High-fidelity qubit readout with parametric protocols and on-chip amplification in a circuit-QED setup.

Recent advancements in parametric techniques have enabled in-situ tunability of the interaction between different resonant modes. A single parametric tone can be used to achieve strong dispersive shifts within a qubit-cavity system [1] or perform gate operations [2]. Using these techniques, we propose a new protocol for high fidelity qubit readout with in-situ on-chip amplification of the readout photons. To implement our protocol, we couple two nearly linear cavity modes (cavity and output modes) and a transmon qubit with a shared SQUID loop. With this system, we developed a pulsed measurement protocol consisting of a sequence of four key processes: (1) preparing the cavity mode with photons, (2) generating qubit-state dependent dispersive shifts to maximally separate the cavity states, (3) on-chip amplification of the cavity photons, and (4) swapping the cavity photons to the output resonator to release the photons to the following amplifiers. With this protocol, we can have complete control over the qubit-cavity dynamics allowing us to optimize qubit readout and possibly remove most limitations of conventional dispersive readout techniques.

[1] T. Noh, Z. Xiao, X. Y. Jin, K. Cicak, E. Doucet, J. Aumentado, L. C. G. Govia, L. Ranzani, A. Kamal, and R. W. Simmonds. Nat. Phys. 19, 1445–1451 (2023).

[2] Jin, X. Y., K. Cicak, Z. Parrott, S. Kotler, F. Lecocq, J. Teufel, J. Aumentado, E. Kapit, and R. W. Simmonds. “Versatile Parametric Coupling between Two Statically Decoupled Transmon Qubits.” arXiv, May 5, 2023.