Record Stability via Improved Laser Coherence in Strontium-87 Optical Clocks

Many-particle clocks are promising candidates for next-generation frequency standards because quantum projection noise scales down with the square root of atom number. Previously, these clocks had been unable to demonstrate stability better than that of single-particle clocks, due to laser noise-induced instability via the Dick effect. We show that a better optical local oscillator with a $10^{-16}$ thermal noise floor directly results in a tenfold improvement in clock stability, now reaching $1\times 10^{-17}$ in 1000 s [1]. Leveraging the superior precision of a many-particle clock, we are working toward a full systematic evaluation of our clock accuracy with a goal of $1\times 10^{-17}$ fractional uncertainty. One of the important systematics inherent in many-particle clocks is the density-dependent frequency shift. In a new system that traps thousands of atoms at low density, we now measure the density shift with a fractional uncertainty of $8.2\times 10^{-19}$ [1]. Additionally, to further improve our clock stability, we have developed a novel technique to evaluate the noise spectrum of our ultra-stable laser using $^{87}$Sr atoms as a quantum reference [2]. \\[4pt] [1] T.L. Nicholson, et al., PRL 109, 23081 (2012).\\[0pt] [2] M. Bishof et al., in preparation (2013).