Ramsey-Borde Matter-Wave Interferometry for Laser Frequency Stabilization at 10-16 Frequency Instability and Below

We demonstrate Ramsey-Borde (RB) atom interferometry for high performance laser stabilization with fractional frequency instability < 2 x 10(-16) for timescales between 10 and 1000s. The RB spectroscopy laser interrogates two counterpropagating Ca-40 beams on the S-1(0) - P-3(1) transition at 657 nm, yielding 1.6 kHz linewidth interference fringes. Fluorescence detection of the excited state population is performed on the (4s4p) P-3(1 )- (4p(2)) P-3(0) transition at 431 nm. Minimal thermal shielding and no vibration isolation are used. These stability results surpass performance from other thermal atomic or molecular systems by 1 to 2 orders of magnitude, and further improvements look feasible.