A b o u t  T i m e

Is it possible to use high frequency mechanical resonators for time measurement to achieve a level of precision that can approach the precision of atomic clocks?

Time is the most precisely measurable quantity and the most powerful metrological variable. Accurate time measurement has been demonstrated to be essential in uncovering a number of fundamental phenomena. Examples include testing of gravitational radiation by observing the shortening of year of the binary pulsar, defining and measuring optical frequencies and the coherence of the optical source.

“The precision of time measurement can be increased essentially without limit by increasing measurement duration and simply counting the increased number of cycles of some regularly-spaced events.” However, for this process to work, it is necessary to have coherence in the source for the entire duration of the measurement. Therefore, optical measurement using coherent laser sources are useful in defining frequency and time standards.

    From our perspective, what is interesting is the following. Using nanomechanical resonators at very high frequencies as the timing source can have fundamental benefits, and potentially, the precision can approach that of atomic clocks. More importantly, the question is, if by cooling the resonator close to the quantum mechanical ground state, we can exploit the quantum coherence of the source itself. Of course, the concept of using time metrology for describing time-invariance or quantum coherence can be fundamentally revolutionary.

    Conventional time measurement in a timing oscillator circuit is achieved by a single resonator in the linear regime to create the reference signal. Traditional sources of noise include the intrinsic dissipation (inverse quality factor) and thermal fluctuations in the resonator and flicker noise the active parts of the oscillator circuit. In spite of relatively high quality factor and thermal stability, it is still challenging to achieve low phase noise and high thermal stability using micromechanical resonators. In an innovative approach to time measurement, we produce multiple reference signals from either a single resonator or multiple coupled resonators. These reference signals are designed to have well-defined phase shifts with common intrinsic phase noise, therefore the final reference signal can have a net phase noise lower than the intrinsic phase noise of the individual resonator. This novel approach will enable realization of accurate time measurement with extremely low phase noise beyond the ultimate limit of a single linear resonator.