Neil Corzo-Trejo, Alberto M. Marino and Paul D. Lett


Optical amplifiers are key devices for implementing continuous-variable quantum communication and quantum information protocols because they can be used as preamplifiers before detection or as a part of a quantum cloner or repeater.  It is important that they add as little noise as possible to the amplified signal. This can be characterized in terms of the noise figure (NF) of the amplifier, which is given by the ratio between the signal-to-noise ratio of the input and the signal-to-noise ratio of the output.  Ideally one would like to use an amplifier with a NF=1.

A phase-insensitive amplifier (PIA) is a linear amplifier whose gain does not depend on the signal phase. This type of amplifier, although it is the most used, always degrades the input signal-to-noise ratio. This problem can be avoided by the proper use of a phase-sensitive amplifier (PSA). A PSA can amplify or deamplify a signal depending on the phase of the input. In the ideal case, it does so without degrading the signal-to-noise ratio of the input.

We implement a PSA through a four-wave mixing process in rubidium vapor. We observe performance near the quantum limit (NF=1) for this type of amplifier over a range of experimental parameters, and compare the results with the ones expected for a PIA.

Additionally, we observe that the amplifier supports multiple spatial-modes (images) without a significant degradation of the signal-to-noise ratio. To confirm the multi-spatial-mode character we study the behavior of the phase-sensitive amplifier for different spatial patterns and different spatial frequencies.