A tiny diamond with defects, nitrogen atoms that replaced carbon, hides an unusual feature. Around each defect, an extra electron spins like a top that can be tilted by radio pulses. On its own, this top is too quiet to reach a superconducting qubit — an artificial atom operating at temperatures colder than outer space. But if you place a mediator between them (a tunable resonator, like a tuning fork with an adjustable pitch) they begin to sing in unison.
In the experiment, the qubit was first prepared in the desired state, like a conductor setting the tone before a concert. Then, through the resonator, this state was transferred to the collective excitation of millions of spin tops in the diamond. The spins remembered the oscillation pattern and held it long enough to later pass it back to the qubit. When the qubit was read out, it reproduced the original state, like a note sung from memory by a choir after a pause.
For the first time, a cloud of spins in a crystal acted as true quantum memory for a superconducting qubit. Such memory is needed to link many qubits into a network: while one waits for a response from another, information is not lost but stored in the long-lived spin cloud. In the future, this could help build quantum processors where computation and memory are separated, as in a classical computer. And then a tiny diamond with defects will cease to be just a crystal: it will become a reliable vault for quantum states.
An NV center in diamond glows red when illuminated with a green laser: that's how it's found under a microscope.
In Neal Stephenson's novel 'The Diamond Age,' diamonds become the foundation of future computing technology.