Weak coherent states as a photon source for quantum cryptography have a limit in secure data rate and transmission distance because of the presence of multiphoton events and loss in transmission line. Two-photon events in a coherent state can be taken out by a two-photon interference scheme. We investigate the security issue of utilizing this modified coherent state in quantum cryptography. A 4-dB improvement in the secure data rate or a nearly twofold increase in transmission distance over the coherent state are found. With a recently proposed and improved encoding strategy, further improvement is possible.

Starting with two weakly coupled antiferromagnetic spinor condensates, we show that by changing the sign of the coefficient of the spin interaction, $U_2$, via an optically induced Feshbach resonance, one can create an entangled state consisting of two anticorrelated ferromagnetic condensates. This state is maximally entangled and a generalization of the Bell state from two anticorrelated spin-$1∕2$ particles to two anticorrelated spin-$N∕2$ atomic samples, where $N$ is the total number of atoms.

It is theoretically shown that tripartite entanglement with different wavelengths can be generated by cascaded nonlinear interaction in an optical parametric oscillator cavity with parametric down conversion and sum-frequency generation. A sufficient inseparability criterion for continuous-variable tripartite entanglement proposed by van Loock and Furusawa was used to evaluate the degree of the quadrature-phase amplitude correlations between the three modes. The dependences of correlation on the cavity parameters and pump intensity are discussed.

Off-diagonal geometric phases have been developed in order to provide information of the geometry of paths that connect noninterfering quantal states. We propose a kinematic approach to off-diagonal geometric phases for pure and mixed states. We further extend the mixed-state concept proposed in [Phys. Rev. Lett. 90, 050403 (2003)] to degenerate density operators. The first- and second-order off-diagonal geometric phases are analyzed for unitarily evolving pairs of pseudopure states.

We analyze mean fidelity between random density matrices of size $N$, generated with respect to various probability measures in the space of mixed quantum states: the Hilbert-Schmidt measure, the Bures (statistical) measure, the measure induced by the partial trace, and the natural measure on the space of pure states. In certain cases explicit probability distributions for the fidelity are derived. The results obtained may be used to gauge the quality of quantum-information-processing schemes.

Information capacities achievable in the multi-parallel-use scenarios are employed to characterize the quantum correlations in unmodulated spin chains. By studying the qubit amplitude damping channel, we calculate the quantum capacity $Q$, the entanglement assisted capacity $C_E$, and the classical capacity $C_1$ of a spin chain with ferromagnetic Heisenberg interactions.

Double ionization of diatomic molecules by short linearly polarized laser pulses is analyzed. We consider the final stage of the ionization process, that is the decay of a highly excited two electron molecule, which is formed after rescattering. The saddles of the effective adiabatic potential energy close to which simultaneous escape of electrons takes place are identified. Numerical simulations of the ionization of molecules show that the process can be dominated by either sequential or nonsequential events. In order to increase the ratio of nonsequential to sequential ionizations very short laser pulses should be applied.

We derive an exact solution to the Thomas-Fermi equation for a Bose-Einstein condensate (BEC) which has dipole-dipole interactions as well as the usual $s$-wave contact interaction, in a harmonic trap. Remarkably, despite the nonlocal anisotropic nature of the dipolar interaction the solution is an inverted parabola, as in the pure $s$-wave case, but with a different aspect ratio. We explain in detail the mathematical tools necessary to describe dipolar BECs with or without cylindrical symmetry. Various properties such as electrostriction and stability are discussed.

The search for particle electric dipole moments (EDM’s) is one of the best places to look for physics beyond the standard model of electroweak interaction because the size of time reversal violation predicted by the standard model is incompatible with present ideas concerning the creation of the baryon-antibaryon asymmetry. As the sensitivity of these EDM searches increases more subtle systematic effects become important. We develop a general analytical approach to describe a systematic effect recently observed in an electric dipole moment experiment using stored particles [J. M. Pendlebury et al., Phys. Rev. A 70, 032102 (2004)]. Our approach is based on the relationship between the systematic frequency shift and the velocity autocorrelation function of the resonating particles. Our results, when applied to well-known limiting forms of the correlation function, are in good agreement with both the limiting cases studied in recent work that employed a numerical and heuristic analysis. Our general approach explains some of the surprising results observed in that work and displays the rich behavior of the shift for intermediate frequencies, which has not been studied previously.

By means of a unitary transformation scheme, the Coulomb-Dirac operator for two electrons in a central potential is transformed into a pseudorelativistic operator which allows for the decoupling of the electron and positron degrees of freedom to arbitrary order $n$ in the potential strength. In case of $n=2$, relative boundedness properties and positivity of the resulting operator are shown for subcritical potential strength.

We show how to achieve perfect quantum state transfer and construct effective two-qubit gates between distant sites in engineered bosonic and fermionic networks. The Hamiltonian for the system can be determined by choosing an eigenvalue spectrum satisfying a certain condition, which is shown to be both sufficient and necessary in mirror-symmetrical networks. The natures of the effective two-qubit gates depend on the exchange symmetry for fermions and bosons. For fermionic networks, the gates are entangling (and thus universal for quantum computation). For bosonic networks, though the gates are not entangling, they allow two-way simultaneous communications. Protocols of entanglement generation in both bosonic and fermionic engineered networks are discussed.

We describe the design for a scalable, solid-state quantum-information-processing architecture based on the integration of GHz-frequency nanomechanical resonators with Josephson tunnel junctions, which has the potential for demonstrating a variety of single- and multiqubit operations critical to quantum computation. The computational qubits are eigenstates of large-area, current-biased Josephson junctions, manipulated and measured using strobed external circuitry. Two or more of these phase qubits are capacitively coupled to a high-quality-factor piezoelectric nanoelectromechanical disk resonator, which forms the backbone of our architecture, and which enables coherent coupling of the qubits. The integrated system is analogous to one or more few-level atoms (the Josephson junction qubits) in an electromagnetic cavity (the nanomechanical resonator). However, unlike existing approaches using atoms in electromagnetic cavities, here we can individually tune the level spacing of the “atoms” and control their “electromagnetic” interaction strength. We show theoretically that quantum states prepared in a Josephson junction can be passed to the nanomechanical resonator and stored there, and then can be passed back to the original junction or transferred to another with high fidelity. The resonator can also be used to produce maximally entangled Bell states between a pair of Josephson junctions. Many such junction-resonator complexes can be assembled in a hub-and-spoke layout, resulting in a large-scale quantum circuit. Our proposed architecture combines desirable features of both solid-state and cavity quantum electrodynamics approaches, and could make quantum-information processing possible in a scalable, solid-state environment.

We propose a class of qubit networks that admit perfect state transfer of any two-dimensional quantum state in a fixed period of time. We further show that such networks can distribute arbitrary entangled states between two distant parties, and can, by using such systems in parallel, transmit the higher-dimensional systems states across the network. Unlike many other schemes for quantum computation and communication, these networks do not require qubit couplings to be switched on and off. When restricted to $N$-qubit spin networks of identical qubit couplings, we show that $2{\log }_{3}N$ is the maximal perfect communication distance for hypercube geometries. Moreover, if one allows fixed but different couplings between the qubits then perfect state transfer can be achieved over arbitrarily long distances in a linear chain. This paper expands and extends the work done by Christandl et al., Phys. Rev. Lett. 92, 187902 (2004).

Electron capture and direct elastic scattering in collisions of ${\mathrm{H}}^{+}$ ions with ${\mathrm{C}}_2{\mathrm{H}}_4$ molecules are studied by using a molecular representation within a semiclassical as well as a fully quantum-mechanical approachs below $10\mathrm{keV}$. Calculations are carried out at three different molecular configurations, in which ${\mathrm{H}}^{+}$ approaches (i) parallel and (ii) perpendicular to the $\mathrm{C}\mathrm{C}$ axis in the molecular plane, and (iii) perpendicular to this plane. We find that electron capture in the (iii) configuration takes place preferentially over that in the (i) and (ii) configurations at scattering angles above 15°, while the results for (i) and (ii) are comparable in magnitude below 10°, although (ii) dominates slightly at still smaller angles. Total capture cross sections for the (iii) and (ii) configurations differ by a factor of 4 above $500\mathrm{eV}$, while those for (i) lie between these values. Below $500\mathrm{eV}$, the results for (i) and (iii) are similar in magnitude, while that for the (ii) configuration sharply decreases.

We report the investigation into the three-body fragmentation of the triatomic molecules $\mathrm{N}{\mathrm{H}}_2$ and $\mathrm{C}{\mathrm{H}}_2$ via the process of dissociative recombination (DR). Recently reported analysis of experiments into the DR of the similar system ${\mathrm{H}}_2\mathrm{O}$ indicated that the DR process is violent, involving a large degree of geometrical change and energy distribution from the initial attachment of the free electron by the parent ion to the final dissociation step. Comparison of data from $\mathrm{N}{\mathrm{H}}_2$ and $\mathrm{C}{\mathrm{H}}_2$ with that of ${\mathrm{H}}_2\mathrm{O}$ gives a similar picture of the DR process, though there are significant differences in the results, not only in the branching fraction between the expected decay channels, but also in the distribution of the available reaction energy over the product hydrogen atoms as well as in the molecular geometry at the point of dissociation.

We created sodium Bose-Einstein condensates in an optically plugged quadrupole magnetic trap. A focused, 532-nm laser beam repelled atoms from the coil center where Majorana loss is significant. We produced condensates containing up to $3\times {10}^7$ atoms, a factor of 60 improvement over previous work and a number comparable to the best all-magnetic traps, and transferred up to $9\times {10}^6$ atoms into a purely optical trap. We compare our observations with a simple model of evaporative cooling, concluding that a laser in the range of 1 W (rather than 5 W) may suffice.

We examine the interaction of a weak probe with $N$ atoms in a $\Lambda$-level configuration under the conditions of electromagnetically induced transparency (EIT). In contrast to previous works on EIT, we calculate the output state of the resultant slowly propagating light field while taking into account the effects of ground state dephasing and atomic noise for a more realistic model. In particular, we propose two experiments using slow light with a nonclassical probe field and show that two properties of the probe, entanglement and squeezing, characterizing the quantum state of the probe field, can be well-preserved throughout the passage.

We investigate the time evolution of two local spins in coupling with itinerant electrons. By interacting with itinerant electrons, the originally unentangled two spins become rapidly entangled, and then the local spins and concurrence oscillate with time. The oscillations contain rapid and slow oscillation components originated from different energies of excitations in the coupled system. The results may shed light on the influence of the itinerant electrons as a part of the environment on the entanglement and decoherence of local spins which may be used as qubit elements.

We have magnetically trapped and evaporatively cooled atomic molybdenum. Using a cryogenic helium buffer gas, $2\times {10}^{10}$ molybdenum atoms are loaded into a magnetic trap at an initial temperature of 500 mK. The molybdenum atoms undergo two-body decay with a measured inelastic rate constant of $g_{\mathrm{in}}=(1.2\pm 0.6)\times {10}^{-12}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ for the initial conditions after loading. The molybdenum atoms are cooled by adiabatic expansion and forced evaporation to 200 mK.

We have measured angular distributions of ion fragments produced in dissociative double ionization of CO, $\mathrm{C}{\mathrm{O}}_2$, and ${\mathrm{C}}_2{\mathrm{H}}_2$ by intense ultrashort laser pulses. This report extends similar recent studies of ${\mathrm{O}}_2$ and ${\mathrm{N}}_2$ to a wider set of molecules. We found that for sub-$10\text{−}\mathrm{fs}$ pulses of sufficiently low intensity the fragments’ angular distributions for all studied molecules are determined by angular dependence of the first ionization step. Those experimental angular distributions were in good agreement with angular dependent ionization probabilities calculated with the molecular tunneling ionization theory. The measured angular distributions directly reflect the symmetry of the corresponding molecular orbitals. For higher laser intensities and longer pulse durations, dynamic alignment and postionization alignment start to affect the angular distributions and ion fragments are preferentially ejected along the laser-polarization direction.