Quantum Optics: Squeezed light generation

[Korak Biswas]

Theoretically, squeezed state can be generated using a process called 'parametric down conversion'. I was going through the literature and found that people performed 'second harmonic generation' before doing 'parametric down conversion'. Can anyone tell me why simple harmonic generation is important for the generation of squeezed states?

 

[Cthugha]

That is a matter of the experiments you want to perform. The best way to characterize a state is quantum state tomography via homodyne detection. For this kind of experiment you need a phase reference with a stable reference phase to the signal and at the same energy. After SPDC, the energy of the signal is (for degenerate SPDC) half of that of the pump beam. So if you use second harmonic generation first, you get a beam at twice the energy of the pump beam. After SPDC, the signal energy is again equal to that of the initial pump beam and this initial beam can be used as a reference beam.