ASSESSMENT OF CURRENT RESULTS AND OUTLOOK ON FUTURE EFFORTS
QUANTUM COMMUNICATION
Quantum communication is the art of transferring a quantum state from one location to another. Quantum cryptography was discovered independently in US and Europe. The American approach, pioneered by Steven Wiesner, was based on coding in non-commuting observables, whereas the European approach was based on correlations due to quantum entanglement. From an application point of view the major interest is Quantum Key Distribution (QKD), as this offers for the first time a provably secure way to establish a confidential key between distant partners. This key is then first tested and, if the test succeeds, used in standard cryptographic applications. This has the potential to solve a long-standing and central security issue in our information based society.
While the realisation of quantum communication schemes is routine work in the laboratory, non-trivial problems emerge in long-distance applications and high bit rate systems. At present, the only suitable system for long-distance quantum communication is photons. Other systems such as atoms or ions are studied thoroughly; however their applicability for quantum communication schemes is not feasible within the near future, leaving photons as the only choice for long-distance quantum communication. One of the problems of photon-based schemes is the loss of photons in the quantum channel. This limits the bridgeable distance for single photons to the order of 100 km with present silica fibers and detectors. Recent quantum cryptography experiments already come close to such distances. In principle, this drawback can eventually be overcome by subdividing the larger distance to be bridged into smaller sections over which entanglement can be teleported. The subsequent application of so-called “entanglement swapping” and “quantum memory” may result in transporting of entanglement over long distances. Additionally, to diminish decoherence effects possibly induced by the quantum channel, quantum purification might be applied to eventually implement a full quantum repeater.
There are two media that can propagate photons: optical fibers and free space. Each of these two possible choices implies the use of the corresponding appropriate wavelength. For optical fibers, the classical telecom choices are 1300 and 1550 nm and any application in the real world of quantum communication in fibers has to stick to this choice. For free space the favored choice is either at shorter wavelengths, around 800 nm, where efficient detectors exist, or at much longer wavelengths, 4-10 microns, where the atmosphere is more transparent.
Recall that quantum physics can deliver «correlations with promises». In particular it can deliver at two locations strictly correlated strings of bits with the promise that no copy of these bits exist anywhere in the universe. This promise is guaranteed by the laws of Nature, they do not rely on any mathematical assumption. Consequently, such two strings of correlated bits provide perfect secure keys ready to be used in standard crypto-systems. However, for quantum physics to holds its promise, truly quantum objects, like photons, have to be send from one location to the other. Since quantum object interacting with the environment lose their quantumness, i.e. become classical object, it is crucial to isolate the photons during their propagation.
Consequently, it is of strategic importance to develop the technology to send photons from one location to a distant one while preserving its truly quantum nature. The test of this quantumness consists in measuring the correlations and proving that they do violate a certain inequality, known as the Bell inequality.
From the present situation, where commercial systems already exist, there are three main directions to be pursued, which we review one after the other.
