Diamond Flaws With Boron and Oxygen Emerge as New Qubit Candidates

A scientist at the Princeton Plasma Physics Laboratory (PPPL) has found a faster way to calculate how tiny flaws in diamond behave as qubits, by treating a cluster of just 16 to 30 atoms around each flaw as if it were a single molecule. The method has already pointed to defects containing boron and oxygen as new qubit candidates, according to Princeton University.

The work, by staff research scientist Mark Martirez at the US Department of Energy lab, appeared in the journal ACS Central Science (DOI: 10.1021/acscentsci.6c00548).

Why flawed diamond matters for quantum devices

A perfect diamond is a regular lattice of carbon atoms, and that alone is not much use for quantum computing. The useful part is the defects: spots where a carbon atom is missing, where another element sits in its place, or both. The atoms around such a defect can act as a qubit, the basic unit that stores and processes quantum information in quantum computers and sensors.

Diamond draws interest because some of these quantum properties stay usable at room temperature. Many other candidate materials only work reliably when chilled to very low temperatures.

The best-studied example is the nitrogen-vacancy center. It forms when a nitrogen atom replaces a carbon atom and the neighboring carbon site is left empty. Under laser light, it can glow red when it is in a particular quantum state.

How the new method works

Modeling the quantum behavior of a defect accurately has usually taken huge amounts of computing power, which made it hard to screen many candidates. Rather than simulate a large chunk of diamond crystal, Martirez carves out the small group of atoms around the defect and treats it as a separate molecule. That lets him use computational methods chemists have refined over decades, without giving up the accuracy the calculations need.

The cost of each defect calculation drops to roughly that of modeling a simple molecule, Martirez said, adding: “This method really opened the floodgates.” The approach lets researchers work out much faster how different qubits respond to laser light and microwave pulses, the two tools used to control qubits and read out what they store.

New colors from boron and oxygen

Using the method, the team flagged defects involving boron and oxygen as possible new types of qubit. Their optical behavior differs from the nitrogen-vacancy center: oxygen defects can give off blue light, and boron defects ultraviolet light.

Because each type reacts to light in its own way, a single device could hold several kinds of qubit, and researchers might address one group without disturbing another. Martirez described this as combining qubits with different responses “so you can basically probe one qubit but not the other.”

What is still unclear

These are computational results. The boron and oxygen defects are candidates identified through calculations, and the reports describe no device built from them. A working quantum computer with many qubits is still a goal, and the claim that such machines could outpace supercomputers applies only to certain complex problems.

What it means for quantum research

The study suggests chemistry could take a bigger role in a field long treated as physics. Martirez thinks qubits might one day be designed for specific jobs in the way chemists design molecules, including compounds for new medicines. He also argues that chemists’ long experience with molecules carries over to defects in solids, and he hopes more of them will join the effort.

He sees a loop as well: a practical quantum computer could make molecular calculations far more accurate, helping scientists describe large biomolecules and proteins in detail and feeding back into pharmaceutical and materials science. The research was funded through PPPL’s Laboratory Directed Research and Development program, with additional support from the DOE Office of Science’s Office of Fusion Energy Sciences.

Source: ScienceDaily