  The accumulation of quantum phase in response to a signal is the central
mechanism of quantum sensing, as such, loss of phase information presents a
fundamental limitation. For this reason approaches to extend quantum coherence
in the presence of noise are actively being explored. Here we experimentally
protect a room-temperature hybrid spin register against environmental
decoherence by performing repeated quantum error correction whilst maintaining
sensitivity to signal fields. We use a long-lived nuclear spin to correct
multiple phase errors on a sensitive electron spin in diamond and realize
magnetic field sensing beyond the timescales set by natural decoherence. The
universal extension of sensing time, robust to noise at any frequency,
demonstrates the definitive advantage entangled multi-qubit systems provide for
quantum sensing and offers an important complement to quantum control
techniques. In particular, our work opens the door for detecting minute signals
in the presence of high frequency noise, where standard protocols reach their
limits.
