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

Experimental test of local oscillator-induced performance limitation for pulse-mode passive frequency standards

G.J. Dick, D. A. Stowers

Published Nov 19, 2002• 2 citations
Publisher

Abstract

An experimental test of Local-Oscillator (L.O.) degradation of time-sequential passive oscillator performance was accomplished using the TAG-controlled crystal oscillator described in an accompanying paper. For this quantitative demonstration, an electronic frequency-counter discriminator replaced the trapped ion physics package in a closed-loop experiment. With a 10 second cycle time and 5 second dead time, L.O. limited stability was demonstrated in excellent agreement with theoretical calculations. The results are also in agreement with computer simulations and confirm previous noise analysis for the case of a passive standard with short pulse "Ramsey" interrogation.

DOI
10.1109/freq.1995.483899

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•Nov 10, 2017•0 citations
An experimental test of Local-Oscillator (L.O.) degradation of time-sequential passive oscillator performance was accomplished using the TAG-controlled crystal oscillator described in an accompanying paper. For this quantitative demonstration, an electronic frequency-counter discriminator replaced the trapped ion physics package in a closed-loop experiment. With a 10 second cycle time and 5 second dead time, L.O. limited stability was demonstrated in excellent agreement with theoretical calculations. The results are also in agreement with computer simulations and confirm previous noise analysis for the case of a passive standard with short pulse "Ramsey" interrogation.
As the performance of passive atomic frequency standards improves, a new limitation is encountered due to frequency fiuctuations in an ancillary local osciktor (L.O.). The efect is due to time variation in the gain of the feedback which compensates L.O. frequenq fluctuations. The high performance promised by new microwave and optical trapped ion standark may be severely compromised by this effect. We present un anabsk uf this performance limitatwn for the case of sequentially interrogated standards. The time dependence oJ the sensitivity of the interrogution process to L.O. frequenq fluctuations is evalu-ated for singde-pulse and double-puke "Ramvey " RF interrogation and also for amplitude moduluted pukes. The e$ect of these various time dependencies on performance of the standard is calculated for an L.O. with frequency fluctuations showing a typical 1 / f spectral density. A limiting 1 1 6 dependent deviation of fre-quencyjluctuutions k calculated as a function of pulse lengths, dead time, and pulse overlap. We also present conceptual und hardwure+riented solutions to this problem which achieve a much more nearly convtant sensitivity to L.O.fluctuatiofiv. Solutionr involve: we of double-puke interrogation; alternute interrogation of mulliple traps so that the "dead time " of one trap can be covered by operution of the other; and the use of double-pulse interrogation for two traps, so that during the time of the RFpulves, the increasing sensitivity of one trup tends to compensate for the decreasing sensitivity of the otht: A solution making use of amplitude-modulated pulses is aLso presented which shows nominally zero time variation.
Active optical frequency standards provide interesting alternatives to their passive counterparts. Particularly, such a clock alone continuously generates highly stable narrow-line laser radiation. Thus, a local oscillator is not required to keep the optical phase during a dead time between interrogations as in passive clocks, but only to boost the active clock's low output power to practically usable levels with the current state of technology. Here we investigate the spectral properties and the stability of active clocks, including homogeneous and inhomogeneous broadening effects. We find that for short averaging times the stability is limited by photon shot noise from the limited emitted laser power and at long averaging times by phase diffusion of the laser output. Operational parameters for best long-term stability were identified. Using realistic numbers for an active clock with 87Sr^{87}\mathrm{Sr}87Sr, we find that optimized stability of ${\ensuremath{\sigma}}_{y}(\ensuremath{\tau})\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}/\sqrt{\ensuremath{\tau}[\mathrm{s}]}$ is achievable.
This article describes the first operational results from the extended linear ion trap frequency standard now being developed at JPL. This new design separates the state selection/interrogation region from the more critical microwave resonance region where the multiplied local oscillator (LO) signal is compared to the stable atomic transition. Hg/sup +/ ions have been trapped, shuttled back and forth between the resonance and state selection traps. In addition, microwave transitions between the Hg/sup +/ clock levels have been driven in the resonance trap and detected in the state selection trap.
A high-performance frequency standard based on (199)Hg(+) ions confined in a hybrid radio frequency (RF)/dc linear ion trap is demonstrated. This trap permits storage of large numbers of ions with reduced susceptibility to the second-order Doppler effect caused by the RF confining fields. A 160-mHz-wide atomic resonance line for the 40.5-GHz clock transition is used to steer the output of a 5-mHz crystal oscillator to obtain a stability of 2 x 10(exp -15) for 24,000-second averaging times. Measurements with a 37-mHz line width for the Hg(+) clock transition demonstrate that the inherent stability for this frequency standard is better than 1 x 10(exp -15) at 10,000-second averaging times.
The purpose of this work is to investigate methods by which the quantum states of trapped ions may be better controlled and measured for the applications of quantum computing, quantum simulation, and the generation of atomic frequency standards. We report on two primary projects: first, we control the relative coupling strengths of carrier and first-order motional sideband interactions of a trapped ion by placing it in a resonant optical standing wave. Our configuration uses the surface of a microfabricated chip trap as a mirror, avoiding the technical challenges of in-vacuum optical cavities. Displacing the ion along the standing wave, we show a periodic suppression of the carrier and sideband transitions, with the cycles for the two cases 180◦ out of phase with each other. This technique allows for the suppression of off-resonant carrier excitations when addressing the motional sidebands, and has applications in quantum computation and quantum simulation. Second, we investigate methods proposed in Ref. [36] which may improve the performance of trapped ion (or atom) frequency standards when the interrogating local oscillator is of limited quality. These methods could be applied to future generations of small-scale frequency standards, which are in demand for applications such as local GPS networks and distributed sensors. We propose a scheme for implementing these methods in a 40Ca+ based frequency standard, and outline the technical requirements for doing so. Finally, we present preliminary results characterizing single-ion coherence times and ion “state destruction” events in our system, and discuss how they will affect the realization of the proposed scheme.