An optical beam frequency reference with 10-14 range frequency instability

The authors report on a thermal beam optical frequency reference with a fractional frequency instability of 9.2x10-14 at 1 s reducing to 2.0x10-14 at 64 s before slowly rising. The 1S0↔3P1 intercombination line in neutral 40Ca 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 Λ-variance of 7x10-14/√(τ).