Vacuum modes in Homodyne detection*

Shot noise in homodyne detection is generally attributed to vacuum fluctuations. Changing the boundary conditions of the electromagnetic field around a detector changes the vacuum modes, and should affect the noise in homodyne detection. We analyze this thought experiment using Glauber's photodetection theory. It is shown that perfect absorbers of the field would not respond to any change in vacuum modes. However, real detectors always scatter some of the light incident on them. We use a generalized detector model to incorporate this emission in the photodetection process, and show that such detectors will detect any change in vacuum modes.. The approach connects photodetection theory with atomic resonance fluorescence techniques, potentially allowing to build detectors sensitive to the quantum vacuum.