Cambridge, MA--A team of Harvard physicists led by Mikhail D. Lukin has achieved the first-ever quantum entanglement of photons and solid-state materials.1 The work marks a key advance toward practical quantum networks, as the first experimental demonstration of a means by which solid-state quantum bits, or "qubits," can communicate with one another over long distances.
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 or other particles 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 can allow one to distribute quantum information over tens of thousands of kilometers, limited only by how fast and how far members of the entangled pair can propagate in space. However, until now, quantum entanglement has been demonstrated only with photons and individual ions or atoms.
'Excellent quantum memory'
The new result builds upon earlier work by Lukin's group in which single-atom impurities in diamonds are used as qubits. Lukin and colleagues previously showed 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 spin degrees of freedom of these impurities make excellent quantum memory.
Lukin and his co-authors now say that these impurities, when excited with a sequence of finely tuned microwave and laser pulses, can emit photons one at a time such that the photons are entangled with quantum memory. The 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."
1. E. Togan et al., Nature, Vol. 466, p. 730, 05 August 2010.