Phys. Rev. Lett. 109, 053602 (2012)

A pulse of light can contain many different quantum states that take on arbitrarily complex shapes in space and time. Constantina Polycarpou and co-workers from Italy, Germany and Brazil have now demonstrated an adaptive technique for measuring ultrashort single-photon pulses with little or no prior information about the shape of the light. In their scheme, measurement of a quantum state of light is achieved when the spatial, temporal and spectral properties of the light are completely matched to that of a reference beam, termed the local oscillator. Perfect overlap of these properties means perfect detection, and the degree of overlap is used as the optimization parameter in a genetic algorithm that mimics biological evolution. The local oscillator is shaped by two spatial light modulators that alter the overlap, and an adaptive correction is sent accordingly to the modulators to ensure that they converge to an optimal shape. The researchers verified their technique by creating complex single-photon pulse shapes and measuring the optimized detection efficiency. The most challenging state created was a single photon in a coherent superposition of two states that are separated in frequency space. The optimized detection efficiency was about 45%. Although only the frequency components were manipulated here, the authors say that their technique may be effectively applied to measure any quantum state, including bright multi-photon states, as long as a suitable optimization parameter is available.