December 1, 2022

Ion beams mean a quantum leap for color-center qubits

Ion beams mean a quantum leap for color-center qubits Ion beams can create chains of closely coupled quantum bits (qubits) based on nitrogen-vacancy “color centers” in diamond for use in quantum computing hardware. The honeycomb pattern in the photo shows the difference between areas exposed to the beam (darker) and masked-off areas. Results indicate it should be possible to create 10,000 coupled qubits over a distance of about the width of a human hair, an unrivaled number and density of qubits. Credit: Susan Brand/Berkeley Lab

A Lawrence Berkeley National Laboratory-led international team of researchers has discovered a way to use ion beams to create long strings of “color center” qubits in diamond.

Creating large numbers of high-quality qubits, in close enough proximity for coupling to each other, is one of the great challenges of quantum computing. The team has been exploring the use of ion beams to create artificial color centers in diamond for use as qubits.

When excited by the rapid energy deposition of a passing ion, Nitrogen-Vacancy (NV) centers can form in the diamond lattice. The electron and nuclear spins of nitrogen-vacancy centers and the adjacent carbon atoms can all function as solid-state qubits, and the crystal lattice can help protect their coherence and mutual entanglement. 

The result is a physically durable system that does not have to be used in a cryogenic environment, which are attractive attributes for quantum sensors and also for qubits in this type of solid-state quantum computer. However, making enough qubits, and making them close enough to each other, has been a challenge.

When swift (high-energy) heavy ions such as the beams this team used—gold ions with a kinetic energy of about one billion electron volts—pass through a material, such as nitrogen-doped diamond, they leave a trail of nitrogen-vacancy centers along their tracks. Color centers were found to form directly, without need for further annealing (heat treatment). What’s more, they formed all along the ion tracks, rather than only at the end of the ion range as had been expected from earlier studies with lower-energy ions. In these straight “percolation chains,” color-center qubits are aligned over distances of tens of microns, and are just a few nanometers from their nearest neighbors.

Results published in the current article show that it will be possible to form quantum registers with up to about 10,000 coupled qubits—two orders of magnitude greater than achieved thus far with the complementary technology of ion-trap qubits—over a distance of about 50 microns (about the width of a human hair). (

Their work has been published in the journal Applied Physics Letters.

Read more.