Secure communications with low-orbit spacecraft using quantum cryptography

Apparatus for generating a key to be used to secure communication between a spacecraft in a low-earth-orbit and an earth station comprising:

• a laser for outputting pulses of light;
• means for splitting said pulses of light into preceding bright pulses and delayed attenuated photon pulses;
• polarizing and beamsplitting means receiving said preceding bright pulses and said delayed attenuated photon pulses for linearly polarizing said preceding bright pulses and said delayed attenuated photon pulses;
• means for generating a string of random numbers;
• a Pockels cell receiving said polarized preceding bright pulses and said polarized delayed attenuated pulses for switching said linear polarization of said delayed attenuated pulses and said string of random numbers so that a first polarized delayed attenuated photon pulse represents a "1," and a second polarized delayed attenuated photon pulse represents a "0," in accordance with said string of random numbers, said first and second polarized delayed attenuated photon pulses having any polarization states which are different, but not orthogonal to each other;
• a first telescope in optical communication with said Pockels cell for transmitting said polarized preceding bright pulses and said first and second polarized delayed attenuated photon pulses into space toward said spacecraft;
• a second telescope at said spacecraft for receiving said transmitted polarized preceding bright pulses and said first and second polarized delayed attenuated photon pulses;
• a first beamsplitter receiving said polarized preceding bright pulses and said polarized delayed attenuated photon pulses for randomly directing said polarized preceding bright pulses and said polarized delayed attenuated pulses onto a longer path, or onto a second shorter path, said shorter path including means for changing said polarization of said polarized preceding bright pulses and said polarized delayed attenuated pulses by a quarter-wave
• a second beamsplitter receiving said longer path and said shorter path for directing said polarized preceding bright pulses and a first portion of said polarized delayed attenuated pulses to a first detector and a second portion of said polarized delayed attenuated pulses to a second detector, said polarized delayed attenuated pulses detected by said second detector being used for determining a key for secure communications between said earth station and said satellite.