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

Autobalanced Ramsey Spectroscopy Improving Clock Accuracy and Testing Fundamental Physics

Christian Sanner, Nils Huntemann, R. Lange, Chr. Tamm, E. Peik

Published Apr 1, 2019• 0 citations
Publisher

Abstract

Everyone is trying to make frequency measurements as accurate as possible. But what if the measurement process itself becomes a major source of error? This issue is particularly relevant in the context of state-of-the-art atomic clocks where errors at the 18th decimal place are not negligible anymore. In order to spectroscopically interrogate an atomic clock transition without compromising the clock's accuracy, we devise a perturbation-immune version of Ramsey's method of separated oscillatory fields. By actively balancing the spectroscopic responses from phase-congruent Ramsey probe cycles of unequal durations, we eliminate a wide variety of interrogation-induced line shifts often encountered in high precision spectroscopy, among them, in particular, light shifts, phase chirps, and transient Zeeman shifts. We experimentally demonstrate autobalanced Ramsey spectroscopy on the light shift prone171Yb+electric octupole optical clock transition and show that interrogation defects are not turned into clock errors. This opens up frequency accuracy perspectives below the 10-18level for the Yb+system and for other types of optical clocks. Further applications of this simple and universal approach include next generation coherent population trapping clocks and perturbation-immune readout of quantum sensors. Future long-term clock comparisons searching for physics beyond the standard model will also benefit from this technique.

DOI
10.1109/fcs.2019.8856113

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Everyone is trying to make frequency measurements as accurate as possible. But what if the measurement process itself becomes a major source of error? This issue is particularly relevant in the context of state-of-the-art atomic clocks where errors at the 18th decimal place are not negligible anymore. In order to spectroscopically interrogate an atomic clock transition without compromising the clock's accuracy, we devise a perturbation-immune version of Ramsey's method of separated oscillatory fields. By actively balancing the spectroscopic responses from phase-congruent Ramsey probe cycles of unequal durations, we eliminate a wide variety of interrogation-induced line shifts often encountered in high precision spectroscopy, among them, in particular, light shifts, phase chirps, and transient Zeeman shifts. We experimentally demonstrate autobalanced Ramsey spectroscopy on the light shift prone171Yb+electric octupole optical clock transition and show that interrogation defects are not turned into clock errors. This opens up frequency accuracy perspectives below the 10-18level for the Yb+system and for other types of optical clocks. Further applications of this simple and universal approach include next generation coherent population trapping clocks and perturbation-immune readout of quantum sensors. Future long-term clock comparisons searching for physics beyond the standard model will also benefit from this technique.
We devise a perturbation-immune version of Ramsey's method of separated oscillatory fields. Spectroscopy of an atomic clock transition without compromising the clock's accuracy is accomplished by actively balancing the spectroscopic responses from phase-congruent Ramsey probe cycles of unequal durations. Our simple and universal approach eliminates a wide variety of interrogation-induced line shifts often encountered in high precision spectroscopy, among them, in particular, light shifts, phase chirps, and transient Zeeman shifts. We experimentally demonstrate autobalanced Ramsey spectroscopy on the light shift prone ^{171}Yb^{+} electric octupole optical clock transition and show that interrogation defects are not turned into clock errors. This opens up frequency accuracy perspectives below the 10^{-18} level for the Yb^{+} system and for other types of optical clocks.
Sophisticated Ramsey-based interrogation protocols using composite laser pulse sequences have been recently proposed to provide next-generation high-precision atomic clocks with a near perfect elimination of frequency shifts induced during the atom-probing field interaction. We propose here a simple alternative approach to the autobalanced Ramsey interrogation protocol and demonstrate its application to a cold-atom microwave clock based on coherent population trapping (CPT). The main originality of the method, based on two consecutive Ramsey sequences with different dark periods, is to sample the central Ramsey fringes with frequency jumps finely adjusted by an additional frequency-displacement concomitant parameter, scaling as the inverse of the dark period. The advantage of this displaced frequency-jump Ramsey method is that the local oscillator (LO) frequency is used as a single physical variable to control both servo loops of the sequence, simplifying its implementation and avoiding noise associated with controlling the LO phase. When tested using a CPT cold-atom clock, the DFJR scheme reduces the sensitivity of the clock frequency to variations of the light shifts by more than an order of magnitude compared with the standard Ramsey interrogation. This simple method can be applied in a wide variety of Ramsey-spectroscopy based applications including frequency metrology with CPT-based and optical atomic clocks, mass spectrometry, and precision spectroscopy.
We present a rigorous justification for the applicability of the recently developed methods of generalized autobalanced Ramsey spectroscopy (GABRS) [V. I. Yudin et al., Phys. Rev. Appl. 9, 054034 (2018)] and combined error signal in Ramsey spectroscopy (CESRS) [ , New J. Phys. 20, 123016 (2018)] for resonances of coherent population trapping (CPT). A protocol of modified combined error signal for CPT resonances is also proposed, in which two successive interrogations of atoms under different times of free evolution are integrated in one cycle. The implementation of the GABRS and CESRS methods in a compact atomic clock based on the CPT effect allows us to significantly suppress the light shift and its fluctuation, which will lead to an improvement in the long-term stability and accuracy of these devices. Moreover, these spectroscopic schemes are insensitive to various distortions of the pulse shape, relaxation processes, errors in the generation of phase jumps, etc., which provides their high reliability and immunity to perturbations.
During precision measurement, the quantity being measured is often perturbed by the measurement process itself. This includes precision frequency measurements for atomic-clock applications using Ramsey spectroscopy. To eliminate probe-induced perturbations, a generalized method is developed in which the frequency control loop is augmented with a second control loop that feeds back to a secondary clock variable, and can perfectly compensate for perturbations of the clock frequency caused by the measurements in the first loop. This universal technique can be used for atomic clocks, high-resolution molecular spectroscopy, two-photon probing schemes, Ramsey mass spectrometry, and beyond.
The next generation of alkali-vapor-cell atomic clocks are under development in labs around the world, with prototypes already demonstrating short-term fractional frequency stabilities one or two orders of magnitude better than for commercial clocks. However, stability is often spoiled at medium time scales (>100 s) by light-power-induced frequency-shift effects. The authors tackle this problem by applying autobalanced Ramsey interrogation, recently demonstrated on a Yb+{}^{+}+ clock, to coherent-population-trapping systems---an approach that could really boost the development of vapor-cell clocks with white-noise-limited frequency stability over greatly extended time scales.
Probing an atomic resonance without disturbing it is an ubiquitous issue in physics. This problem is critical in high-accuracy spectroscopy or for the next generation of atomic optical clocks. Ultra-high resolution frequency metrology requires sophisticated interrogation schemes and robust protocols handling pulse length errors and residual frequency detuning offsets. This review reports recent progress and perspective in such schemes, using sequences of composite laser-pulses tailored in pulse duration, frequency and phase, inspired by NMR techniques and quantum information processing. After a short presentation of Rabi technique and NMR-like composite pulses allowing efficient compensation of electromagnetic field perturbations to achieve robust population transfers, composite laser-pulses are investigated within Ramsey's method of separated oscillating fields in order to generate non-linear compensation of probe-induced frequency shifts. Laser-pulses protocols such as hyper-Ramsey, modified hyper-Ramsey, generalized hyper-Ramsey and hybrid schemes as auto-balanced Ramsey spectroscopy are reviewed. These techniques provide excellent protection against both probe induced light-shift perturbations and laser intensity variations. More sophisticated schemes generating synthetic frequency-shifts are presented. They allow to reduce or completely eliminate imperfect correction of probe-induced frequency-shifts even in presence of decoherence due to the laser line-width. Finally, two universal protocols are presented which provide complete elimination of probe-induced frequency shifts in the general case where both decoherence and relaxation dissipation effects are present by using exact analytic expressions for phase-shifts and the clock frequency detuning. These techniques might be applied to atomic, molecular and nuclear frequency metrology, Ramsey-type mass spectrometry as well as precision spectroscopy.