Ancilla-driven quantum computation for qudits and continuous variables

TL;DR

This paper introduces a universal ancilla-driven quantum computation model applicable to qubits, qudits, and continuous variables, using minimal interactions and measurements, with adaptations for measurement challenges and practical implementations.

Contribution

It proposes a novel ancilla-driven quantum computation framework that is adaptable to various quantum systems and includes measurement-free variants for practical scenarios.

Findings

Universal quantum computation achieved with fixed two-body interactions.

Model supports implementation of Clifford circuits in one quantum layer.

Alternative measurement-free model using basis-prepared ancillas.

Abstract

Although qubits are the leading candidate for the basic elements in a quantum computer, there are also a range of reasons to consider using higher dimensional qudits or quantum continuous variables (QCVs). In this paper we use a general `quantum variable' formalism to propose a method of quantum computation in which ancillas are used to mediate gates on a well-isolated `quantum memory' register and which may be applied to the setting of qubits, qudits (for $d>2$) or QCVs. More specifically, we present a model in which universal quantum computation may be implemented on a register using only: repeated applications of a single fixed two-body ancilla-register interaction gate, ancillas prepared in a single state, and local measurements of these ancillas. In order to maintain determinism in the computation, adaptive measurements via a classical-feedforward of measurement outcomes are used, with the method similar to that in measurement-based quantum computation (MBQC). We show that our model has the same hybrid quantum-classical processing advantages as MBQC, including the power to implement any Clifford circuit in essentially one layer of quantum computation. In some physical settings, high-quality measurements of the ancillas may be highly challenging or not possible, and hence we also present a globally unitary model which replaces the need for measurements of the ancillas with the requirement for ancillas to be prepared in states from a fixed orthonormal basis. Finally, we discuss settings in which these models may be of practical interest.