A team of Harvard physicists led by Mikhail D. Lukin has achieved quantum entanglement of photons and solid-state materials, allowing for communication of qubits over long distances in a quantum network.

Quantum networking applications such as long-distance communication and distributed computing would require the nodes that process and store quantum data in qubits to be connected to one another by entanglement, a state where two different atoms become indelibly linked such that one inherits the properties of the other.

“In quantum computing and quantum communication, a big question has been whether or how it would be possible to actually connect qubits, separated by long distances, to one another,” says Lukin, professor of physics at Harvard and co-author of a paper describing the work in this week’s issue of the journal Nature. “Demonstration of quantum entanglement between a solid-state material and photons is an important advance toward linking qubits together into a quantum network.”

Quantum entanglement has previously been demonstrated only with photons and individual ions or atoms.

“Our work takes this one step further, showing how one can engineer and control the interaction between individual photons and matter in a solid-state material,” says first author Emre Togan, a graduate student in physics at Harvard. “What’s more, we show that the photons can be imprinted with the information stored in a qubit.”

The new result builds upon earlier work by Lukin’s group to use single atom impurities in diamonds as qubits. Lukin and colleagues have previously shown that these impurities can be controlled by focusing laser light on a diamond lattice flaw where nitrogen replaces an atom of carbon. That previous work showed that the so-called spin degrees of freedom of these impurities make excellent quantum memory.

Lukin and his co-authors now say that these impurities are also remarkable because, when excited with a sequence of finely tuned microwave and laser pulses, they can emit photons one at a time, such that photons are entangled with quantum memory. Such a stream of single photons can be used for secure transmission of information.

“Since photons are the fastest carriers of quantum information, and spin memory can robustly store quantum information for relatively long periods of time, entangled spin-photon pairs are ideal for the realization of quantum networks,” Lukin says. “Such a network, a quantum analog to the conventional internet, could allow for absolutely secure communication over long distances.”

The work was supported by the Defense Advanced Research Projects Agency, the Harvard-MIT Center for Ultracold Atoms, the National Science Foundation, the National Defense Science & Engineering Graduate Fellowship, and the Packard Foundation.

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