network withstands noise
Technology Research News
If practical quantum computers are ever
built, chances are that someone will want to link them together. Quantum
computing uses individual particles like atoms to represent the ones and
zeros of digital information, and would theoretically solve certain problems
that are beyond the capabilities of ordinary computers, like cracking
secret codes and searching large databases.
The challenge to linking quantum computers is in building a network capable
of carrying fragile quantum information across not-so-gentle fiber-optic
lines, then reliably transferring the information from one quantum particle
To that end, a team of researchers at the Massachusetts Institute of Technology
and the U.S. Air Force Research Laboratory has proposed a scheme for transmitting
and storing quantum information in a series of quantum network nodes.
The researchers are aiming to space network nodes as far as 10 kilometers
apart, according to Selim M. Shahriar, now an associate professor of physics
at Northwestern University.
A quantum network could theoretically be used for perfectly secure communications,
to transmit quantum information from one quantum computer to another,
or to link logic units within quantum computers.
The researchers' quantum network scheme compensates for errors produced
by weakened signals, failed handoffs between photons and atoms, and false
readings by the system's detectors. "The key advantage of our scheme is
that it is robust against errors," said Shahriar. Under the scheme, errors
do not destroy data, but "only reduce the rate at which we can communicate.
It does not affect the accuracy or fidelity of the communication process,"
The scheme calls for building a series of network nodes that each holds
a single atom, and transferring information represented by the quantum
states of photons, which can travel down fiber-optic lines, to the quantum
states of these atoms. Entangling a pair of photons, sending each to a
separate node in the quantum network, and transferring the photons' quantum
states to the atoms entangles the atoms with each other.
Entanglement, which is one of the weirder traits of quantum physics, is
the critical element in many quantum computing and communications schemes.
When a subatomic particle or atom is undisturbed it enters into the quantum
mechanical state of superposition, meaning it is in some mixture of all
possible states. For example, particles can spin in one of two directions,
up or down. In superposition, however, the particles spin in some mixture
of both directions at the same time.
When two or more particles in superposition come into contact with each
other, they can become entangled, meaning one or more of their properties
are correlated. For example, two entangled photons could have the same
polarizations. When one of the photons is knocked out of superposition
and becomes, say, vertically polarized, the other photon leaves superposition
at the same instant and also becomes vertically polarized, regardless
of the distance between them.
Entanglement lies at the heart of quantum computers' theoretical ability
to solve problems that will always remain beyond the reach of even the
most powerful classical computer because it allows quantum logic operations
to work on many particles at once. A quantum computer can take advantage
of entanglement to check every possible answer to a problem with one series
of operations rather than having to check each possible answer one at
The researchers' scheme is a method for entangling distant atoms. Quantum
information is transmitted between entangled particles via quantum teleportation,
which is akin to faxing quantum particles. A pair of entangled atoms serve
as transmitter and receiver, said Shahriar. "The atom you want to teleport
is then brought close to the transmitter," he said. "A simple set of measurements
is then made on the transmitter end and the observations are sent via
any method, such as a phone call, to the receiver end. A simple operation
on the receiver atom then turns it into a copy of the one we want to teleport."
Using quantum teleportation, qubits could be transmitted across a quantum
"It's an interesting idea," said Paul Kwiat, a professor of physics at
the University of Illinois at Urbana-Champaign. "[But] at the moment it's
not really clear what you would do with a quantum network. It might be
good for hooking together quantum computers, if we had them," he said.
Quantum network nodes could eventually extend quantum cryptography, which
is currently limited to point-to-point communications lines, said Kwiat.
"One could imagine having quantum cryptography over a whole network,"
The researchers are still working on producing the entangled photons and
storing single atoms, said Shahriar. "Once these are ready, we will embark
on demonstrating the teleportation process itself."
The key to making useful quantum network nodes is building a chip with
an array of optical cavities that each hold a single atom at its center,
Shahriar said. It will be at least 10 years before a quantum network could
be used for practical applications, he said.
Shahriar's research colleagues were Seth Lloyd of the Massachusetts Institute
of Technology and Philip Hemmer, now at Texas A&M University. They published
the research in the October 15, 2001 issue of the journal Physical Review
Letters. The research was funded by the Army Research Office and the Air
Force Office of Scientific Research.
Timeline: > 10 years
TRN Categories: Quantum Computing
Story Type: News
Related Elements: Technical paper, "Long Distance, Unconditional
Teleportation of Atomic States via Complete Bell State Measurements,"
Physical Review Letters, October 15, 2001
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