Continuously observing a dynamically decoupled spin-1 quantum gas

We continuously observe dynamical decoupling in a spin-1 quantum gas using a weak optical measurement of spin precession. Continuous dynamical decoupling modifies the character and energy spectrum of spin states to render them insensitive to parasitic fluctuations. Continuous observation measures this new spectrum in a single preparation of the quantum gas. The measured time series contains seven tones, which spectrogram analysis parses as splittings, coherences, and coupling strengths between the decoupled states in real time. With this we locate a regime where a transition between two states is decoupled from magnetic-field instabilities up to fourth order, complementary to a parallel work at higher fields [D. Trypogeorgos et al., preceding paper, Phys. Rev. A 97, 013407 (2018)10.1103/PhysRevA.97.013407]. The decoupled microscale quantum gas offers magnetic sensitivity in a tunable band, persistent over many milliseconds: the length scales, frequencies, and durations relevant to many applications, including sensing biomagnetic phenomena such as neural spike trains.