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arXiv:2104.03511 · Physical Review Applied

Parametric-Resonance Entangling Gates with a Tunable Coupler

Eyob Alebachew Sete, Nicolas Didier, Angela Q. Chen, Shobhan Kulshreshtha, Riccardo Manenti, Stefano Poletto

Published Aug 27, 2021• 91 citations
arXivHTMLPDF

Abstract

High-fidelity parametric gates have been demonstrated with superconducting qubits via rf flux modulation of the qubit frequency. The modulation however leads to renormalization of the bare qubit-qubit coupling, thereby reducing the gate speed. Here, we realize a parametric-resonance gate, which is activated by bringing the average frequency of the modulated qubit in resonance with a static-frequency qubit while approximately retaining the bare qubit-qubit coupling. The activation of parametric-resonance gates does not depend on the frequency of modulation, allowing us to choose the modulation frequencies and avoid frequency collisions. Moreover, we show that this approach is compatible with tunable coupler architectures, which reduce always-on residual couplings. Using these techniques, we demonstrate iswap and controlled-$Z$ gates between two qubits coupled via a tunable coupler with average process fidelities as high as $99.3\mathrm{%}$ and $97.9\mathrm{%}$, respectively. The flexibility in activating parametric-resonance gates combined with a tunable coupler architecture provides a pathway for building large-scale quantum computers.

DOI
10.1103/physrevapplied.16.024050

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High-fidelity parametric gates have been demonstrated with superconducting qubits via rf flux modulation of the qubit frequency. The modulation however leads to renormalization of the bare qubit-qubit coupling, thereby reducing the gate speed. Here, we realize a parametric-resonance gate, which is activated by bringing the average frequency of the modulated qubit in resonance with a static-frequency qubit while approximately retaining the bare qubit-qubit coupling. The activation of parametric-resonance gates does not depend on the frequency of modulation, allowing us to choose the modulation frequencies and avoid frequency collisions. Moreover, we show that this approach is compatible with tunable coupler architectures, which reduce always-on residual couplings. Using these techniques, we demonstrate iswap and controlled-ZZZ gates between two qubits coupled via a tunable coupler with average process fidelities as high as 99.3\mathrm{%} and 97.9\mathrm{%}, respectively. The flexibility in activating parametric-resonance gates combined with a tunable coupler architecture provides a pathway for building large-scale quantum computers.
High-performance two-qubit gates have been reported with superconducting qubits coupled via a single-transmon coupler (STC). Most of them are implemented for qubits with a small detuning since reducing residual ZZ coupling for highly detuned qubits by an STC is challenging. In terms of the frequency crowding and crosstalk, however, highly detuned qubits are desirable. Here, we numerically demonstrate a high-performance parametric gate for highly detuned fixed-frequency qubits using a recently proposed tunable coupler called a double-transmon coupler (DTC). Applying an ac flux pulse, we can perform a maximally entangling universal gate (iSWAP) with an average fidelity over 99.99% and a short gate time of about 24 ns. This speed is comparable to resonance-based gates for slightly detuned tunable qubits. Moreover, using a dc flux pulse alternatively, we can achieve another kind of entangling gate called a CZ gate with an average fidelity over 99.99% and a gate time of about 18 ns. Given the flexibility and feasible settings, we can expect that the DTC will contribute towards realizing a high-performance quantum computer in the near future.
Future quantum information processors require tunable coupling architectures that can produce high-fidelity logical gates between two or more qubits. Parametric coupling is a powerful technique for generating tunable interactions between many qubits. Here, we present a highly flexible parametric coupling scheme with superconducting qubits that provides complete removal of residual Z Z coupling and the implementation of driven SWAP or SWAP-free controlled- Z ( C Z ) gates. Our fully integrated two-dimensional on-chip coupler design is only weakly flux tunable, cancels static linear coupling between the qubits, avoids internal coupler dynamics or excitations, and is extensible to multiqubit circuit-QED systems. Exploring gate fidelity versus gate duration allows us to maximize two-qubit gate fidelity, while providing insights into possible error sources for these gates. Randomized benchmarking over several hours reveals that the parametric SWAP C Z gate achieves an average fidelity of 99.44 ± 0.09 % in a gate duration of 70 ns and that a dispersively driven parametric SWAP-free C Z gate attains an average fidelity of 99.47 ± 0.07 % in only 30 ns. The fidelity remained above this value for over 8 h and peaked twice with a maximum of 99.67 ± 0.14 % . Overall, our parametric approach combines versatility, precision, speed, and high performance in one compact coupler design.
A central challenge in building a scalable quantum computer with superconducting qubits is the execution of high-fidelity two-qubit gates in the presence of many resonant elements. As more elements are added to the architecture, and as the multiplicity of their couplings grows, the design's frequency space becomes crowded, and performance suffers. The authors present a way to address this difficulty: selective activation of interactions between transmon qubits of fixed frequency and those of tunable frequency. This activation depends on both the amplitude and frequency of modulation, and using the amplitude as an additional condition for resonance alleviates frequency crowding.
Tunable couplers have enabled two-qubit gate fidelities in superconducting quantum processors to approach 99.9%99.9\%99.9%, yet simultaneously suppressing residual interactions and maintaining flexible qubit-frequency allocation remain central challenges for scaling. Here, we propose a double-resonator coupler (DRC) consisting of two resonators interconnected by a single Josephson junction and a capacitor. The hybridized resonator modes provide two mediated exchange paths whose interference controls the qubit-qubit interaction. The DRC enables complete cancellation of residual ZZZZZZ interaction for qubit-qubit detunings well outside the straddling regime, even in the absence of direct qubit-qubit coupling, thereby relaxing constraints on frequency allocation and qubit placement. Away from the idle point, the same circuit provides a strong ZZZZZZ interaction of approximately 70 MHz70\,\mathrm{MHz}70MHz, enabling a 20 ns20\,\mathrm{ns}20ns controlled-Z gate with simulated coherent infidelity below 10−510^{-5}10−5. These results establish the DRC as a flexible single-junction coupler architecture for high-fidelity superconducting quantum processors.
As systems for quantum computing keep growing in size and number of qubits, challenges in scaling the control capabilities are becoming increasingly relevant. Efficient schemes to simultaneously mediate coherent interactions between multiple quantum systems and to reduce decoherence errors can minimize the control overhead in next-generation quantum processors. Here, we present a superconducting qubit architecture based on tunable parametric interactions to perform two-qubit gates, reset, leakage recovery, and to read out the qubits. In this architecture, parametrically driven multielement couplers selectively couple qubits to resonators and neighboring qubits, according to the frequency of the drive. We consider a system with two qubits and one readout resonator interacting via a single coupling circuit and experimentally demonstrate a controlled- Z gate with a fidelity of 98.30 ± 0.23 % , a reset operation that unconditionally prepares the qubit ground state with a fidelity of 99.80 ± 0.02 % and a leakage recovery operation with a 98.5 ± 0.3 % success probability. Furthermore, we implement a parametric readout with a single-shot assignment fidelity of 88.0 ± 0.4 % . These operations are all realized using a single tunable coupler and a statically decoupled resonator, demonstrating the experimental feasibility of the proposed architecture and its potential for reducing the system complexity in scalable quantum processors.
Entangling operations are at the heart of all approaches to quantum information processing. Parametric gates, in particular, offer a versatile solution to strongly couple off-resonant superconducting qubits with suppressed parasitic crosstalk to spectator qubits due to frequency-selective activation. In this work, we demonstrate a parametric iSWAP gate between two transmon qubits using the recently developed double transmon coupler (DTC). The DTC supports robust internally-defined cancellation point (``off'' state) for static interactions, while simultaneously mediating a fast parametric coupling between data qubits that can be deployed for high-fidelity two-qubit operations. We use robust phase estimation to calibrate non-commuting error terms in the parametric iSWAP gate, and achieve a 99.827% gate fidelity in 40ns without any numerical optimization. The circuit architecture and calibration techniques developed here are extensible to other gate implementations and qubit modalities, paving the way towards resource-efficient quantum information processing.