James P. Clemens

Enhanced spontaneous emission into the mode of a cavity QED system

We study the light generated by spontaneous emission into a mode of a cavity QED system under weak excitation of the orthogonally polarized mode. Operating in the intermediate regime of cavity QED with comparable coherent and decoherent coupling constants, we find an enhancement of the emission into the undriven cavity mode by more than a factor of 18.5 over that expected by the solid angle subtended by the mode. A model that incorporates three atomic levels and two polarization modes quantitatively explains the observations.

Nonclassical effects of a driven atoms-cavity system in the presence of an arbitrary driving field and dephasing

We investigate the photon statistics of light transmitted from a driven optical cavity containing one or two atoms interacting with a single mode of the cavity field. We treat arbitrary driving fields with emphasis on departure from previous weak field results. In addition effects of dephasing due to atomic transit through the cavity mode are included using two different models. We find that both models show the nonclassical correlations are quite sensitive to dephasing. The effect of multiple atoms on the system dynamics is investigated by placing two atoms in the cavity mode at different positions, therefore having different coupling strengths.

Two-Level Atom in an Optical Parametric Oscillator: Spectra of Transmitted and Fluorescent Fields in the Weak Driving Field Limit

We consider the interaction of a two-level atom inside an optical parametric oscillator. In the weak-driving-field limit, we essentially have an atom-cavity system driven by the occasional pair of correlated photons, or weakly squeezed light. We find that we may have holes, or dips, in the spectrum of the fluorescent and transmitted light. This occurs even in the strong-coupling limit when we find holes in the vacuum-Rabi doublet. Also, spectra with a subnatural linewidth may occur. These effects disappear for larger driving fields, unlike the spectral narrowing obtained in resonance fluorescence in a squeezed vacuum; here it is important that the squeezing parameter N tends to zero so that the system interacts with only one correlated pair of photons at a time. We show that a previous explanation for spectral narrowing and spectral holes for incoherent scattering is not applicable in the present case, and propose an alternative explanation. We attribute these anomalous effects to quantum interference in the two-photon scattering of the system.

Quantum trajectory theory of superradiant emission from randomly distributed atomic samples

We consider the directed superradiant emission from a collection of N two-state atoms with arbitrary spatial locations within the framework of quantum trajectory theory and without a single-mode assumption. The formalism is developed around an unravelling of the master equation in terms of source mode quantum jumps. A modified boson approximation is made to treat the many-atom case, where it is found that strong directional superradiance occurs for a few thousand atoms, even with randomized atomic positions.

Spectra of single-atom lasers

We calculate the output spectrum of a single-atom laser in a microcavity across a wide range of operating conditions. We considered both three-level and four-level atomic level structures. We used a numerical routine to calculate spectra that is more efficient than others used previously. We found that the linewidth of a single-atom laser generally scales as the inverse of the photon number and that there is no pump value at which an abrupt change occurs that might locate a lasing threshold. For a three-level gain atom we found vacuum–Rabi splitting similar to that found by Loffler et al. [Phys. Rev. A 55, 3923 (1997)] and used quantum trajectory theory to obtain a new interpretation of the results. For a four-level gain atom the vacuum–Rabi structure can appear at a small nonzero pump level and is maintained for large pumps, even when the intracavity photon number is larger than unity and the laser is on. We use the quantum trajectory approach to explain these results.

Fidelity of quantum teleportation based on spatially and temporally resolved spontaneous emission

We analyze a quantum teleportation protocol based on spatially and temporally resolved direct photodetection of the collective emission from a pair of atoms, one of which is entangled with a single mode of an optical cavity. We focus on the performance of the protocol as characterized by the fidelity of the teleported state and the success probability. We find that the fidelity approaches unity as the spacing of the atoms becomes much smaller than the emission wavelength with a success probability of 0.25. The fidelity remains above the classical limit of 2/3 for all atomic spacings with the ultimate limit of performance coming from the spatial resolution of the photodetection.

Performance of a quantum teleportation protocol based on collective spontaneous emission

Recently a conditional quantum teleportation protocol has been proposed by Chen et al. [New J. Phys.7, 172 (2005)], which is based on the collective spontaneous emission of a photon from a pair of quantum dots. We formulate a similar protocol for collective emission from a pair of atoms, one of which is entangled with a single mode of an optical cavity. We focus on the performance of the protocol as characterized by the fidelity of the teleported state and the overall success probability. We consider a strategy employing spatially resolved photodetection of the emitted photon in order to distinguish superradiant from subradiant emission on the basis of a single detected photon. We find that fidelity approaches unity as the spacing of the atoms becomes much smaller than the emission wavelength with a success probability of 0.25. The fidelity remains above the classical limit of 2/3 for arbitrary atomic separations with the ultimate limit of performance coming from the spatial resolution of the detectors.

Quantum error correction for various forms of noise

Quantum error correction will be an indispensable ingredient of large-scale quantum computations. Conventional quantum error correction codes (QECC) have been devised with an independent-error model in mind, but one may expect that the noise affecting a system of qubits will, in general, exhibit nonzero correlations in time, or space, or both. This talk will present a brief introduction to the principles of quantum error correction, followed by a discussion of the performance of conventional QECCs in the presence of correlated noise.

Subradiance in a nanofiber mode by an ensemble of a few cold Rb atoms

The excitation-decay from a few cold Rb atoms into the mode of an optical nanofiber shows two distinct time-scales: First the normal lifetime, and then a longer subradiant lifetime that scales linearly with optical density.

Collective Spontaneous Emission and Tripartite Entanglement of Driven, Damped Atoms in Free Space

We calculate the bipartite and tripartite entanglement of three damped two-level atoms in free space with one atom driven. There are two sources of the entanglement: dipole-dipole coupling and collective spontaneous emission.