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Fast, High-Fidelity Transmon Readout with Intrinsic Purcell Protection via Nonperturbative Cross-Kerr Coupling

Guillaume Beaulieu1,2, Jun-Zhe Chen1,2, Marco Scigliuzzo2,3,5, Othmane Benhayoune-Khadraoui4, Alex A. Chapple4, Peter A. Spring6, Alexandre Blais4,7, and Pasquale Scarlino1,2,*

  • *Contact author: pasquale.scarlino@epfl.ch
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PRX Quantum 7, 033017 – Published 27 July, 2026

DOI: https://doi.org/10.1103/m348-gy75

Abstract

Dispersive readout of superconducting qubits relies on a transverse capacitive coupling that hybridizes the qubit with the readout resonator, subjecting the qubit to Purcell decay and measurement-induced state transitions (MIST). Despite the widespread use of Purcell filters to suppress qubit decay and near-quantum-limited amplifiers, dispersive readout often lags behind single- and two-qubit gates in both speed and fidelity. Here, we experimentally demonstrate junction readout, a simple readout architecture that realizes a strong qubit-resonator cross-Kerr interaction without relying on a transverse coupling. This interaction is achieved by coupling a transmon qubit to its readout resonator through both a capacitance and a Josephson junction. By varying the qubit frequency, we show that this hybrid coupling provides intrinsic Purcell protection and enhanced resilience to MIST, enabling readout at high photon numbers. While junction readout is compatible with conventional linear measurement, in this work we exploit the nonlinear coupling to intentionally engineer a large Kerr nonlinearity in the resonator, enabling bifurcation-based readout. Using this approach, we achieve a 99.4% assignment fidelity with a 68 ns integration time and a 98.4% quantum non-demolition (QND) fidelity without an external Purcell filter or a near-quantum-limited amplifier. These results establish the junction readout architecture with bifurcation-based readout as a scalable and practical alternative to dispersive readout, enabling fast, high-fidelity qubit measurement with reduced hardware overhead.

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Accurate qubit measurement remains a major challenge for superconducting quantum computers. Conventional readout relies on capacitively coupling a qubit to a resonator. While this interaction enables the qubit state to be inferred from the resonator response, it also opens unwanted pathways that can cause the qubit to lose energy or undergo transitions during measurement. We experimentally show that adding a single Josephson junction between the qubit and its resonator alongside the conventional coupling suppresses these detrimental effects through destructive interference between the different coupling channels. At the same time, the desired measurement signal remains strong, allowing for fast and accurate readout. Using this approach, we achieve 99.4% readout fidelity in just 68 ns without an external Purcell filter or near-quantum limited amplifier. By simultaneously reducing measurement errors and hardware overhead, this architecture offers a promising and scalable alternative to conventional readout, requiring only minimal modifications.

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PRX Quantum 7, 033017– Published 27 July, 2026
  • Received 9 February 2026
  • Revised 19 May 2026
  • Accepted 28 May 2026
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