  We report a multiple-loop guided atom interferometer in which the atoms make
200 small-amplitude roundtrips, instead of one large single orbit. The approach
is enabled by using ultracold 39K gas 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.
