Entangling unitary gates on distant qubits with ancilla feedback

TL;DR

This paper investigates how feedback mechanisms can optimize the entangling process between distant qubits mediated by an ancilla, improving speed and efficiency in quantum networks.

Contribution

It introduces feedback strategies to accelerate entangling gates between distant qubits using an ancilla, enhancing scalability of quantum computing architectures.

Findings

Feedback reduces expected entangling time.

Stronger coupling speeds up gate generation.

Binary control strategies are nearly as effective as continuous control.

Abstract

By using an ancilla qubit as a mediator, two distant qubits can undergo a non-local entangling unitary operation. This is desirable for when attempting to scale up or distribute quantum computation by combining fixed static local sets of qubits with ballistic mediators. Using a model driven by measurements on the ancilla, it is possible to generate a maximally entangling CZ gate while only having access to a less entangling gate between the pair qubits and the ancilla. However this results in a stochastic process of generating control phase rotation gates where the expected time for success does not correlate with the entangling power of the connection gate. We explore how one can use feedback into the preparation and measurement parameters of the ancilla to speed up the expected time to generate a CZ gate between a pair of separated qubits and to leverage stronger coupling strengths for faster times. Surprisingly, by choosing an appropriate strategy, control of a binary discrete parameter achieves comparable speed up to full continuous control of all degrees of freedom of the ancilla.