Long Distance Photonic
Quantum Communication

QuComm

Popular Science Presentation
(Deliverable D3)

The "spooky action at a distance" of quantum mechanics makes possible the science-fiction dream of teleportation--a way to transfer the complete information to reconstruct quantum objects from one place to another without sending the objects themselves. Quantum mechanics also gives us quantum cryptography--secure information transmission, where any attempt allowed by the laws of physics at secretly intercepting the message can be discovered.

Quantum cryptography and quantum teleportation have already been demonstrated with light particles-photons, both in the laboratory as well as in first trials on telecom fibers, many of the experiments done by members of QuComm. But how far can we extend these quantum effects, and will they become the hardware of a future information technology capitalizing on quantum effects?

In the QuComm - "Long Distance Photonic Quantum Communication" project we try to find answers to some of these questions. The project combines groups from Europe (Royal Institute of Technology-KTH Kista, Univ. München, Univ. Wien, Univ. Oxford, Univ. Geneva, THOMSON-CSF, DERA) and the United States (Los Alamos National Laboratory, New Mexico). All groups have a background either in quantum optics and quantum information or in optical communications and optoelectronics.

The objectives of the project are

• To scale experimental quantum communication protocols, notably quantum teleportation and entanglement based quantum cryptography, towards longer distances, and to explore so called entangled states of multiple photons; such states have no counterpart in classical physics.
• To experimentally demonstrate quantum communication protocols for cryptographic applications in point-to-point and multiparty quantum cryptography, using entangled quantum states to achieve an increased level of security.
• To test optical quantum communication technologies in a "real life" context through various field tests of the developed concepts and technologies.
• To identify and transfer "spin off" results from quantum communication technologies to industries, or to industries-to-be.

Quantum Communication; a primer

In quantum communication, the information to be transmitted is encoded on individual photons that are sent either in free space or through low loss optical fibers. Using non-linear optical effects, we are also generating twin-photons, that are entangled in such a way that any measurement one on photon immediately, irrespective of distance, affects the possible outcomes of measurements on the other. It is like having a pair of dice, miraculously prepared in such a way that whenever one of the two shows a certain number, its twin partner will always show the same number, even if one is in Monte Carlo and the second in Las Vegas. "Spooky action at a distance" was the phrase Albert Einstein used to describe the quantum mechanical correlation in entangled states. In QuComm, entangled twin-photons, or even triple- or quadruple-entangled photons will be used for quantum cryptography and to teleport quantum states. The latter process refers to transferring the full information content of a quantum state from one place to the other without sending the state itself. It should be stressed that two particles in the same quantum state are identical; they are indistinguishable even in principle. Thus if we transfer the quantum state from one particle to another we have indeed transferred-"teleported" the particle itself. The requirement of quantum mechanics in the process is that the "identity"-the quantum state of the original particle is destroyed in the teleportation process. Besides being of fundamental interest, quantum teleportation may also play a role in quantum computation, and could also be a way to transfer quantum information between separated quantum "processors".

One should, however, be clear in saying that going from teleporting the quantum state of a single particle, to teleporting the states of larger-macroscopic- systems, not to say of living beings, is quite unrealistic, even though it is not in principle impossible. The reason for this is that the complexity of the quantum states, and the measurements needed to be performed for the teleportation, scales very unfavorably (exponentially) with the number of particles involved in the quantum state. Also the sensitivity to external perturbations scales very unfavorably with system size. It is for the very same reason that very large-scale quantum computation is judged, from the present understanding, to be a very difficult, if not unrealistic, task.

QuComm technology

Much of the technology used in the project, e.g. laser diodes and optical fiber components is basically the same as that used today in optical communications, the backbone of internet and broadband communications. To generate the entangled twin photons we use either non-linear optical crystals or non-linear frequency conversion in diode laser-like sources specially developed for the project. In QuComm much work is devoted to develop sources that are easy to use -- "plug and play"-- which in turn allows more complex transmission experiments to be performed. To detect light pulses on the level of single photons we use avalanche photodiodes, which are also used in optical communications as low noise receivers. The difference to their standard use is that here we operate them in a "Geiger" regime where even a single incident photon creates an avalanche current pulse that can be sensed by the ensuing electrical circuitry. To get a feel for the light levels involved, single-photon detection sensitivity corresponds to the light flux passing trough a 1- mm pinhole of a 60 W standard light bulb placed some 200 km away.

In QuComm we are planning to do field trial experiments of the developed technology, either in optical fiber networks with distances beyond 50 km, or in free space at distances greater than a kilometer. The ultimate free space experiment would be to use quantum cryptography to send cryptographic keys to Low Earth Orbit Satellites.

There are many interesting spin-offs from the QuComm project. Single-photon detection lies at the heart of optical sensing in a variety of applications, such as optical time domain reflectometry (OTDR) for optical communication, sensitive receivers for wavelength division multiplexing (WDM) systems, range-finding and laser radar detection, and in the life sciences for fluorescence correlation spectroscopy.

What to expect of the future?

At the present exploratory stage of quantum information and communication, no one knows what will be most likely technological applications in a five to ten year perspective. Still, you never get anywhere unless you start. Very much technological progress has also taken place since the field of quantum information started in the early 1990s, and quantum information technologies have already had a profound impact on the way we view information and its relation to fundamental physics.

So please follow QuComm on its mission to boldly go where no one has gone before into Quantum Wonderland!

Back to QuComm startpage