We present an experimental study of the nonclassical correlations of a pair of spatial qubits formed by passing two down-converted photons through a pair of double slits. After confirming the entanglement generated in our setup by quantum tomography using separate measurements of the slit images and the interference patterns, we show that the complete Hilbert space of the spatial qubits can be accessed by measurements performed in a single plane between the image plane and the focal plane of a lens. Specifically, it is possible to obtain both the which-path and the interference information needed for quantum tomography in a single scan of the transversal distribution of photon coincidences. Since this method can easily be extended to multidimensional systems, it may be a valuable tool in the application of spatial qudits to quantum-information processes.

We derive rigorous upper bounds on the distance between quantum states in an open-system setting in terms of the operator norm between Hamiltonians describing their evolution. We illustrate our results with an example taken from protection against decoherence using dynamical decoupling.

On the basis of the Floquet formalism linked with the McCurdy-McNutt stabilization technique, metastable states of helium dimer in intense laser fields were investigated. The potential energy curve of helium dimer is found to become attractive when the laser intensity is sufficiently high, and the previous prediction [T. Yasuike and K. Someda, J. Phys. B 37, 3149 (2004)] based on the Kramers-Henneberger high-frequency approximation is confirmed. Under appropriate laser conditions, the lifetime is found to be sufficiently long to discretize vibrational states, owing to the nonperturbative stabilization effect in photoionization.

Magnetic resonance imaging in the Earth’s magnetic field is achieved using a sensitive atomic magnetometer for detection. We demonstrate images with a submillimeter resolution by recording the flow of two water paths meeting at a $T$-shaped mixer. The high homogeneity of the Earth’s field allows the use of weak gradient fields which circumvent the concomitant-field effect. To distinguish the two input channels, we employed selective polarization, which is a unique and noninvasive labeling method for low-field magnetic resonance imaging. Our technique imposes minimal physical constraints on the object under study, in contrast to conventional high-field magnetic resonance imaging. This technique is applicable for microfluidic imaging in laboratory-on-a-chip devices.

We investigate collisions of solitons of the gap type, supported by a lattice potential in repulsive Bose-Einstein condensates, with an effective double-barrier potential that resembles a Fabry-Perot cavity. We identify conditions under which the trapping of the entire incident soliton in the cavity is possible. Collisions of the incident soliton with an earlier trapped one are considered too. In the latter case, many outcomes of the collisions are identified, including merging, release of the trapped soliton with or without being replaced by the incoming one, and trapping of both solitons.

We explore the behavior of interacting bosonic atoms in an optical lattice subject to a large artificial magnetic field. We extend earlier investigations of this system where the number of magnetic flux quanta per unit cell $\alpha$ is close to a simple rational number [R. N. Palmer and D. Jaksch, Phys. Rev. Lett. 96, 180407 (2006)]. Interesting topological states such as the Laughlin and Read-Rezayi states can occur even if the atoms experience a weak trapping potential in one direction. An explicit numerical calculation near $\alpha =1/2$ shows that the system exhibits a striped vortex lattice phase of one species, which is analogous to the behavior of a two-species system for small $\alpha$. We also investigate methods to probe the encountered states. These include spatial correlation functions and the measurement of noise correlations in time-of-flight expanded atomic clouds. Characteristic differences arise which allow for an identification of the respective quantum Hall states. We furthermore discuss that a counterintuitive flow of the Hall current occurs for certain values of $\alpha$.

We show that the application of atom interferometry techniques to the internal, i.e., rotational-vibrational states of molecules provides a new tool for ultrahigh precision tests of fundamental physics. The measurement principle is based on the fact that the electronic structure of molecules is not spherically symmetric. A diatomic quantum sensor can hence distinguish between the direction along its internuclear axis and the two orthogonal directions and is therefore direction sensitive. As an example we show how a molecular rotational-vibrational quantum interferometer based on the hydrogen deuteride molecular ion $\left({\text{HD}}^{+}\right)$ may be used to detect gravitational waves. We show that a monochromatic gravitational wave of dimensionless amplitude $h={10}^{-19}$ will cause a frequency shift of the order of $30\mu \text{Hz}$ between appropriately prepared quantum states, a frequency difference likely to be detectable with the next generation atom interferometers in 1 s.

The authors call attention to a previous work [M. Feng, Phys. Rev. A 63, 052308 (2001)] for Grover search by trapped ions in decoherence-free subspace (DFS). A scheme for a two-qubit conditional-phase flip gate in DFS is presented as a supplement.

We report the experimental observation of an enhancement in the yield of high-order harmonics using an array of gas jets as the source medium. By comparing the experimental outcome for jet arrays of different spacings with the predicted harmonic intensity in the case of slit sources of equivalent lengths, we clearly show how the periodic modulation of the gas density can improve the harmonic yield. This behavior may be attributed to a quasi-phase-matching effect which increases the length of coherent harmonic buildup during propagation by partially counteracting the dephasing induced by free electrons.

A model system is presented that describes the physical properties of mode-locked lasers. This distributed system incorporates gain and filtering saturated with energy while loss is saturated with power. It is found that general initial pulses evolve to stable localized solutions which exist for wide choices of the parameters, the only requirement being sufficient gain. Moreover, these pulses are essentially solitons of the classical nonlinear Schrödinger (NLS) equation. In the anomalous regime, the additional terms present in the system serve to provide the mode locking mechanism. Consequently, these pulses are approximated by the classical NLS soliton, given by hyperbolic secant functions, in agreement with recent experiments.

In this paper, we present a systematic way based on the nonbinary graph state of constructing good nonbinary quantum codes, both additive and nonadditive, for systems with integer dimensions. With a computer search, which results in many interesting codes including some nonadditive codes meeting the Singleton bounds, we are able to construct explicitly four families of optimal codes, namely, ${\left[\left[6,2,3\right]\right]}_p$, ${\left[\left[7,3,3\right]\right]}_p$, ${\left[\left[8,2,4\right]\right]}_p$, and ${\left[\left[8,4,3\right]\right]}_p$ for any odd dimension $\mathit{p}$ and a family of nonadditive codes ${\left(\left(5,p,3\right)\right)}_p$ for arbitrary $p>3$. In the case of composite numbers as dimensions, we also construct a family of stabilizer codes ${\left(\left(6,2p^2,3\right)\right)}_{2p}$ for odd $\mathit{p}$, whose coding subspace is not of a dimension that is a power of the dimension of the physical subsystem.

We present a measurement of the hyperfine splitting of the $6S_{1/2}$ excited level of rubidium using two-photon absorption spectroscopy in a glass cell. The values we obtain for the magnetic dipole constant $A$ are 239.18(03) MHz and 807.66(08) MHz for ${}^{85}\text{R}\text{b}$ and ${}^{87}\text{R}\text{b}$, respectively. The combination of the magnetic moments of the two isotopes and our measurements show a hyperfine anomaly in this atomic excited state. The observed hyperfine anomaly difference has a value of ${{}_{87}\delta }_{85}=-0.0036\left(2\right)$ due to the finite distribution of nuclear magnetization, the Bohr-Weisskopf effect.

We analyze the stability of nonground nonlinear states of a Bose-Einstein condensate in the mean-field limit in effectively one-dimensional (“cigar-shape”) traps for various types of confining potentials. We find that nonlinear states become, in general, more stable when switching from a harmonic potential to an anharmonic one. We discuss the relation between this fact and the specifics of the harmonic potential which has an equidistant spectrum.

The ground-state phase diagram of Fermi-Fermi mixtures in optical lattices is analyzed as a function of interaction strength, population imbalance, filling fraction, and tunneling parameters. It is shown that population imbalanced Fermi-Fermi mixtures reduce to strongly interacting Bose-Fermi mixtures in the molecular limit, in sharp contrast to homogeneous or harmonically trapped systems, where the resulting Bose-Fermi mixture is weakly interacting. Furthermore, insulating phases are found in optical lattices of Fermi-Fermi mixtures in addition to the standard phase-separated or coexisting superfluid–excess-fermion phases found in homogeneous systems. The insulating states can be a molecular Bose-Mott insulator (BMI), a Fermi-Pauli insulator (FPI), a phase-separated BMI-FPI mixture, or a Bose-Fermi checkerboard.

Trapped ions arranged in Coulomb crystals provide us with the elements to study the physics of a single spin coupled to a boson bath. In this work, we show that optical forces allow us to realize a variety of spin-boson models, depending on the crystal geometry and the laser configuration. We study in detail the ohmic case, which can be implemented by illuminating a single ion with a traveling wave. The mesoscopic character of the phonon bath in trapped ions induces effects such as the appearance of quantum revivals in the spin evolution.

We apply the fidelity metric approach to analyze two recently introduced models that exhibit a quantum phase transition to a topologically ordered phase. These quantum models have a known connection to classical statistical mechanical models; we exploit this mapping to obtain the scaling of the fidelity metric tensor near criticality. The topological phase transitions manifest themselves in divergences of the fidelity metric across the phase boundaries. These results provide evidence that the fidelity approach is a valuable tool to investigate novel phases lacking a clear characterization in terms of local order parameters.

With carefully prepared resonant laser-controlled pulses, the two quantum states of a cold ensemble of sodium atoms were manipulated by pure geometric rotations of the Bloch vector around axes 2 and 3 on the Bloch sphere. The fidelity for two geometric rotations was improved to be 90%. A universal single-qubit operation on the ground-state wave function was demonstrated by a pure geometric operation and the phase shift due to the rotation around axis 3 was measured using a geometric atom interferometer.

The formation of doubly excited He atoms during impact of ${\mathrm{He}}^{2+}$ ions with energies from $64\text{to}500\mathrm{eV}$ on a clean and oxygen-covered Ni(110) surface is studied via Auger electron spectroscopy. We find that the electron spectra are affected by the adsorption of oxygen on the surface, which can be related in a clear-cut manner to shifts of the target work function. Our data are in conflict with recent work on this system where changes in the electron spectra with target temperature are attributed to a high local electron-spin polarization of the Ni(110) surface. We demonstrate that the data do not probe surface magnetism but modifications of the work function caused by surface contaminations.

We have recorded fluorescence spectra of the atomic rubidium D-lines in the presence of several hundreds of bars of buffer gas pressure. The large collisional broadening of this system interpolates between usual atomic physics gas phase and solid-liquid phase conditions. In our buffer gas cell, with additional saturation broadening, a spectral linewidth comparable to the thermal energy of the atoms in the cell is achieved. An observed intensity-dependent blue asymmetry of the spectra is interpreted as evidence for the onset of thermal equilibrium of dressed atom-light states.