Phase estimation is one of the key problems in fundamental physics. The standard tool for phase estimation is interferometer. It requires coherent source, and the precision is dominated by the intrinsic quantum noise. To reduce the influence of the noise, quantum metrology technologies, including  entangled states and squeezed states, have been exploited, while white light is usually deemed to be useless in quantum metrology.

Now,LI Chuanfeng’s group from Key Lab of Quantum Information demonstrates in experiment that by making use of weak measurement, ultrasensitive phase estimation can be achieved with white light even from cheap commercial light-emitting diode (LED). The method is based on a measurement of the imaginary part of the weak value of a polarization operator. The imaginary part of the weak value appeared due to the measurement interaction itself. The sensitivity of this method is equivalent to resolving light pulses of the order of one attosecond and it is robust against chromatic dispersion.

Reference: Xiao-Ye Xu, Yaron Kedem, Kai Sun, Lev Vaidman, Chuan-Feng Li, and Guang-Can Guo, Phase estimation with weak measurement using a white light source, Physical Review Letters 111, 033604 (2013).