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''Many measurement devices utilize quantum properties in order to probe measurements such as atomic clocks, superconducting quantum interference devices, and nuclear magnetic resonance spectroscopy. With new technological advancements, individual quantum systems can be used as measurement devices, utilizing entanglement, superposition, interference and squeezing to enhance sensitivity and surpass performance of classical strategies.''

''The primary advantage of quantum sensors is their hypersensitivity to physical transformations and external disturbances upon an input state, where quantum metrology and quantum discrimination (quantum hypothesis testing) are utilized to distinguish and characterize a quantum channel or process. ''

Advantages of Quantum Sensing
Quantum Sensing utilizes properties of quantum mechanics, such as quantum entanglement, interference and quantum state squeezing, that have optimized precision and beat current limits in sensor technology and evade the Heisenberg uncertainty principle.

Optical Systems
''Photonic quantum sensing leverages entanglement, single photons and squeezed states to perform extremely precise measurements. Optical sensing makes use of continuous variable quantum systems such as different degrees of freedom of the electromagnetic field, vibrational modes of solids, and Bose-Einstein condensates. ''

''These quantum systems can be probed to characterize an unknown transformation between two quantum states. Several methods are in place to improve photonic sensors such quantum illumination of targets which has been used to improve detection of weak signals by the use of quantum correlation. ''

Systems
''Quantum sensing can also be utilized in non-photonic areas such as spin qubits, trapped ions, and flux qubits. 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. ''

''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, and more recently, rotation sensors. ''

Applications of Quantum Sensing
''Injecting squeezed light into interferometers allows for higher sensitivity to weak signals that would be unable to be classically detected. A practical application of quantum sensing is realized in gravitational wave sensing. Gravitational wave detectors, such as LIGO, utilize squeezed light to measure signals below the standard quantum limit .''

Squeezed light has also been used to detect signals below the standard quantum limit in plasmonic sensors and atomic force microscopy .

''Quantum sensing 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. The diminishing projection noise has direct applications in communication protocols and nano-Nuclear Magnetic Resonance. ''

Challenges
''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. ''

''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. [<-confirm]''

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Current Areas of Research
Quantum Sensors have applications in microscopy, positioning systems, communication technology, electric and magnetic field sensors, as well as geophysical areas of research such as mineral prospecting and seismology.