Tunable Inductive Bridge for readout while reducing resonator thermal photons.

Performing sensitive, single-photon-resolution readout of superconducting circuits is a difficult task, balancing the tradeoffs of using amplifiers without increasing thermal photons that could dephase the qubit. Large, magnetic isolators are often used, but this poses a challenge for scaling to large superconducting circuit systems. Active components like the Tunable Inductive Bridge (TIB) [1, 2]  could potentially serve in this role, improving readout without degrading coherence. 

Here, we explore the TIB as a rf-pulsed switch, allowing transmission of photons during readout only, to enhance a 2D superconducting qubit’s readout. We characterize this device using injected thermal noise, observing suppression of  >20dB and reducing the residual resonator photon population, as measured by the qubit’s phase coherence. Looking ahead, it would be desirable to integrate the TIB as an on-chip element, coordinating with a non-reciprocal amplifier as a path towards scalable, high-fidelity quantum readout. 

[1] Chapman, Benjamin J. et al. “General Purpose Multiplexing Device for Cryogenic Microwave Systems.” Appl. Phys. Lett. 108 222602 (2016) 

[2] Zhao, Z. et al. “Integrating planar circuits with superconducting 3D microwave cavities using tunable low-loss couplers.” Appl. Phys. Lett. 123 014001 (2023)