Realizing fault-tolerant quantum computing relies on efficient quantum error correction. Bosonic cat-state qubits offer a highly promising, hardware-efficient approach by intrinsically suppressing bit-flip errors. However, fully unlocking their potential demands fast and high-fidelity quantum nondemolition (QND) measurements. Here, we propose a high-fidelity QND readout protocol for cat-state qubits in a Kerr-nonlinear resonator utilizing an effective longitudinal interaction. By deriving analytical expressions and performing numerical simulations, we demonstrate that longitudinal readout achieves faster signal-to-noise ratio (SNR) growth and shorter measurement times than conventional dispersive readout. Furthermore, we show that injecting squeezed input states can exponentially enhance the SNR, while time-dependent coupling further reduces the measurement time. This scheme provides a robust pathway toward rapid and nondestructive measurement of cat qubits in near-term quantum processors. Reliable quantum computers require error-resistant components like cat-state qubits, along with fast, non-destructive ways to readout them. Here, the authors propose a longitudinal readout protocol for these qubits that achieves faster and higher-fidelity measurements than conventional methods.
Realizing fault-tolerant quantum computing relies on efficient quantum error correction. Bosonic cat-state qubits offer a highly promising, hardware-efficient approach by intrinsically suppressing bit-flip errors. However, fully unlocking their potential demands fast and high-fidelity quantum nondemolition (QND) measurements. Here, we propose a high-fidelity QND readout protocol for cat-state qubits in a Kerr-nonlinear resonator utilizing an effective longitudinal interaction. By deriving analytical expressions and performing numerical simulations, we demonstrate that longitudinal readout achieves faster signal-to-noise ratio (SNR) growth and shorter measurement times than conventional dispersive readout. Furthermore, we show that injecting squeezed input states can exponentially enhance the SNR, while time-dependent coupling further reduces the measurement time. This scheme provides a robust pathway toward rapid and nondestructive measurement of cat qubits in near-term quantum processors. Reliable quantum computers require error-resistant components like cat-state qubits, along with fast, non-destructive ways to readout them. Here, the authors propose a longitudinal readout protocol for these qubits that achieves faster and higher-fidelity measurements than conventional methods.
We show how to realize fast and high-fidelity quantum nondemolition qubit readout using longitudinal qubit-oscillator interaction. This is accomplished by modulating the longitudinal coupling at the cavity frequency. The qubit-oscillator interaction then acts as a qubit-state dependent drive on the cavity, a situation that is fundamentally different from the standard dispersive case. Single-mode squeezing can be exploited to exponentially increase the signal-to-noise ratio of this readout protocol. We present an implementation of this longitudinal parametric readout in circuit quantum electrodynamics and a possible multiqubit architecture.
Fast and high-fidelity qubit measurement is crucial for achieving quantum error correction, a fundamental element in the development of universal quantum computing. For electron spin qubits, fast readout stands out as a major obstacle in the pursuit of error correction. In this work, we explore the dispersive measurement of an individual spin in a semiconductor double quantum dot coupled to a nonlinear microwave resonator. By utilizing displaced squeezed vacuum states, we achieve rapid and high-fidelity readout for semiconductor spin qubits. Our findings reveal that introducing modest squeezing and mild nonlinearity can significantly improve both the signal-to-noise ratio (SNR) and the fidelity of qubit-state readout. By properly marching the phases of squeezing, the nonlinear strength, and the local oscillator, the optimal readout time can be reduced to the sub-microsecond range. With current technology parameters (κ≈2χs, $χ_s\approx 2π\times 0.15 \:\mbox{MHz}$), utilizing a displaced squeezed vacuum state with 30 photons and a modest squeezing parameter r≈0.6, along with a nonlinear microwave resonator charactered by a strength of λ≈−1.2χs, a readout fidelity of 98% can be attained within a readout time of around $0.6\:μ\mbox{s}$. Intriguing, by using a positive nonlinear strength of λ≈1.2χs, it is possible to achieve an SNR of approximately 6 and a readout fidelity of 99.99% at a slightly later time, around $0.9\:μ\mbox{s}$, while maintaining all other parameters at the same settings.
Abstract Fast and high-fidelity qubit measurement is crucial for achieving quantum error correction, a fundamental element in the development of universal quantum computing. For electron spin qubits, fast readout stands out as a major obstacle in the pursuit of error correction. In this work, we explore the dispersive measurement of an individual spin in a semiconductor double quantum dot coupled to a nonlinear microwave resonator. By utilizing displaced squeezed vacuum states, we achieve rapid and high-fidelity readout for semiconductor spin qubits. Our findings reveal that introducing modest squeezing and mild nonlinearity can significantly improve both the signal-to-noise ratio (SNR) and the fidelity of qubit-state readout. By properly marching the phases of squeezing, the nonlinear strength, and the local oscillator, the optimal readout time can be reduced to the sub-microsecond range. With current technology parameters (κ≈2χs, $\chi_s/(2\pi)\approx 0.15 \:\mbox{MHz}$), utilizing a displaced squeezed vacuum state with 30 photons and a modest squeezing parameter r≈0.6, along with a nonlinear microwave resonator charactered by a strength of λ≈−1.2χs, a readout fidelity of 98% can be attained within a readout time of around $0.6\:\mu\mbox{s}$. Intriguing, by using a positive nonlinear strength of λ≈1.2χs, it is possible to achieve an SNR of approximately 6 and a readout fidelity of 99.99% at a slightly later time, around $0.9\:\mu\mbox{s}$, while maintaining all other parameters at the same settings.
The Kerr-cat qubit is a bosonic qubit in which multiphoton Schrödinger cat states are stabilized by applying a two-photon drive to an oscillator with a Kerr nonlinearity. The suppressed bit-flip rate with increasing cat size makes this qubit a promising candidate to implement quantum error correction codes tailored for noise-biased qubits. However, achieving strong light-matter interactions necessary for stabilizing and controlling this qubit has traditionally required strong microwave drives that heat the qubit and degrade its performance. In contrast, increasing the coupling to the drive port removes the need for strong drives at the expense of large Purcell decay. By integrating an effective band-block filter on chip, we overcome this trade-off and realize a Kerr-cat qubit in a scalable 2D superconducting circuit with high coherence. This filter provides 30 dB of isolation at the qubit frequency with negligible attenuation at the frequencies required for stabilization and readout. We experimentally demonstrate quantum nondemolition readout fidelity of 99.6% for a cat with eight photons. Also, to have high-fidelity universal control over this qubit, we combine fast Rabi oscillations with a new demonstration of the X ( π / 2 ) gate through phase modulation of the stabilization drive. Finally, the lifetime in this architecture is examined as a function of the cat size of up to ten photons in the oscillator, achieving a bit-flip time higher than 1 ms and only a linear increase in the phase-flip rate, in good agreement with the theoretical analysis of the circuit. Our qubit shows promise as a building block for fault-tolerant quantum processors with a small footprint. Published by the American Physical Society 2024
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 % 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.
Quantum computing crucially relies on maintaining quantum coherence for the duration of a calculation. Bosonic quantum error correction protects this coherence by encoding qubits into superpositions of noise-resilient oscillator states. In the case of the Kerr-cat qubit (KCQ), these states derive their stability from being the quasi-degenerate ground states of an engineered Hamiltonian in a driven nonlinear oscillator. KCQs are experimentally compatible with on-chip architectures and high-fidelity operations, making them promising candidates for a scalable bosonic quantum processor. However, their bit-flip time must increase further to fully leverage these advantages. Here, we present direct evidence that the bit-flip time in a KCQ is limited by leakage out of the qubit manifold and experimentally mitigate this process. We coherently control the leakage population and measure it to be > 9%, twelve times higher than in the undriven system. We then cool this population back into the KCQ manifold with engineered dissipation, identify conditions under which this suppresses bit-flips, and demonstrate increased bit-flip times up to 3.6 milliseconds. By employing both Hamiltonian confinement and engineered dissipation, our experiment combines two paradigms for Schrödinger-cat qubit stabilization. Our results elucidate the interplay between these stabilization processes and indicate a path towards fully realizing the potential of these qubits for quantum error correction.