We show how to realize high-fidelity quantum non-demolition qubit readout using longitudinal qubit-oscillator interaction. This is realized by modulating the longitudinal coupling atthe cavity frequency. The qubit-oscillator interaction then acts as a qubit-state dependent drive on the cavity, a situation that is fundamentally different from the standard dispersive case. Single-mode squeezing can be exploited to exponentially increase the signal-to-noise ratio of this readout protocol. We present an implementation of this idea in circuit quantum electrodynamics and a possible multi-qubit architecture.
Motivated by recent experimental progress to measure and manipulate Majorana fermions with superconducting circuits, we propose a device interfacing Majorana fermions with circuit quantumelectrodynamics. The proposed circuit acts as a charge parity detector changing the resonance frequency of a superconducting \lambda/4 – resonator conditioned on the parity of charges on nearby gates. Operating at both charge and flux sweet-spots, this device is highly insensitive to environmental noise and enables high-fidelity single-shot quantum non-demolition readout of the state of a pair of Majorana fermions. Additionally, the interaction permits the realization of an arbitrary phase gate on the topological qubit, closing the loop for computational completeness. Away from the charge sweet-spot, this device can be used as a highly sensitive charge detector with a sensitivity smaller than 10^{-4} e / \sqrt{Hz} and bandwidth larger than 1 MHz.