These Perfectly Imperfect Diamonds Are Built for Quantum Physics

 wired.com  2/15/2018 1:00:00 PM 

In the mid-2000s, diamonds were the hot new thing in physics. It wasn’t because of their size, color, or sparkle, though. These diamonds were ugly: Researchers would cut them into flat squares, millimeters across, until they resembled thin shards of glass. Then they would shoot lasers through them.

Probably the most valuable bauble of all was a minuscule diamond mined from the Ural Mountains. “We called it the ‘magic Russian sample,’” says physicist Kai-Mei Fu of the University of Washington. The diamond was extremely pure—almost all carbon, which isn’t common in this messy world—but with a few impurities that gave it strange quantum mechanical properties. “It had been chopped up among academic groups,” says Fu, who worked with a piece. “You know, take a chisel, chip some off. You don’t need much.” Those properties were promising—but the physicists only had a handful of diamonds to study, so they couldn’t run too many experiments.

That’s not a problem any more. These days, Fu can just go online and buy a $500 quantum-grade diamond for an experiment—from the company Element Six, owned by De Beers. They’ve long grown synthetic diamonds for drilling and machining, but in 2007, with funding from the European Union, they started making exactly the kind physicists need. And not just physicists, any more: Today, the supply of synthetic quantum diamonds is so abundant that lots of fields are exploring their possible uses.

The first field to benefit was quantum computing. Quantum computers—which theoretically should compute certain tasks exponentially faster than regular computers—encode information in quantum mechanical properties such as spin or polarization. These properties can be very unstable. But if you encode information inside a diamond by manipulating its impurities with a laser, the gem’s crystal structure actually protects and preserves that information. Physicists are working to make adjacent impurities interact in a controlled way to execute a primitive algorithm.

Element Six grows these perfectly imperfect diamonds in furnaces at nearly 5,000 degrees Fahrenheit. Starting with a seed diamond, the company’s engineers pump gases—something carbon-containing, like methane, along with hydrogen and nitrogen—into the furnace. As the gas molecules heat up, they separate into single atoms, some of which land on the seed diamond. A few choice nitrogen atoms sneak in, and the hydrogen keeps the carbon layer growing in the right crystal structure. “Carbon doesn’t really want to be diamond,” says Matthew Markham, a scientist at Element Six. “It really prefers to be graphite.”

At Harvard University, physics grad student Jenny Schloss programs Element Six diamonds with lasers and measures how nearby magnetic fields interfere. But before she can do that, she has to mess the diamonds up even more.

The diamonds Element Six sells have nitrogen impurities—but what Schloss’s group needs is a hole right next to it, called a nitrogen vacancy. (Disclosure: Schloss is a friend from college.) So they send their diamonds to a small New Jersey company called Prism Gem. Most of its business goes to jewelry companies, who ask them to create colored diamonds by knocking carbon atoms out with beams of high-energy electrons. But physicists can use the same process to create more useful holes in their research diamonds.

Prism Gem will shoot electrons at the diamonds for hours—sometimes days—to create the right number of holes. “Typically, scientists know what technical specifications they’re looking for. They’ll send us information on how many electrons they need per centimeter,” says Ashit Gandhi, Prism Gem’s chief technology officer. “Jewelry is more subjective. They’ll ask for light green, dark green, pink, or whatever.” After sitting under the electron beam, Schloss’s diamond, originally tinted yellow from nitrogen impurities, turns pale blue.

Her group then bakes the diamond again, which causes the holes to migrate next to the nitrogen impurities to create the coveted nitrogen vacancy center. Its final color ranges from clear to pink to red, depending on how many impurities they want.

With the quantum diamond supply chain in place, physicists have been able to study and fiddle with the gems in many iterations of experiments. But it’s been a slow process turning the diamond impurities into connected bits that can compute. “The verdict is still out,” says Fu. “Only two quantum bits [in diamond] have ever been connected. Until things become more scalable, I don’t think anyone can say it’s a definite thing.”

But by understanding the diamonds in more detail, researchers have inadvertently come up with another possible use for them. Harvard physicists Mikhail Lukin and Ronald Walsworth—Schloss’s research advisor—knew that when hit with a laser, a nitrogen vacancy diamond would emit different amounts of light if it was near a magnet. The diamond could function as a type of magnetic sensor—one that wasn’t as bulky as current sensors, which also need to be cooled to temperatures near absolute zero.

So in the early 2010s, Lukin and Walsworth’s research team started using the diamonds to study nerve cells, which emit magnetic fields when stimulated. They started with a squid nerve cell, thicker than a human hair. Grad student Matthew Turner traveled to Woods Hole Marine Biological Laboratory, where he excised long, thin white neurons from fresh squid, put them on ice, and jumped on a bus back to the lab to measure its magnetic field under electric stimulation.

Later, the team switched to studying neurons in marine worms, which they could keep in a tank in the lab. About a year ago, they published a paper on the sensitivity of their diamonds to study those neurons. Now, they’re using the diamonds to study magnetic fields given off by human heart cells.

They’re also collaborating directly with Element Six. In return for grant money, the company sends them diamonds. Recently, the company sent them a round disk the size of a cookie, with four diamonds embedded in it—intended to prevent one diamond from heating up too much when hit by a powerful laser. “I’m not sure why there are four diamonds,” says Schloss. “We haven’t found a good use for it.”

Element Six is the primary supplier of quantum-grade diamonds. “Right now, if it’s not a monopoly, it’s a near monopoly, especially in terms of access,” says Fu. Schloss and Turner’s lab has purchased poorer quality diamonds from eBay for preliminary experiments, but they haven’t worked well.

In the meantime, physicists are working not just on their experiments, but on driving this new technology forward. The Harvard lab has already spun off a small company, Quantum Diamond Technologies, to develop diamond-based imaging devices for medical diagnostics.

Eventually, they’re hoping the diamonds might be useful for imaging inside the human brain, neuron by neuron, something that neuroscientists have yet been unable to do. Or maybe, used in conjunction with other technologies, it’ll illuminate a new corner of the neuroscience puzzle. “I don’t claim to be the best neuroscientist or to have the best tool,” says Turner. “This is just a different tool that I want to understand better.” They don’t know what’s next, but maybe that makes for better science.

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