Quantum non-demolition optical tapping

A quantum non-demolition optical tap comprising:

• a first optical coupler having a first and a second pair of optical communication ports in which substantially equal first signal portions of an optical signal at a first wavelength received at a port of one pair are coupled to the two ports of the other pair of ports;
• an optical waveguide coupling together the second pair of ports and including an interaction section which includes a material having a non-linear refractive index;
• a continuous wave (cw) optical source for providing a cw optical probe signal at the first wavelength optically coupled to a first port of the first pair of ports;
• means for coupling a received optical signal at a second wavelength different from the first wavelength to the interaction section so the received optical signal propagates along it in substantially one direction;
• the said means for coupling including a dichroic coupler for combining the cw probe signal and any received optical signal, the dichroic coupler being coupled to one port of the first pair of optical communications ports;
• the magnitude of non-linearity of the non-linear material being sufficient for the received optical signal to provide a relative phase shift in the first signal portions as they propagate round the optical waveguide.

A method of operating a quantum non-demolition tap comprising:

• providing a first optical coupler having a first and a second pair of optical communication ports;
• coupling together the second pair of ports with an optical waveguide including an interaction section which includes a material having a non-linear refractive index;
• and
• injecting a continuous wave optical signal at a first wavelength to a first port of the pair of ports;
• receiving at one port of said first pair of optical communication ports said continuous wave optical signal at a first wavelength;
• coupling substantially equal first signal portions of said continuous wave optical signal received at said one port to two ports of said second pair of optical communication ports;
• and
• coupling a received optical signal at a secone wavelength different from said first wavelength to said interaction section and propagating said received optical signal along said interaction section in substantially one direction thereby producing a relative phase shift in said first signal portions as they propagate around said optical waveguide.