Opt. Lett. 38, 2949–2952 (2013)

The measurement precision of parameters such as phase, beam deflection, pulse arrival time, Doppler shift and velocity are all fundamentally limited by the so-called Cramér–Rao bound (CRB) of information theory. Now, Gerardo Viza and co-workers from the University of Rochester, USA, and Weizmann Institute of Science, Israel, have developed a 'weak-values' technique for measuring ultraslow velocities. Their scheme combines standard interferometry with time-domain analysis to make it possible to measure velocities as slow as 400 fm s−1. A laser beam with a wavelength of 780 nm was modulated by an acoustic optical modulator to generate a non-Fourier-limited Gaussian pulse. The pulse was sent through a Michelson interferometer with a slowly moving mirror in one arm. The piezoresponse of the moving mirror was 27 pm mV−1. The arms of the interferometer were approximately 1 mm long to ensure long-term phase stability. Calibration was performed by recording the number of photons entering the interferometer using a photon-counting module. Instead of performing a direct spectral measurement, the velocity was obtained by measuring the induced time shift of the non-Fourier-limited pulses. The uncertainties in the experimental measurements of the velocity reached the CRB.