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arXiv · 2501.17439

The quantromon: A qubit-resonator system with orthogonal qubit and readout modes

Kishor V. Salunkhe·Suman Kundu·Srijita Das·Jay Deshmukh·Meghan P. Patankar·R. Vijay
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Abstract

The measurement of a superconducting qubit is implemented by coupling it to a resonator. The common choice is transverse coupling, which, in the dispersive approximation, introduces an interaction term which enables the measurement. This cross-Kerr term provides a qubit-state dependent dispersive shift in the resonator frequency with the device parameters chosen carefully to get sufficient signal while minimizing Purcell decay of the qubit. We introduce a two-mode circuit, nicknamed quantromon, with two orthogonal modes implementing a qubit and a resonator. Unlike before, where the coupling term emerges as a perturbative expansion, the quantromon has intrinsic cross-Kerr coupling by design. Our experiments implemented in a hybrid 2D-3D cQED architecture demonstrate some unique features of the quantromon like weak dependence of the dispersive shift on the qubit-resonator detuning and intrinsic Purcell protection. In a tunable qubit-frequency device, we show that the dispersive shift ($2\chi/2\pi$) changes by only $0.8$ MHz while the qubit-resonator detuning ($\Delta/2\pi$) is varied between $0.398$ GHz - $3.288$ GHz. We also demonstrate Purcell protection in a second device where we tune the orthogonality between the two modes. Finally, we demonstrate a single-shot readout fidelity of $98.3\%$ without using a parametric amplifier which is comparable to the state-of-the-art and suggests a potential simplification of the measurement circuitry for scaling up quantum processors.

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Kishor V. Salunkhe, Suman Kundu, Srijita Das, Jay Deshmukh, Meghan P. Patankar, R. Vijay. 2025-01-29. The quantromon: A qubit-resonator system with orthogonal qubit and readout modes. https://arxiv.org/abs/2501.17439

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