Quantum sensor

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[1] or solid state systems.[2]

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.

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.

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,[3] in order to achieve these goals with optical sensor systems such as lidar.[4][5]

Quantum sensor is also a term used in other settings where entangled quantum systems are exploited to make better atomic clocks[6] or more sensitive magnetometers.[7][8]

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.