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Quantum sensor
Quantum sensor
Quantum sensor
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Within quantum technology, a quantum sensor utilizes properties of quantum mechanics, such as quantum entanglement, quantum interference, and quantum state squeezing. Theoretically such sensor technology would have precision limited only by the uncertainty principle.[1] 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.[2] This can be done with photonic systems[3] or solid state systems.[4]

Characteristics

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In photonics and quantum optics, photonic quantum sensing leverages entanglement, single photons and squeezed 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, and Bose–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 as spin qubits, trapped ions, flux qubits,[4] and nanoparticles.[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 has quantized energy levels, uses quantum 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]

  1. The system has to have discrete, resolvable energy levels.
  2. The sensor can be initialized into a well-known state and its state can be read out.
  3. The sensor can be coherently manipulated.
  4. The sensor interacts with a physical quantity and has some response to that quantity.

Research and applications

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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 and seismology.[4] Many measurement devices utilize quantum properties in order to probe measurements such as atomic clocks, superconducting quantum interference devices, and nuclear magnetic resonance spectroscopy.[4][14] 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.

A good example of an early quantum sensor is an avalanche photodiode (APD). APDs have been used to detect entangled photons. With additional cooling and sensor improvements can be used where photomultiplier 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 on silicon diodes.[15]

The Defense Advanced Research Projects Agency (DARPA) 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,[16] in order to achieve these goals with optical sensor systems such as lidar.[6][17][18][19] The United States judges quantum sensing to be the most mature of quantum technologies for military use, theoretically replacing GPS in areas without coverage or possibly acting with ISR capabilities or detecting submarine or subterranean structures or vehicles, as well as nuclear material.[20]

Photonic quantum sensors, microscopy and gravitational wave detectors

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]

Injecting squeezed light into interferometers allows for higher sensitivity to weak signals that would be unable to be classically detected.[1] A practical application of quantum sensing is realized in gravitational wave sensing.[22] Gravitational wave detectors, such as LIGO, utilize squeezed light to measure signals below the standard quantum limit.[23] Squeezed light has also been used to detect signals below the standard quantum limit in plasmonic sensors and atomic force microscopy.[24]

Uses of projection noise removal

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]

Other uses of entanglement

Entanglement can be used to improve upon existing atomic clocks[29][30][31] or create more sensitive magnetometers.[32][33]

Quantum radars

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]

Neuroimaging

In neuroimaging, the first quantum brain scanner uses magnetic imaging and could become a novel whole-brain scanning approach.[38][39]

Gravity cartography of subterraneans

Quantum gravity-gradiometers that could be used to map and investigate subterraneans are also in development.[40][41]

See also

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References

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Quantum sensor

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from Grokipedia
A quantum sensor is a measurement device that harnesses quantum mechanical principles, including superposition, entanglement, and quantum coherence, to detect and quantify physical quantities such as magnetic and , time, , , and with precision and sensitivity that often exceed the limits of classical sensors. These sensors typically operate using like qubits—two-level quantum states with resolvable levels—that can be initialized, coherently manipulated, and read out to sense external perturbations. The sensitivity of quantum sensors is fundamentally enhanced by their ability to leverage quantum correlations, potentially achieving the Heisenberg limit of precision, which scales as 1/N for N particles, compared to the standard of 1/√N for classical systems. Prominent types of quantum sensors include nitrogen-vacancy (NV) centers in diamond, which enable nanoscale imaging and thermometry through electron spin manipulation; optically pumped atomic magnetometers, utilizing atomic vapors for high-sensitivity magnetic field detection; superconducting quantum interference devices (SQUIDs), capable of attotesla-level measurements; trapped ion or neutral atom ensembles for precision timekeeping and force sensing; and Rydberg atom-based sensors, which enable ultra-sensitive, broadband detection of radio-frequency electric fields from DC to THz using electromagnetically-induced transparency in highly excited atomic states. These platforms offer advantages such as operation at atomic scales, robustness in ambient conditions, and the ability to probe biological processes noninvasively, making them suitable for diverse applications. In biomedicine, quantum sensors facilitate magnetoencephalography for brain activity mapping, single-cell spectroscopy, and disease monitoring, such as early detection of Alzheimer's through neural signal analysis. Beyond healthcare, they support navigation systems independent of GPS, geophysical surveying for resource exploration, fundamental physics experiments like gravitational wave detection, radio-frequency metrology, and communications reception, with commercial examples including atomic clocks integral to global positioning. Ongoing challenges include scaling production, mitigating decoherence, and addressing supply chain limitations for materials like quantum-grade diamonds, yet federal initiatives like the U.S. National Quantum Initiative continue to drive advancements.

Fundamentals

Definition

A quantum sensor is a device that utilizes quantum mechanical phenomena—such as superposition, entanglement, and coherence—to measure physical quantities with precision surpassing that of classical sensors, frequently approaching the fundamental limits imposed by the Heisenberg uncertainty principle. In contrast to classical sensors, which depend on aggregate statistical behaviors of large ensembles, quantum sensors encode signals representing parameters like magnetic fields, electric fields, or temperature into the quantum states of individual particles, atoms, or photons, enabling readout techniques that exploit quantum correlations for superior sensitivity. Key quantum effects underpinning these sensors include quantum interference, which facilitates high-fidelity phase detection in interferometric setups, and squeezed states, which suppress in one observable below the standard shot-noise limit while redistributing it to another, thereby enhancing measurement accuracy. Quantum sensors encompass a broad range of systems, from macroscopic assemblies like optical interferometers to microscopic platforms such as single-atom traps or solid-state defect centers.

Operating Principles

Quantum sensors exploit core quantum mechanical phenomena to achieve measurement sensitivities unattainable by classical devices. Superposition enables the sensor's quantum states to evolve in parallel under the influence of an external parameter, such as a or , allowing simultaneous probing of multiple pathways and enhancing the overall . Entanglement correlates the states of multiple particles or modes, enabling collective measurements where fluctuations in one particle are compensated by others, thereby amplifying sensitivity through shared . Quantum squeezing further refines this by redistributing uncertainty between —reducing noise in the measured quadrature while increasing it in the orthogonal one—in accordance with the Heisenberg , which permits such trade-offs to prioritize the parameter of interest. The fundamental precision limit for these sensors is governed by the Heisenberg uncertainty principle, ΔxΔp≥ℏ2\Delta x \Delta p \geq \frac{\hbar}{2}, where Δx\Delta x and Δp\Delta p represent uncertainties in position and momentum, respectively; quantum sensors approach this bound by optimizing state preparation and readout to minimize excess noise, enabling metrological performance at the quantum limit for parameters like displacement, frequency, or field strength. In multiparticle systems, this manifests as the Heisenberg limit (HL), where estimation precision scales inversely with the number of particles NN (i.e., δθ∼1/N\delta \theta \sim 1/N), surpassing the standard quantum limit (SQL) of δθ∼1/N\delta \theta \sim 1/\sqrt{N}
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