Doped diamond may lead to everyday quantum computers

Quantum computers are still halfway mythical, but they are moving closer to reality step by tiny step. 

One of the most widely favoured structures for building viable quantum computers is a diamond surface dotted with irregularities only a couple of atoms wide.

The problem researchers face, however, is making sure those irregularities – essentially atom-scale holes and accompanying bits of atom-wide foreign material – are drilled into the diamond substrate in exactly the right spot. 

A report by a team from MIT, Harvard University, and Sandia National Laboratories, in the US, covers a new method of doing so, creating the “defects” in the diamond crystal structure within 50 nanometres of their optimal locations.

The precise placement of the irregularities – known as “dopant-vacancies” in the business – is a critical outcome if quantum computers are ever to end up on the market.

This is because the combination of a tiny hole and a couple of atoms of non-diamond matter – nitrogen, for instance – can be engineered to act as a qubit, the fundamental element of quantum computing.

At the heart of a qubit is a subatomic particle that can simultaneously occupy a number of contradictory states – on, off, and a “superposition” of both together, for instance. The combination of the hole, the foreign atoms, and the light refracted through the diamond combine to create an elegant qubit.

At least, theoretically. To date, most experimental work has been done using nitrogen dopant-vacancies. These have the advantage of being able to maintain superposition longer than other candidates, but emit light across a broad range of frequencies, making information retrieval difficult.

The MIT-Harvard-Sandia team, led by Tim Schröder, experimented instead with silicon-based defects, which emit light in a much narrower range. That advantage, however, comes with its own challenge: the silicon dopant-vacancies need to be chilled to within a few thousands of a degree above absolute zero if they are to maintain a superposition for any length of time.

That remains a challenge still to be met, however. The import of the current study, published in the journal Nature Communications, lies in the increase in the accuracy of positioning the defects in the diamond.

To achieve this, scientists at MIT and Harvard first created a sliver of diamond only 200 nanometres thick. Onto this they etched tiny cavities.

The substrate was then sent to the Sandia laboratories, where each cavity was bombarded with 20 to 30 silicon ions. The process led to only about two percent of the cavities attracting silicon residents.

Back at MIT a second new process was employed. The diamond sliver was heated to 1000 ºC, at which temperature its component lattice became malleable, allowing the researchers to align more cavities with more silicon particles – taking the total number of dopant-vacancies to 20%.

Most of the irregularities thus produced were within 50 nanometres of their optimal position, and shone at around 85% of optimal brightness.

A quantum computer in every household is still a long way off, but this study marks a potentially important step in the journey.