Ramsey-comb spectroscopy

Abstract

Summary form only given. We demonstrate the new concept of a “Ramsey comb”, which is based on Ramsey spectroscopy [1] over multiple time zones using a frequency comb laser [2]. In contrast to Ramsey's well-established technique of separated oscillating fields for ultra-high precision metrology [1], the Ramsey-comb technique can measure multiple transitions simultaneously with high accuracy. Moreover, the system is powerful enough to drive multiphoton transitions, and the wavelength range is easily extended by nonlinear optics.A prerequisite for measuring optical Ramsey-fringes are coherent pulse-pairs. We have developed a new system capable of producing phase-coherent, multi-delay pulse pairs at the mJ-level by selective amplification of frequency comb pulses [3]. The macro-delay of these pulse pairs can be changed in steps of ~8 ns, while the exact delay can be fine-adjusted at the attosecond-level and with less than a few mrad of phase error. As a proof of principle, we present kHz-level spectroscopy on four two-photon (5S-7S) transitions in Rb. Fig. 1, upper row, shows part of the fluorescence signal measured with a photo-multiplier after excitation with the amplified comb pulses. The signal is a beating of the four atomic transitions, and was recorded via scanning of the inter-pulse delays by a few femtoseconds, at different macro-delays. In the lower row of Fig. 1 the calculated (via a Fourier-transform) spectrum is shown, looking quite similar to full rep-rate comb excitation.We developed a robust fitting method, which, as opposed to for example traditional frequency comb spectroscopy, does not depend on the exact lineshapes of the transitions. Instead, the position and strengths of the atomic transitions are obtained by only fitting the individual phases of the Ramsey signals. In addition, Ramsey-comb spectroscopy is shown to be largely insensitive to the well known light-shift effect. We achieved an accuracy on the investigated Rb transition frequencies of ~10 kHz (relative accuracy of 1.3x10-11), challenging the most accurate spectroscopic measurements that have been performed on this system. Because of the high pulse energy of the amplified pulse-pairs (1 million times higher then what is typical available from an oscillator), the presented system enables ultra-high precision spectroscopy on transitions that are typically difficult for direct frequency comb excitation. As an example, we measured the weak 6S1/2 - 9S1/2 two-photon transition in Cs, improving the accuracy of the transition frequencies by an order of magnitude.