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

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

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

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 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

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

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

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

Optical Quantum Computing

In 2001 all-optical quantum computing became feasible with the discovery that scalable quantum computing is possible using only single photon sources, linear optical elements, and single photon detectors. Although it was in principle scalable, the massive resource overhead made the scheme practically daunting. However, several simplifications were followed by proof-of-principle demonstrations, and recent approaches based on cluster states or error encoding have dramatically reduced this worrying resource overhead, making an all-optical architecture a serious contender for the ultimate goal of a large-scale quantum computer. Key challenges will be the realization of high-efficiency sources of indistinguishable single photons, low-loss, scalable optical circuits, high efficiency single photon detectors, and low-loss interfacing of these components.Comment: 5 pages, 4 figure

Engineering superpositions of displaced number states of a trapped ion

We present a protocol that permits the generation of a subtle with superposition with 2^(l+1) displaced number states on a circle in phase space as target state for the center-of-mass motion of a trapped ion. Through a sequence of 'l' cycles involving the application of laser pulses and no-fluorescence measurements, explicit expressions for the total duration of laser pulses employed in the sequence and probability of getting the ion in the upper electronic state during the 'l' cycles are obtained and analyzed in detail. Furthermore, assuming that the effective relaxation process of a trapped ion can be described in the framework of the standard master equation for the damped harmonic oscillator, we investigate the degradation of the quantum interference effects inherent to superpositions via Wigner function.Comment: 14 pages, 10 figure