Detecting one or more photons from their interactions with probe photons in a matter system

A device comprising:

• a matter system having a first energy level and a second energy level such that a signal photon couples to a transition between the first energy level and the second energy level;
• a first source providing a first beam that contains photons that couple to a transition between the second energy level and a third energy level of the matter system, the first source directing the first beam to interact with the matter system;
• a second source providing a second beam that contains photons that couple to a transition between the third energy level and a fourth energy level of the matter system, the second source directing the second beam to interact with the matter system;
• and
• a measurement system arranged to measure a change in one of the first beam and the second beam to detect the signal photon in the matter system.