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Bulletin of the American Physical Society

Atomic beam deflection in a quantum field

Lancelot Graham, C. F. Bharucha, F. L. Moore

Published May 1, 1993• 0 citations
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Abstract

Atomic beam deflection in a quantum field is studied theoretically for the case of an atom passing through the mode of a resonant optical cavity. Deflection probability is calculated for a coupling rate g of order g/2{pi}=1 MHz, which is experimentally feasible in a short optical cavity. Atomic velocities are taken in the range of 1-10 m/s, which can be reached with current cooling and trapping techniques. We calculate deflection for a coherent state with mean photon number , and for a number state. For the parameters studied, the predicted scattering is in an intermediate regime between Raman-Nath and Bragg, but is closer to the former. We find significant deflection probability even for =1, indicating potential for a high level of sensitivity. We report on our progress in the experimental realization of this system with laser cooled sodium atoms as the atomic medium, and directions for future work are indicated.

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Atomic beam deflection in a quantum field is studied theoretically for the case of an atom passing through the mode of a resonant optical cavity. Deflection probability is calculated for a coupling rate g of order g/2{pi}=1 MHz, which is experimentally feasible in a short optical cavity. Atomic velocities are taken in the range of 1-10 m/s, which can be reached with current cooling and trapping techniques. We calculate deflection for a coherent state with mean photon number , and for a number state. For the parameters studied, the predicted scattering is in an intermediate regime between Raman-Nath and Bragg, but is closer to the former. We find significant deflection probability even for =1, indicating potential for a high level of sensitivity. We report on our progress in the experimental realization of this system with laser cooled sodium atoms as the atomic medium, and directions for future work are indicated.
We investigate to which extent the quantum nature of light modifies laser-induced atomic beam deflection in a plane standing wave. Our model considers a single two-level atom interacting with a single quantized standing wave mode of the electromagnetic field. We neglect spontaneous emission into other field modes, an approximation valid for short enough times. We find that there is an essential qualitative difference between the cases of a coherent field and a thermal field. For a coherent field, the atomic momentum distribution after interaction has two symmetric peaks on the wings and resembles the semiclassical result of Arimondo et al. [Phys. Rev. A 24, 898 (1981)]. For a thermal field, this distribution has its maximum at p = 0. It is characterized by a constant contribution at p = 0, some sharp interference features, and near-Gaussian wings. The only feature common to both cases is that in the Raman-Nath regime of scattering, the range on transverse momenta grows linearly with time at a rate proportional to 〈 n 〉 , where 〈 n 〉 is the mean number of photons in the field.
We have performed experimental and theoretical studies of the deflection of a sodium atomic beam by a laser tuned to the 3 2{\mathrm{}}^{2}2S1/2{S}_{1/2}S1/2​,F=2--3 2{\mathrm{}}^{2}2P3/2{P}_{3/2}P3/2​,F=3 transition. The intensity profile of the deflected beam was measured at different laser power levels and for different laser polarizations. Calculations using a constant force derived from a two-level atom yield deflections which are too large. The results of more realistic simulation calculations agree very well with the measured deflection. Those calculations incorporate the atomic-beam spatial and velocity distributions as well as the laser frequency distribution and polarization state. Optical pumping, loss of atoms to the nonresonant F=1 ground state, and Doppler shifting of the atoms out of resonance are all considered, as also are the statistics and angular distributions of the spontaneous photons.
This thesis presents a theoretical and experimental investigation of the deflection of an atomic beam by an interaction with a near resonant laser. This investigation focuses on the sensitivity of this force to the initial state of an atom. The term deflection spectroscopy is applied to a specific application of the deflection force as a probe of a collisionally created density matrix. For the purpose of demonstrating the state sensitivity, a method of state preparation is presented that can produce both an and off diagonal density matrix elements in the ground state of sodium, simulating those occurring in a collisional excitation. The atomic collision parameter L1 was determined from a measurement of the deflected momentum of a state prepared atomic beam. A full quantum electrodynamic description of the interaction is used to describe the laser - atom interaction. Incorporated in the description is both the preparation of a ground state and a full description of the momentum exchange. The QED equations are applied to the 32S112 to 32P312 transition of atomic sodium for all possible modes of excitation from the "pure" polarisations, circular (a) and linear (it) to an elliptical polarisation involving combinations and relative phase shifts of these two polarisations. In addition the deflection of a J=2 metastable state is considered, and a set of deflection pseudo -Stokes parameters are developed for both the metastable state and the optically prepared sodium ground state. Comparisons between theory and experiment are made. Both fluorescence measurements and transverse momentum measurements are performed and were compared with theory. All aspects of the interaction were investigated with the theory performing well under all experimental conditions. It was found the deflection was sensitive to the preparation of the atomic ground state of sodium and hence it was demonstrated that atom deflection can be used to determine atomic collision parameters.
A method for the reconstruction of photon statistics and even the Wigner function of a quantized cavity field state is proposed. The method is based on the measurement of momentum distribution of two-level atoms in the Raman–Nath regime. Both the cases of resonant and off-resonant atom–field interaction are considered. The Wigner function is reconstructed by displacing the photon statistics of the cavity field. This reconstruction method is straightforward and does not need much mathematical manipulation of experimental data.
A sodium atomic beam is deflected by two optical standing-wave fields which excite near resonance Raman transitions. Observed deflections are consistent with theoretical predictions of a long-range dc component of the force on a three-level atom in the \ensuremath{\Lambda} configuration. The low laser intensities (and small detunings) used, combined with observed evidence of damping in the three-level \ensuremath{\Lambda} system, may ultimately lead to the design of high-density, all optical traps wherein the atoms would be mostly in the dark state.