Quantum amplifier
In physics, a quantum amplifier is an amplifier that uses quantum mechanical methods to amplify a quantum signal; examples include the active elements of optical lasers and optical re-transmitters.
The main properties of the quantum amplifier are its amplification coefficient and uncertainty. These parameters are not independent; the higher the amplification coefficient, the higher the noise. In the case of lasers, the uncertainty corresponds to the amplified spontaneous emission of the active medium. The unavoidable noise of quantum amplifiers is one of the reasons for the use of digital signals in optical communications and can be deduced from the fundamentals of quantum mechanics.
An amplifier increases the amplitude of whatever goes through it. While classical amplifiers take in classical signals, quantum amplifiers take in quantum signals, such as coherent states. This does not necessarily mean that the output is a coherent state; indeed, typically it is not. The form of the output depends on the specific amplifier design. Besides amplifying the intensity of the input, quantum amplifiers can also increase the quantum noise present in the signal.
The physical electric field in a paraxial single-mode pulse can be approximated with superposition of modes; the electric field of a single mode can be described as
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