Frequency doubling with KNbO3 in an external cavity

Potassium niobate is employed in an external resonator to generate single-frequency tunable radiation near 430 nm. For excitation with 1.35 W of power from a cw titanium-sapphire laser, 0.65 W of blue light is produced. A simple model has been developed to account for thermal lensing in the nonlinear crystal

Distant Entanglement of Macroscopic Gas Samples

One of the main ingredients in most quantum information protocols is a reliable source of two entangled systems. Such systems have been generated experimentally several years ago for light but has only in the past few years been demonstrated for atomic systems. None of these approaches however involve two atomic systems situated in separate environments. This is necessary for the creation of entanglement over arbitrary distances which is required for many quantum information protocols such as atomic teleportation. We present an experimental realization of such distant entanglement based on an adaptation of the entanglement of macroscopic gas samples containing about 10^11 cesium atoms shown previously by our group. The entanglement is generated via the off-resonant Kerr interaction between the atomic samples and a pulse of light. The achieved entanglement distance is 0.35m but can be scaled arbitrarily. The feasibility of an implementation of various quantum information protocols using macroscopic samples of atoms has therefore been greatly increased. We also present a theoretical modeling in terms of canonical position and momentum operators X and P describing the entanglement generation and verification in presence of decoherence mechanisms.Comment: 20 pages book-style, 3 figure

Deterministic atom-light quantum interface

The notion of an atom-light quantum interface has been developed in the past decade, to a large extent due to demands within the new field of quantum information processing and communication. A promising type of such interface using large atomic ensembles has emerged in the past several years. In this article we review this area of research with a special emphasis on deterministic high fidelity quantum information protocols. Two recent experiments, entanglement of distant atomic objects and quantum memory for light are described in detail.Comment: 50 pages (bookstyle) 15 graphs, to be published in "Advances in Atomic, Molecular, and Optical Physics" Vol. 54. (2006)(Some of the graphs here have lower resolution than in the version to be published

Blue-light induced infrared absorption in KNbO3

We have used a high-finesse cavity to measure the cw intensity dependence and dynamics of blue-light-induced infrared absorption (BLIIRA) in KNbO3 crystals for blue-light intensities between 7 x 10^-4 and 2 x 10^4 W/cm^2. We discuss the detrimental effects of BLIIRA on the efficiency of intracavity frequency doubling and the threshold for parametric oscillation

High fidelity teleportation between light and atoms

We show how high fidelity quantum teleportation of light to atoms can be achieved in the same setup as was used in the recent experiment [J. Sherson et.al., quant-ph/0605095, accepted by Nature], where such an inter-species quantum state transfer was demonstrated for the first time. Our improved protocol takes advantage of the rich multimode entangled structure of the state of atoms and scattered light and requires simple post-processing of homodyne detection signals and squeezed light in order to achieve fidelities up to 90% (85%) for teleportation of coherent (qubit) states under realistic experimental conditions. The remaining limitation is due to atomic decoherence and light losses.Comment: 5 pages, 3 figure

Spin squeezing of atomic ensembles by multi-colour quantum non-demolition measurements

We analyze the creation of spin squeezed atomic ensembles by simultaneous dispersive interactions with several optical frequencies. A judicious choice of optical parameters enables optimization of an interferometric detection scheme that suppresses inhomogeneous light shifts and keeps the interferometer operating in a balanced mode that minimizes technical noise. We show that when the atoms interact with two-frequency light tuned to cycling transitions the degree of spin squeezing $\xi^2$ scales as $\xi^2\sim 1/d$ where $d$ is the resonant optical depth of the ensemble. In real alkali atoms there are loss channels and the scaling may be closer to $\xi^2\sim 1/\sqrt d.$ Nevertheless the use of two-frequencies provides a significant improvement in the degree of squeezing attainable as we show by quantitative analysis of non-resonant probing on the Cs D1 line. Two alternative configurations are analyzed: a Mach-Zehnder interferometer that uses spatial interference, and an interaction with multi-frequency amplitude modulated light that does not require a spatial interferometer.Comment: 7 figure

Atoms as nonlinear mixers for detection of quantum correlations at ultrahigh frequencies

Measurements of quantum correlations are reported for a frequency difference of 25 THz between the signal and idler output fields generated by a subthreshold nondegenerate optical parametric oscillator. By simultaneously exciting a two-photon transition in atomic Cs by a combination of signal, idler, and "references oscillator" fields, we record modulation of the excited-state population due to quantum interference between two alternative excitation pathways. The observed phase-sensitive modulation is proportional to the correlation function〈EsEi〉for the quantized signal and idler fields

Quantum interference in two-photon excitation with squeezed and coherent fields

Two-photon excitation of a three-level atom in a ladder configuration (1-->2-->3) by simultaneous illumination with fields in squeezed vacuum and coherent states results in quantum interference for the excitation process. The particular configuration considered here is one for which the signal and idler output fields of a subthreshold nondegenerate optical parametric oscillator are in resonance with the two-stepwise dipole atomic transitions (1-->2,2-->3), while a "reference oscillator" field is in two-photon resonance with the quadrupole transition (1-->3). In an extension of the work of Ficek and Drummond [Phys. Rev. A 43, 6247 (1991)], a theoretical formulation based on the full quantum master equation for the problem is presented. The combined effects of quantum interference and the nonclassical character of the squeezed state are investigated, and offer the potential for a new detection strategy for quantum fluctuations of the electromagnetic field with ultrahigh frequencies (10's-100's THz). Based on the theory developed, we analyze quantum interference in excitation in several special cases relevant to experimental realizations, including the effects of a small focusing angle of the squeezing onto the atoms, and unusual population inversions. Special emphasis is given to identifying intrinsically quantum optical field effects versus classical field effects. Procedures that could distinguish between the two (i.e., classical and nonclassical) are suggested

Single-passage read-out of atomic quantum memory

A scheme for retrieving quantum information stored in collective atomic spin systems onto optical pulses is presented. Two off-resonant light pulses cross the atomic medium in two orthogonal directions and are interferometrically recombined in such a way that one of the outputs carries most of the information stored in the medium. In contrast to previous schemes our approach requires neither multiple passes through the medium nor feedback on the light after passing the sample which makes the scheme very efficient. The price for that is some added noise which is however small enough for the method to beat the classical limits.Comment: 8 pages, 2 figures, RevTeX