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arXiv:2507.03642

High-power readout of a transmon qubit using a nonlinear coupling

Mori, Cyril, Vladimir Milchakov, Francesca d'Esposito, L. M. Ruela, Shelender Kumar, Suresh, Vishnu Narayanan, Waël Ardati, Dorian Nicolas, Giulio Cappelli, Arpit Ranadive, Gwenael Le Gal, Martina Esposito, Quentin Ficheux, Nicolas Roch, Ramos, Tomás, O. Buisson

Published Jul 4, 2025• 0 citations
arXivHTMLPDF

Abstract

The field of superconducting qubits is constantly evolving with new circuit designs. However, when it comes to qubit readout, the use of simple transverse linear coupling remains overwhelmingly prevalent. This standard readout scheme has significant drawbacks: in addition to the Purcell effect, it suffers from a limitation on the maximal number of photons in the readout mode, which restricts the signal-to-noise ratio (SNR) and the Quantum Non-Demolition (QND) nature of the readout. Here, we explore the high-power regime by engineering a nonlinear coupling between a transmon qubit and its readout mode. Our approach builds upon previous work by Dassonneville et al. [Physical Review X 10, 011045 (2020)], on qubit readout with a non-perturbative cross-Kerr coupling in a transmon molecule. We demonstrate a readout fidelity of 99.21% with 89 photons utilizing a parametric amplifier. At this elevated photon number, the QND nature remains high at 96.7%. Even with up to 300 photons, the QNDness is only reduced by a few percent. This is qualitatively explained by deriving a critical number of photons associated with the nonlinear coupling, yielding a theoretical value of $\bar{n}_r^\text{crit} = 377$ photons for our sample's parameters. These results highlight the promising performance of the transmon molecule in the high-power regime, establishing it as a compelling platform for high-fidelity qubit readout.

DOI
10.48550/arxiv.2507.03642

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The field of superconducting qubits is constantly evolving with new circuit designs. However, when it comes to qubit readout, the use of simple transverse linear coupling remains overwhelmingly prevalent. This standard readout scheme has significant drawbacks: in addition to the Purcell effect, it suffers from a limitation on the maximal number of photons in the readout mode, which restricts the signal-to-noise ratio (SNR) and the Quantum Non-Demolition (QND) nature of the readout. Here, we explore the high-power regime by engineering a nonlinear coupling between a transmon qubit and its readout mode. Our approach builds upon previous work by Dassonneville et al. [Physical Review X 10, 011045 (2020)], on qubit readout with a non-perturbative cross-Kerr coupling in a transmon molecule. We demonstrate a readout fidelity of 99.21% with 89 photons utilizing a parametric amplifier. At this elevated photon number, the QND nature remains high at 96.7%. Even with up to 300 photons, the QNDness is only reduced by a few percent. This is qualitatively explained by deriving a critical number of photons associated with the nonlinear coupling, yielding a theoretical value of nˉrcrit=377\bar{n}_r^\text{crit} = 377nˉrcrit​=377 photons for our sample's parameters. These results highlight the promising performance of the transmon molecule in the high-power regime, establishing it as a compelling platform for high-fidelity qubit readout.
To carry out quantum computations, it is necessary to construct a circuit containing several interconnected qubits and be able to readout its states with high fidelity. The transverse dispersive coupling between a qubit and a microwave cavity is the most common technique of qubit readout in cQED. However, despite important progresses, implementing a fast high fidelity readout remains a major challenge. Indeed, inferring the qubit state is limited by the trade-off between speed and accuracy due to Purcell effect and unwanted transitions induced by readout photons in the cavity.This work focuses on an experimental investigation of cross-Kerr coupling and its benefits for qubit state readout, providing high fidelity and maximizing QNDness. To build a new readout scheme based on direct cross-Kerr coupling we employed a qubit architecture called "transmon molecule".The direct cross-Kerr coupling is a key point to our readout scheme since it prevents the qubit from Purcell effect.In order to realise the full potential of this system, thorough measurements were taken of an existing system, and then an improved version was designed and fabricated using a new nano fabrication recipe.To improve the qubit and readout performance, we have optimized different parameters: the circuit geometry has been adjusted to reduce the amplitude of the microwave electrical field at the interfaces; the electric circuit parameters were optimized to reach large detuning between the readout and the transmon frequencies; unwanted dipolar coupling between the transmon qubit and the cavity was reduced to avoid the Purcell effect.The fabrication recipe was also investigated and improved to get rid of residuals of resist on the surface of the substrate so-called "Veil of Death", improve the quality of aluminum and reproducibility of Josephson junctions.The measurements demonstrating the quantum non-demolition readout with fidelity as high as 99.4% and relaxation time 5 times higher than before - 20 microseconds.
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Strong nonlinear coupling of superconducting qubits and/or photons is a critical building block for quantum information processing. Because of the perturbative nature of the Josephson nonlinearity, linear coupling is often used in the dispersive regime to approximate nonlinear coupling. However, this dispersive coupling is weak and the underlying linear coupling mixes the local modes, which, for example, distributes unwanted self-Kerr nonlinearity to photon modes. Here, we use the quarton to yield purely nonlinear coupling between two linearly decoupled transmon qubits. The quarton's zero ϕ^{2} potential enables an ultrastrong gigahertz-level cross-Kerr coupling, which is an order of magnitude stronger compared to existing schemes, and the quarton's positive ϕ^{4} potential can cancel the negative self-Kerr nonlinearity of qubits to linearize them into resonators. This ultrastrong cross-Kerr coupling between bare modes of qubit-qubit, qubit-photon, and even photon-photon is ideal for applications such as single microwave photon detection, ultrafast two-qubit gates, and readout.
Single-shot readout of a quantum state is an essential component in building a large-scale quantum processor, and reducing the readout error is the key challenge. To achieve a high signal-to-noise ratio, quantum-limited low-noise amplifiers such as Josephson parametric amplifiers are typically used. However, a high-fidelity single-shot readout method without a parametric amplifier would be advantageous, as it would reduce system complexity. In this paper, we present a high-fidelity result of frequency-multiplexed, high-power single-shot readout for superconducting qubitswithoutusing a parametric amplifier. We achieved a high readout fidelity of 98.3% with a 400 ns pulse and confirmed that the high-power readout can be used in tomography of an entangled state. We have demonstrated the potential usefulness of this technique in characterizing the final quantum state at the end of quantum operations in a superconducting quantum processor.
Fast, precise, and quantum-non-demolition (QND) readout of superconducting qubits is a fundamental component of high-fidelity quantum sensing and computation. Conventional approaches typically operate in the dispersive regime, where the qubit-resonator coupling ggg is weak compared to the detuning ΔΔΔ. While exhibiting good QND properties, the readout rate is limited to ∼g2N/Δ≪g\sim g^2\sqrt{N}/Δ\ll g∼g2N​/Δ≪g, where NNN is the number of photons in the resonator. QND readout in the nondispersive regime, where the readout rate reaches its full potential ∼g\sim g∼g, relies on parameter sweeps that may encounter resonances, leading to measurement-induced state transitions (MIST). In this work, we study a nondispersive readout protocol that replaces these sweeps by sudden quenches of the coupling constant, using a resonator that is preloaded with photons. We call this protocol bang-bang readout, and show that it realizes single-shot projective measurements. The fidelity and QNDness of the qubit post-measurement are remarkably high, with an error that decreases like 1/N1/N1/N. We show that the protocol can also be implemented without preloading the resonator by instead strongly driving the qubit, e.g., with a classical flux drive.