Every baker and quantum engineer knows how it feels to open an oven and find out they’ve been unsuccessful. The soufflé fell, or in the engineering equivalent, too few, too scattered qubits are formed in bulk diamond to make effective quantum materials.
An innovative approach from the UChicago Pritzker School of Molecular Engineering (PME) High Lab has removed some of the randomness for the engineers at least, pioneering a system of laser-writing qubits—the basic building blocks of powerful quantum computers—into thin diamond films. The new technique was published in the journal Nano Letters.
The technique offers a powerful addition to the many ways researchers at UChicago PME and elsewhere have developed for creating qubits.
The new process pioneered at UChicago PME involves taking a thin diamond film, another recent innovation from the High Lab, and fabricating photonic cavities throughout the material. These cavities—optical structures within the diamond where photons are trapped as if placed between two mirrors—are then excited with a laser. This optical field creates color centers, positioning the qubits where the photonic cavities were placed.
“The incredible thing about this method is that, unlike the typical approach where you throw your diamond in a furnace and you pull it out later and some of these defects have become qubits by bonding with a vacancy, with the laser writing, other research groups have shown you can watch in real time as you write,” said PME Asst. Prof. Alex High, who holds a joint appointment with Argonne National Laboratory. “While we are still fine tuning these processes here, we have demonstrated its potential utility in diamond photonic devices used in quantum sensing and communication.”
Creating qubits out of diamond color centers typically involves implanting foreign atoms such as nitrogen atoms into a diamond, heating the diamond to high temperatures, and seeing what happens. A small percentage of the atoms will bond with a vacancy, creating the qubits needed for computing, sensing and other quantum applications. The new qubits will be both rare and randomly scattered through the diamond.
It’s a more predictable, but also much simpler system, said first author Anchita Addhya, a PhD candidate at UChicago PME.
“You don’t have to take the sample out of the cryostat, do a bunch of processing and then put the sample back in. It's a one-shot process. You make the cavities, you put it in the cryostat, you write the color centers, you read them out,” said Addhya.
Although the placement of the qubits in the diamond film isn't yet fully deterministic, co-author F. Joseph Heremans, staff scientist at Argonne and affiliated scientist at UChicago PME, said it’s a very nice proof of concept for predictable placement of qubits.
“We're still working on controlling this process more precisely,” Heremans said. “We can pulse the light, observe the creation of these different defects and know they’re localized within the nanopillar, but there remain a lot of exciting dynamics to study.”
Without a singular reliable system, many quantum researchers are moving toward Hybrid approaches, Addhya noted.
“Superconducting qubits, cold atoms, and other techniques for making qubits have their own benefits, but they also have drawbacks,” Addhya said. “People are looking for a platform where they can essentially sandwich of all the good things from each platform together.”
The ultimate goal is to create a reliable method, or methods, for creating the building blocks for future quantum computers. This new method moves researchers one step closer.
Citation: “Photonic-Cavity-Enhanced Laser Writing of Color Centers in Diamond,” Addhya et al, Nano Letters, August 29, 2024. DOI: 10.1021/acs.nanolett.4c02639
Funding: This work was supported by the U.S. Department of Energy Office of Science National Quantum Information Science Research Centers as part of the Q-NEXT center.