Making a Silicon Quantum Processing Chip Requires More Than Silicon and Phosphorus

A high-performance silicon-based quantum processor has been developed in which the spins of nine phosphorus nuclei — divided into two groups, or registers — and two shared electrons act as quantum bits (qubits).

Nature - An 11-qubit atom processor comprising two precision-placed nuclear spin registers of phosphorus in silicon is shown to achieve state-of-the-art Bell-state fidelities of up to 99.5%.

Fault-tolerant quantum computation offers one of the viable paths, necessitating the encoding and processing of information within logical qubits to curb such errors. Although substantial progress has been achieved recently in building silicon quantum computers, logical operations still haven’t been realized in silicon. Here we demonstrate a logical quantum processor using a phosphorus donor cluster in silicon.

In order to realise such a audacious quantum computer architecture however we require the ability to (i) place individual phosphorus dopants in a silicon lattice with atomic precision, (ii) encapsulate the atomically created phosphorus array ...

Weak localization thickness measurements of embedded phosphorus delta layers in silicon produced by PH3 dosing · Atom-by-atom construction of a cyclic artificial molecule in silicon · Electron-electron interactions in low-dimensional Si:P delta layers · Atomic-scale control of tunneling in donor-based devices · Atom-by-Atom Fabrication of Single and Few Dopant Quantum Devices

(B) Si-MOS structure. (C) Ohmic contacts are formed by phosphorus diffusion: a top layer of high-quality oxide is thermally grown, followed by the definition of an aperture for ion implantation.

This error detection strategy, presented in a paper published in Nature Electronics, was found to successfully detect quantum errors in silicon qubits, while also preserving entanglement after their detection. "In this work, we used the nuclear spins of phosphorus donors in a silicon cluster to encode quantum information, with the atomic-scale device functioning as a quantum information processor," Prof.

While logical qubits and operations have been successfully demonstrated in platforms such as superconducting circuits, neutral atoms, nitrogen-vacancy centers and trapped ions, their implementation in silicon-based spin qubits poses notable technical challenges," the study authors write. However, the team was capable of overcoming these challenges. To create the logical quantum processor, the research team used five phosphorus nuclear spins in a silicon donor cluster as qubits.

Detection of the movement of single electrons between small, dense clusters of phosphorus donors has also been achieved. While this group remains optimistic that a practical, large-scale Kane quantum computer can be built, others believe that the idea needs to be modified. In 2020, Andrea Morello and others demonstrated that an antimony nucleus (with eight spin states) embedded in silicon could be controlled using an electric field, rather than a magnetic field.