Quantum Cryptography's Reach Extended

Foundation laid for device that will make unbreakable codes usable at any distance

1 August 2003—Code makers dream of communicating using quantum states, such as the polarization of photons. Those quantum characteristics can encode and distribute the keys to unbreakable cryptography without fear of their falling prey to an eavesdropper [see ”Making Unbreakable Code,” IEEE Spectrum, May 2002, pp. 40–45]. While that dream has spawned several experimental demonstrations and even near-commercial products, quantum cryptography, as it stands, is severely limited in the distances it can bridge. In recent weeks, however, a number of breakthroughs have extended the distance we can send quantum messages beyond the few tens of kilometers of years past. Even more important to quantum cryptography’s future, physicists are laying the groundwork for so-called quantum repeaters, devices that will allow those messages to be transmitted around the world.

Earlier this year, Andrew Shields and his colleagues at Toshiba Research Europe Ltd. (Cambridge, UK) carried out quantum cryptography by encoding information in the polarization of individual photons sent over 100 km of optical fiber, breaking an earlier record by about 40 km. Photons are less and less likely to be detectable the farther they have to travel. So Toshiba designed new, highly sensitive detectors that can pick out individual photons from background noise. Shields says his detectors makes quantum cryptography possible over metropolitan distances.

But if quantum cryptography is going to span the globe, it will have to take a different form and rely on the ghostly quantum phenomenon of entanglement. In the quantum world, two or more particles can share the same quantum existence, known as a wave function, giving particles that are joined at the hip in this way extraordinary properties. Measuring one of these quantum twins instantaneously determines the quantum state of the other, even if they are on opposite sides of the universe.

For instance, entangled photons can be made so that if the polarization of one measures 45 degrees, the polarization of the other will instantly become –45 degrees. In recent years, physicists have learnt how to exploit this very deep connection to tackle Shannon’s problem of sending quantum states from one point in the universe to another, using entanglement as the medium of transmission.

But beaming entangled photons from one point to another is even more difficult than sending ordinary photons. In practice, it is not possible to distribute entangled particles more than a few tens of kilometers before noise from the environment ruins their link. Earlier this year, Markus Aspelmeyer at the University of Vienna (Austria) managed to beam entangled photons a few hundred meters over the Danube. But even at these short distances, the researchers had to work at night because the signal is overwhelmed by sunlight, although the group is working on ways around this. Without some way of periodically boosting the signal, however, quantum communications will only ever work at these very short distances.

But back in 2001, a group of physicists from the University of Innsbruck (Austria) and Harvard University (Cambridge, Mass.) proposed a way of generating long-distance entanglement. Their idea is to break down the transmission distance into segments, each short enough to allow an entangled link to be created between its ends. The range of the link is then extended by entangling photons in adjacent segments until the message spans the entire transmission distance—from New York City to London, say.

The challenge with this method is to develop the ”quantum repeaters” that sit at the ends of each segment. These devices must first allow the entanglement to be set up within the segment and then be capable of extending that entanglement to the neighboring segment. In the last two months, two groups have reported their preliminary work, making this idea a reality.

In theory, a repeater can consist of a single atom of cesium, for example, which first absorbs an entangled photon, becoming entangled with the photon’s twin. This is done by holding the atom in a mirrored cavity in which the photon can bounce back and forth many millions of times. The repeated interaction between the atom and photon allows the entanglement to be swapped from one to the other.

In effect, the atom temporarily stores the quantum state of the photon. The atom can then pass on this state by re-emitting the photon. However, this is a tricky process because of the difficulty in building cavities that perfectly reflect photons. If the mirrors aren’t close to perfection, they will absorb the photon instead of allowing the atom to do so.

In June, Alex Kuzmich and colleagues at the California Institute of Technology (Caltech) in Pasadena announced that they got around this problem by replacing the single atom with an ensemble of many millions of atoms. So, instead of the photon interacting with a single atom many millions of times, it interacts once with many millions of atoms. The result is that the photon swaps its state with the entire ensemble of atoms, and the entire ensemble becomes entangled. The process is reversible, says Kuzmich, making his ensemble of atoms a pretty good building block from which to develop a quantum repeater.

A similar approach was used by a group at Harvard University. But instead of dealing with single photons, the team used rubidium atoms and pulses of light. The quantum repeater is their ultimate goal, too. ”This is now only the beginning of a long road,” says Mikhail Lukin, who leads the Harvard group and was one of the physicists who originally suggested the quantum repeater approach in 2001.

Both the Caltech and Harvard groups admit that their work is at the proof-of-principle stage and that they are a long way from making a working telecommunications device. But such is the demand for unbreakable encryption that commercial products are already hitting the market, albeit working only over the shorter distances currently possible without a repeater.

Two companies focusing on quantum physics have formed. In 2001, four researchers from the University of Geneva (Switzerland) started a company called ID Quantique [see ”Quantum Physics Spin Off Marketable Products,” IEEE Spectrum, May 2002, pp. 21–22]. Grègoire Ribordy, the physicist who runs the company, says it is currently working on integrating a test system into the existing communications networks of a number of pilot customers in the financial services and information technology industries.

MagiQ Technologies, a quantum mechanics technology start-up based in New York City, began beta testing a similar system in March. The company is working with customers both in the financial services industry and in academic and government labs.

For the moment, the systems of both these companies have a range of only a few tens of kilometers. ”Within metropolitan areas, that is usually all you need,” says Ribordy.

It may be some time before the range of these systems can be extended. Harvard’s Lukhin is confident that the work on quantum repeaters will continue apace. ”At the moment, it’s just exciting science, but we should be able to demonstrate and study elements of repeater physics in the next few years,” he says.

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