The resulting degeneracy of the energy levels, together with the spontaneously generated coherence, generate a steady-state entanglement between the cavity modes. It is shown that the condition for the maximal entanglement coincides with the collapse of the atomic system into a pure trapping state. We also show that the creation of entanglement depends strongly on the mutual polarization of the transition atomic dipole moments.

The authors call attention to a previous work [Lin and He, Phys. Rev. A 80, 042310 (2009)] on the realization of multiqubit logic gates with controlled-path and merging gates. We supplement the work by showing how to efficiently implement quantum algorithms in this approach and by providing guide rules for the task.

Propagation of an asymmetric Gaussian beam in a medium with saturated nonlinear refractive index is analyzed using a “collective variable approach” to solve the general nonlinear Schrödinger equation and compared with that of a symmetric beam in both lossless and lossy media. For a lossless medium, we construct a diagram which defines regions of oscillatory and diffractive propagation of an asymmetric beam and compare it with that of a symmetric beam. We detect breathing dynamics of the widths and amplitude of the asymmetric beam in the oscillatory regime of propagation and identify two different types of width and amplitude beating, Type $1$ and $2$, depending on the initial beam energy and saturation constant of the medium. This is in contrast to a cubic-quintic medium where only one type of beating is obtained.

We reveal that there exists an indirect interaction between two single-mode microcavities which have no direct coupling. This indirect interaction is mediated by coupling the two microcavities to a common waveguide, and it plays a key role in obtaining the asymmetric Fano line shape of the transport spectrum of the coupled cavity system. Finally, we show that this sharp Fano line shape can contribute to highly sensitive sensing, which is immune to most environmental variations.