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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

The "atomic candle:" progress towards a smart rubidium atomic clock [and application to microwave dielectric measurements]

James C. Camparo

Published Nov 8, 2002• 3 citations
Publisher

Abstract

Just as it is possible to stabilize the frequency of an electromagnetic field to an atomic resonance between energy eigenstates, so it is possible to stabilize the intensity or brightness of a field to an atomic Rabi-resonance. We term such a device an "atomic candle." We describe the operation of a microwave atomic candle based on the 0-0 ground state hyperfine transition in /sup 87/Rb, and our experiments examining its stability and sensitivity to various experimental parameters. Specifically, we have measured the microwave power stability of our candle, relative to the peak of the Rabi-resonance, and obtained /spl sigma//sub /spl Delta/P/P/(/spl tau/)=9/spl times/10/sup -7/+10/sup -7//spl radic//spl tau/. Additionally, we examined the Rabi-resonance shift as a function of various experimental parameters including microwave detuning from the 0-0 resonance and resonance-cell temperature. The paper concludes with a discussion of a novel application of the atomic candle: the precision measurement of a material's complex dielectric constant.

DOI
10.1109/freq.2000.887440

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IEEE Journal of Quantum Electronics•Apr 1, 1987•61 citations
Just as it is possible to stabilize the frequency of an electromagnetic field to an atomic resonance between energy eigenstates, so it is possible to stabilize the intensity or brightness of a field to an atomic Rabi-resonance. We term such a device an "atomic candle." We describe the operation of a microwave atomic candle based on the 0-0 ground state hyperfine transition in /sup 87/Rb, and our experiments examining its stability and sensitivity to various experimental parameters. Specifically, we have measured the microwave power stability of our candle, relative to the peak of the Rabi-resonance, and obtained /spl sigma//sub /spl Delta/P/P/(/spl tau/)=9/spl times/10/sup -7/+10/sup -7//spl radic//spl tau/. Additionally, we examined the Rabi-resonance shift as a function of various experimental parameters including microwave detuning from the 0-0 resonance and resonance-cell temperature. The paper concludes with a discussion of a novel application of the atomic candle: the precision measurement of a material's complex dielectric constant.
Just as it is possible to stabilize the frequency of an electromagnetic field to an atomic resonance between energy eigenstates, it is possible to stabilize the intensity or brightness of a field to an atomic Rabi resonance. For ease of reference, such a stabilized field is termed an ``atomic candle.'' Here, we describe a microwave atomic candle based on the (F=2,(F=2,(F=2, mF=0){m}_{F}=0)mF​=0)--(F=1,(F=1,(F=1, mF=0){m}_{F}=0)mF​=0) ground-state hyperfine transition of 87Rb{}^{87}\mathrm{Rb}87Rb (6834.7 MHz) as observed in an optically pumped vapor. We discuss the atomic candle's operation and the intensity stability of the field it produces (relative to the Rabi-resonance peak). We then discuss our use of the candle to investigate Rabi-resonance shifts. Specifically, we find that the Rabi resonance depends on the ac Stark shift of the hyperfine transition, the microwave field-atom detuning, and the alkali-metal vapor temperature.
Just as it is possible to stabilize the frequency of an electromagnetic field to an atomic resonance between energy eigenstates, so too is it possible to stabilize the amplitude (or 'brightness') of a field to atomic parametric resonances, the so-called Rabi-resonances. For ease of reference, and by analogy to the atomic clock, we have coined the term 'atomic candle' for this quantum-mechanical, amplitude-stabilization system. Though the atomic candle was originally developed to stabilize microwave power in gas-cell atomic clocks, thereby eliminating a source of timekeeping instability in these devices, the atomic candle's applications extend well beyond the area of precise timekeeping. Basically, the atomic-candle provides a means for detecting and controlling subtle amplitude changes in electromagnetic fields at very low Fourier frequencies (i.e., f < 0.1 Hz). In the present work, we discuss a number of atomic candle applications: laser stabilization, absorption/refractive-index measurements, observations of cavity mode stability over very long time scales (i.e., 50 days), and measurements of low-frequency absorption/scattering fluctuations along a propagation path.
Across a broad range of disciplines, the accurate determination of an electromagnetic wave's amplitude (either absolute or relative) has considerable relevance. Here, we demonstrate a novel and potentially very precise method for making intensity measurements based on the atomic stabilization of electromagnetic field-strength. For ease of reference, and by analogy to atomic clocks, we refer to this field-strength stabilization system as an atomic candle. While the candle's original purpose was to create a field with long-term intensity stability, its very nature makes it ideal for detecting subtle amplitude changes in strong electromagnetic fields; a problem that is fundamentally different from detecting weak signals in the presence of noise. In this work, we discuss proof-of-principle experiments demonstrating the atomic candle's ability to make precise measurements of absorption coefficients and indices of refraction.
The coupling between microwave fields and atoms (or atomlike systems) is inherently weaker than for optical fields, making microwave signal manipulation for applications like quantum information processing technically challenging. In order to better understand this coupling and to develop tools for measuring it, we explore the microwave coupling to atoms using the ``atomic candle'' technique, and push it beyond the bounds of the small-signal regime in order to deliver a larger signal. In familiar two-level systems, responses beyond the usual Rabi oscillations can arise when a single-tone drive is phase modulated, causing the steady-state populations to oscillate at integer harmonics of the modulation frequency. Resonant behavior of the first two harmonics for frequencies near the Rabi frequency, known as $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ Rabi resonances, is widely used for microwave-field magnetometry and as a power standard known as the atomic candle. Here, we explore Rabi resonances beyond the small-signal approximation and report upon experimental observations of higher-order-harmonic population response for microwave hyperfine transitions in cold 87Rb^{87}\mathrm{Rb}87Rb atoms, which we compare to numerical simulations.
Across a broad range of disciplines, the accurate determination of an electromagnetic wave's amplitude (either absolute or relative) has considerable relevance. We demonstrate a novel and potentially very precise method for making intensity measurements based on the atomic stabilization of electromagnetic field-strength. For ease of reference, and by analogy to atomic clocks, we refer to this field-strength stabilization system as an atomic candle. While the candle's original purpose was to create a field with long-term intensity stability, its very nature makes it ideal for detecting subtle amplitude changes in strong electromagnetic fields, a problem that is fundamentally different from detecting weak signals in the presence of noise. In this paper, we discuss proof-of-principle experiments demonstrating the atomic candle's ability to make precise measurements of absorption coefficients and indices of refraction.
Experiments were carried out to evaluate the performances of a semiconductor laser pumped rubidium (87Rb) atomic clock. Two kinds of Rb gas cells were used and their performances were compared [gas cell A (natural rubidium (87Rb/85Rb =\frac{3}{7}) and buffer gases) and gas cell B (87Rb and buffer gases)]. The highest microwave frequency stabilities were estimated as3.4 \times 10^{-12} \tau^{-1/2}and2.7 \times 10^{-12} \tau^{-1/2}at the optimal gas cell temperatures of 60°C and 48°C for the gas cellsAandB, respectively (τ: integration time). The light shift, i.e., microwave frequency shift induced by laser light, was measured as -0.50 Hz/MHz and -0.11 Hz/MHz for the gas cellsAandBat their optimal operating conditions given above. As an improved experiment by utilizing high temporal coherence of the laser, a novel double resonance spectral line shape with a drastically narrower linewidth was demonstrated. A technique, similar to FM laser spectroscopy, was employed for this purpose by utilizing laser FM sidebands which are induced by microwave frequency modulation and nonlinear susceptibility of three-level87Rb atoms. The minimum linewidth obtained was 20 Hz, which can be used as a sensitive frequency discriminator for an improved87Rb atomic clock.