Quantum sensing beyond the standard quantum limit with 2D arrays of trapped ions

Quantum sensing protocols using trapped-ions can enable detection of weak electric fields (<1 nV/m) by sensing displacements surpassing the Standard Quantum Limit (SQL) – the sensitivity achievable with a coherent state. We present experiments and theory investigating the limits of electric field sensing via the excitation of the center-of-mass (COM) motion of 100s of ions in a 2D crystal. By employing spin-dependent optical dipole forces to couple mechanical motion of the ions to their spin states, the displacement of the ion crystal can be sensitively read out through measurements of the spin state [1].

Recent experiments conducted far off-resonance from the COM mode imply a measurement imprecision, free from thermal and frequency noise, 40x below the zero point fluctuations of the COM ground state. Probing on-resonance with the COM mode provides the maximum sensitivity to electric fields, but thermal and frequency noise limits the detectable displacement. Currently displacements about 7dB below the SQL are detected in a single measurement, limited by 50Hz frequency fluctuations of the COM mode. With future improvements electric field sensitivities of about 1 nV/m may be possible, which may enable searches for dark matter.

[1] K. Gilmore et al. PRL 118, 263602 (2017).