Novel Sensing Schemes for Frequency Tracking and Resolution

Precise time-keeping is critical to measurements of energy, distance, frequency and fundamental constants. In spectroscopy, time-keeping precision defines the spectral resolution. Ultimately, measurement accuracy is limited by the stability of the measuring clock. In quantum metrology, where the phase of a qubit is used to detect external fields, the qubit coherence time defines the clock stability, and therefore the measurement linewidth and precision. In this talk I will present a quantum sensing protocol for classical fields where the measurement linewidth goes beyond the sensor coherence time and is limited by the stability of a classical oscillator. Using this technique it is possible to observe a precision in frequency estimation which scales as T^{-3/2}. The high spectral resolution diamond magnetometer was applied to sensing of nanoscale magnetic fields with an intrinsic frequency resolution of 600µHz with single quantum coherent spins in diamond.