QUIE2T Quantum Information Entanglement-Enabled Technologies

Approach and perspectives

The field of quantum communication is still very young, having been essentially unknown until 15 years ago. As such, one should expect new ideas and leave open space for fundamental research. From the theoretical point of view, there are several problems that have to be considered in the context of quantum communication. First of all, since the field is still very young, one should expect new applications related to both efficiency and secrecy in communication. Examples of the first can be connected to secret voting protocols, digital signatures, or fingerprinting. Examples of the second field could be, for example, connected to dense coding, or agenda protocols. Apart from that, there are still many open theoretical questions of crucial importance for quantum cryptography. These are related to the tolerance to noise of current protocols (both with one and two way communication), the connection between single photon and continuous variable protocols, and the search for more efficient and faster ways of distributing keys and quantifying their security.

For some quantum communication applications it can be useful to operate in a larger dimension Hilbert space. This can be obtained by preparing two photons entangled in more than one degree of freedom (hyper-entangled) for increasing the number of qubits or making more efficient measurements. Other proposals concern the generation of d-level quantum systems (qudits) by using different degrees of freedom. Quantum communication protocols can be often understood as entanglement manipulation protocols. An important class of these protocols delivers classical data with properties derived from the underlying quantum state. For this class the question arises whether one can replace the quantum manipulation and subsequent measurement by another two-step procedure that first measures the quantum states and then performs classical communication protocols on the resulting data to complete the task. Such an approach would be preferential in real implementations, as is illustrated in the case of quantum key distribution. It is important to study under which circumstances such a replacement can be done. The adaptation and demonstration of device independent QKD will also be important for future secure networks. A relatively new idea could be using quantum memories to perform local operations and store the results while the classical communication is going on in communication protocols, which require local operations and classical communication (LOCC) are required. Transforming ideas of percolation to quantum networks has been a relatively new concept but one that opens some fascinating possibilities for network distribution of entanglement.

European groups working in this field include: S. Massar & N. Cerf (Brussels, B), A. Acin (ICFO, E), N. Gisin (Geneva, CH), M. Plenio (Ulm, D), J Eisert (Potsdam, D), R. Renner & S. Wolf (Zurich, CH), R. Werner (Braunschweig, D), H. Buhrman (Amsterdam, NL).

State of the art

Important progress has been made in developing new protocols for quantum repeater architectures. A key concept that was recently introduced was the multimode capacity of quantum memories, which allows orders of magnitude increases in distribution rates [1]. Combining this with approaches that serialise distribution [2] may hold the potential for high rates and long distance. The possibility of a cheat sensitive quantum protocol to perform a private search on a classical database [3] have also been proposed with potential for experimental demonstrations foreseen. A return to some of the foundational concepts of QIFT has seen Bell inequalities find renewed importance for so-called “Device Independent” security [4].

Challenges

The main challenges for new applications and protocols are:

• Explore new verification strategies of single and multipartite quantum information links;
• Realize new modes of teleportation as a quantum communication primitive;
• Security proofs need to be optimised to cope with a wide range of experimental parameters (e.g. excess noise). The quantum channel can be verified by effective entanglement measures and / or Bell inequalities;
• CV QKD protocols should be optimised to reduce the impact of decoherence and/or noise in the channel;
• It is known that existing classical communication procedures and security proofs do not make optimal use of the correlations that are generated in the physical set-up and can be improved. Further improvement in secure key rate can follow from a scenario of trusted sending and receiving devices, which cannot be manipulated by an eavesdropper. It would also be valuable to have security proofs easier to understand for classical cryptographers;
• Develop new quantum repeater protocols that are robust with respect to loss & low component efficiencies;
• Lab demonstration of device-indpendent QKD;
• Break with the paradigm that noise is necessarily harmful and innovate tools of state engineering and quantum information protocols based on noise and measurements.

[1] C. Simon, et al., Phys. Rev. Lett., 98, 190503 (2007)
[2] W.J. Munro et al., arXiv:0910.4038v1 (2009)
[3] V. Giovannetti, et al., Phys. Rev. Lett., 100, 230502 (2008)
[4] A. Acín, et al., Phys. Rev. Lett., 98, 230501 (2007)