A superconducting quantum processor architecture for fast, crosstalk-free gates between far-detuned fixed-frequency qubits

Background

Quantum computing, particularly with superconducting qubits, is a rapidly advancing field that promises to revolutionize computation by solving certain problems exponentially faster than classical computers. Superconducting qubits, such as transmons, are favored for their relatively high coherence times and compatibility with existing fabrication technologies. As the field pushes toward building larger, more powerful quantum processors, a key requirement is the ability to implement fast, high-fidelity two-qubit gates between many qubits. Achieving this at scale necessitates strong, controllable interactions between qubits while maintaining high coherence and minimizing errors. However, as the number of qubits increases, so does the complexity of managing interactions and preventing unwanted crosstalk, which can degrade computational accuracy and limit the scalability of quantum systems.

Current approaches to coupling qubits and suppressing crosstalk face significant challenges. Many existing methods rely on using multiple coupling elements or require qubits to have closely spaced frequencies, which leads to frequency crowding—a situation where available frequency space becomes saturated, reducing device yield and making large-scale integration difficult. Alternatively, tunable qubits can be used to suppress crosstalk, but these typically suffer from lower coherence compared to fixed-frequency qubits, compromising overall processor performance. Furthermore, conventional crosstalk mitigation strategies often introduce additional complexity, require precise calibration, or do not fully eliminate unwanted interactions such as static ZZ coupling. These limitations hinder the realization of scalable, high-coherence quantum processors capable of executing deep quantum circuits with high fidelity.

Technology

This technology is a quantum computing architecture that introduces a flux-tunable coupler with anharmonicity opposite in sign to that of the two fixed-frequency transmon qubits it connects. The coupler, implemented as a generalized flux qubit with four shunting Josephson junctions, is positioned between the qubits and is designed to leverage interference from higher energy levels to cancel static ZZ crosstalk—a common source of error in superconducting quantum processors. By tuning the coupler’s frequency between those of the qubits and applying parametric flux modulation at the qubit-qubit detuning frequency, the system enables the execution of high-fidelity, entangling two-qubit gates in under 20 nanoseconds. This approach supports the use of high-coherence, fixed-frequency components and mitigates frequency crowding, which is a major bottleneck in scaling up quantum processors.

What differentiates this solution is its ability to combine the coherence advantages of fixed-frequency qubits with strong, on-demand coupling between qubits that are far apart in frequency. Traditional methods for crosstalk suppression often require closely spaced qubit frequencies or multiple coupling elements, leading to frequency crowding and reduced device yield. Alternatively, tunable qubits can suppress crosstalk but at the cost of lower coherence. By employing a coupler with opposite anharmonicity, this architecture achieves crosstalk-free, ultra-fast gates without sacrificing coherence or scalability. Experimental validation—including spectroscopy and population dynamics—confirms the suppression of unwanted interactions and the realization of fast, high-fidelity gates. The use of advanced modeling techniques, such as Floquet theory, further optimizes device performance and gate protocols, making this architecture a significant step forward in the development of scalable, reliable superconducting quantum computers.

Benefits

• Enables fast, high-fidelity two-qubit entangling gates in under 20 ns
• Suppresses static ZZ crosstalk between qubits, improving gate fidelity
• Supports coupling between far-detuned, fixed-frequency qubits, reducing frequency crowding
• Maintains high coherence by using fixed-frequency qubits rather than tunable qubits
• Increases scalable quantum processor circuit depth and device yield
• Utilizes a flux-tunable coupler with opposite anharmonicity to qubits for precise control
• Facilitates efficient simulation and optimization of gate parameters via Floquet theory
• Enables practical implementation of larger, more reliable superconducting quantum computers

Applications

• Scalable superconducting quantum processors
• High-fidelity quantum gate operations
• Large-scale quantum algorithm execution
• Quantum error correction architectures
• Quantum simulation of complex systems

Additional info

This quantum architecture utilizes a flux-tunable coupler with anharmonicity—the difference between 0-1 and 1-2 energy transitions—opposite in sign to its linked fixed-frequency transmon qubits. By employing a generalized flux qubit with four shunting Josephson junctions, the system leverages higher energy level interference to eliminate static ZZ crosstalk. This design facilitates high-speed entangling gates under 20 ns via parametric flux modulation, mitigating frequency crowding in scalable superconducting processors.