In-loop squeezing is like real squeezing to an in-loop atom

Electro-optical feedback can produce an in-loop photocurrent with arbitrarily low noise. This is not regarded as evidence of "real" squeezing because squeezed light cannot be extracted from the loop using a linear beam splitter. Here I show that illuminating an atom (which is a nonlinear optical element) with "in-loop" squeezed light causes line narrowing of one quadrature of the atom's fluorescence. This has long been regarded as an effect that can only be produced by squeezing. Experiments on atoms using in-loop squeezing should be much easier than those with conventional sources of squeezed light. [S0031-9007(98)07512-7].