Withinquantum technology, aquantum sensor utilizes quantum mechanical phenomena, such asquantum superposition,quantum entanglement, andquantum squeezing, to measure things. If a quantum system is measurable, and it interacts with its environment in a known way, then measurements of that system can provide information about its environment. Theoretically suchsensor technology would have precision limited only by theuncertainty principle.[1]The field of quantum sensing deals with the design and engineering of quantum mechanical systems and measurements with potential for better performance than any classical strategy in a number of technological applications.[2] Of the wide range of quantum mechanical systems that can be used as a quantum sensor, most can be classified asphotonic systems[3] orsolid state systems.[4]
Inphotonics andquantum optics, photonic quantum sensing leveragesentanglement, single photons andsqueezed states to perform extremely precise measurements. Optical sensing makes use of continuously variable quantum systems such as different degrees of freedom of the electromagnetic field, vibrational modes of solids, andBose–Einstein condensates.[5] These quantum systems can be probed to characterize an unknown transformation between two quantum states. Several methods are in place to improve photonic sensors'quantum illumination of targets, which have been used to improve detection of weak signals by the use of quantum correlation.[6][7][8][9][10]
Quantum sensors are often built on continuously 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 optical states of light, often involving quantum mechanical properties such as squeezing or two-mode entanglement.[3] These states are sensitive to physical transformations that are detected by interferometric measurements.[5]
Quantum sensing can also be utilized in non-photonic areas such asspin qubits,trapped ions,flux qubits,[4] andnanoparticles.[11] These systems can be compared by physical characteristics to which they respond, for example, trapped ions respond to electrical fields while spin systems will respond to magnetic fields.[4]Trapped Ions are useful in their quantized motional levels which are strongly coupled to the electric field. They have been proposed to study electric field noise above surfaces,[12] and more recently, rotation sensors.[13]
In solid-state physics, a quantum sensor is a quantum device that responds to a stimulus. Usually this refers to a sensor, which hasquantized energy levels, usesquantum coherence or entanglement to improve measurements beyond what can be done with classical sensors.[4] There are four criteria for solid-state quantum sensors:[4]
The system has to have discrete, resolvable energy levels.
The sensor can be initialized into a well-known state and its state can be read out.
The sensor can be coherently manipulated.
The sensor interacts with a physical quantity and has some response to that quantity.
Quantum sensors have applications in a wide variety of fields including microscopy, positioning systems, communication technology, electric and magnetic field sensors, as well as geophysical areas of research such as mineral prospecting andseismology.[4] Many measurement devices utilize quantum properties in order to probe measurements such asatomic clocks,Atomic radio receiver,superconducting quantum interference devices, andnuclear magnetic resonance spectroscopy.[4][14] With new technological advancements, individual quantum systems can be used as measurement devices, utilizingentanglement,superposition, interference andsqueezing to enhance sensitivity and surpass performance of classical strategies.
A good example of an early quantum sensor is anavalanche photodiode (APD). APDs have been used to detect entangledphotons. With additional cooling and sensor improvements can be used wherephotomultiplier tubes (PMT) in fields such as medical imaging. APDs, in the form of 2-D and even 3-D stacked arrays, can be used as a direct replacement for conventional sensors based onsilicon diodes.[15]
For photonic systems, current areas of research consider feedback and adaptive protocols. This is an active area of research in discrimination and estimation of bosonic loss.[21]
Quantum sensing also has the capability to overcome resolution limits, where current issues of vanishing distinguishability between two close frequencies can be overcome by making the projection noise vanish.[25][26] The diminishing projection noise has direct applications in communication protocols and nano-Nuclear Magnetic Resonance.[27][28]
Quantum radar is also an active area of research. Current classical radars can interrogate many target bins while quantum radars are limited to a single polarization or range.[34] A proof-of-concept quantum radar or quantum illuminator using quantum entangled microwaves was able to detect low reflectivity objects at room-temperature – such may be useful for improved radar systems, security scanners and medical imaging systems.[35][36][37]
^abAdesso, Gerardo; Ragy, Sammy; Lee, Antony R. (June 2014). "Continuous Variable Quantum Information: Gaussian States and Beyond".Open Systems & Information Dynamics.21 (1n02): 1440001.arXiv:1401.4679.doi:10.1142/S1230161214400010.S2CID15318256.
^Maze, J. R.; Stanwix, P. L.; Hodges, J. S.; Hong, S.; Taylor, J. M.; Cappellaro, P.; Jiang, L.; Dutt, M. V. Gurudev; Togan, E.; Zibrov, A. S.; Yacoby, A. (October 2008). "Nanoscale magnetic sensing with an individual electronic spin in diamond".Nature.455 (7213):644–647.Bibcode:2008Natur.455..644M.doi:10.1038/nature07279.PMID18833275.S2CID136428582.
