4.1 Quantum Communication

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Quantum communication is the art of transferring a quantum state from one location to another, in this way information, or resources such as entanglement, can be distribributed between distant locations [1]. The communication of qubits will be an important ingredient in taking full advantage of what is possible with quantum technologies, from quantum computing and simulation to secure communication based on quantum key distribution (QKD). 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 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 -- relying solely on the laws of quantum physics and the ability to implement the protocol as defined by the theory. This has the potential to solve long-standing and central security issues 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, long-distance applications and as the network complexity increases. One of the emerging areas of interest for quantum communication schemes is in connecting the nodes within quantum simulators, which can either be all located in the one lab, or more interestingly, in distributed scenarios -- the tools from quantum communication playing the role of wiring circuits for these quantum computers. While there remains many challenges for proof-of-principle laboratory demonstrations, the transition to deployment in real-world environments defines a new set of challenges in the QIPC 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 as we work towards a quantum internet. To succeed this needs to target both the underlying technologies, ranging from fundamental aspects of engineering quantum systems to integrating these quantum systems with fast (classical) opto-electrial systems, as well as the end-user applications themselves. In particular the following need to be addressed:

  1. Point-to-point Quantum Communication: Improving detection and electronics, or even new protocols for practical, real-world, systems and to reinforce the commercialisation of QKD technologies. New protocols, beyong QKD, are needed to extend the benefits of quantum security to other applications;
  2. Quantum Networks: Demonstration of trusted-node or repeater architectures will be essential for increasing distances to continental, and even global, scales. Quantum repeater concepts will also be critical in the context of computation and simulation, both for short distance scales (local) or long, (distributed), processing systems. This requires hybrid systems linking quantum sources, interfaces, memories and detectors with performance significantly greater than the current state of the art.
  3. Implementation & Security: Increasingly complex quantum networks of disparate technologies require new approaches for ensuring security. Quantum hacking is necessary for improving the system technologies although Device Independent & Self Testing systems provide a new perspective with the potential to also minimise security assumptions and hence simplify the security of real-world quantum communication systems.

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 (~4 Mbps for commercial devices) quantum random number generators (QRNGs)- see Section 4.4. 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. In free space, these distances are even lower. There are two possible solutions to overcome this limitation: use satellite configurations, i.e. free space systems; or, use quantum repeaters, a theoretical concept proposed in 1998 the analogue of fibre optical amplifiers that made global fibre communication feasible. The first clearly requires satellites and systems are currently being developed and tested to meet the associated demands. Several countries already have planned missions to launch quantum systems for further testing. This however only addresses increasing point to point distances. Quantum repeaters require quantum interfaces or memories for the inter-conversion from photonic (distribution) to atomic (storage) systems and while increasing distances, it also opens up the possibility of more complex network structures. This is perhaps one of the most active areas of QIPC research.

One of the great advantages of quantum physics is that it can deliver "correlations with promises". In particular it can deliver strictly correlated strings of bits to two locations 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 provably 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. It is of strategic importance to not only develop the technologies to distribute quantum resources, such as entanglement from one location to another one but to be able to ensure that its truly quantum nature is preserved and that these systems are will be described by their abstract security proofs. The gap between theory and experiment has to be reduced. This needs to take several paths: the theory needs to consider the practical implementation and hence the experimental parameters. Experimentalists need to better engineer these systems to avoid potential loopholes. the Another alternative arises from a key test of the quantumness, which consists in measuring correlations and proving that they violate a certain inequality, known as the Bell inequality. This approach has lead to the idea of "Device Independent" security proofs that provides the possibility of 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 increasing rates and distances to solutions extending point-to-point QKD towards complex quantum networks for the distribution of quantum resources and for performing new protocols.

Key references
[1] N. Gisin and R. T. Thew, Nature Phot 1, 165 (2007)