Quantum-limited measurement of spin qubits via curvature coupling to a cavity (and more)

There are still new physics to discover in circuit-QED-like systems. We investigate coupling an encoded spin qubit to a superconductor resonator via qubit energy level curvature versus gate voltage [1]. This approach enables quantum non-demolition readout with strength of tens to hundred MHz all while the qubit stays at its full sweet-spot to charge noise, with zero dipole moment. A "dispersive-like" spin readout approach similar to circuit-QED but avoiding the Purcell effect is proposed. With the addition of gate voltage modulation, selective longitudinal readout and n-qubit entanglement-by-measurement are possible. We consider the origin of the transverse, longitudinal, and curvature couplings in "lumped-element" spin qubit system to a superconducting cavity and make connections with superconducting qubits.

[1] R. Ruskov and C. Tahan, https://arxiv.org/abs/1704.05876