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A quantum sensor is a quantum device that responds to a stimulus. Usually this refers to a sensor which has quantized energy levels, uses quantum coherence to measure physical quantity, or uses entanglement to improve measurements beyond what can be done with classical sensors. There are 4 criteria for quantum sensors: 
- The system has to have discrete, resolvable energy levels.
- You can initialize the sensor and you can perform readout (turn on and get answer).
- You can coherently manipulate the sensor.
- The sensor interacts with a physical quantity and has some response to that quantity.
The Defense Advanced Research Projects Agency has recently[when?] launched a research program in optical quantum sensors that seeks to exploit ideas from quantum metrology and quantum imaging, such as quantum lithography and the NOON state, in order to achieve these goals with optical sensor systems such as lidar.
A good example of an early quantum sensor is an APD avalanche photodiode as these have been used to detect entangled photons and in fact with additional cooling and sensor improvements can be used where PMTs once ruled the market such as medical imaging. APDs in the form of 2-D and even 3-D stacked arrays as a direct replacement for conventional sensors based on silicon diodes.
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- DARPA Quantum Sensor Program.
- BROAD AGENCY ANNOUNCEMENT (BAA) 07-22 Quantum Sensors
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