One Second Interrogation Time in a 200 Round-Trip Waveguide Atom Interferometer*

We report a multiple-loop guided atom interferometer in which the atoms make 200 small-amplitude round-trips, instead of one large single orbit. The approach is enabled by using an ultracold ³⁹K Nose-Einstein condensate and a magnetic Feshbach resonance that can tune the s-wave scattering length across zero to significantly reduce the atom loss from cold collisions. This scheme is resilient against noisy environments, achieving 0.9 s interrogation time without any vibration noise isolation or cancellation. A form of quantum lock-in amplification can be used with the device to measure localized potentials with high sensitivity. We used this technique to measure the dynamic polarizability of the ³⁹K ground state at 1064 nm. The interferometer may also be a useful approach to building a compact multiple-loop Sagnac atom interferometer for rotation sensing.