What is squeezed light?

The word ‘squeezed’ describes a property of the light’s quantum noise. A squeezed-light laser beam shows less quantum noise than a conventional laser beam, even less quantum noise than a ‘laser beam’ having no photons at all! That sounds surprising, doesn’t it? You should know, that according to Heisenberg’s Uncertainty Relation (HUR) the electric field of a laser beam can’t be precisely zero everywhere – this is also true for laser beams without any photons. For zero photons, a light beam is in its quantum mechanical ground state. Here, the uncertainty of the field strength does not show any variation, neither with time nor along the light’s propagation axis. For squeezed states of light, or in short ‘squeezed light’, the situation is different. The strength of the uncertainty varies periodically around the mean ground state uncertainty. For example, squeezed light may show a low uncertainty whenever the electric field strength is maximal. (According to the HUR, then the uncertainty must be high in the nodes of the electric field). In this case, the light beam also shows a reduced (squeezed) fluctuation of its power, which is a useful property for optical measuring devices as well as for optical communication. ‘Squeezing’ of light is strongly linked to entanglement. Splitting squeezed light on beam splitter results in two output beams in an entangled state.


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