Quantum crypto network debuts
Technology Research News
Quantum cryptography has the potential
to guarantee perfectly secure communications, but until now all of the
prototype systems have been point-to-point links rather than networks
that share connections.
BBN Technologies, Harvard University and Boston University researchers
have built a six-node quantum cryptography network that operates continuously
to provide a way to exchange secure keys between BBN and Harvard, which
is about 10 kilometers away. "Any node in the network can act as a relay,"
said Chip Elliott, a principal scientist at BBN Technologies. The researchers
will soon move one of the network nodes across town to link Boston University
into the network, said Elliott.
Because the Defense Advanced Research Projects Agency (DARPA)
Quantum Network is switched, the organizations can share the costs of
the fiber infrastructure, Elliott said. "BBN and Harvard can both talk
to BU by sharing a single fiber, rather than each requiring its own fiber."
The network is resilient because any node in the network can act
as a relay to connect two other nodes. Because there are multiple connections
to and from any given node, "failure of a link or node does not mean that
we have lost quantum cryptography," said Elliott.
Quantum cryptography schemes allow a pair of correspondents to
securely exchange a one-time pad, or key that will unlock a scrambled
message. The schemes call for transferring each bit of information using
a single photon. The systems are potentially very secure because the quantum
state of a particle cannot be observed without altering it. If the random
string of bits that make up the key have been observed, it will be obvious
to the sender and receiver and the key can be discarded.
The quantum network uses secure point-to-point connections between
nodes and allows a given node to relay secure cryptographic keys between
two other nodes. "The relays develop one-time pad keys with their nearest
neighbors, and then use these one-time pads to protect end-to-end cryptographic
material as it makes its way through the network, hop-by-hop," said Elliott.
The six nodes that make up the network are dubbed Alice, Anna,
Ali, Bob, Boris and Baba. In its current setup, Alice-Anna can share keys
through Bob and Boris, Bob-Boris can share keys through Alice and Anna,
and Ali and Baba can share keys with all other nodes through Alice, said
The network also contains a optical switch that can change the
way the nodes are connected.
Four of the nodes are connected via fiber-optic cable and two
nodes use wireless optics. The network is limited to metropolitan areas
and will require the development of quantum repeaters to span greater
distances. Because the quantum properties of photons are lost if they
are observed, they cannot be copied, but making copies of light signals
is the way signals are boosted along ordinary telecommunications lines.
Quantum repeaters, which are under development at several research
labs around the world, would instead transfer the quantum state of one
photon to another through interactions with atoms or through the strange
quantum phenomenon of entanglement, which allows traits of two or more
particles to be linked regardless of the distance between them.
The network's photon sources are currently heavily filtered lasers,
which are extremely dim and sometimes emit more than one photon at a time.
This makes them relatively inefficient and bits that are represented by
more than one photon are not invulnerable to eavesdroppers. The Boston
University researchers are working on more efficient photon sources that
would emit entangled pairs of photons, one of which can be transmitted
so two parties can share the pair. The sources would be brighter and would
avoid the risk of multiple photons.
The quantum cryptography network works with Internet protocols
including the secure Internet Protocol (IPsec) and creates a type of virtual
private network, which provides secure communications over unsecured networks
like the Internet at large. The idea is that even if an eavesdropper is
able to listen in on a line, he would be unable to learn much about the
communications traversing it.
The network is ready for practical applications today, said Elliott.
Elliott's research colleagues were Alex Colvin, Chris Lirakis,
David Pearson, Oleksiy Pikalo, John Schlafer, Greg Troxel and Henry Yeh
of BBN Technologies, Tai Tsun Wu, John Myers, Dionisios Margetis, F. Hadi
Madjid and Margaret Owens of Harvard University, and Alexander Sergienko,
Bahaa Saleh, Malvin Teich, Gregg Jaeger, Gianni Di Giuseppe and Hugues
De Chatellus of Boston University. The research was funded by the Defense
Advanced Research Projects Agency (DARPA).
TRN Categories: Quantum Computing and Communications; Cryptography
Story Type: News
Related Elements: Technical paper, "Quantum Cryptography
in Practice," posted on the arxiv physics archive at http://arxiv.org/abs/quant-ph/0307049
July 14/21, 2004
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