Quantum network withstands noise

By Eric Smalley, 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 another.

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," he said.

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 a time.

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 network.

"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," he said.

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
Funding:   Government
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


January 30, 2002

Page One

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Quantum network withstands noise

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