Sending quantum information over long distances

Ever since the idea of quantum communication was proposed, scientists have considered quantum communication systems as completely separate from classical communication systems. However, an international group of scientists is challenging this idea with a quantum communications scheme that incorporates existing classical telecommunications structures.

“This resulting system is more powerful,” explains Peter van Loock, one of the authors of the June 19th letter in Physical Review Letters, “in addition to classical communication, we’d be able to communicate quantum information.”

In their letter, titled “Hybrid Quantum Repeater Using Bright Coherent Light,” van Loock and his colleagues from the National Institute of Informatics in Tokyo, the Nanoelectronics Collaborative Research Center in Tokyo, Stanford University, and the Hewlett-Packard Laboratories in Bristol, England explain how it could be possible to use quantum repeaters in tandem with existing fiber optics to send quantum information over long distances using a series of repeaters.

“The idea of quantum repeaters is not new,” van Loock explains to PhysOrg.com. Scientists have recognized for seven or eight years that repeaters would be necessary in order to prevent the break down of quantum information over long distances. Sending the information over a distance of more than 10 or 20 kilometers could result in the accumulation of so much noise that the original information is gone by the time it reaches the end. A repeater distills the information and then passes it to the next station in a process that would allow information to travel more than 1,000 kilometers.

Van Loock says that other quantum communication schemes concentrate only on using discrete degrees of freedom; entangled matter qubits are distributed by detecting discrete photons or polarization of light. For this, rather than using strong laser pulses, weak pulses are used. While this results in high quality entanglement, it also takes quite a bit of time. Efficiency is low, as there is a great deal of waiting for a successful result.

The theory expressed in the PRL letter written by van Loock and his peers works differently. “We have a hybrid system in that we use both discrete and continuous degrees of freedom. The discrete variable describes the spin in the qubits, and the continuous variables describe the light.” A continuous variable, says van Loock, deals more with light amplitudes and phases than with photon numbers. “It is a more natural way to work with light, similar to what people do in classical optics” he says.

Rather than using a weak laser pulse to carry the entangled qubits from repeater station to repeater station, van Loock and his colleagues propose a bright light pulse carrying 10⁴ photons. To detect the phase of the light and thus produce the entangled spins, this scheme would use homodyne detection, which is generally used in quantum optics, but not incorporated into current schemes of long-distance quantum information transport.

“The main advantages of our system,” explains van Loock, “are that we have high efficiency of detection and high efficiency of entanglement generation. Even though the entanglement is initially slightly worse [than other methods], it is sufficiently good that we can bring it up to near-perfect entanglement.” This is done by a process of distillation.

Van Loock points out that even though the initial fidelity of the system is modest, it works at a much higher speed. Additionally, calculations show that the success rate is at about 40 percent—higher than the success rates offered by other systems.

Right now, such a scheme is theory. However, van Loock says that his Stanford co-authors are already working on ways to experimentally realize the idea. Quantum information transport work is already underway at Stanford, and van Loock believes that once experiments that can establish conditional phase shifts with lasers are realized, it should be possible to experiment with different distribution schemes. The use of bright light, as proposed in the letter, could be tested.

Quantum information transport in this manner would be useful for a variety of future applications, says van Loock: “You could connect quantum computers over long distances, allowing them to exchange information.” He also points out that quantum communication is the only unconditionally secure way to send sensitive information. Such a set-up would allow for transmission of secret information without fear of interception.

However, he does point out that there are some applications for which quantum information is unnecessary. “It’s more practical to keep with classical communications for many things. Much like in quantum computing, it is only for the more complex things that such a scheme is practical.”

And that’s the beauty of a hybrid system. It would allow for classical fiber optic communication systems to remain in place, but also be compatible for quantum information transport when needed. “We believe our scheme is more practical than other schemes,” says van Loock. “It takes the best of both the classical and the quantum worlds.”

By Miranda Marquit, Copyright 2006 PhysOrg.com

Related stories:

Physicists Transmit Light through Opaque Materials
No matter how thick an opaque "scattering material" is, physicists have shown how to weave light through tiny open channels in the material, so that the light passes through on the other side.
World's Largest Quantum Bell Test Spans Three Swiss Towns
In an attempt to rule out any kind of communication between entangled particles, physicists from the University of Geneva have sent two entangled photons traveling to different towns located 18 km apart – the longest distance for this type of quantum measurement. The distance enabled the physicists to completely finish performing their quantum measurements at each detector before any information could have time to travel between the two towns.
How foamy is spacetime?
Maybe not as foamy as some scientists thought, as a fresh look at a quasar first observed in 1998 by the Hubble Space Telescope (HST) shows. Physicists observed a diffraction pattern called an Airy ring around the image of a distant quasar-like object. This ring persuades physicists that the light from this distant object has traveled through a relatively calm – rather than extremely frothy – spacetime.
Engineers Solve Chaos Mystery in Use of High-Tech Atomic Force Microscope
Mechanical engineers at Purdue University have proven that the same sort of "deterministic chaos" behind the baffling uncertainties of the stock market and long-term weather conditions also interferes with measurements taken with a commonly used scientific instrument.
Physicists Achieve Quantum Entanglement Between Remote Ensembles of Atoms
Physicists have managed to "entangle" the physical state of a group of atoms with that of another group of atoms across the room. This research represents an important advance relevant to the foundations of quantum mechanics and to quantum information science, including the possibility of scalable quantum networks (i.e., a quantum Internet) in the future.
Fundamental limitation to quantum computers
Quantum computers that store information in so-called quantum bits (or qubits) will be confronted with a fundamental limitation. This is the claim made by Dutch theoretical physicists from the Foundation for Fundamental Research on Matter (FOM) and Leiden University in an article recently published in the journal Physical Review Letters.
Scientist of the University of Ghent discovers natural atom antihydrogen
On February 15, 2005 of the Physics/Einstein Year, the complete story of the discovery of natural atom antihydrogen, started in 1985, was published on-line.
The antihydrogen problem has become a highly mediatic issue, both in the specialized physics and the more general press [1]. A real hype started at the end of 2002 when rivalling CERN-based groups ATHENA and ATRAP both claimed the production of large quantities of artificial antihydrogen. Scientists, wondering about a signature for the presence of this mysterious species antihydrogen, were disappointed as no direct signature whatever was presented. In fact, a spectral identification of antihydrogen is impossible since measuring its spectrum is exactly the goal of ATHENA and ATRAP collaborations.
Ecma International creates TC44 to standardize Holographic Information Storage systems
Ecma has created Technical Committee 44 (TC44) to develop a standardization strategy for Holographic Information Storage (HIS) systems, initially based upon the Collinear Technologies of Optware Corporation, a leading developer of Holographic Versatile Disc (HVD) storage products. TC44 was set up at the request of Optware Corporation and several of its partners, including CMC Magnetics Corporation, Fuji Photo Film Co., Ltd., Pulstec Industrial Co., Ltd., Strategic Media Technology and Toagosei Co., Ltd.

News discussion:

Entangled Atomic Particle Communication in Physics news