Quantum functionalities via feedback amplification

TL;DR

This paper introduces a quantum feedback amplification method to create various quantum functionalities like filters, oscillators, and amplifiers, enhancing quantum communication and measurement systems.

Contribution

It develops a quantum version of feedback amplification, enabling the construction of quantum differentiators, integrators, and active filters with practical applications.

Findings

Design of quantum differentiator and integrator

Implementation of a quantum active filter for gravitational-wave detection

Enhanced measurement and protection of superconducting qubits

Abstract

Feedback amplification is a key technique for synthesizing various important functionalities, especially in electronic circuits involving op-amps. This paper presents a quantum version of this methodology, where the general phase-preserving quantum amplifier and coherent (i.e., measurement-free) feedback are employed to construct various type of systems having useful functionalities: quantum versions of differentiator, integrator, self-oscillator, and active filters. The class of active filters includes the Butterworth filter, which can be used to enhance the capacity of an optical quantum communication channel, and the non-reciprocal amplifier, which enables measurement of a superconducting qubits system as well as protection of it by separating input from output fields. A particularly detailed investigation is performed on the active phase-cancelling filter for realizing a broadband gravitational-wave detector; that is, the feedback amplification method is used to construct an active filter that compensates the phase delay of the signal and eventually recovers the sensitivity in the high frequency regime.