Transmon-qubit readout using an in situ bifurcation amplification in the mesoscopic regime

• Published in: Physical review applied 2023-10, Vol.20 (4), Article 044050
• Main Authors: Dassonneville, R., Ramos, T., Milchakov, V., Mori, C., Planat, L., Foroughi, F., Naud, C., Hasch-Guichard, W., García-Ripoll, J.J., Roch, N., Buisson, O.
• Format: Article
• Language: English
• Subjects: Physics; Quantum Physics
• ISSN: 2331-7019; 2331-7019
• DOI: 10.1103/PhysRevApplied.20.044050

We demonstrate a transmon-qubit readout based on the nonlinear response to a drive of polaritonic meters in situ coupled to the qubit. Inside a three-dimensional readout cavity, we place a transmon molecule consisting of a transmon qubit and an ancilla mode interacting via nonperturbative cross-Kerr-coupling. The cavity couples strongly only to the ancilla mode, leading to hybridized lower and upper polaritonic meters. Both polaritons are anharmonic and dissipative, as they inherit a self-Kerr nonlinearity U from the ancilla and effective decay κ from the open cavity. Via the ancilla, the polariton meters also inherit the nonperturbative cross-Kerr-coupling to the qubit. This results in a high qubit-dependent displacement 2χ>κ,U that can be read out via the cavity without causing Purcell decay. Moreover, the polariton meters, being nonlinear resonators, present bistability, and bifurcation behavior when the probing power increases. In this work, we focus on the bifurcation at low power in the few-photon regime, called the mesoscopic regime, which is accessible when the self-Kerr and decay rates of the polariton meter are similar, U∼κ. Capitalizing on a latching mechanism by bifurcation, the readout is sensitive to transmon-qubit relaxation error only in the first tens of nanoseconds. We thus report a single-shot fidelity of 98.6% while having an integration time of 500 ns and no requirement for an external quantum-limited amplifier.