A universal gate for fixed-frequency qubits via a tunable bus (1604.03076v3)

Abstract: A challenge for constructing large circuits of superconducting qubits is to balance addressability, coherence and coupling strength. High coherence can be attained by building circuits from fixed-frequency qubits, however, leading techniques cannot couple qubits that are far detuned. Here we introduce a method based on a tunable bus which allows for the coupling of two fixed-frequency qubits even at large detunings. By parametrically oscillating the bus at the qubit-qubit detuning we enable a resonant exchange (XX+YY) interaction. We use this interaction to implement a 183ns two-qubit iSWAP gate between qubits separated in frequency by 854MHz with a measured average fidelity of 0.9823(4) from interleaved randomized benchmarking. This gate may be an enabling technology for surface code circuits and for analog quantum simulation.

• The paper presents a tunable bus mechanism that enables a universal iSWAP gate between fixed-frequency qubits via parametric modulation.
• It achieves a 183 ns gate operation with 0.9823 fidelity despite an 854 MHz detuning, showcasing robust coherence preservation.
• The approach minimizes qubit dephasing by shifting tunability to the bus, offering scalability for complex quantum circuits.

Overview of "A universal gate for fixed-frequency qubits via a tunable bus"

In the quest for scalable quantum computing, addressing the challenge of implementing high-fidelity gates with superconducting qubits remains a significant hurdle. This paper introduces a novel approach leveraging a tunable bus to facilitate a universal iSWAP gate between fixed-frequency qubits, even amid large detunings. This method addresses fundamental issues regarding coherence, coupling strength, and addressability, providing a pathway toward utilizing fixed-frequency qubits while retaining high coherence.

The core innovation lies in the use of a parametric modulation approach of a tunable bus to achieve the resonant exchange interaction (XX+YY) necessary for the iSWAP operation. Specifically, the authors have implemented a 183 ns iSWAP gate between two qubits, which are intentionally detuned by 854 MHz. Utilizing interleaved randomized benchmarking, they report an appreciable gate fidelity of 0.9823 with an associated error of 0.0177, demonstrating the robustness of their approach compared to existing methods. This result is particularly striking, considering the prevalent challenge of operational fidelity as circuit complexity increases in quantum architectures.

The implementation's efficacy stems from displacing the tunability responsibility from the qubits themselves to the coupling degree of freedom provided by the tunable bus. This shift reduces the exposure of computational qubits to dephasing noise typically introduced by conventional tunability channels, such as flux noise. In this configuration, the qubits maintain high coherence, as evidenced by $T_1=26.3 μs$ and $T_2=12.1 μs$ for qubit Q1 reported in the paper.

Implications and Future Directions

Practically, this methodology presents significant implications for large-scale quantum circuits utilizing the surface code or those intending to execute analog quantum simulations. The scalability to multiple qubits without significant fidelity degradation could potentially facilitate the implementation of more complex quantum algorithms and error correction schemes, vital for achieving fault-tolerant quantum computing.

From a theoretical perspective, this work promotes further exploration into flux-tunable bus architectures, especially concerning device noise dynamics and multi-qubit interactions. The capability to couple more than two qubits through a single tunable bus while maintaining low crosstalk and high addressability will require extensive investigation. Moreover, optimizing the design parameters, such as enhancing the coherence and minimizing leakage effects during fast gate operations, could lead to even lower error rates.

In summary, the implementation of a universal two-qubit gate using a tunable bus for fixed-frequency qubits marks a significant step toward developing scalable superconducting qubit technologies. This approach effectively balances critical parameters of coherence, interaction strength, and addressability, paving the way for more intricate quantum computing applications. Future research could focus on extending this architecture to handle multiple qubits and overcoming any experimental constraints to further reduce error rates, thus aligning superconducting qubit technology with the aspirations of quantum information processing's scalability and fault tolerance.