Perrella, C.Light, P.S.Anstie, J.D.Baynes, F.N.White, R.T.Luiten, A.N.2020-01-192020-01-192019Physical Review Applied, 2019; 12(5):054063-1-054063-62331-70192331-7019http://hdl.handle.net/2440/122778We demonstrate an optical frequency standard based on the 5S1/2→5D5/2 two-photon transition of rubidium. The transition is interrogated in a Doppler-free arrangement by two lasers at 780 and 776 nm, with the sum frequency of the two lasers locked to the two-photon transition. We measure the fractional frequency stability of the frequency standard to be 1.5×10−13 at 1 s, reaching 4.9×10−14 at approximately 100s, by comparison with an optical frequency comb. Performance limitations are presented and a clear pathway to an order-of-magnitude improvement is discussed. This platform has the potential to deliver a compact, robust standard for commercial and industrial applications.en© 2019 American Physical Society.Journal article100001005010.1103/PhysRevApplied.12.0540630004988977000022-s2.0-85076397670511627Perrella, C. [0000-0002-6140-9323]Light, P.S. [0000-0003-3873-7991]Luiten, A.N. [0000-0001-5284-7244]