We propose an optical parametric down-conversion (PDC) scheme that does not suffer a trade-off between the state purity of single-photon wave packets and the rate of packet production. This is accomplished by modifying the PDC process by using a microcavity to engineer the density of states of the optical field at the PDC frequencies. The high-finesse cavity mode occupies a spectral interval much narrower than the bandwidth of the pulsed pump laser field, suppressing the spectral correlation, or entanglement, between signal and idler photons. Spectral filtering of the field occurs prior to photon creation rather than afterward as in most other schemes. Operator-Maxwell equations are solved to find the Schmidt-mode decomposition of the two-photon states produced. Greater than 99% pure-state packet production is predicted to be achievable.

We discuss the influence of phase-changing collisions on the group velocities in Doppler-broadened, cycling, degenerate two-level systems where $F_e=F_g+1$ and $F_g>0$, interacting with pump and probe lasers, that exhibit electromagnetically induced absorption (EIA). Two model systems are considered: the $N$ system where the pump and probe are polarized perpendicularly, and EIA is due to transfer of coherence (TOC), and the double two-level system (TLS) where both lasers have the same polarization, and EIA is due to transfer of population (TOP). For the case of Doppler-broadened EIA TOC, which occurs at low pump intensity, there is a switch from positive to negative dispersion and group velocity, as the rate of phase-changing collisions is increased. For the case of EIA TOP at low pump intensity, the dispersion and group velocity remain negative even when the collision rate is increased. Pressure-induced narrowing, accompanied by an increase in the magnitude of the negative dispersion and a decrease in the magnitude of the negative group velocity, occurs in both EIA TOC and EIA TOP, at low pump intensity. When the pump intensity is increased, a switch from negative to positive dispersion and group velocity, with increasing collision rate, also occurs in the double TLS system. However, the effect is far smaller than in the case of the $N$ system at low pump intensity.

We show that two ways of manipulating quantum entanglement—namely, entanglement-assisted local transformation [D. Jonathan and M. B. Plenio, Phys. Rev. Lett. 83, 3566 (1999)] and multiple-copy transformation [S. Bandyopadhyay, V. Roychowdhury, and U. Sen, Phys. Rev. A 65, 052315 (2002)]—are equivalent in the sense that they can asymptotically simulate each other’s ability to implement a desired transformation from a given source state to another given target state with the same optimal success probability. As a consequence, this yields a feasible method to evaluate the optimal conversion probability of an entanglement-assisted transformation.

We investigate the Yuen 2000 (so-called Y-00)-protocol, which can realize a randomized stream cipher with high bit rate (Gbit/s) for long distances (several hundreds km). The randomized stream cipher with randomization by quantum noise based on the Y-00 protocol is called a quantum stream cipher in this paper, and it may have security against known plaintext attacks which has no analog with any conventional symmetric key ciphers. We present a simple cryptanalysis based on an attacker’s heterodyne measurement and a quantum unambiguous measurement to make clear the strength of the Y-00 protocol in real communication. In addition, we give a design for the implementation of an intensity-modulation scheme and report an experimental demonstration of $1\mathrm{Gbit}∕\mathrm{s}$ quantum stream cipher through a $20\text{−}\mathrm{km}$-long transmission line.

The quantum key distribution protocol without public announcement of bases is equipped with a two-way classical communication symmetric entanglement purification protocol. This modified key distribution protocol is unconditionally secure and has a higher tolerable error rate of 20%, which is higher than previous scheme without public announcement of bases.

We present a quantum protocol utilizing a tripartite three-level state. The state used in this scheme contains entanglement even after one system is traced out and as a result can be used for both a secret-sharing protocol among the three parties and a quantum-key-distribution protocol between any two parties. We show how to utilize this residual entanglement for quantum-key-distribution purposes, and explore a possible realization of the scheme using entanglement of orbital-angular-momentum states of photons.

We present a way for symmetric multiparty-controlled teleportation of an arbitrary two-particle entangled state based on Bell-basis measurements by using two Greenberger-Horne-Zeilinger states, i.e., a sender transmits an arbitrary two-particle entangled state to a distant receiver, an arbitrary one of the $n+1$ agents, via the control of the others in a network. It will be shown that the outcomes in the cases that $n$ is odd or is even are different in principle as the receiver has to perform a controlled-NOT operation on his particles for reconstructing the original arbitrary entangled state in addition to some local unitary operations in the former. Also we discuss the applications of this controlled teleporation for quantum secret sharing of classical and quantum information. As all the instances can be used to carry useful information, its efficiency for qubit approaches the maximal value.

We present here triple differential cross sections for ionization of hydrogen atoms by electron impact at 1, 0.5, and $0.3\mathrm{eV}$ energy above threshold, calculated in the hyperspherical partial wave theory. The results are in very good agreement with the available semiclassical results of Deb and Crothers [Phys. Rev. A 65, 052721 (2002)] for these energies. With this, we are able to demonstrate that the hyperspherical partial-wave theory yields good cross sections from $30\mathrm{eV}$ [Phys. Rev. A 67, 042717 (2003)] down to near threshold for equal-energy-sharing kinematics.

The properties of spin-1 Bose gases with ferromagnetic interactions in the presence of a nonzero magnetic field are studied. The equation of state and thermodynamic quantities are worked out with the help of a mean-field approximation. The phase diagram besides Bose-Einstein condensation contains a first-order transition where two values of the magnetization coexist. The dynamics is investigated with the help of the random phase approximation. The soft mode corresponding to the critical point of the magnetic phase transition is found to behave like in conventional theory.

A detailed theoretical and experimental study of the atom-field interaction starting from first principles was made possible by the realization of the single-atom micromaser. The situation is very close to the ideal case of a single two-level atom interacting with a single quantized mode of a superconducting cavity. In spite of the considerable amount of work devoted to the study of the micromaser, it still remains to be established how the system behaves under coherent pumping. We present a comprehensive study of the coherently pumped micromaser; we develop an analytic method to obtain the steady-state solution of the master equation governing the time evolution of the cavity field. We illustrate this method with a simple example and present results for the photon distribution and purity of the steady state of the coherently pumped micromaser.

A reliable single photon source is a prerequisite for linear optical quantum computation and for secure quantum key distribution. A criterion yielding a conclusive test of the single photon character of a given source, attainable with realistic detectors, is therefore highly desirable. In the context of heralded single photon sources, such a criterion should be sensitive to the effects of higher photon number contributions, and to vacuum introduced through optical losses, which tend to degrade source performance. In this Rapid Communication we present, theoretically and experimentally, a criterion meeting the above requirements.

The Casimir interaction between two thick parallel plates, one made of metal and the other of a dielectric, is investigated at nonzero temperature. It is shown that in some temperature intervals the Casimir pressure and the free energy of a fluctuating field are nonmonotonic functions of temperature and the corresponding Casimir entropy can be negative. A physical interpretation for these conclusions is given. At the same time we demonstrate that the entropy vanishes when the temperature goes to zero, i.e., in the Casimir interaction between a metal and a dielectric the Nernst heat theorem is satisfied. The investigation is performed both analytically, by using the model of an ideal metal and dilute dielectric or dielectric with a frequency-independent dielectric permittivity, and numerically for a real metal (Au) and dielectrics with different behaviors of the dielectric permittivity along the imaginary frequency axis (Si and $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$).

In this paper, we present a method to construct full separability criterion for tripartite systems of qubits. The spirit of our approach is that a tripartite pure state can be regarded as a three-order tensor that provides an intuitionistic mathematical formulation for the full separability of pure states. We extend the definition to mixed states and give out the corresponding full separability criterion. As applications, we discuss the separability of several bound entangled states, which shows that our criterion is feasible.

We study the measurement-induced non-Gaussian operation on the single- and two-mode Gaussian squeezed vacuum states with beam splitters and on-off type photon detectors, with which mixed non-Gaussian states are generally obtained in the conditional process. It is known that the entanglement can be enhanced via this non-Gaussian operation on the two-mode squeezed vacuum state. We show that, in the range of practical squeezing parameters, the conditional outputs are still close to Gaussian states, but their second order variances of quantum fluctuations and correlations are effectively suppressed and enhanced, respectively. To investigate an operational meaning of these states, especially entangled states, we also evaluate the quantum dense coding scheme from the viewpoint of the mutual information, and we show that non-Gaussian entangled state can be advantageous compared with the original two-mode squeezed state.

Cross sections for electron-impact excitation from the $5p{}^{5}6s$ $J=2$ metastable level of xenon to the lowest six levels of the $5p{}^{5}6p$ configuration have been measured. The cross sections generally have very large magnitudes $\left({10}^{-15}{\mathrm{cm}}^2\right)$ and scale with the corresponding optical oscillator strengths. The substantial variations observed in the energy dependence of the cross sections for the six levels can also be related to the optical oscillator strengths. Cross sections for excitation out of the $J=2$ metastable level into the upper four levels of the $5p{}^{5}6p$ configuration are much smaller. The large disparity in cross sections between the upper and lower groups is explained in terms of the electronic structure of the excited states of xenon.

We solve the perturbative electron-positron pair production exactly by calculating the second-order Feynman diagrams. We compare our result with Born methods that include Coulomb corrections. We find that a small-momentum approximation is not adequate to obtain exact Coulomb corrections and higher-order terms should also be included. We also compare the impact parameter dependence cross sections.

Quantum control aims at the manipulation of atomic- and molecular-scale dynamics phenomena. An important objective in this regard is the understanding of dynamical control within a family of related quantum systems. To explore this issue, diffeomorphic changes in the system Hamiltonian $\mathcal{H}(s,t)$ are introduced by scanning over a homotopy parameter $s$ and then monitoring the control field response needed to maintain the value of a specified target observable. This operation is implemented through a procedure referred to as diffeomorphic modulation under observable-response-preserving homotopy (D-MORPH). The governing D-MORPH differential equation determining the control laser field $E(s,t)$ is shown to explicitly allow for innumerable solutions, with each characterized by the choice of an arbitrary function $f(s,t)$ of $s$ and time $t$. The presence of $f(s,t)$ in the D-MORPH differential equation makes clear the origin of multiple control fields that produce the same observable objective. A stable algorithm is presented for practical execution of D-MORPH with the only criterion that the Hamiltonian $\mathcal{H}(s,t)$ permit reaching the objective over the full domain of $s$ being sampled. Both analytic and numerical examples are presented to illustrate the D-MORPH concept.

We studied the focusing of atoms by multiple layers of standing light waves in the context of atom lithography. In particular, atomic localization by a double-layer light mask is examined using the optimal squeezing approach. Operation of the focusing setup is analyzed both in the paraxial approximation and in the regime of nonlinear spatial squeezing for the thin-thin, as well as thin-thick, atom lens combinations. It is shown that the optimized double light mask may considerably reduce the imaging problems, improve the quality of focusing, and enhance the contrast ratio of the deposited structures.