Designer diamonds could one day help build a quantum internet

A new kind of artificial diamond is a cut above the rest for quantum memory.

Unlike other synthetic diamonds, which could either store quantum information for a long time or transmit it clearly, the new diamond can do both. This designer crystal, described in the July 6 Science, could be a key building block in a quantum internet. Such a futuristic communications network would allow people to send supersecure messages and connect quantum computers around the world (SN: 10/15/16, p. 13).
Synthetic diamond can serve as quantum storage thanks to a type of flaw in its carbon lattice, where two neighboring carbon atoms are replaced with one noncarbon atom and an empty space (SN: 4/5/08, p. 216). This pairing exhibits a quantum property known as spin, which can be in an “up” state, a “down” state or both at once. Each of those states reflects a bit of quantum data, or qubit, that may be 1, 0 or both at once. A diamond transmits qubits by encoding them in light particles, or photons, that travel through fiber-optic cables.

Qubit-storing diamond defects are typically made with nitrogen atoms, which can store quantum data for milliseconds — a relatively long time in the quantum realm (SN: 4/23/11, p. 14). But nitrogen defects can’t communicate that data clearly. They emit light particles at a broad range of frequencies, which muddles the quantum information written into the photons.

Defects made with silicon atoms emit light more precisely, but until now haven’t been able to store qubits for longer than several nanoseconds due to their electrical interactions with nearby particles, explains Nathalie de Leon, an electrical engineer at Princeton University.

De Leon and colleagues got around this problem by forging silicon defects in a diamond infused with boron. This extra chemical ingredient shielded the delicate silicon defects from electrical interactions with nearby particles, extending the defects’ quantum memory. The boron-infused crystal nearly rivaled the long-term quantum memory of nitrogen defects, storing qubits for about a millisecond. And it gave a clean photon readout, emitting about 90 percent of its photons at the exact same frequency — compared to just 3 percent of photons spat out by nitrogen defects.
Tweaking the environment of the silicon defects was “an extremely creative way” to help keep a better grip on qubits, says Evelyn Hu, an applied physicist and electrical engineer at Harvard University not involved in the work.

This new artificial diamond could be used to construct devices called quantum repeaters for long-distance quantum communications, says David Awschalom, a physicist and quantum engineer at the University of Chicago who wasn’t involved in the work. Qubit-carrying photons can travel only up to about 100 kilometers through optical fiber before their signal gets scrambled (SN: 9/30/17, p. 8). Quantum repeaters that catch, store and re-emit photons could serve as stepping stones between fiber-optic cables to extend the reach of future networks.