Dispersive readout in circuit QED

Abstract: The speed and fidelity of dispersive readout of superconducting qubits should improve by increasing the amplitude of the measurement drive. Experiments show, however, that beyond some drive amplitude there is always a saturation or drop in fidelity, often associated with a decrease in qubit energy relaxation time T1. A simple Lindblad master equation does not capture the latter effect. More involved approaches based on effective master equations rely on strong assumptions about the spectra of the system and the bath and only partially agree with observations.  

In the first part of this talk, we will present a first-principles simulation of the full unitary dynamics of dispersive readout by considering the circuit QED Hamiltonian coupled to a microscopic model for the measurement transmission line, allowing for its arbitrary spectrum, including filters. This allows us to study the dependence of qubit energy relaxation time T1 on readout drive amplitude and its sensitivity to the details of the bath spectrum. [1]  

In the second part of the talk, I will present the theoretical analysis for a recently proposed quantum non-demolition readout scheme that promises to render the qubit immune to the effect of readout photons up to large drive amplitudes. We demonstrate, using arguments pertaining to the classical dynamics and a branch analysis, the robustness of the quantum non-demolition property of measurement as the photon number in the readout mode is increased. To further characterize this, by tuning in and out of the regime of pure dispersive interaction, we explore the possible measurement-induced state transitions (MIST). [2]  

[1] Angela Riva, Prakritish Gogoi, Nicolas Gheeraert, Serge Florens, Alex W. Chin, Alain Sarlette, and Alexandru Petrescu 2604.11722

[2] Cyril Mori, Francesca D Esposito, Alexandru Petrescu, Lucas Ruela, Shelender Kumar, Vishnu Narayanan Suresh, Wael Ardati, Dorian Nicolas, Giulio Cappelli, Arpit Ranadive, Gwenael Le Gal, Martina Esposito, Quentin Ficheux, Nicolas Roch, and Olivier Buisson 2509.05126, accepted in PRX Quantum.