High-fidelity single-shot Toffoli gate via quantum control

Ehsan Zahedinejad, Joydip Ghosh, Barry C. Sanders

Chad T. Rigetti, Jerry M. Chow, Antonio D. Corcoles +8 more

Paul Bailey, Jean-François S. Van Huele

Ehsan Zahedinejad, Joydip Ghosh, Barry C. Sanders

A single-shot Toffoli, or controlled-controlled-not, gate is desirable for classical and quantum information processing. The Toffoli gate alone is universal for reversible computing and, accompanied by the Hadamard gate, forms a universal gate set for quantum computing. The Toffoli gate is also a key ingredient for (nontopological) quantum error correction. Currently Toffoli gates are achieved by decomposing into sequentially implemented single- and two-qubit gates, which require much longer times and yields lower overall fidelities compared to a single-shot implementation. We develop a quantum-control procedure to construct a single-shot Toffoli gate for three nearest-neighbor-coupled superconducting transmon systems such that the fidelity is 99.9% and is as fast as an entangling two-qubit gate under the same realistic conditions. The gate is achieved by a nongreedy quantum control procedure using our enhanced version of the differential evolution algorithm.

Xiao‐Qiang Shao, Hong‐Fu Wang, Li Chen +2 more

Vladimir M. Stojanović

We investigate the feasibility of single-shot Toffoli- and Fredkin-gate realizations in qubit arrays with Heisenberg-type exchange interactions between adjacent qubits. As follows from the Lie-algebraic criteria of controllability, such an array is rendered completely controllable---equivalent to allowing universal quantum computation---by a Zeeman-like control field with two orthogonal components acting on a single ``actuator'' qubit. Adopting this local-control setting, we start our analysis with piecewise-constant control fields and determine the global maxima of the relevant figure of merit (target-gate fidelity) by combining the multistart-based clustering algorithm and quasi-Newton type local optimization. We subsequently introduce important practical considerations, such as finite frequency bandwidth of realistic fields and their leakage away from the actuator. We find the shortest times required for high-fidelity Toffoli- and Fredkin-gate realizations and provide comparisons to their respective two-qubit counterparts---controlled-not and exponential-swap. In particular, the Toffoli-gate time compares much more favorably to that of controlled-not than in the standard decomposition-based approach. This study indicates that the use of the single-shot approach can alleviate the burden on control-generating hardware in future experimental realizations of multiqubit gates.