Mediated gates between spin qubits

TL;DR

This paper introduces mediated gates for spin qubits that enable nonlocal operations via intermediate ancillae, reducing circuit depth and facilitating the generation of complex quantum states and gates.

Contribution

It develops a new class of mediated gates acting on nonproximal spins through ancilla qubits, improving quantum circuit efficiency for spin-based quantum computing.

Findings

Mediated gates can generate arbitrary quantum states and gates.

Circuit depths are often shorter than swap-based circuits.

An explicit experimental implementation in triple-quantum-dot systems is proposed.

Abstract

In a typical quantum circuit, nonlocal quantum gates are applied to nonproximal qubits. If the underlying physical interactions are short-range (e.g., exchange interactions between spins), intermediate swap operations must be introduced, thus increasing the circuit depth. Here we develop a class of "mediated" gates for spin qubits, which act on nonproximal spins via intermediate ancilla qubits. At the end of the operation, the ancillae return to their initial states. We show how these mediated gates can be used (1) to generate arbitrary quantum states and (2) to construct arbitrary quantum gates. We provide some explicit examples of circuits that generate common states [e.g., Bell, Greenberger-Horne-Zeilinger (GHZ), W, and cluster states] and gates (e.g.,square-root-SWAP, SWAP, CNOT, and Toffoli gates). We show that the depths of these circuits are often shorter than those of conventional swap-based circuits. We also provide an explicit experimental proposal for implementing a mediated gate in a triple-quantum-dot system.