Quantum networds advance with entanglement of photons, solid-state qubits

"In quantum computing and quantum communication, a big question has been whether or how it would be possible to actually connect qubits, eparated by long distances, to one another," says Mikhail Lukin, senior author of the new study.

A team of Harvard physicists led by Mikhail D. Lukin
has achieved the first-ever quantum entanglement of photons and
solid-state materials. 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 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."

Quantum entanglement, famously termed "spooky action at a distance" by a
skeptical Albert Einstein , is a fundamental property of quantum
mechanics. It allows 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.

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."

Lukin and Togan’s co-authors on the paper are Yiwen Chu, Alexei
Trifonov, Jeronimo Maze, and Alexander S. Zibrov , all at Harvard; Liang
Jiang of Harvard and the California Institute of Technology ; Lilian I.
Childress of Harvard and Bates College ; M.V. Gurudev Dutt of Harvard and
the University of Pittsburgh ; A.S. Sorensen at the University of
Copenhagen ; and Phillip R. Hemmer of Texas A&M University. 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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