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Quantum optics with cold atoms--nonlinear spectroscopy and road toward single-atom trap

Z. Hu

Published Oct 12, 2007• 1 citation
Publisher

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Investigations of optical processes for laser cooled and trapped atoms are described. Fluorescence from Cs atoms in a magneto-optical trap is detected under conditions of very low atomic density. Discrete steps are observed in the fluorescent signal versus time and are associated with the arrival and departure of individual trapped atoms. Histograms of the frequency of occurrence of a given level of fluorescence exhibit a series of uniformly spaced peaks that are attributed to the presence of N = 0, 1, 2 atoms in the trap. In addition, numerical absorption and emission spectra for three-level [...], [...], and V systems under intense radiations are calculated. Absorption spectra for a [...] system is used to explain the probe-wave amplification and absorption spectra recorded for Cs atoms cooled and confined in a magneto-optical trap, in which novel spectral features of narrow frequency widths with single-pass gain exceeding 20% are observed. The consequence of the optical gain is demonstrated to lead to negative radiation pressure, which is investigated together with other mechanical forces in the trap. Various alternative trapping schemes in three-level and two-level atoms are proposed as possible means to compress an atomic sample and demonstrated for a two-level magneto-optical trap.

DOI
10.7907/ykpq-pa09

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NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Investigations of optical processes for laser cooled and trapped atoms are described. Fluorescence from Cs atoms in a magneto-optical trap is detected under conditions of very low atomic density. Discrete steps are observed in the fluorescent signal versus time and are associated with the arrival and departure of individual trapped atoms. Histograms of the frequency of occurrence of a given level of fluorescence exhibit a series of uniformly spaced peaks that are attributed to the presence of N = 0, 1, 2 atoms in the trap. In addition, numerical absorption and emission spectra for three-level [...], [...], and V systems under intense radiations are calculated. Absorption spectra for a [...] system is used to explain the probe-wave amplification and absorption spectra recorded for Cs atoms cooled and confined in a magneto-optical trap, in which novel spectral features of narrow frequency widths with single-pass gain exceeding 20% are observed. The consequence of the optical gain is demonstrated to lead to negative radiation pressure, which is investigated together with other mechanical forces in the trap. Various alternative trapping schemes in three-level and two-level atoms are proposed as possible means to compress an atomic sample and demonstrated for a two-level magneto-optical trap.
Nonlinear spectroscopy of cold atoms has recently been reported for a sample of cesium atoms confined in a magnetooptical trap.1,2 In particular, we report measurements of the spectral response of the cold sample to a weak probe beam for a range of detunings and power for the saturating trapping beams. Transmission spectra for the probe beam reveal the presence of broad absorption and gain features as well as narrow resonances. The power dependence of the broad part of the spectrum can be reasonably well described by earlier work for two-level atoms in a strong field.3 However, narrow dispersive shaped features around the frequency of the trapping beams are also observed and can have appreciable single pass gain (>20%). An exploration of the correlation between the narrow spectroscopic features and the mechanical properties of the atomic sample leads to the observation of both positive and negative radiation pressure associated with stimulated absorption from and emission to the probe beam.
A variety of experiments are underway in the Quantum Optics Group at Caltech which investigate the quantum nature of atom-field interactions at the level of individual atoms and quanta. A recent technical advance in support of this research is the observation of cooling and trapping of single neutral cesium atoms in magneto-optical trap. Discrete steps are recorded in the fluorescence signal from the trap and are associated with the arrival and departure of individual trapped atoms. Such a spatially localized sample of a single atom with small kinetic energy is an enabling advance for diverse studies in quantum optics, including the possibility of spectroscopy with squeezed and other forms of nonclassical light and cavity quantum electrodynamics with strong coupling of an atom to the field of an optical cavity.
A spectroscopic investigation is reported for a sample of Cesium atoms which are cooled and confined in a Zeeman shift spontaneous force optical trap.(1'2) From measured absorption spectra for a weak probe beam both in steady state and under transient conditions, we infer that the sample consists of approximately 106 atoms trapped in a spherical volume of radius 400µm and cooled to roughly 300µK. To explore the spectral response of this sample under conditions of strong excitation by the trapping laser beams ("pump" waves), we record transmission as a function of frequency for a low intensity "probe" wave focussed through the trap. As might be expected, we observe probe spectra with broad regions of absorption and amplification characteristic of the Autler-Townes splitting for a "dressed" two-state atom. However, additional narrow features around the pump frequency have linewidths of several hundred kilohertz (well-below the natural linewidth of 5MHz) and exhibit appreciable single pass gain (≥30%) . Our preliminary attempts to describe these features theoretically provide limited qualitative agreement with experiment. In an effort to correlate the spectroscopic characteristics of the trap with its mechanical properties, we have observed recoil of the trapped atoms either along or opposite to the direction of propagation of the probe beam depending upon whether the probe experiences absorption or gain. The direction of recoil can be switched with a change in probe detuning of less than 300kHz.
Absorption spectra of cold Cs atoms in a magneto-optical trap under the influence of a strong external pump laser field with arbitrary detuning and intensity have been systematically investigated. The spectra featured the Rabi sidebands of each transition, and their behavior was nicely fitted by the dressed-atom theory. A pronounced dispersion-type profile with sub-natural linewidth at the pump frequency may be attributable to the stimulated Raman process by sublevels that are lightshifted and optically pumped by the pump laser.