Friedrich-Alexander-Universität Erlangen-Nürnberg

Free Space Quantum Key Distribution

Free Space Quantum Key Distribution

Fig. 1: Experimental Setup: Alice generates a laser beam of a particular polarization, that later serves as the local oscillator (LO) for Bob’s measurements. The weak signal states originate from a modulation of the LO using two electro optical modulators. The beam containing signal states and the LO is expanded and sent through a 1.6 km urban link. Bob then performs a Stokes measurement.

Quantum key distribution (QKD) is the process of establishing a secret key shared between two parties, traditionally named Alice and Bob. The security is based on the laws of quantum mechanics, whereas in classical schemes security relies only on the unproven lack of efficient mathematical algorithms.

We have developed a QKD protocol which is particularly suitable for atmospheric transmission: we employ a local oscillator to perform an optical homodyne detection of weak coherent signal states. Alice utilizes polarization states to combine signal and local oscillator in a single beam. As a consequence, Bob’s detection is very efficient and perfectly shielded against any stray light.

We have successfully demonstrated the experimental feasibility of this protocol over a distance of 100m on the roof of the building of the Max Planck Institute for the Science of Light (MPL) [1,2]. Thus, we subsequently established a quantum link of length 1.6km in an urban environment between the MPL building and the University computer centre. [3]. Currently, we are implementing further tests and improvements of our protocol including different modulation schemes over this atmospheric channel. Furthermore, as low losses are a crucial point for quantum communication protocols, we have developed an active beam stabilization in order to increase the overall channel transmission.

In parallel, our quantum link will also be used to transmit continuous variable squeezed and entangled states of light.



[1] B. Heim, D. Elser, T. Bartley, M. Sabuncu, C. Wittmann, D. Sych, C. Marquardt and G. Leuchs, Appl. Phys. B, 98 (4), 635-640 (2010).

[2] D. Elser, T. Bartley, B. Heim, C. Wittmann, D. Sych and G. Leuchs, New Journal of Physics 11, 045014 (2009).

[3] B. Heim, C. Peuntinger, C. Wittmann, C. Marquardt, and G. Leuchs, "Free Space Quantum Communication using Continuous Polarization Variables" in Applications of Lasers for Sensing and Free Space Communications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper LWD3



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