Demonstration of a Two-Photon Atomic Clock with Light Shift Suppression using Two-Colour Magic Wavelengths

Both the University of Adelaide and AFRL have been constructing vapour-cell atomic optical frequency standards which aim to provide frequency stability performance matching or exceeding that of the highest-stability commercial frequency standards (e.g. a Microsemi MHM-2010 H-Maser), but do it with a approx. 10-fold reduction in SWaP. The fundamental approach is based around frequency locking a laser source to a two-photon 51/2 --> 55/2 transition in a heated Rb vapour cell. The AFRL clock takes the approach of a single colour excitation of this transition using two 778nm photons, while the Adelaide group uses two different wavelength photons at 780nm and 776nm for the excitation. In both cases, the excitation is performed using counter-propagating fields to provide a high level of suppression of Doppler broadening. In the AFRL case, this yields a transition bandwidth of around approx. 300kHz, while the Adelaide group obtains a degraded linewidth of 4MHz because of the use of the two colours. However, on the other hand, the more complex dual-colour scheme has an advantage in that it produces a 1000-fold increase in the strength of the two-photon interaction. The principal technical benefit of this increased interaction strength is to create a similar signal-to-noise ratio with a 1000-fold reduction in excitation power. Both frequency standards detect 420nm fluorescence to ensure that the two-photon excitation is frequency locked to the atomic transition. To that end, this project sought to reduce false light shift signals and improve the overall stability of the Adelaide clock by increasing the fluorescence signal which will significantly increasing the signal-to-noise ratio and minimize technical noise sources; experimentally eliminate light shifts and demonstrate a reduction in sensitivity to driving optical powers; and demonstrate that the experimental measurements are consistent with the theoretical expectations.