We use an optical cavity in the regime of intermediate coupling between atom and cavity mode to detect single moving atoms. Degenerate polarization modes allow excitation of the atoms in one mode and collection of spontaneous emission in the other while keeping separate the two sources of light; we obtain a higher confidence and efficiency of detection by adding cavity-enhanced Faraday rotation. Both methods greatly benefit from coincidence detection of photons, attaining fidelities in excess of 99% in less than $1\mu \text{s}$. Detailed studies of the second-order intensity autocorrelation function of light from the signal mode reveal evidence of antibunched photon emissions and the dynamics of single-atom transits.

Quantum optical input-output models are described for a class of optical switches based on cavity quantum electrodynamics (QED) with a single multilevel atom (or comparable bound system of charges) coupled simultaneously to several resonant field modes. A recent limit theorem for quantum stochastic differential equations is used to show that such models converge to a simple scattering matrix in a type of strong-coupling limit that seems natural for nanophotonic systems. Numerical integration is used to show that the behavior of the prelimit model approximates that of the simple scattering matrix in a realistic regime for the physical parameters and that it is possible in the proposed cavity-QED configuration for low-power optical signals to switch higher-power signals at attojoule energy scales.

Recent model calculations by Gallup et al. [Phys. Rev. A79, 042710 (2009)] suggest that low-energy dissociative electron attachment to formic acid can be explained solely in terms of a very short-lived ${\sigma }^{\ast }$ anion state and that no ${\sigma }^{\ast }/{\pi }^{\ast }$ coupling is required. We argue that this interpretation of the experimental data, which is at odds with our earlier study, is flawed.

Rescigno et al. [Phys. Rev. A80, 046701 (2009)] criticized our theoretical treatment of dissociative electron attachment in formic acid in which we show that this process can be explained by electron attachment into the temporary negative ion state formed by occupation of the ${\sigma }^{\ast }\left(\text{OH}\right)$ orbital. We argue that their objections do not hold up to scrutiny.

We propose an experiment that would produce and measure a large Aharonov-Casher (AC) phase in a solid-state system under macroscopic motion. A diamond crystal is mounted on a spinning disk in the presence of a uniform electric field. Internal magnetic states of a single nitrogen-vacancy $\left(\text{N-}V\right)$ defect, replacing interferometer trajectories, are coherently controlled by microwave pulses. The AC phase shift is manifested as a relative phase, of up to 17 radians, between components of a superposition of magnetic substates, which is two orders of magnitude larger than that measured in any other atom-scale quantum system.

Frequency-stabilized cavity ring-down spectroscopy was utilized to measure electric quadrupole transitions within the ${{}^{16}\text{O}}_2$ $A$ band, $b{{}^1\Sigma }_g^{+}\leftarrow X{{}^3\Sigma }_g^{-}\left(0,0\right)$. We report quantitative measurements (relative uncertainties in intensity measurements from 4.4% to 11%) of nine ultraweak transitions in the ${}^{N}O$, ${}^{P}O$, ${}^{R}S$, and ${}^{T}S$ branches with line intensities ranging from $3\times {10}^{-30}$ to $2\times {10}^{-29}\text{cm}\text{molec}{.}^{-1}$. A thorough discussion of relevant noise sources and uncertainties in this experiment and other cw-cavity ring-down spectrometers is given. For short-term averaging $\left(t<100\text{s}\right)$, we estimate a noise-equivalent absorption of $2.5\times {10}^{-10}{\text{cm}}^{-1}{\text{Hz}}^{-1/2}$. The detection limit was reduced further by co-adding up to 100 spectra to yield a minimum detectable absorption coefficient equal to $1.8\times {10}^{-11}{\text{cm}}^{-1}$, corresponding to a line intensity of $\sim 2.5\times {10}^{-31}\text{cm}\text{molec}{.}^{-1}$. We discuss calculations of electric quadrupole line positions based on a simultaneous fit of the ground and upper electronic state energies which have uncertainties $<3\text{MHz}$, and we present calculations of electric quadrupole matrix elements and line intensities. The electric quadrupole line intensity calculations and measurements agreed on average to 5%, which is comparable to our average experimental uncertainty. The calculated electric quadrupole band intensity was $1.8\left(1\right)\times {10}^{-27}\text{cm}\text{molec}{.}^{-1}$ which is equal to only $\sim 8\times {10}^{-6}$ of the magnetic dipole band intensity.

Electron spectra from ${\text{Ar}}_n$ $\left(n\le 24000\right)$ and doped ${\text{Ar-CS}}_2$ clusters are studied experimentally at laser intensities of about ${10}^{15}\text{W}{\text{cm}}^{-2}$. More than an order of magnitude enhancement in the electron yield is observed in the case of doped ${\text{Ar-CS}}_2$ clusters. The enhancement observed is argued to be because of the presence of species with low ionization potential, ${\text{CS}}_2$, in the cluster. Similar enhancement is also observed in $K$-shell emission when comparison is done between pure ${\text{Ar}}_n$ and mixed $\text{Ar-}\left({\text{CS}}_2\right)$ clusters under identical experimental conditions of laser irradiation. We also address the effect of doping ${\text{CS}}_2$ clusters with dopant, Ar, which has higher ionization energy. We demonstrate a simple method for the characterization of doping level based on the Rayleigh scattering measurements.

The optimum conditions for producing ${\text{Cs}}_2$ molecular condensates from Cs atomic condensates with high transfer efficiency by stimulated Raman adiabatic passage are presented. Under the extended “two-photon” resonance condition, including the two-photon process, the mean-field correction, and the tunneling coupling between two upper excited molecular levels, a high and stable conversion efficiency is realized. The high conversion efficiency could be achieved by following two methods under experimentally less demanding conditions (relatively small effective Rabi frequency for pump laser pulse). One is adjusting the detuning difference between two laser pulses for same effective Rabi frequencies with up to 87.2% transfer efficiency. Another one is adjusting the effective Rabi frequency, the detuning of dump laser for given effective Rabi frequency, and the detuning of pump laser with up to 80.7% transfer efficiency.

In this paper, we construct an effective field theory for a condensate of cold Fermi atoms whose scattering is controlled by a narrow Feshbach resonance. We show how, from first principles, it permits a hydrodynamic description of the BEC-BCS crossover from which the equation of state, intimately related to the speed of sound, can be derived. Specifically, we stress the nonuniversal behavior of the equation of state at the unitary limit of infinite scattering length that arises when either, or both, of the range of the interatomic force and the scale of the molecular field become large.

Bipartite entanglement entropies are calculated for the ground state of the two-excitation subspace in a two-site coupled cavity model. Each region in the phase diagram (atomic insulator, polaritonic insulator, photonic superfluid, and polaritonic superfluid) is found to be characterized by unique entanglement properties. In particular, the polaritonic superfluid region exhibits genuine 4-partite entanglement among the two atoms and two-cavity fields. The difference in entanglement properties between the small- and large-hopping limits provides further evidence that the transitions in the two limits are qualitatively different and that the large-hopping limit cannot be described by a simple Bose-Hubbard-type model.

We investigate the property of optical bistability in a photonic-crystal one-atom laser when nonlinear microcavity is present. The physical system consists of a coherently driven two-level light emitter strongly coupled to a high-quality microcavity which is embedded within a photonic crystal and another coherent probing field which has incident into the microcavity. In our case, the microcavity is fabricated by nonlinear material and placed as an impurity in photonic crystal. This study reveals that such a system can exhibit optical bistability. The dependence of threshold value and hysteresis loop on the photonic band gap of the photonic crystal, driving field Rabi frequency and dephasing processes, are studied. Our results clearly illustrate the ability to control optical bistability through suitable photonic-crystal architectures and external coherent driving field, and this study suggests that in a photonic-crystal nonlinear microcavity, the one-atom laser acts as an effective controllable bistable device in the design of all-light digital computing systems in the near future.

We investigate the spontaneous emission from an inverted $\mathsf{Y}$-type atomic system coupled by three coherent fields. We use the Schrödinger equation to calculate the probability amplitudes of the wave function of the system and derive an analytical expression of the spontaneous emission spectrum to trace the origin of the spectral features. Quantum interference effects, such as the spectral line narrowing, spectrum splitting, and dark resonance, are observed. The number of spectral components, the spectral linewidth, and relative heights can be very different depending on the physical parameters. A variety of spontaneous emission spectral features can be controlled by the amplitude of the coupling fields and the preparation of the initial quantum state of the atom. We propose an ultracold atomic ${}^{87}\text{R}\text{b}$ system for experimental observation.

We address several different Casimir experiments where theory and experiment disagree. First out is the classical Casimir force measurement between two metal half spaces; here both in the form of the torsion pendulum experiment by Lamoreaux and in the form of the Casimir pressure measurement between a gold sphere and a gold plate as performed by Decca et al.; theory predicts a large negative thermal correction, absent in the high precision experiments. The third experiment is the measurement of the Casimir force between a metal plate and a laser irradiated semiconductor membrane as performed by Chen et al.; the change in force with laser intensity is larger than predicted by theory. The fourth experiment is the measurement of the Casimir force between an atom and a wall in the form of the measurement by Obrecht et al. of the change in oscillation frequency of a ${}^{87}\text{R}\text{b}$ Bose-Einstein condensate trapped to a fused silica wall; the change is smaller than predicted by theory. We show that saturation effects can explain the discrepancies between theory and experiment observed in all these cases.

A scheme for fault-tolerant quantum computation (FTQC) with probabilistic two-qubit gates is proposed. The error thresholds for FTQC are estimated by numerical simulations, where the conditional error probability (the error probability under the condition of “success”) is taken into account in addition to the failure probability. As a result, it turns out that FTQC is possible even if the failure probability is as high as 10% (5%) under the condition that the conditional error probability is lower than about 0.03% (0.3%). These results mean that the present scheme has a great advantage in using probabilistic two-qubit gates.

We demonstrate a chaotic microcavity laser whose quality is comparable to typical nonchaotic microcavity lasers, yet it has the unique characteristic of unidirectional output. The cavity shape is a disk with the boundary defined by a curve called limaçon of Pascal. For a lasing mode of volume less than $0.2\mu {\text{m}}^3$, the measured quality factor is $23000$. The record-high quality and small modal volume result in extremely low-lasing threshold. The spontaneous emission coupling efficiency is approximately 6% and is the reported value for a chaotic microcavity laser. All the lasing modes have one output beam in the same direction with a divergence angle less than $40°$. This universal directionality is determined by the chaotic ray dynamics in the open cavity.

With the density-matrix variational method, the ground-state energy of a many-electron system is calculated by minimizing the energy expectation value subject to the selected representability conditions for the second-order reduced-density matrix (2-RDM). There is a lack of size extensivity under the $P$, $Q$, and $G$ conditions of this method by applying to $M$ noninteracting atoms and molecules up to $M=32$; ${\text{Be}}_M$, ${\left({\text{CH}}_4\right)}_M$, and ${\left({\text{N}}_2\right)}_M$. The energy per molecule $\left(E/M\right)$ monotonically decreases as $M$ increases, showing the lack of size extensivity in this method. The inextensive contributions to energies are $3\times {10}^{-4}$ and $3\times {10}^{-3}\text{a}\text{.u}\text{.}$ for ${\left({\text{CH}}_4\right)}_M$ and ${\left({\text{N}}_2\right)}_M$ using the STO-6G basis set, respectively. In the examples studied, $E/M$ approaches a finite value as $M\to \infty$. The intermolecular elements of 2-RDMs do not satisfy the cumulant expansion.

We consider a type of $\left(M+N\right)$-mode entangled coherent states and propose a simple deterministic scheme to generate these states that can fly freely in space. We then exploit such free-flying states to teleport certain kinds of superpositions of multimode coherent states. We also address the issue of manipulating size and type of entangled coherent states by means of linear optics elements only.

We consider the effect of broadband decoherence on the performance of refocusing sequences, having in mind applications of dynamical decoupling in concatenation with quantum error correcting codes as the first stage of coherence protection. Specifically, we construct cumulant expansions of effective decoherence operators for a qubit driven by a pulse of a generic symmetric shape and for several sequences of $\pi$ and $\pi /2$ pulses. While, in general, the performance of soft pulses in decoupling sequences in the presence of Markovian decoherence is worse than that of the ideal $\delta$ pulses, it can be substantially improved by shaping.

We introduce an arbitrary number of accessible modes when analyzing bipartite entanglement degradation due to Unruh effect between two partners Alice and Rob. Under the single mode approximation (SMA) a fermion field only had a few accessible levels due to Pauli exclusion principle conversely to bosonic fields which had an infinite number of excitable levels. This was argued to justify entanglement survival in the fermionic case in the SMA infinite acceleration limit. Here we relax SMA. Hence, an infinite number of modes are excited as the observer Rob accelerates, even for a fermion field. We will prove that, despite this analogy with the bosonic case, entanglement loss is limited. We will show that this comes from fermionic statistics through the characteristic structure it imposes on the infinite dimensional density matrix for Rob. Surprisingly, the surviving entanglement is independent of the specific maximally entangled state chosen, the kind of fermionic field analyzed, and the number of accessible modes considered. We shall discuss whether this surviving entanglement goes beyond the purely statistical correlations, giving insight concerning the black hole information paradox.

We present a scheme for scalable quantum information processing with atomic ensembles and flying photons. Using the Rydberg blockade, we encode the qubits in the collective atomic states, which could be manipulated fast and easily due to the enhanced interaction in comparison to the single-atom case. We demonstrate that our proposed gating could be applied to generation of two-dimensional cluster states for measurement-based quantum computation. Moreover, the atomic ensembles also function as quantum repeaters useful for long-distance quantum state transfer. We show the possibility of our scheme to work in bad cavity or in weak coupling regime, which could much relax the experimental requirement. The efficient coherent operations on the ensemble qubits enable our scheme to be switchable between quantum computation and quantum communication using atomic ensembles.