Quantum Op-amp and Functionalities

Quantum amplifier is an essential component in quantum information technology. In [Phys. Rev. Applied, 5, 044012, 2016], I presented the control scheme for improving the sensitivity of a general phase-insensitive amplifier via coherent (i.e., measurement-free) feedback. In this presentation I show that this feedback amplification scheme can be further extended and applied to construct systems that produce several useful functionalities; this is exactly a quantum version of op-amp in electrical circuits. For example in quantum optics, by feedback-connecting a high-gain non-degenerate parametric amplifier with an optical cavity, one can devise an integrator for itinerant fields, which can be used to improve the detection efficiency of those fields. Also I show that a similar feedback amplification scheme yields a quantum version of active filters such as a Butterworth filter; thanks to the steep roll-off characteristic in frequency, such an active filter enhances the capacity of a quantum communication channel. Another example is that a feedback connection of some high-gain amplifiers produces a robust nonreciprocal amplifier, which enables measurement of the state of a superconducting system while protecting it from the back-action noise of the amplifier.