Correlation Spectroscopy Between Two ²⁷Al⁺ Quantum-Logic Clocks

Correlation spectroscopy is a technique for performing coherent differential frequency measurements between two atomic clocks with an interrogation time beyond the coherence time of the local oscillator. This technique was initially demonstrated for two co-trapped ions [1-3]. Here, we present a demonstration of correlation spectroscopy between two independent ²⁷Al⁺ optical atomic clocks separated by a few meters. We discuss limitations caused by differential noise sources and techniques that can be used to mitigate these effects. Correlation spectroscopy allows us to extend the interrogation time to 8 seconds, beyond the capability of cavity stabilized lasers and approaching the lifetime limit of the Al⁺ atomic transition. From this increase in the interrogation time we obtain a fractional measurement instability below 4x10^-16 at 1 s, a factor of ~10 improvement from previous Al⁺ clock comparisons.
[1] M. Chwalla et al., Appl. Phys. B, 89, 483, (2007)
[2] S. Olmschenk et al., Phys. Rev. A, 76, 052314, (2007)
[3] C.W. Chou et al., Phys. Rev. Lett., 106, 160801, (2011)