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Dispersive Qubit Readout with Intrinsic Resonator Reset

Markus Jerger, Felix Motzoi, Yuan Gao, Christian Dickel, Lukas F. Buchmann, F. A. Cárdenas-López, Andreas Bengtsson, Giovanna Tancredi, Christopher Warren, Jonas Bylander, David Peter Divincenzo, R. Barends, Pavel A. Bushev

Published Jul 8, 2026• 1 citation
Publisher

Abstract

Measurement and readout speed impose limits for logical qubit and qudit performance in quantum computing. Here, we experimentally demonstrate a universal analytical method for designing bandwidth-efficient dispersive readout pulses that intrinsically reset the resonator and extend it to multiple quantum states. The approach is based on universal analytical pulses and requires knowledge of the qubit and resonator parameters, but needs no direct optimization of the pulse shape, even when accounting for the nonlinearity of the system. When correcting for the Kerr nonlinearity, we find even more precise solutions to the speed-versus-photon-number trade-off, achieving 10 2 photon readout with reset to 10 − 2 within 3 κ − 1 duration and amplifier efficiency- and T 1 -limited assignment error around 0.6%. Results on qubits and qutrits validate the approach.

DOI
10.1103/z5x1-2mr9

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Measurement and readout speed impose limits for logical qubit and qudit performance in quantum computing. Here, we experimentally demonstrate a universal analytical method for designing bandwidth-efficient dispersive readout pulses that intrinsically reset the resonator and extend it to multiple quantum states. The approach is based on universal analytical pulses and requires knowledge of the qubit and resonator parameters, but needs no direct optimization of the pulse shape, even when accounting for the nonlinearity of the system. When correcting for the Kerr nonlinearity, we find even more precise solutions to the speed-versus-photon-number trade-off, achieving 10 2 photon readout with reset to 10 − 2 within 3 κ − 1 duration and amplifier efficiency- and T 1 -limited assignment error around 0.6%. Results on qubits and qutrits validate the approach.
A key challenge in quantum computing is speeding up measurement and initialization. Here, we experimentally demonstrate a dispersive measurement method for superconducting qubits that simultaneously measures the qubit and returns the readout resonator to its initial state. The approach is based on universal analytical pulses and requires knowledge of the qubit and resonator parameters, but needs no direct optimization of the pulse shape, even when accounting for the nonlinearity of the system. Moreover, the method generalizes to measuring an arbitrary number of modes and states. For the qubit readout, we can drive the resonator to ∼102\sim 10^2∼102 photons and back to ∼10−3\sim 10^{-3}∼10−3 photons in less than 3κ−13 κ^{-1}3κ−1, while still achieving a T1T_1T1​-limited assignment error below 1\%. We also present universal pulse shapes and experimental results for qutrit readout.
Rapid reset and readout of qubit states are essential for quantum error correction, yet accelerating these operations through stronger coupling to a dissipative environment inevitably increases qubit decay via the Purcell effect. Here we present a broadband Purcell filter that decouples the reset and readout paths, enabling both operations to be independently optimized without compromising qubit coherence. The filter employs two engineered notches - an intrinsic notch and a bandstop notch - to provide broadband Purcell protection, together with an additional reset stub that creates a reset mode below the protected band. To enable fast reset while suppressing filter-mediated interactions between qubits, we couple each qubit to a dedicated reset resonator. We experimentally demonstrate Purcell-limited relaxation times exceeding 1 ms across a 1.2 GHz bandwidth, simultaneously with 500 ns readout without a Josephson parametric amplifier and 100 ns reset with 99.6% efficiency. The reset resonator is designed with a deliberate kappa-chi mismatch, which suppresses photon-shot-noise-induced dephasing by a factor of 70 compared to the readout resonator. Our work provides a scalable hardware solution that resolves the traditional trade-off between fast qubit operations and qubit protection, advancing the prospects for fault-tolerant quantum computing.
A key challenge in quantum computing is speeding up measurement and initialization. Here, we experimentally demonstrate a dispersive measurement method for superconducting qubits that simultaneously measures the qubit and returns the readout resonator to its initial state. The approach is based on universal analytical pulses and requires knowledge of the qubit and resonator parameters, but needs no direct optimization of the pulse shape, even when accounting for the nonlinearity of the system. Moreover, the method generalizes to measuring an arbitrary number of modes and states. For the qubit readout, we can drive the resonator to ∼102\sim 10^2∼102 photons and back to ∼10−3\sim 10^{-3}∼10−3 photons in less than 3κ−13 κ^{-1}3κ−1, while still achieving a T1T_1T1​-limited assignment error below 1\%. We also present universal pulse shapes and experimental results for qutrit readout.
Efficient qubit reset and leakage reduction are essential for scalable superconducting quantum computing, particularly in the context of quantum error correction. However, such operations often require additional on-chip components. Here, we propose and experimentally demonstrate a hardware-efficient approach to qubit reset and readout using a multi-mode Purcell filter in a superconducting quantum circuit. We exploit the inherent multi-mode structure of a coplanar waveguide resonator, using its fundamental and second-order modes for qubit reset and readout, respectively, thereby avoiding additional components. Implemented in a flip-chip architecture, our device achieves unconditional reset with residual excitation below 1\% in 220 ns, and a leakage reduction unit that selectively resets the second excited state within 62 ns with a residual ∣f⟩|f\rangle∣f⟩ population of 6.1\%, accounting for the readout error. Despite the qubits being directly coupled to the filter in our configuration, the measured relaxation times are not degraded owing to intrinsic Purcell protection provided by an auxiliary mode. To our knowledge, this is the first experimental trial that exploits different-order modes of a microwave resonator for distinct qubit operations, representing a new direction toward scalable, hardware-efficient quantum processor design.
Designing quantum systems with the measurement speed and accuracy needed for quantum error correction using superconducting qubits requires iterative design and testing informed by accurate models and characterization tools. We introduce a single protocol, with few prerequisite calibrations, to measure the dispersive shift, resonator linewidth, and drive power used in the dispersive readout of superconducting qubits. We find that the resonator linewidth is poorly controlled, with a factor of 2 variation between the maximum and minimum measured values, and this variation is likely to require focused attention in future quantum error correction experiments. We also introduce a protocol for measuring the efficiency of the readout system using the same power levels as are used in typical qubit readout, and without the need to measure the qubit dephasing. We routinely run these protocols on chips with tens of qubits to a hundred qubits, driven by automation software with little human interaction. Using the extracted system parameters, we find that a model based on these parameters predicts the readout signal-to-noise ratio to within 10% over a device with 54 qubits.
The ability to perform rapid, high fidelity readout of a qubit state is an important requirement for quantum algorithms and, in particular, for enabling operations such as mid-circuit measurements and measurement-based feedback for error correction schemes on large quantum processors. The growing interest in fluxonium qubits, due to their long coherence times and high anharmonicity, merits further attention to reducing the readout duration and measurement errors. We find that this can be accomplished by exploiting the flux tunability of fluxonium qubits. In this work, we experimentally demonstrate flux-pulse-assisted readout, as proposed in Phys. Rev. Applied 22, 014079 (https://doi.org/10.1103/PhysRevApplied.22.014079), in a setup without a quantum-limited parametric amplifier. Increasing the dispersive shift magnitude by almost 20% through flux pulsing, we achieve an assignment fidelity of 94.3% with an integration time of 280 ns. The readout performance is limited by state initialization, but we find that the limit imposed only by the signal-to-noise ratio corresponds to an assignment fidelity of 99.9% with a 360 ns integration time. We also verify these results through simple semi-classical simulations. These results constitute the fastest reported readout of a fluxonium qubit, with the prospect of further improvement by incorporation of a parametric amplifier in the readout chain to enhance measurement efficiency.