Simple optics make quantum relay

By Eric Smalley, Technology Research News

If it weren't for repeaters, the light pulses that carry information over fiber-optic long distance lines would fade before they got much further than 100 kilometers.

Quantum cryptography devices and networks, which transport photons whose properties can be used to represent the 1s and 0s of digital information, could also benefit from repeaters. Today's prototype quantum cryptography systems provide theoretically perfect security, but these systems can't carry information over long distances.

Researchers from the NASA-Caltech Jet Propulsion Laboratory have found a way to make a quantum repeater using ordinary optical equipment. Practical quantum repeaters could boost the reach of quantum cryptography systems, and eventually enable quantum networks. The device would allow for an exponential improvement in the distance quantum bits can be transmitted, said Jonathan Dowling, a principal scientist at at the Jet Propulsion Laboratory.

The challenge was finding a way to preserve entanglement.

Particle properties like polarization can become entangled when two or more particles come into contact with each other or simultaneously interact with a third entity like another particle or a laser beam. Entanglement keeps properties like polarization linked, regardless of the distance between entangled particles. A photon's electric field can be polarized, or oriented, in one of four directions. Pairs of directions can represent binary numbers.

Entanglement is the basic ingredient of many quantum computing, quantum cryptography and quantum communications schemes. Sharing entangled particles between locations makes theoretically perfectly secure communications possible because the traits of a series of particles can form a random string of bits that can be used to encrypt messages. It is impossible for an eavesdropper to copy or intercept the particles without disrupting the entanglement, which would reveal the security breach.

Shared entanglement would also make it possible to network quantum computers. "Many quantum communication protocols rely on shared entanglement between two distant parties," said Pieter Kok, one of the Jet Propulsion Laboratory researchers who is now at Hewlett-Packard Laboratories.

But because photons must be in the same place when they are initially entangled, using entangled particles for communication means finding a way to transport them, he said. This is difficult because particles can't be copied without destroying their quantum information, which means ordinary repeaters, which produce copies of fading signals, can't be used for quantum communications.

The researchers' linear optical quantum repeater uses optical elements like mirrors, beam splitters and photodetectors to purify and transfer entanglement among photon pairs. Entanglement purification makes two or more partially entangled states into one fully entangled state. Entanglement swapping converts entanglement: entanglements between particles A and B and particles C and D can be converted to an entanglement between A and D.

Beam splitters direct photons in one of two directions based on the photons' polarization, and photodetectors at each output of a beam splitter determine a photon's polarization. The repeater is made up of a network of beam splitters and photodetectors that route photons based on whether specific photodetectors detect other photons. The combination of the right paths and detection-triggered routing is enough to carry out entanglement purification and swapping.

To use the system to initiate quantum communications, a sender, Alice, would entangle photons A and B, keep A, and send B to a receiver, Bob. A repeater in the network between Alice and Bob would generate a new pair of entangled photons, C and D, and bring together B and C. This would destroy B and C and in the process leave A entangled with D. The device would then send photon D on to Bob, giving Alice and Bob a shared pair of entangled photons. Rather than copying photons, the quantum repeater transfers entanglement.

In practice, there are degrees of entanglement, and in order to transmit entangled states of high enough purity, quantum communications schemes typically distill multiple entangled pairs down to a single pair of fully entangled photons. In the researchers' repeater, the purification step takes place before the entanglement swapping.

The linear optical quantum repeater was inspired by the landmark theoretical demonstration of linear optical quantum computing by Emanuel Knill, Raymond Laflamme and Gerard Milburn in 2001, said Dowling. "Since a repeater is just a very simple type of quantum computer, logic dictated it would be possible, but the devil was in the details," he said.

Other research teams have devised quantum repeaters that tap the interactions of photons with gas atoms. In these schemes, fading photons that enter a repeater transfer their quantum states to atoms, which can briefly store the state information until it can be transferred to fresh photons that are transmitted over the next leg of the network. Light-matter interactions are difficult to carry out, however, especially with equipment that could be used in practical communications networks.

A third approach uses nonlinear optical quantum repeaters that use complicated equipment to cause photons to interact with each other; these may be harder to make than the linear design, said Dowling.

The researchers' goal is to develop simple devices that prove the utility of their linear optical approach, and eventually use the approach to build a full-scale quantum computer, said Dowling.

A reliable source of entangled photons is a top priority, said Kok. "It not only has to be able to make high-quality entanglement, it also needs to do this reproducibly," he said. "Two sources must produce almost indistinguishable photon pairs in order for the interference to work." Another key component that needs to be developed is quantum memory so that, for instance, Alice can hold onto her half of the original entangled photon pair.

And the system eventually has to be miniaturized into a quantum optoelectronic chip, according to Dowling.

Such systems could eventually be used in quantum cryptography systems, for quantum telecommunications, and for distributed quantum computing, said Dowling. It will be 20 years before the method can be used practically, he said.

Dowling and Kok 's research colleague was Colin P. Williams. The work appeared in the August 1, 2003 issue of Physical Review A. The research was funded by the National Aeronautics and Space Administration (NASA), and The Advanced Research and Development Activity (ARDA), the National Security Agency (NSA), the Office of Naval Research (ONR), and the Defense Advanced Research Projects Agency (DARPA).

Timeline:   20 years
Funding:   Government
TRN Categories:   Quantum Computing and Communications; Physics; Cryptography and Security; Optical Computing, Optoelectronics and Photonics
Story Type:   News
Related Elements:  Technical paper, "Construction of a Quantum Repeater with Linear Optics," Physical Review A, August 1, 2003.


February 12, 2004

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