  Dynamical decoupling (DD) is normally ineffective when applied to DC
measurement. In its straightforward implementation, DD nulls out DC signal as
well while suppressing noise. This work proposes a phase relay method (PRM)
that is capable of continuously interrogating the DC signal over many DD
cycles. We illustrate its efficacy when applied to measurement of a weak DC
magnetic field with an atomic spinor Bose-Einstein condensate. Sensitivities
approaching standard quantum limit (SQL) or Heisenberg limit (HL) are
potentially realizable for a coherent spin state (CSS) or a squeezed spin state
(SSS) of 10,000 atoms respectively, while ambient laboratory level noise is
suppressed by DD. Our work offers a practical approach to mitigate the
limitations of DD to DC measurement and will like find other applications for
resorting coherence in quantum sensing and quantum information processing
research.

Quantum Physics