ABSTRACT

Contents 2.1 A Geneva-Biased Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.2 Time-Bin Qubits and Higher Dimensions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.3 Faint Laser Quantum Cryptography: The Plug & Play

Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.3.1 Basics of Faint Laser Quantum Key Distribution . . . . . . . . . . . . . . 23 2.3.2 A Practical Realization: The Plug & Play

Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.4 Two-Photon Quantum Cryptography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.4.1 Single-Photon Based Realizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.4.2 Entanglement-Based Realizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.4.2.1 Long-Distance Quantum Correlation . . . . . . . . . . . . . . . 29 2.4.2.2 Quantum Key Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.5 The Future of Quantum Cryptography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 2.5.1 Quantum Cryptography and Entanglement . . . . . . . . . . . . . . . . . . 35 2.5.2 PNS Attacks and Countermeasures . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 2.5.3 Three-and Four-Photon Quantum

Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 2.5.4 Conclusions and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Abstract This chapter reviews experimental and theoretical achievements of the Group of Applied Physics (GAP) at University of Geneva in the domain of quantum communication. All work presented can be motivated by the goal to render experimental quantum key distribution simple and robust, and to

and

devise means to extend the maximum transmission distance in spite of technical imperfections like lack of single-photon sources, lossy quantum channels and non-perfect detectors. In detail, we present an auto-aligning “plug & play” system for quantum key distribution based on faint laser pulses, two entanglement-based systems, teleportation in quantum relay configuration and finally entanglement swapping. All experiments take advantage of photons at telecommunication wavelengths and optical fibers, and use timebin encoding which enables us to demonstrate the different protocols over distances of a few kilometers to several tens of kilometers. In addition, we developed a new protocol for quantum key distribution which also enables extending the maximum transmission distance in spite of so-called photon number splitting eavesdropper attacks and non-ideal faint laser pulses instead of true single photons.