An optical beam frequency reference with 10~(-14) range frequency instability

摘要

The authors report on a thermal beam optical frequency reference with a fractional frequency instability of 9.2×10~(-14) at 1 s reducing to 2.0× 10~(-14) at 64 s before slowly rising. The ~1S_0←→~3P_1 intercombination line in neutral ~(40)Ca is used as a frequency discriminator. A diode laser at 423 nm probes the ground state population after a Ramsey-Borde sequence of 657 nm light-field interactions on the atoms. The measured fractional frequency instability is an order of magnitude improvement on previously reported thermal beam optical clocks. The photon shot-noise of the read-out produces a limiting square root A-variance of 7 × 10~(-14)/ τ~(1/2).