High-fidelity single- and two-qubit gates are essential building blocks for a fault-tolerant quantum computer. While there has been much progress in suppressing single-qubit gate errors in superconducting qubit systems, two-qubit gates still suffer from error rates that are orders of magnitude higher. One limiting factor is the residual ZZ-interaction, which originates from a coupling between computational states and higher-energy states. While this interaction is usually viewed as a nuisance, here we experimentally demonstrate that it can be exploited to produce a universal set of fast single- and two-qubit entangling gates in a coupled transmon qubit system. To implement arbitrary single-qubit rotations, we design a new protocol called the two-axis gate that is based on a three-part composite pulse. It rotates a single qubit independently of the state of the other qubit despite the strong ZZ-coupling. We achieve single-qubit gate fidelities as high as 99.1% from randomized benchmarking measurements. We then demonstrate both a CZ gate and a CNOT gate. Because the system has a strong ZZ-interaction, a CZ gate can be achieved by letting the system freely evolve for a gate time $t_g=53.8$ ns. To design the CNOT gate, we utilize an analytical microwave pulse shape based on the SWIPHT protocol for realizing fast, low-leakage gates. We obtain fidelities of 94.6% and 97.8% for the CNOT and CZ gates respectively from quantum progress tomography.High-fidelity single- and two-qubit gates are essential building blocks for a fault-tolerant quantum computer. While there has been much progress in suppressing single-qubit gate errors in superconducting qubit systems, two-qubit gates still suffer from error rates that are orders of magnitude higher. One limiting factor is the residual ZZ-interaction, which originates from a coupling between computational states and higher-energy states. While this interaction is usually viewed as a nuisance, here we experimentally demonstrate that it can be exploited to produce a universal set of fast single- and two-qubit entangling gates in a coupled transmon qubit system. To implement arbitrary single-qubit rotations, we design a new protocol called the two-axis gate that is based on a three-part composite pulse. It rotates a single qubit independently of the state of the other qubit despite the strong ZZ-coupling. We achieve single-qubit gate fidelities as high as 99.1% from randomized benchmarking measurements. We then demonstrate both a CZ gate and a CNOT gate. Because the system has a strong ZZ-interaction, a CZ gate can be achieved by letting the system freely evolve for a gate time $t_g=53.8$ ns. To design the CNOT gate, we utilize an analytical microwave pulse shape based on the SWIPHT protocol for realizing fast, low-leakage gates. We obtain fidelities of 94.6% and 97.8% for the CNOT and CZ gates respectively from quantum progress tomography.

arXiv:2103.12305Mar 23, 20217 citations

High-fidelity iSWAP gate with Double Transmon Coupler

Tarush Tiwari, Sudhir K. Sahu, Guilhem Ribeill +6 more

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.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.

quant-pharXiv:2604.27080May 1, 20260 citations

Transmon qubits coupled to compact resonators

Shyam Shankar, K. Geerlings, E.V. Edwards +3 more

Bulletin of the American Physical SocietyMar 23, 20110 citations

Stabilization of entanglement between remote transmon qubits

Felix Motzoi, Mohan Sarovar, K. Birgitta Whaley

Bulletin of the American Physical SocietyMar 4, 20140 citations

Control of the ZZ coupling between Kerr cat qubits via transmon couplers

Takaaki Aoki, Taro Kanao, Hayato Goto +2 more

Kerr cat qubits are a promising candidate for fault-tolerant quantum computers owing to the biased nature of their errors. The $ZZ$ coupling between the qubits can be utilized for a two-qubit entangling gate, but the residual coupling called $ZZ$ crosstalk is detrimental to precise computing. In order to resolve this problem, we propose a tunable $ZZ$ -coupling scheme using two transmon couplers. By setting the detunings of the two couplers at opposite values, the residual $ZZ$ couplings via the two couplers cancel each other out. We also apply our scheme to the ${R}_{ZZ}(\mathrm{\ensuremath{\Theta}})$ gate ( $ZZ$ rotation with angle $\mathrm{\ensuremath{\Theta}}$), one of the two-qubit entangling gates. We numerically show that the fidelity of the ${R}_{ZZ}(\ensuremath{-}\ensuremath{\pi}/2)$ gate is higher than 99.9% in a case of 16-ns gate time and without decoherence.Kerr cat qubits are a promising candidate for fault-tolerant quantum computers owing to the biased nature of their errors. The $ZZ$ coupling between the qubits can be utilized for a two-qubit entangling gate, but the residual coupling called $ZZ$ crosstalk is detrimental to precise computing. In order to resolve this problem, we propose a tunable $ZZ$-coupling scheme using two transmon couplers. By setting the detunings of the two couplers at opposite values, the residual $ZZ$ couplings via the two couplers cancel each other out. We also apply our scheme to the ${R}_{ZZ}(\mathrm{\ensuremath{\Theta}})$ gate ($ZZ$ rotation with angle $\mathrm{\ensuremath{\Theta}}$), one of the two-qubit entangling gates. We numerically show that the fidelity of the ${R}_{ZZ}(\ensuremath{-}\ensuremath{\pi}/2)$ gate is higher than 99.9% in a case of 16-ns gate time and without decoherence.

arXiv:2303.16622Physical Review AppliedJan 18, 202415 citations

t_g=53.8

ns. To design the CNOT gate, we utilize an analytical microwave pulse shape based on the SWIPHT protocol for realizing fast, low-leakage gates. We obtain fidelities of 94.6% and 97.8% for the CNOT and CZ gates respectively from quantum progress tomography.

Kerr cat qubits are a promising candidate for fault-tolerant quantum computers owing to the biased nature of their errors. The

ZZ

coupling between the qubits can be utilized for a two-qubit entangling gate, but the residual coupling called

ZZ

crosstalk is detrimental to precise computing. In order to resolve this problem, we propose a tunable

ZZ

-coupling scheme using two transmon couplers. By setting the detunings of the two couplers at opposite values, the residual

ZZ

couplings via the two couplers cancel each other out. We also apply our scheme to the ${R}_{ZZ}(\mathrm{\ensuremath{\Theta}})$ gate (

ZZ

rotation with angle $\mathrm{\ensuremath{\Theta}}$), one of the two-qubit entangling gates. We numerically show that the fidelity of the ${R}_{ZZ}(\ensuremath{-}\ensuremath{\pi}/2)$ gate is higher than 99.9% in a case of 16-ns gate time and without decoherence.