Phys. Rev. Lett. 108, 083601 (2012)

Optical frequency combs, perfect tools for studies in the field of high-precision metrology, could now benefit tasks such as quantum computation. Oliver Pinel and co-workers from France, China and Germany now claim to have realized a multimode non-classical frequency comb with a singly resonant synchronously pumped optical parametric oscillator (SPOPO). They sent 120 fs pulses from a Ti:sapphire mode-locked laser and its second harmonic (wavelength of 397 nm) into a cavity, whose length was locked using the Pound–Drever–Hall technique. They confirmed the non-classicality of the generated field by measuring a noise intensity below the standard quantum limit, and revealed the multimode nature of the comb by studying the distribution of quantum intensity fluctuations in the frequency comb over its optical spectrum. In particular, they conclude that two of the modes of the SPOPO are in a squeezed (quantum-enhanced) state with a normalized intensity noise of <1.