Longitudinal Coupling for Fast QND Measurement: Numerical Study

Dispersive qubit readout in circuit QED relies on transverse coupling to a microwave resonator. This type of coupling, however, leads to mixing between qubit and resonator eigenstates. In turn, this leads to Purcell decay and to a breakdown of the quantum non-demolition (QND) aspect of the measurement associated with the critical number photons. An alternative approach avoiding these two issues is longitudinal qubit-resonator coupling [1,2,3]. In this talk, we consider circuit realizations of this idea and perform realistic numerical simulations of the longitudinal measurement. We show that the measurement remains QND at higher power than can be expected under transverse coupling. Moreover, comparing transverse and longitudinal qubit readouts, we show that the latter leads to fast and high-fidelity QND measurements.

[1] Nicolas Didier, Jérôme Bourassa, and Alexandre Blais, Phys. Rev. Lett. 115, 203601, (2015)
[2] P.-M. Billangeon, J. S. Tsai, and Y. Nakamura, Phys. Rev. B 91, 094517, (2015)
[3] Susanne Richer and David DiVincenzo, Phys. Rev. B 93, 134501, (2016)