Quantum Properties of The Generated Light Beams
The OPO is the physical system most widely used to generate squeezed coherent states and entangled states of light in the continuous variables regime. Many demonstrations of quantum information protocols for continuous variables were realized using OPOs.
The downconversion process really occurs in the single photon regime: each pump photon that is annihilated inside the cavity gives rise to a pair of photons in the signal and idler intracavity modes. This leads to a quantum correlation between the intensities of signal and idler fields, so that there is squeezing in the subtraction of intensities, which motivated the name "twin beams" for the downconverted fields. The highest squeezing level attained was 10.12 ± 0.15 dB.
It turns out that the phases of the twin beams are quantum correlated as well, leading to entanglement, theoretically predicted in 1988. Below threshold, entanglement was measured for the first time in 1992, and in 2005 above threshold.
Above threshold, the pump beam depletion makes it sensitive to the quantum phenomena happening inside the crystal. The first measurement of squeezing in the pump field after parametric interaction was done in 1997. It has been recently predicted that all three fields (pump, signal and idler) must be entangled.
Not only intensity and phase of the twin beams share quantum correlations, but also do their spatial modes. This feature could be used to enhance signal to noise ratio in image systems.
The OPO is being employed nowadays as a source of squeezed light tuned to atomic transitions, in order to study how the atoms interact with squeezed light.
It is also recently demonstrated that a degenerate OPO can be used as an all-optical quantum random number generator that does not require post processing.
Read more about this topic: Optical Parametric Oscillator
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