Phys. Rev. Lett. 107, 133603 (2011)

How do you amplify a single photon? By making it act like many photons. Amir Feizpour and colleagues have now shown how this might be possible using so-called weak measurements.

Boosting a conventional signal to improve the ratio of signal to noise is a crucial element of any computation or communication system. In photonic systems, this is often achieved using a material with nonlinear optical properties. But at the single-photon level, these effects are tiny. Feizpour et al. now propose that 'weakly' probing a photon in the nonlinear medium with a second optical beam can amplify the effect.

Weak measurements can interrogate a quantum state without disturbing it. It requires a process of post-selection in which only photons with a specific property are retained. The authors show that the improvement in the signal-to-noise ratio using their technique outweighs the loss of photons during post-selection. This idea could have a role in the development of optical quantum logic gates.