Quasi-one-dimensional outflow from a dilute gas Bose-Einstein condensate reservoir is a promising system for the creation of analog Hawking radiation. We use numerical modeling to show that stable sonic horizons exist in such a system under realistic conditions, taking into account the transverse dimensions and three-body loss. We find that loss limits the analog Hawking temperatures achievable in the hydrodynamic regime, with sodium condensates allowing the highest temperatures. A condensate of $30000$ atoms, with transverse confinement frequency ${\omega }_{\perp }=6800\times 2\pi \mathrm{Hz}$, yields horizon temperatures of about $20\mathrm{nK}$ over a period of $50\mathrm{ms}$. This is at least four times higher than for other atoms commonly used for Bose-Einstein condensates.

The collapse of a trapped boson-fermion mixture with attraction between bosons and fermions is investigated in the framework of the effective Hamiltonian for the Bose system. The properties of an ${}^{87}\mathrm{Rb}$ and ${}^{40}\mathrm{K}$ mixture are analyzed quantitatively at $T=0$. We find numerically solutions of the modified Gross-Pitaevskii equation which continuously go from the stable to the unstable branch. We discuss how the conditions for the onset of collapse are related to the macroscopic properties of the system. A comparison with the case of a Bose condensate of the atomic ${}^7\mathrm{Li}$ system is given.

The phase sensitivity of interferometers is limited by the so-called Heisenberg limit, which states that the optimum phase sensitivity is inversely proportional to the number of interfering particles $N$, a $1∕\sqrt{N}$ improvement over the standard quantum limit. We have used simulated annealing, a global optimization strategy, to systematically search for quantum interferometer input states that approach the Heisenberg-limited uncertainty in estimates of the interferometer phase shift. We compare the performance of these states to that of other nonclassical states already known to yield Heisenberg-limited uncertainty.

Cascaded nonlinear phase shifts via second-harmonic generation are theoretically studied for quasi-phase-matched (QPM) structures. Both the effective phase mismatch and the structure itself contribute to the resultant nonlinear phase shift. Depending on the sign of the effective phase mismatch, structural contribution may enhance or depress the nonlinear phase shift. The fundamental wave will experience a nonlinear phase shift even if the effective phase matching is satisfied. By properly designing the effective phase mismatch, QPM structures with large material-based phase mismatch may provide large nonlinear phase shifts before saturation.

We report on a theoretical study of the influence of incoherent optical pumping and beam profiling on slow light propagation in three-level atomic vapor. The pumping reduces the residual unwanted absorption of the signal while destroying the atomic coherence and increasing the group velocity of light. We show that the pumping enhances the resulting group delay for a certain range of parameters. We also examine changing the cross section of the beam of light along the propagation direction by, e.g., focusing the beam, for the purpose of maintaining the drive intensity to effectively counter its absorption.

Quantum memories based on the photon-echo principle (with controlled reversible inhomogeneous broadening) allow in principle perfect reconstruction of the stored light. In the retrieval process, the envelope of the absorbed wave packet is reversed in time, but the evolution of the phase of the carrier wave is unchanged. We discuss the consequences of this fact for the relative phase of pulses with a certain time delay, and thus for the storage of time-bin qubits. As an illustration, we show that the combination of photon-echo-based memories and unbalanced interferometers leads to a counterintuitive interference effect, allowing one to measure a path length difference $\Delta L$ using pulses that are much shorter than $\Delta L$.

We investigate the economical Gaussian cloning of coherent states with the known phase, which produces $M$ copies from $N$ input replica and can be implemented with degenerate parametric amplifiers and beam splitters.The achievable fidelity of single copy is given by $2M\sqrt{N}∕\left[\sqrt{N}\right(M-1)+\sqrt{(1+N)(M^2+N)}]$, which is bigger than the optimal fidelity of the universal Gaussian cloning. The cloning machine presented here works without ancillary optical modes and can be regarded as the continuous variable generalization of the economical cloning machine for qudits.

We show how the slow light propagation through a four-level $N$-type system can be significantly influenced by the application of the Kerr field under the condition of electromagnetically induced transparency. The change due to the Kerr field under fairly general conditions can be about 10–15 %.

In a recent theoretical study on the photodetachment cross section of ${\mathrm{H}}^{-}$ in a static electric field near an elastic wall, Yang et al. found significant difference between the result of closed-orbit theory and a quantum approach [Phys. Rev. A 73, 043413 (2006)]. They attributed the difference to the semiclassical approximations in closed-orbit theory. We have examined the application of closed-orbit theory to the same problem and found the disagreement is caused by the incorrectly assigned phases and amplitudes associated with closed-orbits. After the phases and amplitudes are corrected, the closed-orbit theory is shown to be in excellent agreement with the quantum result.

Interacting quantum systems evolving from an uncorrelated composite initial state generically develop quantum correlations—entanglement. As a consequence, a local description of interacting quantum systems is impossible as a rule. A unitarily evolving (isolated) quantum system generically develops extensive entanglement: the magnitude of the generated entanglement will increase without bounds with the effective Hilbert space dimension of the system. It is conceivable that coupling of the interacting subsystems to local dephasing environments will restrict the generation of entanglement to such extent that the evolving composite system may be considered as approximately disentangled. This conjecture is addressed in the context of some common models of a bipartite system with linear and nonlinear interactions and local coupling to dephasing environments. Analytical and numerical results obtained imply that the conjecture is generally false. Open dynamics of the quantum correlations is compared to the corresponding evolution of the classical correlations and a qualitative difference is found.

We study the resources needed to construct topological two-dimensional stabilizer codes as a way to estimate in part their efficiency, and this leads us to perform a comparative study of surface codes and color codes. This study clarifies the similarities and differences between these two types of stabilizer code. We compute the topological error-correcting rate $C≔n∕d^2$ for surface codes $C_s$ and color codes $C_c$ in several instances. On the torus, typical values are $C_s=2$ and $C_c=3∕2$, but we find that the optimal values are $C_s=1$ and $C_c=9∕8$. For planar codes, a typical value is $C_s=2$, while we find that the optimal values are $C_s=1$ and $C_c=3∕4$. In general, a color code encodes twice as many logical qubits as does a surface code.

We study dressed Bose-Einstein condensates in an atom chip radio-frequency trap. We show that in this system sufficiently strong dressing can be achieved to cause the widely used rotating-wave approximation (RWA) to break down. We present a full calculation of the atom-field coupling which shows that the non-RWA contributions quantitatively alter the shape of the emerging dressed adiabatic potentials. The non-RWA contributions furthermore lead to additional allowed transitions between dressed levels. We use rf spectroscopy of Bose-Einstein condensates trapped in the dressed state potentials to directly observe the transition from the RWA to the beyond-RWA regime.

We analyze the scattering of bright solitons in dipolar Bose-Einstein condensates placed in unconnected layers. Whereas for short-range interactions unconnected layers are independent, a remarkable consequence of the dipole interaction is the appearance of nonlocal interlayer effects. In particular, we show that both for one- and two-dimensional solitons the interlayer interaction leads to an effective molecular potential between disconnected solitons, which induces a complex scattering physics between them, that includes inelastic fusion into soliton molecules, and strong inelastic resonances. In addition, contrary to the short-range interacting case, a two-dimensional soliton scattering is possible, in which inelastic spiraling occurs, resembling phenomena in photorefractive materials.

We report a combined experimental and computational study of scandium doped copper clusters. The clusters are studied with time-of-flight mass spectrometry after laser fragmentation. Enhanced stabilities for specific cluster sizes in the mass abundance spectra are discussed using both electronic (shell closing) and geometric (symmetry) arguments. The exceptional stability observed for ${\mathrm{Cu}}_{16}{\mathrm{Sc}}^{+}$ is investigated in detail computationally. Density functional geometry optimizations at the Becke-Perdew 1986–LANL 2-double-zeta (BP86/LANL2DZ) level result in a Frank-Kasper tetrahedron, encapsulating a scandium atom in a highly coordinated position. The high stability is therefore interpreted in terms of extremely stable dopant encapsulated structures featuring a closed electron shell. The thermodynamic stability, as indicated by the stable backbone and large binding energy per atom, the relatively small ionization energy, and the moderate electron affinity of the neutral ${\mathrm{Cu}}_{16}\mathrm{Sc}$ cluster show that it has a superatom character, chemically similar to the alkaline-metal atoms.

In this paper we investigate stabilizer quantum error correction codes using controlled phase rotations of strong coherent probe states. We explicitly describe two methods to measure the Pauli operators that generate the stabilizer group of a quantum code. First, we show how to measure a Pauli operator acting on physical qubits using a single coherent state with large average photon number, displacement operations, and photon detection. Second, we show how to measure the stabilizer operators fault-tolerantly by the deterministic preparation of coherent quantum superposition (“cat”) states along with one-bit teleportations between a qubitlike encoding of coherent states and physical qubits.

The density-functional correlation potential of a finite system is shown to asymptotically approach a nonzero constant along a nodal surface of the energetically highest occupied orbital and zero everywhere else. This nonuniform asymptotic form of the correlation potential exactly cancels the nonuniform asymptotic behavior of the exact exchange potential discussed by Della Sala and Görling [Phys. Rev. Lett. 89, 33003 (2002)]. The sum of the exchange and correlation potentials therefore asymptotically tends to $-1∕r$ everywhere, consistent with the asymptotic form of the Kohn-Sham potential as analyzed by Almbladh and von Barth [Phys. Rev. B 31, 3231 (1985)].

The momentum-space coupled-channel optical method has been applied to positron-helium scattering. The ionization continuum and the positronium formation channels are included via a complex equivalent-local optical potential. The positronium formation cross sections at the energy range from the threshold to $400\mathrm{eV}$ and the total scattering cross sections at the energies from 17 to $500\mathrm{eV}$ are reported and compared with available experimental measurements and theoretical calculations. The agreements between the present results and the corresponding measurements are satisfactory.

The spontaneous emission from a degenerate $\Lambda$-type three-level atom, embedded in a photonic crystal, is studied. The emitted field, as a function of time and position, is calculated by solving the three coupled differential equations governing the amplitudes. We show that the spontaneously emitted field is characterized by three components (as in the case of two-level and $V$-type atoms): a localized part, a traveling part, and a $t^{-3∕2}$ decaying part. Our calculations indicate that under specific conditions the atoms do not emit propagating fields, while the localized field, having shorter localization length and time, is intensified. As a consequence, the population of the upper level, after a short period of oscillations, approaches a constant value. It is also shown that this steady value, under the same conditions, is much larger than its counterpart in $V$-type atoms.

Atom and molecule currents in a Fermi gas in the neighborhood of a Feshbach resonance are studied in a one-dimensional optical ring lattice by directly diagonalizing small models. A rotational analogy of flux quantization is used to show that fraction of the current is carried by particles with twice the mass of an atom, which suggests pairing and superfluidity.