Method and apparatus for free-space quantum key distribution in daylight

Apparatus for securely generating a key to be used for secure transmission between a sender and a receiver connected by an atmospheric transmission link, comprising:

• at least one first laser for outputting timing bright light pulses;
• second and third lasers for outputting first and second polarized optical data states;
• fourth and fifth lasers for outputting third and fourth polarized optical data states;
• a random bit generator for randomly enabling either the second and third lasers or the fourth and fifth lasers;
• output optics effective to transmit output light from the first, second, third, fourth and fifth lasers;
• receiving optics effective to receive output light of the output optics from the first, second, third, fourth and fifth lasers;
• a first beam splitter for receiving light from the receiving optics, where a received bright light pulse is directed to a delay circuit for establishing a window for receiving light from the second, third, fourth or fifth lasers and where an optical data state from either the second, or third, or fourth or fifth laser has a probability of being either transmitted by the beam splitter or reflected by the beam splitter
• a first polarizer for receiving transmitted optical data states to output one data bit value;
• a second polarizer for receiving reflected optical data states to output a second data bit value;
• and
• a computer receiving pulses representing receipt of a bright timing pulse and the first and second data bit values, where receipt of the first and second data bit values is indexed by the bright timing pulse.