A faster way of finding the best diamond-based materials for quantum computers has been developed by a researcher at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL). The new approach has already identified promising materials for practical quantum sensors and computers that could one day transform scientific research as a whole. The research was recently published in the journal ACS Central Science and has already garnered attention from other experts in the field.
Many types of materials are currently being considered for use in quantum technology, but most can only hold a stable quantum state when chilled to extreme temperatures. Diamond-based materials, however, offer promising alternatives because they are functional at room temperature. This is just one way that PPPL continues to advance strategic national priorities, such as creating the technology of the future. Quantum technology is critical as the sensors would be more sensitive and accurate than anything currently available, and quantum computers would be able to handle highly complex problems far faster than any traditional supercomputer on Earth today.
But pure diamond, which is made entirely of carbon atoms, won't work. Defects in the diamond are essential for storing and processing information. A defect might mean a carbon atom is missing from its usual spot, a different kind of atom has taken its place, or both. The clusters of atoms closest to a defect form the fundamental information processing unit of a quantum computer or sensor, known as a qubit.
Treating Qubits as Molecules to Speed up Simulations
Simulating qubits has traditionally been a computational nightmare. Mark Martirez, a staff research scientist at PPPL, has solved this by "carving out" a small cluster of just 16 to 30 atoms around the defect. By treating this cluster as an isolated molecule, he can apply highly accurate tools that chemists have been using for decades to study simple compounds.
"Because we're treating the qubits as molecules, the cost of calculating these defects is equivalent to the cost of just modeling a simple molecule," Martirez said. "This method really opened the floodgates" in terms of how qubits can be assessed.
Using the new approach, the researchers were able to simulate how qubits would respond to light and microwave pulses far faster than ever before. This led them to discover new boron and oxygen-based defects that could make useful qubits.
Creating Qubits of Many Colors
Scientists shine laser light on qubits and pulse microwaves at them to read back information stored within. Most quantum diamond research has focused on the nitrogen-vacancy defect, in which one carbon atom is replaced by a nitrogen atom, and a nearby spot - normally holding another carbon atom - is empty. When scientists shine a laser on this particular kind of qubit, it emits a red light if it is in a particular quantum state. The oxygen-based defects Martirez is studying shine blue, while the boron-based defects emit an ultraviolet light. The different color centers could potentially be used in different ways in a single quantum device.
"One could imagine, then, combining different qubits of different responses, so you can basically probe one qubit but not the other," Martirez said.
Fully understanding the chemistry of qubits could also transform the concept of what a qubit structure should be, Martirez said. Scientists often treat diamond-based qubits as a physics problem. This new research shows that they can also be understood through chemistry, enabling scientists to design and build qubits the way a chemist designs custom molecules, such as a new medication, for a specific purpose.
Research Could Allow Quantum Chemistry to Enrich Itself
When Martirez thinks about the future of his field, he said it's interesting to note that quantum chemistry may help develop tools to "enrich itself" in the decades to come.
Martirez said a practical multi-qubit system would vastly improve the level of accuracy of quantum chemistry calculations. "We could describe larger molecules, biomolecules, proteins and everything like that in a more accurate way, and that could improve all the chemical-adjacent industries, such as pharmaceutical science and materials science. Even our fundamental understanding of molecular systems could be enriched."
Martirez also encouraged other chemists to consider research in solid-state physics. "At this moment in time, there's a great emphasis on building the first state of the art qubit computer," he said. "But it's not just physicists who should be participating in this research. Chemists have decades of developed intuition about chemical systems that could be easily translated into understanding defects in solids as well.
Funding for this research comes from PPPL's Laboratory Directed Research and Development under prime contract number DE-AC02-09CH11466, as well as additional support from DOE Office of Science, Office of Fusion Energy Sciences.