Inside Quantum Technology

Harvard’s Lukin Discusses His Group’s Work in Quantum Memory and Quantum Repeaters

( Mikhail Lukin of Harvard University recently presented how his group is working to extend the range of secure quantum communications. In particular, he described how diamond-based quantum memory can boost the efficiency of quantum key distribution (QKD) involving Bell-state measurements.
QKD systems have been available commercially for some time but remain the subject of intense research. In particular, secure communications cannot be carried out over more than a few hundred kilometers owing to the absorption and scattering of photons as they travel along optical fiber. Classically, such losses can be overcome using amplifiers. Doing a similar thing with a quantum signal, however, would impair its security.
Lukin explained how the quantum memory can improve QKD and provide a key ingredient for quantum repeaters—devices that in future could allow quantum communications to be extended over arbitrarily long distances.
Lukin and his team are focusing on so-called silicon–vacancy centers—atom-like defects created when two carbon atoms in a diamond lattice are replaced by a silicon atom and a gap. Such a defect, integrated inside a nanophotonic diamond resonator, yields a solitary spin that can remain in a coherent quantum state for more than two milliseconds when chilled to within a fraction of a degree above absolute zero.
Looking ahead, the Harvard researchers will attempt to carry out a similar feat with light at telecom wavelengths. And at the same time, said Lukin, they hope to make “serious inroads” into the development of quantum repeaters.

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