Mon, 2010-03-01 16:30 - Daniele Binosi

Quantum Communication is the art of transferring quantum states from one place to another. The general idea is that quantum states encode quantum information: hence quantum communication also implies transmission of quantum information and the distribution of quantum resources such as entanglement. Quantum Communication covers aspects of basic physics as well as of practical relevance. Additionally, it will take care of the whole “wiring” inside a quantum computer, i.e., contribute to the quantum interface. Already now, one of its outstanding results is the emerging technology of quantum cryptography, which promises absolute secure transmission of the key codes that are essential to encrypt messages with tamper proof security. More specifically, any encryption scheme entails the distribution of a secret key among legitimate users; as the key must be transmitted between sender and recipient, it is susceptible to interception by an eavesdropper. For a secret key made of classical bits, none of the two parties will ever know that their communication has been intercepted. Not so if the key is carried out by a quantum communication channel. Qubits, unlike classical bits, do not possess definite values, such as the 0 or 1; rather, they represent a so-called coherent superposition of physical states (e.g., the polarisations of a photon). The laws of quantum mechanics imply that the mere act of observing a quantum bit modifies it, causing it to change its quantum state. The eavesdropper’s attempt to intercept the secret key made of qubits will therefore be manifest to both parties.

Quantum cryptography is now developing from the initial approach known as point-to-point Quantum Key Distribution (QKD), towards the management of quantum-based security over many-node networks, that are running in various places worldwide (Europe, Japan). Presently, technical problems are controlled well enough so that secure transmissions over a few tens of kilometres can be implemented. However, non-trivial problems emerge for really long-distance communication (hundreds to thousand of kilometres), and in the quest for higher bit rates. High-flux single photon sources as well as entangled photon sources should be developed in order to enhance secure medium range quantum communication. At present photons are the only suitable system for medium-distance quantum communication, as they maintain a robust quantum state throughout transmission, can be detected efficiently and with low levels of noise (other systems, such as atoms or ions, can be used for building quantum memories but not to propagate qubits over long distances).

Nonetheless, even light signals, whether viewed classically or quantum-mechanically, are dampened exponentially with distance in both optical fibres and free space. Both fundamental and more applied efforts are needed to address the problems facing the production, detection and distribution of qubits. In classical optical telecommunication, this problem is solved by using simple devices known as repeaters that amplify and reshape the transmitted signal. However, these are of no use for quantum communication: they are intrinsically noisy and create so many errors that any quantum key being transmitted would not survive. This is related to the fact that a classical repeater breaks down quantum entanglement, a purely quantum phenomenon associated with very strong, non-classical correlations between the states of two widely separated qubits. In parallel, novel protocols (for instance based on entangled qudits), that could enhance the fault-tolerance of quantum communication schemes, need further investigation. Entanglement is a crucial element in quantum communication schemes, which allows one to ‘teleport’ qubits directly to their destination, avoiding transmission losses. So quantum communication must reinvent the repeater concept, using quantum hardware that preserves entanglement. A further motivation for entanglement-based schemes comes from security based on Bell Inequalities, so called “Device Independent” security proofs that need to be studied and demonstrated experimentally.

**Real world medium-distance quantum communication**. If Quantum Communication is to become, on the 5 to 10 year time-scale, an established technology, backing up the quantum cryptography “boxes” which are already commercialised, several scientific as well as technological gaps have to be filled. While in recent years we have seen free space quantum communication over 144km and fibre demonstrations over 200km, both in field trials, many barriers remain. In particular, when demonstrating the feasibility of ‘real world’ medium-distance quantum communication both in optical fibres and in free space, a significant increase in the qubit transfer rate by several orders of magnitude will be required. These goals, together with the one of realising long-distance secure quantum networks will be significantly advanced by developing quantum repeaters Achieving these goals will require facing a number of non-trivial challenges, needing very strong interaction between fundamental and applied research.

**Quantum repeaters**. In the long term a quantum repeater would actually be a small, dedicated, quantum processor, incorporating quantum memories, which, whilst feasible, requires a significant effort and is perhaps the most important technological hurdle facing QIFT. So far we have seen some first experimental steps towards elements needed for a quantum repeater, but there is much work to be done. Some of the basic elements that need to be developed and demonstrated are: medium range entanglement between memories, teleportation between different memories. The exact number of qubits that would have to be stored and processed in such a repeater, to ensure high-fidelity quantum communication over thousands of kilometres, is an open issue and highly dependent on the protocol. Nonetheless, it is likely to be in the range of tens or hundreds – much lower than the number required for a fully-fledged quantum computer. Therefore it is more likely that we will have secure global quantum communication before quantum code breaking.