Quantum key distribution system

A key distribution method comprising:

• (a) emitting a plurality of light signal pulses, wherein each light signal pulse is emitted with a corresponding encoding and the plurality of light signal pulses includes signal pulses with at least four different corresponding encodings, and wherein each light signal pulse is emitted with the same wavelength;
• (b) combining the plurality of light signal pulses into a combined pulse sequence;
• (c) filtering the combined pulse sequence through a spatial filter having an aperture diameter that is less than or equal to the specified wavelength to generate a filtered light signal sequence;
• and
• (d) collimating the filtered light signal sequence into a collimated pulse sequence.

An optical transmitter comprising:

• (a) a plurality of light sources, each light source configured to emit a corresponding light signal with the same specified wavelength, wherein each light source element is configured to emit the corresponding light signal with a unique encoding;
• and
• (b) a spatial filter having a filter aperture with a diameter less than or equal to the specified wavelength;
• wherein the optical transmitter is configured to output a plurality of pulses of light that are generated by the light sources and directed through the spatial filter, wherein the plurality of pulses of light average at most one photon per pulse.