Real-time detection of individual atoms falling through a high-finesse optical cavity

The enhanced coupling between atoms and photons inside a high-finesse optical cavity provides a novel basis for optical measurements that continuously monitor atomic degrees of freedom. We describe an experiment in which cavity quantum-electrodynamic effects are utilized for real-time detection of individual atoms falling through an optical cavity after being dropped from a magneto-optical trap. Our technique permits experiments that are triggered by the presence of a single optimally coupled atom within the cavity mode volume

Nonlinear spectroscopy in the strong-coupling regime of cavity QED

A nonlinear spectroscopic investigation of a strongly coupled atom-cavity system is presented. A two-field pump-probe experiment is employed to study nonlinear structure as the average number of intracavity atoms is varied from N̅≈4.2 to N̅≈0.8. Nonlinear effects are observed for as few as 0.1 intracavity pump photons. A detailed semiclassical simulation of the atomic beam experiment gives reasonable agreement with the data for N̅≳2 atoms. The simulation procedure accounts for fluctuations in atom-field coupling which have important effects on both the linear and nonlinear probe transmission spectra. A discrepancy between the simulations and the experiments is observed for small numbers of atoms (N̅≲1). Unfortunately, it is difficult to determine if this discrepancy is a definitive consequence of the quantum nature of the atom-cavity coupling or a result of the severe technical complications of the experiment

Quantum non-demolition measurement of nonlocal variables and its application in quantum authentication

Quantun non-demolition (QND) variables are generlized to the nonlocal ones by proposing QND measurement networks of Bell states and multi-partite GHZ states, which means that we can generate and measure them without any destruction. One of its prospective applications in the quantum authentication system of the Quantum Security Automatic Teller Machine (QSATM) which is much more reliable than the classical ones is also presented.Comment: 5 Pages, 3 Figure

Quantum phase gate for photonic qubits using only beam splitters and post-selection

We show that a beam splitter of reflectivity one-third can be used to realize a quantum phase gate operation if only the outputs conserving the number of photons on each side are post-selected.Comment: 6 pages RevTex, including one figur

Two-photon nonlinearity in general cavity QED systems

We have investigated the two-photon nonlinearity at general cavity QED systems, which covers both weak and strong coupling regimes and includes radiative loss from the atom. The one- and two-photon propagators are obtained in analytic forms. By surveying both coupling regimes, we have revealed the conditions on the photonic wavepacket for yielding large nonlinearity depending on the cavity Q-value. We have also discussed the effect of radiative loss on the nonlinearity.Comment: 8 pages, 5 figure

Near-field imaging with two transmission gratings for submicrometer localization of atoms

We show theoretically that an atomic pattern with period d can be obtained with 100% visibility even for an infinitely extended source by sending atoms through two transmission gratings with periods d and d/2, respectively, and separated by half the Talbot length LT/2=d^2/2λdB, where λdB is the atomic wavelength and the source is infinitely far away. For a finite source distance, as would be attainable in any real experiment, a small correction to the grating periods and separations restores the period-d pattern. This effect is closely related to the Talbot and Lau effects in classical optics and can be used to localize atoms to a submicrometer scale without a compromise in atomic flux. We first derive compact analytical formulas for the idealized case of a monochromatic source and large gratings and then verify numerically that a finite grating size and velocity dispersion in the beam do not decrease the fringe visibility considerably. Finally, we briefly present an experiment in preparation to exhibit this localization

Trapped-Ion Quantum Logic Utilizing Position-Dependent ac Stark Shifts

We present a scheme utilizing position-dependent ac Stark shifts for doing quantum logic with trapped ions. By a proper choice of direction, position and size, as well as power and frequency of a far-off-resonant Gaussian laser beam, specific ac Stark shifts can be assigned to the individual ions, making them distinguishable in frequency-space. In contrast to previous all-optical based quantum gates with trapped ions, the present scheme enables individual addressing of single ions and selective addressing of any pair of ions for two-ion quantum gates, without using tightly focused laser beams. Furthermore, the decoherence rate due to off-resonant excitations can be made negligible as compared with other sources of decoherence.Comment: 5 pages, 4 figures. Submitted to Physical Review Letter

Nonperturbative and perturbative treatments of parametric heating in atom traps

We study the quantum description of parametric heating in harmonic potentials both nonperturbatively and perturbatively, having in mind atom traps. The first approach establishes an explicit connection between classical and quantum descriptions; it also gives analytic expressions for properties such as the width of fractional frequency parametric resonances. The second approach gives an alternative insight into the problem and can be directly extended to take into account nonlinear effects. This is specially important for shallow traps.Comment: 12 pages, 2 figure

Adiabatic quantum computation with Cooper pairs

We propose a new variant of the controlled-NOT quantum logic gate based on adiabatic level-crossing dynamics of the q-bits. The gate has a natural implementation in terms of the Cooper pair transport in arrays of small Josephson tunnel junctions. An important advantage of the adiabatic approach is that the gate dynamics is insensitive to the unavoidable spread of junction parameters.Comment: 18 pages, 3 figures not supplied by autho

Measurement of conditional phase shifts for quantum logic

Measurements of the birefringence of a single atom strongly coupled to a high-finesse optical resonator are reported, with nonlinear phase shifts observed for intracavity photon number much less than one. A proposal to utilize the measured conditional phase shifts for implementing quantum logic via a quantum-phase gate (QPG) is considered. Within the context of a simple model for the field transformation, the parameters of the "truth table" for the QPG are determined.Comment: 4 pages in Postscript format, including 4 figures (attached as uuencoded version of a gzip-file