To improve the performance of a quantum-key-distribution (QKD) system, high speed, low dark count single photon detectors (or low-noise homodyne detectors) are required. However, in practice, a fast detector is usually noisy. Here, we propose a dual-detector method to improve the performance of a practical QKD system with realistic detectors: the legitimate receiver randomly uses either a fast (but noisy) detector or a quiet (but slow) detector to measure the incoming quantum signals. The measurement results from the quiet detector can be used to bound the eavesdropper’s information, while the measurement results from the fast detector are used to generate a secure key. We apply this idea to various QKD protocols. Simulation results demonstrate significant improvements of the secure key rate in the lower loss regime in both Bennett-Brassard 1984 (BB84) protocol with ideal single photon source and Gaussian-modulated coherent states protocol; while for decoy-state BB84 protocol with weak coherent source, the improvement is moderate. We also discuss various practical issues in implementing the dual-detector scheme.

Absolute cross sections for electron-impact excitation of the dipole-allowed transitions ${\mathrm{S}}^{4+}$ $(3s^2{}^{1}S\to 3s3p{}^1\mathrm{P})$ and ${\mathrm{S}}^{5+}$ $(3s{}^{2}S\to 3p{}^{2}P)$ were measured near threshold using the merged electron-ion beams energy-loss technique. Although the magnitudes of the measured cross sections are in reasonable agreement with available theoretical data, the experimental data indicate that the contributions of dielectronic resonances in the near-threshold region are underestimated by these calculations.

In this paper we present a single-particle scattering approach for the angular correlation between a photoelectron and the subsequent Auger electron from atomic targets. This method is proposed as an alternative approach with respect to the usual density matrix formalism, since it is more convenient for extension to the solid state case. Such an extension is required by the great progress made in the field of coincidence spectroscopy in condensed matter systems. We derived a tensor expression for the cross section and an equivalent expression in terms of convenient angular functions has been treated for the case of linearly polarized light. Numerical calculations are performed for the $L_3{M}_{2,3}{M}_{2,3}$ transition in argon, in the single configuration Dirac-Fock scheme. Results are compared with experimental data for different final angular momentum states of the doubly charged ion and for different kinematical conditions.

Dipolar Bose and Fermi gases, which are currently being studied extensively experimentally and theoretically, interact through anisotropic, long-range potentials. Here, we replace the long-range potential by a zero-range pseudopotential that simplifies the theoretical treatment of two dipolar particles in a harmonic trap. Our zero-range pseudopotential description reproduces the energy spectrum of two dipoles interacting through a shape-dependent potential under external confinement very well, provided that sufficiently many partial waves are included, and readily leads to a classification scheme of the energy spectrum in terms of approximate angular momentum quantum numbers. The results may be directly relevant to the physics of dipolar gases loaded into optical lattices.

We present calculated cross sections for resonance-enhanced dissociative excitation and vibrational excitation in the $1–15\text{−}\mathrm{eV}$ energy region. In this region, the cross sections are dominated by a series of doubly excited resonance states converging to the first excited state of the ion. The resonance states and parameters were previously calculated using the complex Kohn variational method and are used as input to a time-dependent treatment of the nuclear dynamics. The dissociative recombination cross section for the ${}^4\mathrm{He}{}^4\mathrm{He}^{+}$ ion is calculated and compared to the previously published cross section for the mixed isotopologue. The calculated cross sections for dissociative excitation and vibrational excitation, including excitation into individual vibrational states, are reported for the ${}^3\mathrm{He}{}^4\mathrm{He}^{+}$ and ${}^4\mathrm{He}{}^4\mathrm{He}^{+}$ molecular ions and compared to recent experiments.

Proton ejection during Coulomb explosion is studied for several structure-related organic molecules (anthracene, anthraquinone, and octahydroanthracene) subjected to $800\mathrm{nm}$, $60\mathrm{fs}$ laser pulses at intensities from 0.50 to $4.0\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$. The proton kinetic energy distributions are found to be markedly structure specific. The distributions are bimodal for anthracene and octahydroanthracene and trimodal for anthraquinone. Maximum (cutoff) energies of the distributions range from $50\mathrm{eV}$ for anthracene to $83\mathrm{eV}$ for anthraquinone. The low-energy mode $(\sim 10\mathrm{eV})$ is most pronounced in octahydroanthracene. The dependence of the characteristic features of the distributions on the laser intensity provides insights into molecular specificity of such strong-field phenomena as (i) nonadiabatic charge localization and (ii) field-mediated restructuring of polyatomic molecules polarized by a strong laser field.

Reabsorption, the multiple scattering of spontaneously emitted photons in optically thick gases, is a major limitation to efficient optical pumping and laser cooling in ultracold gases. We report mitigation of reabsorption using spatial and frequency modulation of laser light illuminating such gases. We developed a semiclassical model that successfully describes the reabsorption process when frequency-modulated light is present. It was necessary to extend the treatment in the model beyond a simple two-atom picture in order to reproduce our experimental results.

We present a multiscale quantum-defect theory for two identical atoms in a symmetric harmonic trap that combines the quantum-defect theory for the van der Waals interaction [B. Gao, Phys. Rev. A 64, 010701(R) (2001)] at short distances with a quantum-defect theory for the harmonic trapping potential at large distances. The theory provides a systematic understanding of two atoms in a trap, from deeply bound molecular states and states of different partial waves, to highly excited trap states. It shows, e.g., that a strong $p$-wave pairing can lead to a lower energy state around the threshold than a $s$-wave pairing.

The limitations on the delay-bandwidth product (DBP) in an electromagnetically induced transparency medium are investigated analytically by studying the susceptibility of the system, derived through Lindblad’s master equation, including dephasing. The effect of inhomogeneous broadening is treated. It is shown that the DBP for a given material is fundamentally limited by the frequency-dependent absorption, while the residual absorption limits the penetration length of a pulse. Simple expression for the optimal choice of parameters to maximize the DBP are derived. Also, the length of a device is presented as a function of DBP and control-field Rabi frequency. Supporting these results, numerical calculations are carried out through the Maxwell-Bloch equations in the slowly varying envelope approximation. The results are scalable, hence they apply to the case of atoms or molecules in a gas as well as quantum dots and wells.

We present a theory for fourfold quantum interference of photons generated from independent spontaneous parametric down-conversion processes. Closed-form expressions for fourfold quantum interference patterns and visibility are found. The theoretical result for fourfold quantum interference patterns is in good agreement with experimental data reported. Detailed numerical calculations for the dependence of fourfold quantum interference visibility on experimentally controllable parameters are carried out. It is found out that higher visibility can be achieved for small biphoton width, short pump pulse coherence time, and narrow bandwidth of spectral filters. The optimal condition for obtaining at the same time higher fourfold interference visibility and intensity is also discussed.

We find an upper bound on the rate at which entanglement can be unlocked by classical bits. In particular, we show that, for quantum information sources that are specified by ensembles of pure bipartite states, one classical bit can unlock at most one ebit.

We propose a scheme to generate polarization-entangled multiphoton Greenberger-Horne-Zeilinger states with weak cross-Kerr nonlinearity based on controlled bus rotation and subsequent homodyne measurement. Our method is simple in operation and has high success probabilities with near perfect fidelities in an ideal case.

The formation of a self-guided beam pattern via optical trapping in a suspension of Rayleigh scatterers is studied. An analytic self-guided solution is presented that is valid for low intensities. A numerical solution of the coupled diffusion and optical-wave equations is presented, which shows an evolution toward a steady state that is independent of the initial optical beam shape. At higher intensities, the numerical solution shows the influence of higher-order nonlinearities, which lead to instability. We define a critical optical intensity, in terms of the particle size, refractive index contrast, and thermal energy, that characterizes the onset of higher-order effects. Instability is found that is consistent with past parametric studies of solitons.

We demonstrate a simple, all-optical technique to prepare and determine the desired internal quantum states in multi-Zeeman-sublevel atoms. By choosing appropriate coupling and pumping laser beams, atoms can be easily prepared in a desired Zeeman sublevel with high purity or in any chosen ground-state population distributions (spin-polarized quantum-state engineering). The population distributions or state purities of such prepared atomic states can be determined by using a weak, circularly polarized probe beam due to differences in transition strengths among different Zeeman sublevels. This technique will have potential impact on quantum-information processing in multilevel atomic systems.

We show how to do decoy-state quantum key distribution efficiently with large random errors in the intensity control. We present a theorem for efficiently calculating the lower bound of single-photon counts with many undetermined parameters. In the calculation of the single-photon counts of our protocol, the linear terms of the intensity fluctuation disappear and only the quadratic terms take effect. Given that the intensity fluctuation is upper bounded by $\pm 5\%$, $\pm 10\%$, and $\pm 15\%$, the verified lower bound of the percentage of untagged bits from our protocol is as large as 99.7%, 99.0%, and 97.9% of that from an ideal protocol where the light intensity is exactly controlled.

We derive an experimentally observable lower bound on concurrence of mixed quantum states in terms of an entanglement witness, relating measurements on single states with those on two copies.

Building on the previous results of the Weyl chamber steering method, we demonstrate how to generate high-fidelity controlled-NOT (CNOT) gates by direct application of certain physically relevant Hamiltonians with fixed coupling constants containing Rabi terms. Such Hamiltonians are often used to describe two superconducting qubits driven by local rf pulses. It is found that in order to achieve 100% fidelity in a system with capacitive coupling of strength $g$, one Rabi term suffices. We give the exact values of the physical parameters needed to implement such CNOT gates. The gate time and all possible Rabi frequencies are found to be $t=\pi ∕\left(2g\right)$ and ${\Omega }_1∕g=\sqrt{64n^2-1},n=1,2,3,\dots$. Generation of a perfect CNOT gate in a system with inductive coupling, characterized by additional constant $k$, requires the presence of both Rabi terms. The gate time is again $t=\pi ∕\left(2g\right)$, but now there is an infinite number of solutions, each of which is valid in a certain range of $k$ and is characterized by a pair of integers $(n,m)$, $\frac{{\Omega }_{1,2}}{g}=\sqrt{16n^2-{\left(\frac{k-1}{2}\right)}^2}\pm \sqrt{16m^2-{\left(\frac{k+1}{2}\right)}^2}$. We distinguish two cases, depending on the sign of the coupling constant: (i) the antiferromagnetic case $(k⩾0)$ with $n⩾m=0,1,2,\dots$ and (ii) the ferromagnetic case $(k⩽0)$ with $n>m=0,1,2,\dots$. We conclude with consideration of fidelity degradation by switching to resonance. Simulation of time evolution based on the fourth-order Magnus expansion reveals characteristics of the gate similar to those found in the exact case, with slightly shorter gate time and shifted values of the Rabi frequencies.

An electron momentum spectroscopy study on ionization-excitation processes of He is reported. The symmetric noncoplanar $(e,2e)$ cross sections for transitions to excited ion states have been measured relative to that to the ground ion state at impact energies of 1240 and $4260\mathrm{eV}$. The experimental results exhibit a marked dependence on the impact energy. This provides strong evidence that higher-order effects are involved. Second-Born-approximation calculations are performed and their results are compared with the experimental ones. It is shown that the two-step mechanisms play crucial roles in the ionization-excitation processes.

Charge exchange and fragmentation in the collision systems ${\mathrm{He}}^{2+}+{\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{He}}^{+}+{\mathrm{H}}_2\mathrm{O}$ are theoretically investigated at projectile energies of a few keV. The calculations are based on the electron nuclear dynamics (END) method which solves the time-dependent Schrödinger equation. Total and differential cross sections for charge exchange are evaluated by averaging over 10 orientations of the ${\mathrm{H}}_2\mathrm{O}$ molecule. Summed total electron capture cross sections are found to be in good agreement with available experimental data. Projectile scattering was studied in the full angular range with respect to the incident beam direction. The theory provides a description of the fragmentation mechanisms such as Coulomb explosion and binary collision processes. For impact parameters below $3.5\mathrm{a.u.}$, we find that single and double electron capture occurs, resulting always in full fragmentation of ${\mathrm{H}}_2\mathrm{O}$ independent of the target orientation for ${}^3\mathrm{He}^{2+}$ ions. Hydrogen and oxygen fragments and its respective ions, are studied as a function of emission angle and energy. In the binary collisions regime the theoretical results are found to be in excellent agreement with previous experimental data. In the Coulomb explosion regime the theoretical data are found to peak at specific angles including 90°, which is consistent with the experiment.