A quantum stream cipher by Yuen 2000 protocol (so-called Y00 protocol or $\alpha \eta$ scheme) consisting of linear feedback shift register of short key is very attractive in implementing secure 40 Gbits/s optical data transmission, which is expected as a next-generation network. However, a basic model of the Y00 protocol with a very short key needs a careful design against fast correlation attacks as pointed out by Donnet et al. This Brief Report clarifies an effectiveness of irregular mapping between running key and physical signals in the driver for selection of $M$-ary basis in the transmitter, and gives a design method. Consequently, quantum stream cipher by the Y00 protocol with our mapping has immunity against the proposed fast correlation attacks on a basic model of the Y00 protocol even if the key is very short.

It is shown that the principal results of a recent work by Khalilov [Phys. Rev. A 71, 012105 (2005)] are in conflict with earlier work of the present author [Phys. Rev. Lett. 64, 503 (1990)] and also with a well known result concerning helicity conservation in a magnetic field. This can be attributed to the fact that Khalilov requires wave function regularity at the origin even when the relevant potential is singular at that point.

Recently Munday and Capasso [Phys. Rev. A 75, 060102(R) (2007)] claimed that they have performed a precision measurement of the Casimir force between a sphere and a plate coated with Au, both immersed in ethanol. The measurement results were claimed to be consistent with the Lifshitz theory. We demonstrate that the calculation of the Casimir force between the smooth surfaces of the test bodies following the authors prescription has a discrepancy up to 25% with respect to the authors result. We also show that the attractive electrostatic force only due to the surface potential differences was underestimated by a factor of 590. The resulting disagreement with the experimental data might be partially decreased by the effect of the charge double layer screening which was not taken into account. All this leads to the conclusion that the results of this experiment are in fact uncertain.

We have reviewed the Comment of Geyer et al. [preceding paper, Phys. Rev. A 77, 036102 (2008)] concerning our recent work [Phys. Rev. A 75, 060102(R) (2007)], and while we disagree with their criticisms, we acknowledge them for giving us the opportunity to add interesting addition material and a more detailed description of our experiment. We further describe our calculation and explain why a more sophisticated model is not warranted. We also present detailed experiments on the effects of electrostatic forces in our measurements and show that the contribution due to work function differences is small and that the residual electrostatic force is dominated by trapped charges and external fields within the apparatus. Finally, we estimate the effect of double-layer interactions. These additional calculations and measurements support our original conclusion that the experimental results are “consistent with Lifshitz’s theory.”

We consider the dynamics of a vibrating and rotating end mirror of an optical Fabry-Pérot cavity that can sustain Laguerre-Gaussian modes. We demonstrate theoretically that since the intracavity field carries linear as well as angular momentum, radiation pressure can create bipartite entanglement between a vibrational and a rotational mode of the mirror. Further we show that the ratio of vibrational and rotational couplings with the radiation field can easily be adjusted experimentally, which makes the generation and detection of entanglement robust to uncertainties in the cavity manufacture. This is a proposal to demonstrate entanglement between two qualitatively different degrees of freedom of the same macroscopic object.

We investigate the stability and the free expansion of a trapped dipolar Fermi gas. We show that stabilizing the system relying on tuning the trap geometry is generally inefficient. We further show that the expanded density profile always gets stretched along the attractive direction of dipolar interaction. We also point out that by switching off the dipolar interaction simultaneously with the trapping potential, the deformation of momentum distribution can be directly observed.

The information rates achievable with a photon-gas model of electromagnetic radiation are studied. At any frequency, information rates over the photon-gas model essentially coincide with the Shannon capacity when the signal-to-noise ratio is below a threshold. Only above the threshold does the photon gas incur in a significant loss in information rates; the loss can amount to half of the capacity. The threshold exceeds $40\mathrm{dB}$ for radio frequencies and vanishes at higher frequencies.

We investigate the optimal control problem for a non-Markovian open, dissipative quantum system. Optimal control using the Pontryagin maximum principle is specifically derived. The influences of ohmic reservoir with Lorentz-Drude regularization are numerically studied in a two-level system under the following three conditions: ${\omega }_0⪡{\omega }_c$, ${\omega }_0\approx {\omega }_c$, or ${\omega }_0⪢{\omega }_c$, where ${\omega }_0$ is the characteristic frequency of the quantum system of interest, and ${\omega }_c$ the cutoff frequency of the ohmic reservoir. The optimal control process shows its remarkable influences on the decoherence dynamics. The temperature is a key factor in the decoherence dynamics. We analyze the optimal decoherence control in high temperature, intermediate temperature, and low temperature reservoirs, respectively. It implies that designing some engineered reservoirs with the controlled coupling and state of the environment can slow down the decoherence rate and delay the decoherence time. Moreover, we compare the non-Markovian optimal decoherence control with the Markovian one and find that with non-Markovian the engineered artificial reservoirs are better than with the Markovian approximation in controlling the open, dissipative quantum system’s decoherence.

We propose an unconditionally secure quantum key distribution protocol, which uses a relatively strong signal pulse. While our protocol shares similar security bases as the Bennett 1992 protocol with a strong reference pulse, our scheme uses a smaller number of detectors and it is robust against Rayleigh scattering in an optical fiber. We derive a lower bound of secret key generation rate of our protocol and show that our protocol can cover relatively long distances, assuming precise phase modulations and stable interferometers.

Here is analyzed the dynamics of two-qubit entanglement, when the two qubits are initially in a mixed extended Werner-like state and each of them is in a zero temperature non-Markovian environment. The dependence of entanglement dynamics on the purity and degree of entanglement of the initial states and on the amount of non-Markovianity is also given. This extends the previous work about non-Markovian effects on the two-qubit entanglement dynamics for initial Bell-like states [Bellomo et al., Phys. Rev. Lett. 99, 160502 (2007)]. The effect on the two-qubit entanglement dynamics of nonzero temperature in Markovian environments is finally studied.

Calculations of the leading quantum electrodynamics effects in few-electron systems involve singular matrix elements of interelectronic distances of the form $1/r_i^3$ and $1/r_{ij}^3$. Integrals that result when the nonrelativistic wave function is represented by a Hylleraas basis are studied. Recursion relations for various powers of the electron coordinates and the master integrals are derived in a form suited for high-precision numerical evaluations.

We analyze the effects of Zeeman degeneracies on the long-range interactions between like Rydberg atoms, with particular emphasis on applications to quantum-information processing using the van der Waals blockade. We present a general analysis of how degeneracies affect the primary error sources in blockade experiments, emphasizing that blockade errors are sensitive primarily to the weakest possible atom-atom interactions between the degenerate states, not the mean interaction strength. We present explicit calculations of the van der Waals potentials in the limit where the fine-structure interaction is large compared to the atom-atom interactions. The results are presented for all potential angular momentum channels involving $\mathit{s}$, $\mathit{p}$, and $\mathit{d}$ states. For most channels there are one or more combinations of Zeeman levels that have extremely small dipole-dipole interactions and are therefore poor candidates for effective blockade experiments. Channels with promising properties are identified and discussed. We also present numerical calculations of Rb and Cs dipole matrix elements and relevant energy levels using quantum defect theory, allowing for convenient quantitative estimates of the van der Waals interactions to be made for principal quantum numbers up to 100. Finally, we combine the blockade and van der Waals results to quantitatively analyze the angular distribution of the blockade shift and its consequence for angular momentum channels and geometries of particular interest for blockade experiments with Rb.

In a one-dimensional model we consider the dependence of the magneto-optical force on the elliptical polarization of laser beams for different types of closed atomic transitions $J_g\to J_e$. The linear dependencies on atomic velocity and magnetic field of the magneto-optical force are found analytically. These results establish the possibility (for $J_g>0$) to capture atoms in a magneto-optical trap even at zero detuning (i.e., at the exact resonance of atoms with laser field) in certain $\left(ϵ-\theta -\overline{ϵ}\right)$ field configurations as well as in an anomalous blue-detuned region. The nonlinear dependencies of the force on atomic velocity and magnetic field are studied numerically. The capture velocity and the number of trapped atoms are estimated.

We explore stimulated photoassociation in the context of attosecond pump-probe schemes of atomic matter. An attosecond pulse—the probe—is used to induce photoassociation of an electronic wave packet which had been created before, typically with an attosecond pump pulse at an atomic center different from the one of photoassociation. We will show that the electron absorption is maximal for a certain delay between the pulses. Two ways of enhancing and controlling stimulated photoassociation are proposed, namely, using an additional infrared pulse to steer the electronic wave packet and using a train of attosecond pulses instead of a single pair. A direct application of ultrafast stimulated photoassociation is the measurement of atomic distances.

Analyzers comprised of an angular phase plate and a single-mode fiber have recently been introduced to study the angular profile of optical fields. Here, we quantify the number of degrees of freedom, or modes, that such an analyzer can resolve. Its performance is described by means of an angular coherence function and we introduce a dimensionality that gives the effective number of modes that a given analyzer can probe. This quantity can, as we show experimentally, easily be retrieved from a dual analyzer setup.

The nonlinear propagation of two coupled pulses in an optical amplifier has been studied in the case of positive group-velocity dispersion. Three separate self-similar coupled propagating regimes have been found: the asymptotical parabolic, the asymptotical quasiparabolic, and the intermediate quasiparabolic. The asymptotic propagation equations governing these coupled regimes have been formulated, and similariton solutions have been found. The conditions for the existence of these solutions have been found, and the analytical solutions for the first two cases have been shown to be asymptotically (at $z\to \infty$) exact. Numerical simulations have confirmed the predicted power distributions and chirp functions for the solutions.

We introduce an elementary optical gate for expanding polarization entangled $W$ states in which every pair of photons is entangled alike. The gate is composed of a pair of 50:50 beamsplitters and ancillary photons in the two-photon Fock state. By seeding one of the photons in an $n$-photon $W$ state into this gate, we obtain an $(n+2)$-photon $W$ state after postselection. This gate gives a better efficiency and a simpler implementation than previous proposals for $W$-state preparation.

Repeated measurements of a quantum particle to check its presence in a region of space was proposed long ago [G. R. Allcock, Ann. Phys. 53, 286 (1969)] as a natural way to determine the distribution of times of arrival at the orthogonal subspace, but the method was discarded because of the quantum Zeno effect: in the limit of very frequent measurements the wave function is reflected and remains in the original subspace. We show that by normalizing the small bits of arriving (removed) norm, an ideal time distribution emerges in correspondence with a classical local-kinetic-energy distribution.