Statistical Tests of Randomness on Quantum Keys Distributed Through a Free-Space Channel Coupled to Daylight Noise

A suite of random number tests was used to evaluate the randomness of raw cryptographic keys generated using quantum key distribution (QKD). The cryptographic channel was established by coupling an entanglement source over a free-space link during both day and night for a period of 48 h. External noise from sunlight both scattered in the atmosphere and directly reflected within the field-of-view of the receiving optic reduced the randomness of the raw cryptographic keys. Systematic influences on the supposed random key were observed due to both imbalances in the coupling of the source and detectors through the free-space QKD channel, as well as the introduction of noise counts into the raw key from which the final key was distilled. The introduction of noise in the key exchange may drastically reduce the rate of secure key bits extracted from the raw keys using privacy amplification. Synchronization limits for key generation using photon timing correlations were established by experimentally controlling the coupling ratio of unbiased noise to the coincidence counts. The influence of external optical noise on the quantum bit error ratio is discussed, whereby the suppression of external noise, a balance of optical hardware, and active coupling of the free-space link were identified for optimization of the extracted QKD key generation rates.