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arXiv:0704.1686 · Physical Review A

Effect of atomic beam alignment on photon correlation measurements in cavity QED

Levente Horvath, H. J. Carmichael

Published Oct 16, 2007• 8 citations
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Abstract

Quantum trajectory simulations of a cavity QED system comprising an atomic beam traversing a standing-wave cavity are carried out. The delayed photon coincident rate for forward scattering is computed and compared with the measurements of Rempe et al. [Phys. Rev. Lett. 67, 1727 (1991)] and Foster et al. [Phys. Rev. A 61, 053821 (2000)]. It is shown that a moderate atomic beam misalignment can account for the degradation of the predicted correlation. Fits to the experimental data are made in the weak-field limit with a single adjustable parameter---the atomic beam tilt from perpendicular to the cavity axis. Departures of the measurement conditions from the weak-field limit are discussed.

DOI
10.1103/physreva.76.043821

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Quantum trajectory simulations of a cavity QED system comprising an atomic beam traversing a standing-wave cavity are carried out. The delayed photon coincident rate for forward scattering is computed and compared with the measurements of Rempe et al. [Phys. Rev. Lett. 67, 1727 (1991)] and Foster et al. [Phys. Rev. A 61, 053821 (2000)]. It is shown that a moderate atomic beam misalignment can account for the degradation of the predicted correlation. Fits to the experimental data are made in the weak-field limit with a single adjustable parameter---the atomic beam tilt from perpendicular to the cavity axis. Departures of the measurement conditions from the weak-field limit are discussed.
Ab initio quantum trajectory simulations of a cavity QED system comprising an atomic beam traversing a coherently driven standing-wave cavity are carried out. The intensity correlation function in transmission is computed and compared with the experimental measurements of Rempe et al. [Phys. Rev. Lett. 67, 1727 (1991)] and Foster et al. [Phys. Rev. A 61, 053821 (2000)]. It is shown that atomic beam density fluctuations induced by the motion of the atoms can account for the reported disagreement of the experimental results with theory (by an overall scale factor of 2 to 4). Moderate misalignments of the atomic beam produce large intracavity photon number fluctuations which significantly degrade the quantum correlations. One parameter fits to the experimental data are made in the weak-field limit with the adjustable parameter being the atomic beam tilt. Departures of the experimental conditions from the weak-field limit are discussed.
We use the quantum trajectory (stochastic wave function) approach to simulate the dynamics of a cavity QED system. The system consists of one atom interacting on resonance with one mode of a coherently driven optical cavity. We simulate a variety of different measurements on this system. The simulation of direct photoelectric detection produces trajectories that illustrate the collapse of the wave function in delayed photon coincidence measurements. It also allows us to calculate waiting-time distributions, pho­toelectron counting distributions, and the mean transmitted intensity as a function of the excitation frequency and field strength. A simulation with frequency filtering of the stochastic wave fuction gives results for optical spectra. From a simulation of homodyne detection we obtain trajectories showing the fluctuations in the quadrature phase amplitudes of the intracavity field and the atomic dipole. These simulations are used to calculate spectra of squeezing. We show that when the cavity is excited near one of the "vacuum" Rabi resonances, the composite atom-cavity system behaves like a two-state system. Under these conditions the optical spectrum shows Mollow sidebands caused by a dynamic Stark shift of the Jaynes-Cummings eigenstates. Thus, in this measurement we are observing the "dressing" of the (one-quantum) dressed states.
We study the time evolution of the quantum field inside a cavity coupled to a beam of two-level atoms of temperature T, given that each atom, after having crossed the cavity, interacts with a classical field and finally with a detector measuring its state. It is shown numerically that, if the atom-quantum field coupling is weak and % is large, for any given realization of the measurements, any initial state of the cavity field localizes after some time into a squeezed state. The center a of the squeezed state moves randomly on a line in the complex plane, but the squeezing parameters r and Φ show very little fluctuation. Their mean values r and Φ are independent of the realization, the initial state, and the atom-field coupling constant. The time evolution of r and Φ is computed in the limit of weak atom-field coupling and large. It is found that r increases with T; i.e., the squeezing is enhanced by increasing the temperature of the atomic beam. The limitation on the degree of squeezing that can be reached is discussed.
One of the canonical questions in quantum optics is the nature of the radiative properties of an atom when the normal vacuum fluctuations of the electromagnetic reservoir are replaced by the asymmetric, reduced fluctuations of a squeezed vacuum. While the basic radiative linewidth-narrowing effect has been known for over a decade [C. W. Gardiner, Phys. Rev. Lett. 56, 1917 (1986)], experimental realizations with operationally definable definitive manifestations of the quantum nature of the squeezed reservoir have been largely lacking from subsequent investigations. This paper presents measurements on an experimentally realized atom--squeezed-light system, in which the squeezed-light output of a subthreshold optical parametric oscillator illuminates an atom strongly coupled to a high-finesse optical resonator. Transmission of a weak probe field incident on the atom-cavity system is investigated both theoretically and experimentally. Alteration of the transmitted probe spectrum has been observed, as has a transmission modulation that depends on the phase of the squeezed field relative to a saturating coherent field (displaced squeezing). In certain parameter regimes, properties unique to the quantum nature of the squeezed light have been identified in the theoretical treatment, but complications in the experiment prevent their unequivocal measure. It is found that the observed effects of the squeezed light are dramatically reduced relative to the predictions of an idealized theory. This is quantitatively attributed to the effects of atomic beam fluctuations and a simple modeling of the atomic beam as an additional loss mechanism in the theory leads to reasonable agreement with the data.
Summary form only given. The second order intensity correlation function can provide evidence of the non-classical nature of an electric field, as well as information about the dynamics which produce the field. In practice, this is a conditional measurement of the probability to detect a photon given that another photon was detected in coincidence or some time before. We have measured the correlation function of the photons escaping from a strongly coupled cavity QED system consisting of a high finesse interferometer traversed by a beam of optically pumped Rb atoms. In this regime, the rate of reversible exchange of energy between the atoms and the cavity is larger than the cavity decay and spontaneous emission rates, and the saturation photon number is less than one. The correlations exhibit the non-classical features of antibunching and sub-Poissonian statistics, and an oscillation related to the exchange of energy taking place in the coupled system of atoms and cavity field. We view the initial photon detection as causing a projection of the system into a well defined initial state from which it coherently evolves back to steady state. A full characterization of this conditional evolution requires the measurement of the electric field as it evolves from the initial state.
We present an alternative route to track the motion of a single atom traversing a high-finesse optical cavity by means of the coherence statistics of photon antibunching. Here the single atom strongly couples to the high-order transverse Laguerre-Gaussian (LG) mode (e.g., LG01{\text{LG}}_{01}LG01​) of optical cavity, instead of the high-order transverse Hermite-Gaussian (HG) mode, which is beneficial to the single-atom trajectory measurement utilizing such a photon antibunching effect. With this aim, we characterize the position-dependent quantum correlations of the transmitted light in such a cavity quantum electrodynamics (QED) system subject to various kinds of the LG modes, finding that the photon antibunching effect is closely related to the position of the atom in the transverse plane and therefore the degree of the antibunching carries the information about the position of the atom within the cavity. The numerical results of the second-order correlation function agree well with the analytical calculations. Thanks to the tilted transverse LG01{\text{LG}}_{01}LG01​ mode of the cavity, which is inclined to the vertical direction by an angle of $\ensuremath{\sim}{15}^{\ensuremath{\circ}}$, the LG-mode cavity QED architecture helps us to eliminate the degenerate trajectory of the single atom falling through the cavity and to obtain a unique atomic trajectory. In a $10\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{s}$-long time interval as an example, our in-depth analysis displays that the atomic position with the spatial resolution of $\ensuremath{\sim}4.0\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ in the vertical direction (axis xxx) can be achieved. The present study is useful for well understanding and researching both the robust generation of single-photon source and the precision measurement of single-atom trajectory.