Double-transmon coupler demonstrated for superconducting quantum computers
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Toshiba Corporation has confirmed a breakthrough technology that promises to advance progress toward the development of higher-performance quantum computers. Experiments conducted by a joint research group from Toshiba and RIKEN, one of Japan’s largest comprehensive research institutions, have successfully realized a Double-Transmon Coupler, a tunable coupler comprised of two transmon-type superconducting qubits for superconducting quantum computers initially proposed by Toshiba.
There are multiple approaches to implementation of superconducting circuits in quantum computers. For instance, there are now several types of qubits, including the standard transmon qubit and the relatively new fluxonium qubit. The transmon qubit remains the simplest and is currently the most widely used. There are also different methods for coupling qubits, from direct capacitive coupling to those that involve a tunable coupler between the qubits.
Toshiba’s Double-Transmon Coupler is a tunable coupler comprised of two transmon-type superconducting qubits. It can both turn off coupling and execute high-speed two-qubit gate operations for fixed-frequency transmon qubits with significantly different frequencies. Its effective functioning requires a sufficiently long coherence time, longer than the gate operation time, and this can only be achieved if the superconducting materials used, the surrounding circuit design, and the fabrication processes are all carefully considered. At the same time, for high-speed gate operations, a strong coupling strength between the qubits is crucial.
Toshiba and RIKEN’s joint research group successfully conducted the world’s first experimental demonstration of this method and demonstrated that it has high performance fidelity.
The researchers achieved a world-class fidelity of 99.90% for a two-qubit gate. Fidelity is a standard performance indicator for quantum gates, quantifying how close an operation is to the ideal in a range from 0% to 100%, with higher percentages indicating greater accuracy in the quantum gate’s operation.
Originally proposed by Toshiba in a paper from September 2022, the Double-Transmon Coupler is a tunable coupler that holds the key to improving the performance of superconducting quantum computers. In successful experimental realization, Toshiba and RIKEN have confirmed its theoretical superiority over conventional tunable couplers in suppressing the long-standing problem of unnecessary residual coupling and enabling high-speed, high-fidelity two-qubit gates.
To improve the performance of two-qubit gates, the coherence time, the period for which the quantum superposition state can be maintained — critical in quantum computers — must be extended. Gates must also be executed quickly and the strength of residual coupling must be suppressed to reduce the errors it causes. The Toshiba-RIKEN team achieved a world-class coherence time for the transmon qubit, a short gate time of 48 ns, and reduced the residual coupling strength to as low as 6 kHz, thereby achieving a fidelity of 99.90%.
Quantum computers that adopt the Double-Transmon Coupler can also utilize the fixed-frequency transmon qubit, which is highly stable with a simple structure that is relatively easy to manufacture. This will further advance the fast-developing field of quantum computing and points the way to the large-scale quantum computers essential for practical applications, such as realizing carbon neutrality and new drug development.
The research is published at DOI: https://doi.org/10.1103/PhysRevX.14.041050
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