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Physical Review Letters

Atomic Homodyne Detection of Weak Atomic Transitions

Mevan Gunawardena, Daniel S. Elliott

Published Jan 22, 2007• 18 citations
Publisher

Abstract

We have developed a two-color, two-pathway coherent control technique to detect and measure weak optical transitions in atoms by coherently beating the transition amplitude for the weak transition with that of a much stronger transition. We demonstrate the technique in atomic cesium, exciting the 6s(2)S(1/2) --> 8s(2)S(1/2) transition via a strong two-photon transition and a weak controllable Stark-induced transition. We discuss the enhancement in the signal-to-noise ratio for this measurement technique over that of direct detection of the weak transition rate, and project future refinements that may further improve its sensitivity and application to the measurement of other weak atomic interactions.

DOI
10.1103/physrevlett.98.043001

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We have developed a two-color, two-pathway coherent control technique to detect and measure weak optical transitions in atoms by coherently beating the transition amplitude for the weak transition with that of a much stronger transition. We demonstrate the technique in atomic cesium, exciting the 6s(2)S(1/2) --> 8s(2)S(1/2) transition via a strong two-photon transition and a weak controllable Stark-induced transition. We discuss the enhancement in the signal-to-noise ratio for this measurement technique over that of direct detection of the weak transition rate, and project future refinements that may further improve its sensitivity and application to the measurement of other weak atomic interactions.
We describe a technique based upon the principles of two-pathway coherent control that allows one to amplify the magnitude of a weak atomic or molecular transition. In our initial demonstrations, we amplify a weak Stark-induced transition (the $6s\text{ }{^{2}S}_{1∕2}\ensuremath{\rightarrow}8s\text{ }{^{2}S}_{1∕2}$ transition in atomic cesium) by coherently beating it with a stronger two-photon transition. We describe the measurements in detail, provide a quantitative discussion of the effects of beam overlap and alignment on the depth of modulation, and discuss the enhancement in the signal-to-noise ratio for this measurement technique over that of direct detection of the weak transition rate.
This thesis demonstrates the experimental implementation of a homodyne interference technique to detect weak atomic transitions. The demonstration of the technique uses two pathways to excite the cesium atom from its ground state to an excited state. One is a Stark-induced transition that is regulated by controlling a dc field across the cesium atoms. This is excited by a 411 nm laser field. A two-photon transition excited by an 822 nm laser field concurrently excites the same transition. The simultaneous excitation of the two pathways results in a quantum interference of the transition amplitudes. Phase modulating one of the beams gives rise to an amplitude modulation of the net excitation rate resulting in an amplitude modulated fluorescence signal. We observe the fluorescence at 794.6 nm due to the decay of the excited state to the ground state via an intermediate state. The strength of the amplitude modulation of the fluorescence signal is a measure of weak signal transition amplitude which in this demonstration is the controllable Stark-induced transition. Results of this work have shown improvements of two to three orders of magnitude in signal to noise over direct detection of the weak interaction. With possible improvements that are discussed it is projected to be much better.
We have developed a two-pathway Coherent Control technique for measurements of weak optical transition moments. We demonstrate this technique through a measurement of the transition moment of the highly-forbidden magnetic dipole transition between the 6s2S1/21/2 and 7s2S1/21/2 states in atomic Cesium. The experimental principle is based on a two-pathway excitation, using two phase-coherent laser fields, a fundamental field at 1079 nm and its second harmonic at 539.5 nm. The IR field induces a strong two-photon transition, while the 539.5 nm field drives a pair of weak one-photon transitions: a Stark-induced transition of controllable strength as well as the magnetic dipole transition. Observations of the interference between these transitions for different Stark-induced transition amplitudes, allow a measurement of the ratio of the magnetic dipole to the Stark-induced moment. The interference between the transitions is controlled by modulation of the phase-delay between the two optical fields. Our determination of the magnetic dipole moment is at the 0.4% level and in good agreement with previous measurements, and serves as a benchmark for our technique and apparatus. We anticipate that with further improvement of the apparatus detection sensitivity, the demonstrated scheme can be used for measurements of the very weak Parity Violation transition moment on the Cesium 6s2S1/21/2→7s2S1/21/2 transition.
We present a new technique based on two-pathway optical coherent control for the sensitive detection and precise measurement of highly-forbidden transitions in atomic systems. Specifically, we show that (ωωω, 2ωωω) coherent control can be applied to the measurement of the magnetic dipole and electroweak parity nonconserving amplitudes in atomic cesium, with the principal advantage of reduced systematic errors related to field reversals often encountered in previous measurements of these effects. We present a complete analysis in one specific geometry, and discuss prospects for improved laboratory determinations of these weak transition amplitudes.
We present a technique based upon two-pathway coherent control for the measurement of a weak transition moment. In our approach, we use two coherent optical beams, one the second harmonic of the other, to drive a transition by means of three distinct optical interactions. The interference between these interactions allows a determination of one moment relative to another. In this work, we demonstrate the approach by applying it to an experimental determination of the magnetic dipole moment for the $6s{\phantom{\rule{0.222222em}{0ex}}}^{2}{S}_{1/2}\ensuremath{\rightarrow}7s{\phantom{\rule{0.222222em}{0ex}}}^{2}{S}_{1/2}$ transition in atomic cesium. Our results are in excellent agreement with previous single-beam measurements. We also discuss prospects for extending this measurement technique to a new determination of the weak-force induced parity-nonconserving moment on this same transition.
We report a new method of two-pathway coherent control using three narrow-band cw laser sources, phase locked in an optical phase-lock loop, to maintain the high degree of optical coherence required for the coherent control process. In addition, we derive expressions for two-photon transition amplitudes and demonstrate their dependence on the polarization of the field components. This phase-locking technique expands the set of interactions to which coherent control techniques may be applied. It also allows for a constant low-frequency offset between the optical interactions, producing a continuous and constant phase ramp between the interactions, facilitating phase-sensitive detection of the modulating atomic signal. We illustrate this technique with two-photon vs.~one-photon excitation of a ΔF=1ΔF = 1ΔF=1 component, and alternatively a ΔF=0ΔF = 0ΔF=0 component, of the 6s 2S1/2→7s 2S1/26s \: ^2S_{1/2} \rightarrow 7s \: ^2S_{1/2}6s2S1/2​→7s2S1/2​ transition of atomic cesium.