Quibbling Over Qubits: New Discovery Marks Milestone in Quantum Computer Science

Quantum bits, or qubits, are considered the basic building blocks of quantum computers, the futuristic machines that, once developed, promise to answer problems too complex for today's technology. And now, a team of researchers at the University of New South Wales has discovered a new way to distinguish between those small blocks, even when they are microscopic distances apart.

In order to be most effective, leader of the study Michelle Simmons explains that a qubit must be bound to a phosphorus atom within a silicon chip where its spins can be measured. And herein lies the magic: unlike today's computers that can only be in two states (1 or 0) at one time, the spins of a qubit can combine both of its states, up or down, at the same time, allowing exponentially larger amounts of information to be stored and processed in parallel.

"However, to be able to couple electron-spins on single atom qubits, the qubits need to be placed with atomic precision, within just a few tens of nanometers of each other," Simmons said in a press release.

This then poses the technical and operational problem, she explained, of controlling them independently even when they are right next to each other.

"If each electron spin-qubit is hosted by a single phosphorus atom, every time you try to rotate one qubit, all the neighbouring qubits will rotate at the same time -- and quantum computation will not work," lead author of the study Holger Buch said.

To solve this problem, the team, in collaboration with theorists at Sandia National Laboratories in New Mexico, worked together to develop what Simmons calls "an elegant and satisfying piece of work."

The group created a tiny device in which they deposited a layer of hydrogen on a silicon wafer and used a scanning tunneling microscope to create a pattern on the surface in an ultra-high vacuum.  They then exposed it to phosphine gas and annealed at extremely high temperatures so the phosphorus atoms became incorporated precisely into the silicon. Finally, they buried the device in another layer of silicon.

The result, according to the scientists, is the qubits are hosted by a different number of phosphorus atoms, causing them to respond to different electromagnetic fields. And when this happens, they can be distinguished from their neighbors.

"This first demonstration that we can maintain long spin lifetimes of electrons on multi-donor systems is very powerful," Simmons said. "It offers a new method for addressing individual qubits, putting us one step closer to realizing a practical, large-scale quantum computer."