QUROPE Quantum Information Processing and Communication in Europe

Quantum communication is the art of transferring a quantum state from one location to another. The communication of qubits will be an important ingredient in taking full advantage of what is possible with quantum technologies, from quantum computing to unconditionally secure communication based on quantum key distribution The first application, quantum cryptography, was discovered independently in the 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 has focused on 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 basic quantum communication schemes is becoming routine work in the laboratory, non-trivial problems emerge in high bit rate systems and long-distance applications. The transition from proof-of-principle laboratory demonstrations to deployment in real-world environments defines a new set of challenges in the QIFT domain. The issues of scale, range, reliability, and robustness that are critical in this transition cannot be resolved by incremental improvements, but rather need to be addressed by making them the focal point of the research and technology development agenda. This needs to target both the underlying technologies:

• Detectors
• Sources
• Quantum Memories and Interfaces

as well as their integration for specific applications, such as:

• High rate, fibre or free space quantum communication
• Quantum Repeaters
• Satellite-based communication links

There are key technological limitations for high-speed quantum communication and fundamental roadblocks for long distances. A significant speed limitation on the distribution of true randomness, a resource for many security protocols including QKD, is due to relatively slow (~4Mbps) quantum random number generators (QRNGs) – see Appendix A. Novel schemes and advanced entanglement enabled technologies, using and possibly combining both discrete and continuous variable encoding aspects, will be required for the next generation devices to surpass current rate limitation. The distances over which quantum information can be communicated face fundamental limitations due to transmission losses in the quantum channels, both free space and fibre. In fibre, this limit is a few hundred kilometres. Recent quantum cryptography experiments already come close to such distances but with impractically low distribution rates. There are two possible solutions to overcome this limitation: use free space systems in satellite configurations; or, use quantum repeaters, a theoretical concept proposed in 1998 the analogue of fibre optical amplifiers that made global fibre communication feasible. The later requires quantum interfaces or memories for the inter-conversion from photonic (distribution) to atomic (storage) systems.

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 exists anywhere in the universe. This promise is guaranteed by the laws of Nature and does not rely on any mathematical assumption. Consequently, these strings of correlated bits provide perfectly secure keys ready to be used in standard crypto-systems. However, for quantum physics to hold its promise, the quantumness of these distributed systems needs to be ensured. Consequently, it is of strategic importance to not only develop the technology to distribute quantum resources, such as entanglement from one location to a distant one but to be able to ensure that its truly quantum nature is preserved. A key test of this quantumness consists in measuring the correlations and proving that they violate a certain inequality, known as the Bell inequality. Following on from this is the idea of “Device Independent” security proofs that provide one possibility for characterising the quantum nature of a system. Practical and feasible schemes to test these device independent approaches and ensure the quantum nature of systems will be crucial as communication links and networks become more complex.

From the present situation, where commercial systems already exist, we briefly review the underlying foundational technologies and more generally, quantum communication from the perspective of high-rate and long-distance solutions and how to characterise and optimise these systems and resources.