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The field of quantum sensing deals with the design and engineering of quantum sources (e.g., entangled) and quantum measurements that are able to beat the performance of any classical strategy in a number of technological applications. This can be done considering photonic systems or solid state systems.

Solid States
In solid-state physics, 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 a physical quantity, or uses entanglement to improve measurements beyond what can be done with classical sensors. There are 4 criteria for solid-state quantum sensors:


 * 1) The system has to have discrete, resolvable energy levels.
 * 2) You can initialize the sensor and you can perform readout (turn on    and get answer).
 * 3) You can coherently manipulate the sensor.
 * 4) The sensor interacts with a physical quantity and has some response to that  quantity.

Photonics
In photonics and quantum optics, quantum sensors are often built on continuous variable systems, i.e., quantum systems characterized by continuous degrees of freedom such as position and momentum quadratures. The basic working mechanism typically relies on using optical states of light which have squeezing or two-mode entanglement. These states are particularly sensitive to record physical transformations that are finally detected by interferometric measurements.

Absolute Photon Sources
Many of the procedures for executing quantum metrology require certainty in the measurement of light. An absolute photon source is knowing the origin of the photon which helps determine which measurements relate for the sample being imaged. The best methods for approaching an absolute photon source is through spontaneous parametric down-conversion (SPDC). Coincidence measurements are a key component for reducing noise from the environment by factoring in the amount of the amount of incident photons registered with respect to the photon number. However, this not a perfected system as error can still exist through inaccurate detection of the photons.

Quantum Ellipsometry
Classical ellipsometry is a thin film material characterization methodology used to determine reflectivity, phase shift, and thickness resulting from light shining on a material. Though, it can only be effectively used if the properties are well know for the user to reference and calibrate. Quantum ellipsometry has the distinct advantage of not requiring the properties of the material to be well-defined for calibration. This is because any detected photons will already have a relative phase relation with another detected photon assuring the measured light if from the material being studied.

Quantum Optical Coherence Tomography (QOCT)
Optical coherence tomography uses Michelson interferometry with a distance adjustable mirror. Coherent light passes through a beam splitter where one path hits the mirror then the detector and the other hits a sample then reflects into the detector. The quantum analogue uses the same premise with entangle photons and a Hong–Ou–Mandel interferometer. Coincidence counting of the detected photons permits more recognizable interference leading to less noise and higher resolution.

Future
The Defense Advanced Research Projects Agency has recently 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.

Quantum sensor is also a term used in other settings where entangled quantum systems are exploited to make better atomic clocks or more sensitive magnetometers.

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.