Minimal Non-orthogonal Gate Decomposition for Qubits with Limited Control

TL;DR

This paper presents optimal decomposition methods for single-qubit gates with non-orthogonal rotations under physical constraints, reducing gate time and providing explicit construction procedures for quantum-dot and topological qubits.

Contribution

It introduces complete solutions for minimal non-orthogonal gate decomposition, including explicit procedures, criteria, and code, improving efficiency over previous methods.

Findings

Analytic explanation for four-gate EO qubit decomposition

50% reduction in gate time for ST qubits compared to previous methods

Extension of approach to topological qubits via braiding operations

Abstract

In quantum control theory, a question of fundamental and practical interest is how an arbitrary unitary transformation can be decomposed into minimum number of elementary rotations for implementation, subject to various physical constraints. Examples include the singlet-triplet (ST) and exchange-only (EO) qubits in quantum-dot systems, and gate construction in the Solovay-Kitaev algorithm. For two important scenarios, we present complete solutions to the problems of optimal decomposition of single-qubit unitary gates with non-orthogonal rotations. For each unitary gate, we give the criteria for determining the minimal number of pieces, the explicit gate construction procedure, as well as a computer code for practical uses. Our results include an analytic explanation to the four-gate decomposition of EO qubits, previously determined numerically by Divincenzo et al [Nature, 408, 339 (2000)]. Furthermore, compared with the approaches of Ramon sequence and its variant [Phys. Rev. Lett., 118, 216802 (2017)], our method can reduce about 50% of gate time for ST qubits. Finally, our approach can be extended to solve the problem of optimal control of topological qubits, where gate construction is achieved through the braiding operations.