Spatial Modes of a Squeezed Vacuum Field

We prepared a quantum noise suppressed squeezed vacuum field by propagating a beam with a wavelength of 795nm through a hot Rb cell. Observation of the quadrature noise showed that we achieved a noise suppression of -2.0 dB below the quantum noise limit. When a spatial mask was applied to the beam after its interaction with atoms, we observed that the detected quantum noise suppression strongly depended on the shape of the mask. An exploration of the spatial distribution of noise in the squeezed field illustrated that the squeezed field was in a different spatial mode from the pump field used as a local oscillator. Our research showed that the squeezed field consisted of several spatial modes with various squeezing parameters. If a pure squeezed mode could be extracted, it would enhance the signal to noise ratio, which would impact precision metrology and quantum memory applications.