Figure 1: Experimental setup: Alice’s laser emits a linearly polarized CW beam which later serves as a local oscillator for Bob’s measurements. In terms of Stokes parameters, the local oscillator is S 1 polarized. Alice’s magneto-optical modulator generates a weak signal in the S 2 component. The beam is expanded and sent to a retro-reflector at a distance of 50 m. Bob characterizes the reflected beam by homodyne measurements of the S 2 and S 3 components. The intensity of the local oscillator can be monitored by an S 1 measurement. Free Space Quantum Key Distribution with Coherent Polarization States Dominique Elser (1), Tim Bartley (1,2), Bettina Heim (1), Christoffer Wittmann (1), Denis Sych (1) and Gerd Leuchs (1) 1: Institute of Optics, Information and Photonics, Max Planck Research Group, University of Erlangen-Nuremberg, Günther-Scharowsky-Str. 1, Building 24, 91058 Erlangen, Germany delser@optik.uni-erlangen.de 2: Physics Department, Blackett Laboratory, Imperial College, London SW7 2BZ, United Kingdom Abstract We present an experimental demonstration of Free Space Quantum Key Distribution using Continuous Variables. A local oscillator, inherent in the setup, also acts as spatial and spectral filter thus allowing unrestrained daylight operation. Our prepare-and-measure setup uses binary encoding on coherent polarization states. The quantum states are transmitted over a 100 m free space channel on the roof of our institute’s building. We employ simulta- neous homodyne detection on two conjugate Stokes parameters. Signal and local oscillator are combined in a single spatial mode which auto-compensates atmospheric fluctuations and results in an excellent interference. Introduction In classical telecommunication, free space optics (FSO) can help bridging the “last mile” between network nodes and users where installing a glass fiber is often time-consuming and cost-intensive. Furthermore, FSO is utilized for satellite communica- tion. In the domain of quantum key distribution (QKD) [1] FSO offers an additional benefit: Since fiber losses limit the maximum link range, FSO using ground-to- satellite links is the only feasible way to accomplish QKD over large distances. After the first demonstration of free space QKD in 1996 [2] several prepare-and-measure [2-6] and entanglement-based [7-10] systems have been implemented. Currently, the world record in distance is 144 km [6,9] and satellite QKD is in the starting phase [11,12]. A common feature of all systems up to now is the use of single-photon detectors which, however, are blinded already at low background light intensities. Spatial, spectral and/or temporal filtering has to be employed to reduce background light and daylight operation is still challenging. In our system, we use an alternative approach: With the help of a bright local oscillator (LO) we perform homodyne measurements on weak coherent states. Interestingly, apart from enabling the homodyne measurement, the LO fulfills additional functions in FSO: • Spatial filtering: Only photons which are spatially mode-matched to the LO are detected. Any other stray light is effectively filtered out. • Spectral filtering: Only photons within a frequency range given by the LO and the detector’s electron- ics are detected. • Spatial tracking: Atmospheric beam wander and distortions can easily be monitored in order to compensate for them. • Timing generation: Atmospherically induced time jitter can be determined by applying a temporal modulation to the LO. For a homodyne detection, a good interference of signal and LO is crucial. Stabilizing this interference would be a problem if, as usual, signal and LO were propagating as two separate beams. In our setup, however, we use polarization states which allow for co-propagation of signal and LO in one single beam. Thus the interference is intrinsically excellent and furthermore phase fluctuations in the channel are auto-compensated.