A number of authors have recently pointed out inconsistencies of results obtained with the Huang-Yang multipolar pseudopotential for low-energy scattering [K. Huang and K. C. Yang, Phys. Rev. 105, 767 (1957); later revised by Huang, Statistical Mechanics (Wiley, New York, 1963)]. The conceptual validity of their original derivation has been questioned. Here I show that these inconsistencies are rather due to an algebraic mistake made by Huang and Yang. With the corrected error, I present the revised version of the multipolar pseudopotential.

In a recent paper [Phys. Rev. A 70, 025803 (2004)] we presented a scheme to teleport an entanglement of zero- and one-photon states from a bimodal cavity to another one, with 100% success probability. Here, inspired by recent results in the literature, we have modified our previous proposal to teleport the same entangled state without using Bell-state measurements. For comparison, the time spent, the fidelity, and the success probability for this teleportation are considered.

In the paper by ] the nonexistence of the quantum Zeno effect caused by indirect measurements has been claimed. It is shown here that the pertinent proof is incorrect and that the claim is unfounded.

In this paper, we study decoherence in Grover’s quantum search algorithm using a perturbative method. We assume that each two-state system (qubit) that belongs to a register suffers a phase-flip error (${\sigma }_z$ error) with probability $p$ independently at every step in the algorithm, where $0⩽p⩽1$. Considering an $n$-qubit density operator to which Grover’s iterative operation is applied $M$ times, we expand it in powers of $2Mnp$ and derive its matrix element order by order under the large-$n$ limit. [In this large-$n$ limit, we assume $p$ is small enough, so that $2Mnp$ can take any real positive value or zero. We regard $x\equiv 2Mnp$ $(⩾0)$ as a perturbative parameter.] We obtain recurrence relations between terms in the perturbative expansion. By these relations, we compute higher orders of the perturbation efficiently, so that we extend the range of the perturbative parameter that provides a reliable analysis. Calculating the matrix element numerically by this method, we derive the maximum value of the perturbative parameter $x$ at which the algorithm finds a correct item with a given threshold of probability $P_{\mathrm{th}}$ or more. (We refer to this maximum value of $x$ as $x_{\mathrm{c}}$, a critical point of $x$.) We obtain a curve of $x_{\mathrm{c}}$ as a function of $P_{\mathrm{th}}$ by repeating this numerical calculation for many points of $P_{\mathrm{th}}$ and find the following facts: a tangent of the obtained curve at $P_{\mathrm{th}}=1$ is given by $x=(8∕5)(1-P_{\mathrm{th}})$, and we have $x_{\mathrm{c}}>-(8∕5){\log }_e{P}_{\mathrm{th}}$ near $P_{\mathrm{th}}=0$.

Feshbach resonances of trapped ultracold alkali-metal atoms allow to vary the atomic scattering length $a$. At very large values of $a$ the system enters an universal strongly coupled regime in which its properties—the ground-state energy, pressure, etc.—become independent of $a$. We discuss the transport properties of such systems. In particular, the universality arguments imply that the shear viscosity of ultracold Fermi atoms at the Feschbach resonance is proportional to the particle number density $n$ and the Plank constant $\hslash$: $\eta =\hslash n{\alpha }_{\eta }$, where ${\alpha }_{\eta }$ is a universal constant. Using Heisenberg uncertainty principle and Einstein’s relation between diffusion and viscosity we argue that the viscosity has the lower bound given by ${\alpha }_{\eta }⩽{\left(6\pi \right)}^{-1}$. We relate the damping of low-frequency density oscillations of ultracold optically trapped ${}^6\mathrm{Li}$ atoms to viscosity and find that the value of the coefficient ${\alpha }_{\eta }$ is about 0.3. We also show that such a small viscosity cannot be explained by kinetic theory based on binary scattering. We conclude that the system of ultracold atoms near the Feshbach resonance is a near-ideal liquid.

We report an experiment in which optical pulses stored in an atomic system can be controllably released into two different photonic channels. By controllably turning on the retrieve control pulses at either 795 or $780\mathrm{nm}$ to read the stored optical pulses in a four-level double $\Lambda$-type atomic medium, we can obtain the released probe pulse at 795 or $780\mathrm{nm}$, respectively. These readout pulses can be further separated spatially and directed into different optical propagation channels through a grating. Such controlled release of stored optical pulses may extend the capabilities of the quantum information storage technique, and can have applications in multichannel all-optical switching, all-optical routing, quantum information processing, and image storage systems.

We show how it is possible to improve the efficiency of a laser system by adding a classical heat engine cycle to the laser’s gaseous working media that can recover both thermal and internal waste energy from the gas. Experimental implementation of a pulsed $\mathrm{C}{\mathrm{O}}_2$ laser combined with a gas expansion cycle is discussed.

When various observers obtain information in an independent fashion about a classical system, there is a simple rule which allows them to pool their knowledge, and this requires only the states-of-knowledge of the respective observers. Here we derive an equivalent quantum formula. While its realm of applicability is necessarily more limited, it does apply to a large class of measurements, and we show explicitly for a single qubit that it satisfies the intuitive notions of what it means to pool knowledge about a quantum system. This analysis also provides a physical interpretation for the trace of the product of two density matrices.

We present a mean-field model to describe condensates in the BEC-BCS crossover regime based on pairs of fermionic atoms. By introducing an effective potential, the mean-field equation allows us to calculate the chemical potential, the equation of states, and the atomic correlation function. The results agree surprisingly well with recent many-body calculations. We show that the smooth crossover from the bosonic mean-field repulsion between molecules to the Fermi pressure among atoms is associated with the evolution of the atomic correlation function.

We consider the $g$ factor of a spin-$1∕2$ particle (electron or muon) bound by an arbitrary central field. We present an approach that allows one to express the relativistic $g$ factor in terms of the binding energy. We derive the general expression for the correction to the $g$ factor caused by a deviation of the central potential from the Coulomb one. As the application of this method, we consider the corrections to the $g$ factor due to the finite nuclear size, including vacuum polarization radiative correction. The effect of the anomalous magnetic moment is also taken into account.

We introduce methods for clock synchronization that make use of the adiabatic exchange of nondegenerate two-level quantum systems: ticking qubits. Schemes involving the exchange of $N$ independent qubits with frequency $\omega$ give a synchronization accuracy that scales as ${\left(\omega \sqrt{N}\right)}^{-1}$—i.e., as the standard quantum limit. We introduce a protocol that makes use of $N_c$ coherent exchanges of a single qubit at frequency $\omega$, leading to an accuracy that scales as ${\left(\omega N_c\right)}^{-1}\ln N_c$. This protocol beats the standard quantum limit without the use of entanglement, and we argue that this scaling is the fundamental limit for clock synchronization allowed by quantum mechanics. We analyze the performance of these protocols when used with a lossy channel.

We discuss the use of Rydberg blockade techniques for entanglement of single-atom qubits with collective $N$-atom qubits. We show how the entanglement can be used to achieve fast readout and transmission of the state of single-atom qubits without the use of optical cavities.

We introduce a generalized theory of decoherence-free subspaces and subsystems (DFSs), which do not require accurate initialization. We derive a set of conditions for the existence of DFSs within this generalized framework. By relaxing the initialization requirement we show that a DFS can tolerate arbitrarily large preparation errors. This has potentially significant implications for experiments involving DFSs, in particular for the experimental implementation, over DFSs, of the large class of quantum algorithms which can function with arbitrary input states.

Quantum teleportation of a squeezed state is demonstrated experimentally. Due to some inevitable losses in experiments, a squeezed vacuum necessarily becomes a mixed state which is no longer a minimum uncertainty state. We establish an operational method of evaluation for quantum teleportation of such a state using fidelity and discuss the classical limit for the state. The measured fidelity for the input state is $0.85\pm 0.05$, which is higher than the classical case of $0.73\pm 0.04$. We also verify that the teleportation process operates properly for the nonclassical state input and its squeezed variance is certainly transferred through the process. We observe the smaller variance of the teleported squeezed state than that for the vacuum state input.

Using the translational energy-gain spectroscopy technique, we have measured the energy-gain spectra and absolute total cross sections for single-electron capture (SEC) in collisions of ${\mathrm{He}}^{2+}$ ions with ${\mathrm{O}}_2$, $\mathrm{N}{\mathrm{H}}_3$, ${\mathrm{N}}_2$, and $\mathrm{C}{\mathrm{O}}_2$ at laboratory impact energies between 25 and $400\mathrm{eV}∕\mathrm{amu}$. The measured spectra for the ${\mathrm{He}}^{2+}-{\mathrm{N}}_2$ and $\mathrm{C}{\mathrm{O}}_2$ collision systems show that the dominant reaction channel is due to dissociative transfer ionization (i.e., SEC accompanied by ionization of the molecular target ion). In the case of the ${\mathrm{He}}^{2+}-\mathrm{N}{\mathrm{H}}_3$ collision system, nondissociative single-electron capture into $n=2$ states of ${\mathrm{He}}^{+}$ with production of $\mathrm{N}\mathrm{H}_3{}^{+}$ in the ground state is predominantly populated. These processes are observed to be the dominant reaction channels over the entire impact energy region studied and at laboratory scattering angles between 0° and 8°. The energy dependence of total cross sections for SEC are also measured and found to slowly increase with increasing impact energies. The measured cross sections are also compared with the available measurements and theoretical results based on the Demkov and Landau-Zener models.

Collisions between 16-200 eV electrons and ${\mathrm{H}}_2\mathrm{O}$ target have been investigated experimentally. The fragment ions originating from single, double, and triple ionization were energetically analyzed at an observation angle of 90° with respect to the incident beam direction. Fragments with energies as low as $0.1\mathrm{eV}$ were detected. From the spectra, cross sections for double and triple ionization, relative to single ionization cross sections, are derived. The present results are compared with previous experimental results involving several molecular targets, and with formulas that have been developed previously for electron-atom collisions.

We analyze the performance of nonlinear detection schemes surpassing the Heisenberg limit with semiclassical input states when using realistic detectors having nonunit detection efficiency.

Within the frame of lowest-order perturbation theory, the van der Waals potential of a ground-state atom placed within an arbitrary dispersing and absorbing magnetodielectric multilayer system is given. Examples of an atom situated in front of a magnetodielectric plate or between two such plates are studied in detail. Special emphasis is placed on the competing attractive and repulsive force components associated with the electric and magnetic matter properties, respectively, and conditions for the formation of repulsive potential walls are given. Both numerical and analytical results are presented.

In the present paper we propose a method of quantization of mechanical systems that is applicable, inter alia, to systems acted on by nonconservative forces, such as dissipative systems. First, the correspondence between classical and quantum mechanics is considered and we make some important observations that can be used in defining the rules of quantization. Next, the quantization rules are presented and we give a few examples of the application of these rules to mechanical systems that are difficult to quantize within any other systemetic approach. Finally, it is shown that for any quantum-mechanical system there are stationary solutions of the evolution equations, and that the existence of these solutions is directly related to the proposed quantization rules.