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These Perfectly Imperfect Diamonds Are Built for Quantum Physics

These Perfectly Imperfect Diamonds Are Built for Quantum Physics
From Wired - February 15, 2018

In the mid-2000s, diamonds were the hot new thing in physics. It wasnt 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 purealmost all carbon, which isnt common in this messy worldbut 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 dont need much. Those properties were promisingbut the physicists only had a handful of diamonds to study, so they couldnt run too many experiments.

Thats not a problem any more. These days, Fu can just go online and buy a $500 quantum-grade diamond for an experimentfrom the company Element Six, owned by De Beers. Theyve 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 computerswhich theoretically should compute certain tasks exponentially faster than regular computersencode 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 gems 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 companys engineers pump gasessomething carbon-containing, like methane, along with hydrogen and nitrogeninto 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 doesnt 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 impuritiesbut what Schlosss 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 hourssometimes daysto create the right number of holes. Typically, scientists know what technical specifications theyre looking for. Theyll send us information on how many electrons they need per centimeter, says Ashit Gandhi, Prism Gems chief technology officer. Jewelry is more subjective. Theyll ask for light green, dark green, pink, or whatever. After sitting under the electron beam, Schlosss 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.

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