Squeezing more out of a laser

Machida and Yamamoto used negative feedback from a conventional photodetector to stabilize a laser's intensity and thus to obtain sub-shot-noise photon statistics within the feedback loop. Out-of-Ioop light extracted at a beam splitter, however, displayed super-shot-noise statistics. This behavior can be understood both quantum mechanically and semiclassically. Quantum mechanically the culprit is the vacuum field that enters the beam splitter’s second input port. I describe a technique for extracting sub-shot-noise light by replacing the culprit vacuum field with squeezed light generated by a nonlinear device—a squeezer—that is pumped by a portion of the laser light. Moreover, even if the unstabilized laser is shot-noise-limited, the extracted light can have a better intensity SNR; this signal-to-noise improvement requires both feedback and squeezing.

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