Guided atom clock interferometry is a new paradigm in atom interferometry. This is so for two reasons. 1) the interferometer arms are not determined by the free-fall of the atomic clouds, but by a matter-wave-guide, and 2) the beam splitter is either a π/2 pulse of the Bragg scheme (which splits the wavepacket in momentum components via laser radiation) or a π/2 pulse of the Ramsey scheme (which splits the wavepacket in spin components via, usually, microwave radiation). π-pulses can be used analogously to mirrors in both cases. Figure 1 shows a sketch of how this would work
We have developed fundamental tools to get, in particular, the atom interferometry Ramsey scheme to measure the Sagnac phase (briefly: this is the phase that two waves travelling along opposite paths along a closed loop develop if this loop is placed in a rotating frame). The first of this tools is state-dependent manipulation of two spin states: under the same experimental sequence, each spin state travels along the ring in an opposite direction (we did this together with T. Fernholz at Nottingham). Figure 2 shows both a simulation and experimental images of state-dependent manipulation in the ring waveguide.
But this is not enough! A Ramsey interferometer relies on the preservation of the superposition state created after the first pulse for as long as the duration of the free-evolution time (which, in this case, consists of, at least, one round trip along the ring waveguide). In Rubidium 87 atoms, two spin-states, the hyperfine Zeeman sub-levels and are well regarded for this job, because of their “magic field” (no, really, this link). At this magic field, a superposition state is robust against magnetic field fluctuations.
However, things get complicated when these spin states enter the ring potential. The reason is that the trapping is based in Radio-Frequeny (RF) dressing. RF-dressing results in non-trivial spectra when driving transitions between the two hyperfine manifolds F=1 and F=2, and it also ends up broadening the magic transition, which shortens the dephasing time of the superposition state.
We have narrowed down the not-anymore-a-clock-transition in an RF-dressed potential by one order of magnitude and interferometry will be coming soon!