We present a quantitative experimental and theoretical study of intensity fluctuations in the emitted light of a random laser that has different realizations of disorder for every pump pulse. A model that clarifies these intrinsic fluctuations is developed. We describe the output versus input power graphs of the random laser with an effective spontaneous emission factor ($\beta$ factor).

Recently, atomic ensemble and single photons were successfully entangled by using collective enhancement [D. N. Matsukevich et al., Phys. Rev. Lett. 95, 040405 (2005)], where atomic internal states and photonic polarization states were correlated in a nonlocal manner. Here, we experimentally clarified that in an ensemble of atoms and a photon system, there also exists an entanglement concerned with spatial degrees of freedom. Generation of higher-dimensional entanglement between remote atomic ensemble and an application to condensed matter physics are also discussed.

We report differential and momentum transfer cross sections for elastic scattering of low-energy electrons by trans formic acid. Our calculations employed the Schwinger multichannel method with pseudopotentials and were performed in the static-exchange and in the static-exchange plus polarization approximations. We found a shape resonance around $1.9\mathrm{eV}$ belonging to the $A^{″}$ symmetry of the $C_s$ group. This result agrees with recent calculations and experiments. Our differential cross sections show good agreement with experiment, but disagree with another theoretical calculation at higher scattering angles.

In this work we show that, by frequent measurements of adequately chosen observables, a complete suppression of the decay in an exponentially decaying two-level system interacting with a squeezed bath is obtained. The observables for which the effect is observed depend on the squeezing parameters of the bath. The initial states that display total Zeno effect are intelligent states of two conjugate observables associated to the electromagnetic fluctuations of the bath.

We present detailed derivations, various improvements and application to concrete experimental data of spin squeezing inequalities formulated recently by some of us [Phys. Rev. Lett. 95, 120502 (2005)]. These inequalities generalize the concept of the spin squeezing parameter and provide necessary and sufficient conditions for genuine two-, or three-qubit entanglement for symmetric states, and sufficient condition for general $N$-qubit states. We apply our method to theoretical study of Dicke states, and, in particular, to $\mathrm{W}$ states of $N$ qubits. Then, we analyze the recently experimentally generated seven- and eight-ion $\mathrm{W}$ states [Nature 438, 643 (2005)]. we also present some details concerning this experiment. Finally, we improve criteria for detection of genuine tripartite entanglement based on entanglement witnesses.

When discriminating between two pure quantum states, there exists a quantitative trade-off between the information retrieved by the measurement and the disturbance caused on the unknown state. We derive the optimal trade-off and provide the corresponding quantum measurement. Such an optimal measurement smoothly interpolates between the two limiting cases of maximal information extraction and no measurement at all.

We study a coupling (decoupling) method between a superconducting qubit and a data bus that uses a controllable time-dependent electromagnetic field (TDEF). As in recent experiments, the data bus can be either an $LC$ circuit or a cavity field. When the qubit and the data bus are initially fabricated, their detuning should be made far larger than their coupling constant, so these can be treated as two independent subsystems. However, if a TDEF is applied to the qubit, then a “dressed qubit” (i.e., qubit plus the electromagnetic field) can be formed. By choosing appropriate parameters for the TDEF, the dressed qubit can be coupled to the data bus and, thus, the qubit and the data bus can exchange information with the assistance of the TDEF. This mechanism allows the scalability of the circuit to many qubits. With the help of the TDEF, any two qubits can be selectively coupled to (and decoupled from) a common data bus. Therefore, quantum information can be transferred from one qubit to another.

Recently, there has been growing interest in using adiabatic quantum computation as an architecture for experimentally realizable quantum computers. One of the reasons for this is the idea that the energy gap should provide some inherent resistance to noise. It is now known that universal quantum computation can be achieved adiabatically using two-local Hamiltonians. The energy gap in these Hamiltonians scales as an inverse polynomial in the number of quantum gates being simulated. Here we present stabilizer codes which can be used to produce a constant energy gap against one-local and two-local noise. The corresponding fault-tolerant universal Hamiltonians are four-local and six-local, respectively, which are the optimal result achievable within this framework.

The $B$-spline $R$-matrix method is used to investigate electron scattering from neutral calcium and photodetachment of ${\mathrm{Ca}}^{-}$ in the low-energy range from threshold to $4\mathrm{eV}$. The multiconfiguration Hartree-Fock method with nonorthogonal orbital sets is employed for an accurate representation of the target wave functions. The close-coupling expansion includes 39 bound states of neutral calcium, covering all states from the ground state to $4s8s{}^{1}S$. The present calculations yield good agreement with the few available experimental data for both elastic electron scattering and photodetachment of ${\mathrm{Ca}}^{-}$. The prominent resonance structure in the low-energy region is analyzed and discussed.

We investigate how and to what extent molecular alignment can be controlled in a dissipative medium by a suitable train of laser pulses. We focus primarily on the extension of a scheme of control originally constructed for unitary evolution. The procedure is applied to control the alignment of CO molecules in an Ar gas. The parameters of the train of kicks—i.e., the intensity of each kick and the delay between them—are either obtained by a systematic procedure (maximum strategy) or by optimization by evolutionary algorithms.

We propose a scheme for making a Bose-Einstein condensate (BEC) of molecules from a BEC of atoms in a strongly confining two-dimensional optical lattice and a weak one-dimensional optical lattice in the third dimension. The stable solutions obtained for the order parameters take the form of a different type of gap soliton, with both atomic and molecular BECs, and also standard gap solitons with only a molecular BEC. The strongly confining dimensions of the lattice stabilize the BEC against inelastic energy transfer in atom-molecule collisions. The solitons with atoms and molecules may be obtained by starting with an atomic BEC, and gradually tuning the resonance by changing the external magnetic-field strength until the desired atom-molecule soliton is obtained. A gap soliton of a BEC of only molecules may be obtained nonadiabatically by starting from an atom-only gap soliton, far from a Feshbach resonance and adjusting the magnetic field to near Feshbach resonance. After a period of time in which the dimer field grows, change the magnetic field such that the detuning is large and negative and Feshbach effects wash out, turn off the optical lattice in phase with the atomic BEC, and turn on an optical lattice in phase with the molecules. The atoms disperse, leaving a gap soliton composed of a molecular BEC. Regarding instabilities in the dimension of the weak optical lattice, the solitons which are comprised of both atoms and molecules are sometimes stable and sometimes unstable—we present numerically obtained results. Gap solitons comprised of only molecules have the same stability properties as the standard gap solitons: stable from frequencies slightly below the middle of the band gap to the top, and unstable below that point. Instabilities are only weakly affected by the soliton velocities, and all instabilities are oscillatory.

We investigate bright matter-wave solitons in the presence of a spatially varying scattering length. It is demonstrated that a soliton can be confined due to the inhomogeneous collisional interactions. Moreover, we observe the enhanced transmission of matter-wave solitons through potential barriers for suitably chosen spatial variations of the scattering length. The results indicate that the manipulation of atomic interactions can become a versatile tool to control matter-wave dynamics.

We present the design and experimental proof of principle of a low threshold optical parametric oscillator (OPO) that continuously oscillates over a large bandwidth allowed by phase matching. The large oscillation bandwidth is achieved with a selective two-photon loss that suppresses the inherent mode competition, which tends to narrow the bandwidth in conventional OPOs. Our design performs pairwise mode locking of many frequency pairs, in direct equivalence to passive mode locking of ultrashort pulsed lasers. The ability to obtain high powers of continuous and broadband down-converted light enables the optimal exploitation of the correlations within the down-converted spectrum, thereby strongly affecting two-photon interactions even at classically high power levels, and opening new venues for applications such as two-photon spectroscopy and microscopy and optical spread spectrum communication.

We analyze the dynamical symmetries and the selection rules relevant to the process of production of high-order harmonics as a result of irradiating an atom with strong, continuous bichromatic laser radiation of similar frequencies $\omega$ and $\omega +\delta \omega$. We show that when the two laser frequencies are close to one another (small $\delta \omega$), the spectrum becomes very dense and new selection rules are obtained. Such a dense spectrum of harmonics could be a source for the production of attosecond light pulses. We demonstrate our results numerically on a one-dimensional model Hamiltonian of a Xe atom and show how a pulse of length $700\text{−}\mathrm{as}$ could be produced.

The nature of two-photon interference is a subject that has aroused renewed interest in recent years and is still under debate. In this paper we report the observation of two-photon interference with independent pseudothermal sources in which subwavelength interference is observed. The phenomenon may be described in terms of optical transfer functions and the classical statistical distribution of the two sources.

We introduce a figure of merit for a quantum memory which measures the preservation of entanglement between a qubit stored in and retrieved from the memory and an auxiliary qubit. We consider a general quantum memory system consisting of a medium of two level absorbers, with the qubit to be stored encoded in a single photon. We derive an analytic expression for our figure of merit taking into account Gaussian fluctuations in the Hamiltonian parameters, which, for example, model inhomogeneous broadening and storage time dephasing. Finally we specialize to the case of an atomic quantum memory where fluctuations arise predominantly from Doppler broadening and motional dephasing.

We derive an analytical lower bound for the concurrence of tripartite quantum mixed states. A functional relation is established relating concurrence and the generalized partial transpositions.

We introduce a protocol for steady-state entanglement generation and protection based on detuning modulation in the dissipative interaction between a two-qubit system and a bosonic mode. The protocol is a global-addressing scheme which only requires control over the system as a whole. We describe a postselection procedure to project the register state onto a subspace of maximally entangled states. We also outline how our proposal can be implemented in a circuit-quantum electrodynamics setup.

A scheme is proposed to obtain three-qubit gates, such as quantum-phase gate and $C^2$-NOT gate operations, in a cavity QED system where highly detuned cavity-field modes interact with a four-level system in an inverted-Y configuration. The influence of the Stark shift is also included in such proposed gate operations. Since only the metastable lower levels are involved in the gate operations, the gates are not affected by the atomic decay rates. The potential application of such gates to realize Grover’s algorithm is also discussed.

The properties of the quasirelativistic Hartree-Fock equations are investigated. Different ways of forming the equations are discussed. The relativistic corrections employed in the traditional quasirelativistic Hartree-Fock equations are compared with the ones used within the Breit-Pauli approach. The two-electron contact interactions are revised and the new versions of the quasirelativistic Hartree-Fock equations are considered. The solutions of these equations are compared among themselves and also with those obtained within the Dirac-Fock and nonrelativistic Hartree-Fock approaches. Solution data are provided for some heliumlike, neonlike, and mercurylike ions.