News from the lab

High-Bandwidth Transfer-Locking

Controlling the relative phase between two lasers, so that they oscillate with a precisely-known frequency relationship, is a valuable experimental capability.  It allows the manipulation of coherent superpositions of several atomic internal states (by matching the frequency differences between lasers to the energy differences between states) and it means that a single carefully-stabilized laser can serve as a reference for others, which can then share its stability while being much simpler, cheaper and more compact.  For lasers operating at the same wavelength, whose frequencies differ by at most a few GHz, this can be accomplished simply by overlapping light from the two lasers and measuring the beat note with a fast photo-detector.  But what if the lasers are of completely different colours, separated in frequency by hundreds of THz?
We’ve adapted and extended the transfer-oscillator technique, a method for comparing the frequencies of lasers at widely separated wavelengths using an optical frequency comb to bridge the spectral gap between them.  Unlike the traditional implementation, our version is immune to even fast (µs-scale) phase fluctuations of the optical frequency comb, so that even at short times it lets us measure the frequency relationship between the lasers we are interested in without interference from the noisy frequency comb that connects them.  We now use this scheme to phase-lock all the important lasers in the experiment, from 267 nm to 866 nm, to a single stable reference laser at 1542 nm.  Among other benefits, this lets us perform coherent manipulations involving three different electronic levels in Ca⁺ at once.

For further information you can read the paper or this more accessible introduction to the scheme.