We report the generation of polarization-entangled photons by cavity-enhanced spontaneous parametric down-conversion. The bandwidth of entangled photons is reduced to that of the optical parometric oscillator and the conversion rate is enhanced by cavity. The time interval between the conjugate two photon is prolongated so that it can be directly measured by a common detector. The time correlation function of the entangled-photons pair shows a time-oscillatory characteristic. To demonstrate the entanglement, we experimentally obtain two-photon quantum interference for the polarization variable.

We demonstrate a trimodal waveguide architecture in a three-dimensional (3D) photonic-band-gap (PBG) material, in which the local electromagnetic density of states (LDOS) within and adjacent to the waveguide exhibits a forklike wavelength filter characteristic. This facilitates the control and switching of one laser beam with other laser beams ( $\sim 1\mu \mathrm{W}$ steady-state holding power and $\sim 5\mathrm{nW}$ switching power) through mutual coherent resonant interaction with quantum dots. Two waveguide modes provide narrow spectral windows where the electromagnetic LDOS is enhanced by a factor of 100 or more relative to the background LDOS of a third air-waveguide mode with nearly linear dispersion. This “engineered vacuum” can be used for frequency-selective, atomic population inversion and switching (by coherent resonant optical pumping) of an inhomogeneously broadened collection of “atoms” situated adjacent to the waveguide channel. The “inverted” atomic system can then be used to coherently amplify fast optical pulses propagating through the third waveguide mode. This coherent “control of light with light” occurs without recourse to microcavity resonances (involving long cavity buildup and decay times for the optical field). Our architecture facilitates steady-state coherent pumping of the atomic system (on the lower-frequency LDOS peak) to just below the gain threshold. The higher-frequency LDOS peak is chosen to coincide with the upper Mollow sideband of the same atomic resonance fluorescence spectrum. The probing laser is adjusted to the lower Mollow sideband, which couples to the linear dispersion (high group velocity part) of the third waveguide mode. This architecture enables rapid modulation (switching) of light at the lower Mollow sideband frequency through light pulses conveyed by the linear dispersion mode at frequencies corresponding to the central Mollow component (lower LDOS peak). We demonstrate that LDOS jumps of order 100 can occur on frequency scales of $\Delta \omega \approx {10}^{-4}{\omega }_c$ (where ${\omega }_c$ is the frequency of the jump) in a finite-size 3D photonic crystal (PC) consisting of only $10\times 10\times 20$ unit cells. When the semiconductor backbone of the PC has a refractive index of 3.5 and ${\omega }_c$ corresponds to a wavelength of $1.5\mu \mathrm{m}$, this vacuum engineering may be achieved in a sample whose largest dimension is about $12\mu \mathrm{m}$.

Studies of quantum computer implementations suggest cellular quantum computer architectures. These architectures can simulate the evolution of quantum cellular automata, which can possibly simulate both quantum and classical physical systems and processes. It is however known that except for the trivial case, unitary evolution of one-dimensional homogeneous quantum cellular automata with one qubit per cell is not possible. Quantum cellular automata that comprise two qubits per cell are defined and their evolution is studied using a quantum computer simulator. The evolution is unitary and its linearity manifests itself as a periodic structure in the probability distribution patterns.

We show that a multiply quantized vortex line core can be energetically stable in a harmonically trapped two-component atomic Bose-Einstein condensate. The structure, in which the condensate component with the vortex line is surrounded by the second component forming a vortex ring, can be identified as a particlelike skyrmion with a multiply quantized winding number, and closely resembles cosmic vortons.

We write explicitly a general protocol for faithful teleportation of a $d$-state particle (qudit) via a partially entangled pair of (pure) $n$-state particles. The classical communication cost (CCC) of the protocol is ${\log }_2\left(nd\right)$ bits, and it is implemented by a projective measurement performed by Alice, and a unitary operator performed by Bob (after receiving from Alice the measurement result). We prove the optimality of our protocol by a comparison with the concentrate and teleport strategy. We also show that if $d>n∕2$, or if there is no residual entanglement left after the faithful teleportation, the CCC of any protocol is at least ${\log }_2\left(nd\right)$ bits. Furthermore, we find a lower bound on the CCC in the process transforming one bipartite state to another by means of local operation and classical communication.

Using the functional integral formulation of nonequilibrium quantum many-body theory we develop a regular description of a Fermi system with a strong attractive interaction in the presence of an external time-dependent potential. In the strong coupling limit this fermionic system is equivalent to a nonequilibrium dilute Bose gas of diatomic molecules. We also consider nonequilibrium strongly coupled Bardeen-Cooper-Schrieffer theory and show that it reduces to the full nonlinear time-dependent Gross-Pitaevski equation, which determines the evolution of the condensate wave function.

A method of suppression of losses due to excited-state absorption (ESA) in laser crystals is proposed, based on the well-known phenomenon of electromagnetically induced transparency. Typical situations in which ESA prevents lasing are considered. Theoretical analysis shows that by using an additional driving laser field one can inhibit ESA and make lasing possible. On the basis of available spectroscopic information estimates for rare-earth-ion-doped laser materials are presented. They show the feasibility of obtaining optical gain at parity-allowed $4f^n\leftrightarrow 4f^{n-1}5d$ transitions of rare-earth ions in the ultraviolet spectral region after suppression of ESA at an emission wavelength into the conduction band of a host material.

It is proposed that single attosecond pulses be generated by gating high-order harmonic emission with fields whose ellipticity varies rapidly with time. The laser pulse with a time-dependent ellipticity is created from two $5\text{−}\mathrm{fs}$ laser pulses centered at $750\mathrm{nm}$ and separated by $5\mathrm{fs}$. One of the laser pulses is left-circularly polarized and the other is right-circularly polarized. Numerical simulations show that when neon atoms are driven by such laser pulses, the generated high-order harmonic spectrum in the 25th to the 85th orders are a supercontinuum that corresponds to single attosecond pulses. A simple analytical expression is derived for estimating the high-harmonic radiation time.

Three-photon entangled states such as a Greenberger-Horne-Zeilinger state can be produced by mixing a weak coherent state with parametric down-conversion. Quantum interference between the two states is responsible for the cancellation of unwanted photon states. Two schemes of interference are considered and both are shown to produce the entangled state.

We study the possibility of revealing a weak coherent force by using a pendular mirror as a probe, and coupling this to a radiation field, which acts as the meter, in a cavityless configuration. We determine the sensitivity of such a scheme and show that the use of an entangled meter state greatly improves the ultimate detection limit. We also compare this scheme with that involving an optical cavity.

We report the observation of three $p$-wave Feshbach resonances of ${}^6\mathrm{Li}$ atoms in the lowest hyperfine state $f=1∕2$. The positions of the resonances are in good agreement with theory. We study the lifetime of the cloud in the vicinity of the Feshbach resonances and show that, depending on the spin states, two- or three-body mechanisms are at play. In the case of dipolar losses, we observe a nontrivial temperature dependence that is well explained by a simple model.

We report the generation of polarization entangled photon pairs in the 1550-nm wavelength band using spontaneous four-wave mixing in a dispersion-shifted fiber loop. The use of the fiber-loop configuration made it possible to generate polarization entangled states very stably. With accidental coincidences subtracted, we obtained coincidence fringes with $>90\%$ visibilities, and observed a violation of Bell’s inequality by seven standard deviations. We also confirmed the preservation of the quantum correlation between the photons even after they had been separated by 20 km of optical fiber.

For the Stokes tensor parametrization of a multiqubit density operator, we provide an explicit formulation of the corresponding unitary dynamics at the infinitesimal level. The main advantage of this formalism (clearly reminiscent of the ideas of “coherences” and “coupling Hamiltonians” of spin systems) is that the pattern of correlation between qubits and the pattern of infinitesimal correlation are highlighted simultaneously and can be used constructively for qubit manipulation. For example, it allows us to compute explicitly Rodrigues’ formula for the one-parameter orbits of nonlocal Hamiltonians. The result is easily generalizable to orbits of Cartan subalgebras and allows us to express the Cartan decomposition of unitary propagators as a linear action directly in terms of the infinitesimal generators.

A conditional unitary transformation ($90°$ polarization rotation) is performed at single-photon level. The transformation is realized by rotating polarization for one of the photons of a polarization-entangled biphoton state (signal photon) by means of a Pockel cell triggered by the detection of the other (idler) photon after polarization selection. As a result, the state of the signal photon is losslessly changed from being completely unpolarized to being partially polarized, so that the final polarization degree is given by the idler detector quantum efficiency. This experiment can be used for developing a different method of absolute quantum efficiency calibration.

We discuss entanglement in the spin-1∕2 anisotropic ferromagnetic Heisenberg chain in the presence of a boundary magnetic field generating domain walls. By increasing the magnetic field, the model undergoes a first-order quantum phase transition from a ferromagnetic to a kink-type phase, which is associated to a jump in the content of entanglement available in the system. Above the critical point, pairwise entanglement is shown to be nonvanishing and independent of the boundary magnetic field for large chains. Based on this result, we provide an analytical expression for the entanglement between arbitrary spins. Moreover, the effects of the quantum domains on the gapless region and for antiferromagnetic anisotropy are numerically analyzed. Finally multiparticle entanglement properties are considered, from which we establish a characterization of the critical anisotropy separating the gapless regime from the kink-type phase.

The scalar and tensor dynamic polarizabilities of the first excited ${}^{2}P_{1∕2,3∕2}$ states of the alkali-metal atoms $\mathrm{Na}$, $\mathrm{K}$, and $\mathrm{Rb}$ are calculated using relativistic many-body perturbation theory. Comparison with experimental measurements of the static polarizabilities suggests that the errors in the theoretical predictions do not exceed 1%. The dynamic polarizabilities at imaginary frequencies are employed to calculate the long-range van der Waals coefficients for the interaction of ground-state helium atoms with the excited alkali-metal atoms to a probable error of less than 2%.

We present results of the pressure broadening and shift of the rubidium $5s{}^{2}S_{1∕2}\text{–}6p{}^{2}P_{3∕2}$ (420.2 nm) spectral line by argon and helium buffer gases at pressures below 10 torr. High-resolution absorption measurements were performed with a single-mode external cavity diode laser, having 20-GHz tuning range and a linewidth of about 1 MHz. Pressure broadening and shift parameters of Lorentzian hyperfine components have been determined. We obtained the redshift of hyperfine spectral lines for He and the blueshift for Ar.

Coherent optical spectroscopy has been performed on the narrow $5s{}^{2}S_{1∕2}–4d{}^{2}D_{5∕2}$ quadrupole transition in a single Doppler-cooled and trapped ${}^{88}\mathrm{Sr}^{+}$ ion. High-contrast optical Ramsey spectra have been observed with fringe visibilities up to $\sim 90\%$. The visibility decreased as the free precession period was increased, and was limited by the interrogation laser’s coherence. The effect of varying the relative phase of the second Ramsey pulse was investigated. By measuring the difference in excitation probability on reversing a $90°$ relative phase shift between the two Ramsey pulses, we have demonstrated Ramsey’s anti-symmetric line shape in the optical domain. A significant advantage of this line shape is the zero crossing at line center and the independence of this center frequency on drifts in signal amplitude. This optical Ramsey line shape is suitable for stabilizing a local oscillator to an atomic reference transition in an optical frequency standard. All observed optical Ramsey signals are well described by a model using the optical Bloch equations.

We report the observation of impulsive alignment of $\mathrm{C}{\mathrm{O}}_2$ molecules produced through their interaction with a nonresonant, strong laser pulse. The periodic alignment is monitored using a polarization technique generally employed in optical Kerr effect experiments; the birefringence produced by alignment of the molecular sample is measured with a weak pulse, time-delayed with respect to the alignment pulse. The technique provides a signal proportional to $⟨{\cos }^2\theta ⟩-\frac{1}{3}$, where $\theta$ is the polar angle between the molecular axis and the strong-field polarization axis. Experiments are conducted at room and at low temperatures. Two methods of analysis are presented. The first one consists in comparing the signal with the prediction of the time-dependent Schrödinger equation. From a fine analysis of the temporal signal shape one can then deduce the value of $⟨{\cos }^2\theta ⟩$. The second one allows us to extract $⟨{\cos }^2\theta ⟩$ through a calibration of the birefringence signal obtained by performing the experiment in a reference atomic gas sample. Both analyses are compared and found in good agreement for the different laser intensities investigated. Saturation of the alignment process is observed at a laser intensity that agrees with the ionization saturation intensities of $\mathrm{C}{\mathrm{O}}_2$.

We report on a series of measurements of decoherence and wave-packet dephasing between two colliding, strongly coupled, identical Bose-Einstein condensates. We measure, in the strong-excitation regime, a suppression of the mean-field shift, compared to the shift which is observed for a weak excitation. This suppression is explained by applying the Gross-Pitaevskii energy functional. By selectively counting only the nondecohered fraction in a time-of-flight image we observe oscillations for which both inhomogeneous and Doppler broadening are strongly suppressed. If no post-selection is used, the decoherence rate due to collisions can be extracted and is in agreement with the local density average calculated rate.