| A free-space quantum key distribution system is described, in which binary information (the key) is encoded onto polarization states of single photons. In this type of system, any potential eavesdropper will induce measurable changes in polarization state, and therefore can be detected. Designs of state-of-the-art free-space systems are reviewed, along with their performance in terms of bit error rates. Our system design for operation in daylight is described, including the use of spectral, temporal, and spatial filtering to reduce the effects of unwanted ambient photons. Experimental results are given, showing soft bit error rates in the range of 1-5%, which are typical for current systems. However, this type of implementation has seen to be adversely affected by ambient temperature changes; these effects are also discussed. Therefore, a new design for a free-space quantum cryptography system is given, which is overall less complex, more stable, has potentially higher data rates, and lower cost than previous implementations. Experimental results for this system are given, and performance is compared to previous implementations. |
