Novel Applications of Nonreciprocal Quantum Amplifiers

The implementation of quantum-limited amplifiers has been a necessity for the high-fidelity processing of signals involving few weak photons, commonly encountered in experiments involving superconducting microwave electronics. The need to protect sensitive quantum systems during these experiments demands the use of devices capable of directionally routing the noisy signals from these amplifiers away from the quantum system under observation, which is accomplished by breaking the symmetry of reciprocity. To reduce losses and improve fidelity, both characteristics may be realised in a single device: the nonreciprocal quantum amplifier. In this dissertation, we explore uses for nonreciprocal quantum amplifiers that go beyond these signal processing applications. These applications will depend on the quantum nature of the amplifier, and so cannot be realised using components operating in the classical regime. Two different utilisations are considered in this work. We first examine the ability of nonreciprocal amplifiers to entangle their output signals and find that not only does directional signal transmission not inhibit the formation of entangled states, but it simultaneously allows for the routing of unwanted noise away from these outputs. ... mehrThis allows for the generation of high-fidelity propagating entangled states in noisy channels. The second project is motivated by a desire to improve the efficiency of measuring the state of a superconducting transmon qubit. The measurement requires the use of a microwave resonator to convert information about the qubit state into a microwave photon signal, which must then be passed to a quantum-limited preamplifier. While it has been demonstrated that using a nonreciprocal preamplifier improves the measurement efficiency, it is still reduced due to the presence of extra wiring with the resonator. We therefore consider a case where the same device acts as both the measurement resonator and the nonreciprocal preamplifier. We demonstrate that the amplification allows for a robust and highly efficient qubit measurement, while the nonreciprocity concurrently prevents excess backaction on the qubit.