We present total absolute collision cross sections for neon in the ${}^{3}P_2$ metastable state with ground state, thermal atoms and molecules using a recently developed experimental technique. The technique utilizes a magneto-optical trap (MOT) and involves the measurement of MOT population dynamics to determine the cross section. Collision cross section measurements are presented for metastable neon in the ${}^{3}P_2$ state with He, Ne, Ar, ${\mathrm{H}}_2$, ${\mathrm{O}}_2$, and ${\mathrm{N}}_2$. The average thermal energy of the collision ranges from $11\text{to}27\mathrm{meV}$. The measurements made using this technique have small errors, of the order of 10% of the measured cross section; the technique is capable of producing benchmark collision cross sections.

We have realized a one-dimensional optical lattice for individual atoms with a lifetime $>300\mathrm{s}$, which is $5\times$ longer than previously reported. In order to achieve this long lifetime, it is necessary to laser-cool the atoms briefly every $20\mathrm{s}$ to overcome heating due to technical fluctuations in the trapping potential. Without cooling, we observe negligible atom loss within the first $20\mathrm{s}$ followed by an exponential decay with a $62\text{−}\mathrm{s}$ time constant. We obtain quantitative agreement with the measured fluctuations of the trapping potential and the corresponding theoretical heating rates.

We predict solitary vortices in quasiplanar condensates of dipolar atoms, polarized parallel to the confinement direction, with the effective sign of the dipole-dipole interaction inverted by means of a rapidly rotating field. Energy minima corresponding to vortex solitons with topological charges $\ell =1$ and 2 are predicted for moderately strong dipole-dipole interaction, using an axisymmetric Gaussian Ansatz. The stability of the solitons with $\ell =1$ is confirmed by full three-dimensional simulations, whereas their counterparts with $\ell =2$ are found to be unstable against splitting into a set of four fragments (quadrupole).

We present a detailed quantum-statistical analysis of superresolution for optical systems with circular symmetry based on our previous paper [Opt. Commun. 264, 9 (2006)].We consider as example an imaging system with two Fourier lenses; however, our approach can be easily generalized for more complex paraxial systems with circular symmetry. We present an original and very effective algorithm for numerical evaluation of the circular prolate spheroidal functions, which are the eigenmodes of our system. We investigate in detail the two-dimensional reconstruction point-spread function and provide examples of superresolution for rotationally symmetric and angularly dependent objects. Finally, we investigate the role of the local quantum fluctuations in the superresolution process and evaluate the local noise figure for the reconstruction process.

Nonlinear Floquet states associated with a symmetry-breaking driving field are exploited to control the dynamics of a Bose-Einstein condensate in a double-well potential. The population imbalance between the two wells is shown to be controllable by slowly tuning system parameters along a closed path. The results extend symmetry-breaking-based quantum control to many-body systems on the mean-field level and extend navigation of linear Floquet states to navigation of nonlinear Floquet states.

We extend the concept of classicality in quantum optics to spin states. We call a state “classical” if its density matrix can be decomposed as a weighted sum of angular momentum coherent states with positive weights. Classical spin states form a convex set $\mathcal{C}$, which we fully characterize for a spin $1∕2$ and a spin 1. For arbitrary spin, we provide “nonclassicality witnesses.” For bipartite systems, $\mathcal{C}$ forms a subset of all separable states. A state of two spins $1∕2$ belongs to $\mathcal{C}$ if and only if it is separable, whereas for a spin $1∕2$ coupled to a spin 1, there are separable states which do not belong to $\mathcal{C}$. We show that in general the question whether a state is in $\mathcal{C}$ can be answered by a linear programming algorithm.

Degenerate quantum codes are codes that do not reveal the complete error syndrome. Their ability to conceal the complete error syndrome makes them powerful resources in certain quantum-information processing tasks. In particular, the most error-tolerant way to purify depolarized Bell states using one-way communication known to date involves degenerate quantum codes. Here we study three closely related purification schemes for depolarized Greenberger-Horne-Zeilinger states shared among $m⩾3$ players by means of degenerate quantum codes and one-way classical communications. We find that our schemes tolerate more noise than all other one-way schemes known to date, further demonstrating the effectiveness of degenerate quantum codes in quantum-information processing.

Differential phase shift quantum key distribution systems have a high potential for achieving high speed key generation. However, its unconditional security proof is still missing, even though it has been proposed for many years. Here, we prove its security against collective attacks with a weak coherent light source in the noiseless case (i.e., no bit error). The only assumptions are that quantum theory is correct, the devices are perfect and trusted and the key size is infinite. Our proof works on threshold detectors. We compute the lower bound of the secret key generation rate using the information-theoretical security proof method. Our final result shows that the lower bound of the secret key generation rate per pulse is linearly proportional to the channel transmission probability if Bob’s detection counts obey the binomial distribution.

Using high-quality gas phase electron scattering calculations and multiple scattering theory, we attempt to gain insights into the radiation damage to DNA induced by secondary low-energy electrons in the condensed phase, and to bridge the existing gap with the gas phase theory and experiments. The origin of different resonant features (arising from single molecules or diffraction) is discussed and the calculations are compared to existing experiments in thin films.

We have studied two-photon transition rates of atomic lithium within model potential by using the pseudostate summation approach. Transition amplitudes and absorption coefficients have been computed for $2s\text{−}3s$, $2s\text{−}4s$, and $2s\text{−}3d$ transitions with various Debye lengths. In the case of $2s\text{−}3s$ transition, two-photon transparency vanishes for smaller Debye length. Reverse incident occurred for $2s\text{−}3d$ transition. Here, we have found a two-photon transparency for lower values of Debye length. These two situations occur due to energy level shifting in the presence of a plasma environment. A detailed description of resonance enhancement frequency and transition amplitude has been analyzed.

We propose an improved photoassociation scheme to produce ultracold molecules in their vibronic ground state for the generic case in which nonadiabatic effects facilitating transfer to deeply bound levels are absent. Formation of molecules is achieved by short laser pulses in a Raman-like pump-dump process where an additional near-infrared laser field couples the excited state to an auxiliary state. The coupling due to the additional field effectively changes the shape of the excited-state potential and allows for efficient population transfer to low-lying vibrational levels of the electronic ground state. Repetition of many pump-dump sequences together with collisional relaxation allows for accumulation of molecules in $v=0$.

Transfer functions are of fundamental importance in analyzing the image formation in optical imaging systems. For the recently proposed lensless ghost-imaging systems, when a pointlike or bucket detector is used in the measurement, the corresponding amplitude transfer functions or optical transfer functions are derived. Defocusing effects are also investigated. With the help of these transfer functions, we can quantitatively study the imaging ability of lensless ghost-imaging systems.

We investigate the Maxwell-Bloch equations representing the propagation of electric fields through a three-level atomic system. We then generate soliton solutions for the field variables and study the interaction of solitons. We show how the signatures of electromagnetically induced transparency are manifested in the collision of solitons which can then be used for quantum-information processing.

We construct a simplex for multipartite qubit states of even number $n$ of qubits, which has the same geometry concerning separability, mixedness, kind of entanglement, amount of entanglement, and nonlocality as the bipartite qubit states. We derive the entanglement of the class of states which can be described by only three real parameters with the help of a multipartite measure for all discrete systems. We prove that the bounds on this measure are optimal for the whole class of states and that it reveals that the states possess only $n$-partite entanglement and not, e.g., bipartite entanglement. We then show that this $n$-partite entanglement can be increased by stochastic local operations and classical communication to the purest maximal entangled states. However, pure $n$-partite entanglement cannot be distilled; consequently all entangled states in the simplex are $n$-partite bound entangled. We study also Bell inequalities and find the same geometry as for bipartite qubits. Moreover, we show how the (hidden) nonlocality for all $n$-partite bound entangled states can be revealed.

We propose a scheme for generating $N$-qubit entangled states which can teleport an unknown state perfectly. By switching on the exchange interaction $\left(J\right)$ between the qubits one can get the desired states periodically. A multielectron quantum dot can be a possible realization for generating such $N$-qubit states with high fidelity. It is shown that, in the limit of $N\to \infty$, there exists a unique time $t=\frac{2}{3J}{\cos }^{-1}(-1∕8)$, where the Hamiltonian dynamics gives the $N$-qubit state that can carry out perfect teleportation. We also discussed the effect of the nuclear spin environment on the entanglement of the $N$-qubit teleportation channels.

We have calculated the double-electron-capture cross sections for ${\mathrm{He}}^{2+}$, ${\mathrm{Li}}^{3+}$, and ${\mathrm{B}}^{5+}$ collisions with He atoms in the intermediate-to-high energies in the framework of four-body formalism of the post form of the boundary corrected continuum intermediate state approximation. In this formalism, the intermediate continuum state of each of the active electrons with the projectile ion has been taken into account because of the asymmetric nature of collision when the projectile charge is greater than the target nuclear charge. As a consequence, the contribution to the capture cross section from excited states is significant. However, for symmetric collision, we have compared the present results in the post form with our earlier investigation in the prior form of the same approximation for the same process. It is observed that the discrepancies for contributions from excited states to the total double electron capture cross sections are small for symmetric collision in both forms. However, the present calculated results for symmetric and asymmetric cases have been compared with the available theoretical and experimental results. We find that our computed results are in better agreement with the experimental observations in comparison to other theoretical findings below $2\mathrm{MeV}$.

Using a laser-cooled ion trapping apparatus, we have investigated laser-induced chemical reactions between cold trapped ${\mathrm{Ba}}^{+}$ ions and several neutral molecular gases at room temperature, ${\mathrm{O}}_2$, $\mathrm{C}{\mathrm{O}}_2$, and ${\mathrm{N}}_2\mathrm{O}$, leading to the production of cold trapped $(\approx 20\mathrm{mK})$ $\mathrm{Ba}{\mathrm{O}}^{+}$ ions. The $\mathrm{Ba}{\mathrm{O}}^{+}$ ions were converted back to ${\mathrm{Ba}}^{+}$ ions via reaction with room-temperature CO. Reaction rates were determined by employing molecular dynamics simulations. The cold mixed-species ion ensembles produced were used for studying the efficiency of sympathetic cooling, by variation of the ratio of laser-cooled to sympathetically cooled ion numbers. In one extreme case, 20 laser-cooled ${}^{138}\mathrm{Ba}^{+}$ ions were capable of maintaining the translational temperature of 120 sympathetically cooled barium isotopes $\left({}^{135–137}\mathrm{Ba}^{+}\right)$ and 430 ${}^{138}\mathrm{Ba}^{16}{\mathrm{O}}^{+}$ molecules at approximately $25\mathrm{mK}$.

A model of photodetection using a Bose-Einstein condensate in an atom-chip-based microtrap is analyzed. Atoms absorb photons from the incident light field, receive part of the photon momentum, and leave the trap potential. Upon counting escaped atoms within predetermined time intervals, the photon statistics of the incident light is mapped onto the atom-count statistics. Whereas traditional photodetection theory treats the emission centers of photoelectrons as distinguishable, here the centers of escaping atoms are condensed and thus indistinguishable atoms. From this, an enhancement of the photon-number resolution as compared to the commonly known counting formula is derived for the case of low saturation.

The momentum distribution of an expanding cloud of one-dimensional hard-core anyons is studied by an exact numerical approach, and shown to become indistinguishable from that of a noninteracting spin-polarized Fermi gas for large enough times (dynamical fermionization). We also consider the expansion of one-dimensional anyons with strongly attractive short-range interactions suddenly released from a parabolic external potential, and find that the momentum distribution approaches that of its dual system, the ideal Bose gas (dynamical bosonization). For both processes the characteristic time scales are identified, and the effect of the initial confinement is analyzed comparing the dynamics associated with both harmonic and hard-wall traps.