Quantum sensing exploits the properties of quantum mechanics to develop ultra-sensitive technology that can detect changes in electric and magnetic fields, and motion.
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A quantum object is characterized by its quantum mechanical behavior and properties. For example, the energy levels of a quantum object are quantized. This can be electronic, magnetic, or vibrational levels of atoms or molecules or spin states in superconductors. Another quantum characteristic is quantum coherence.
This describes the ability of the quantum states to maintain their wave-like superposition over time, withstanding any environmental interference. Quantum entanglement is also a quantum mechanical feature that describes a quantum object. Entanglement refers to generating two or more entangled particles that have identical quantum characteristics regardless of the distance between them.
What is Quantum Sensing?
Quantum sensing is achieved when a quantum object is used to measure a physical quantity. Any of the quantum properties described above can be implemented for detection. Changes in a physical quantity can be precisely measured by quantum coherence, quantum entanglement, or quantum states.
The physical parameter that a quantum sensor responds to will determine the type of quantum technology platform required. For example, trapped ions are sensitive to electric fields and will be an ideal probe for electric field detection. Spin-based quantum sensors respond primarily to magnetic fields. Some of the different quantum technology platforms and their applications in sensing are described below.
Neutral Atoms
Spin properties of neutral alkali atoms in their ground state are used in quantum sensing. The requisite conditions required for sensing can be prepared and read out by lasers.
A thermal vapor of atoms at room temperature can be used as a magnetic probe. The Zeeman splitting of the atomic energy levels is used to detect weak magnetic fields. Magnetoencephalography (MEG) is a medical testing method that uses atomic vapor to measure magnetic fields produced by the brain's neural activity. In high-energy physics, atomic vapor-based sensing promises to enhance the detection of elementary particles.
Laser-cooled atoms that free-fall inside a vacuum tube are used in gravimetry. The matter-wave property of quantum particles is used to calculate acceleration by atom interferometry. The free-falling atoms are probed by lasers and the phase shift in the laser beam caused by the atoms is measured.
Gravimeters have the ability to detect gravity at a given location with very high sensitivity. An application where a gravity sensor has major implications is in construction projects. Infrastructure development is often delayed and costly because of unforeseen hidden features underground. Quantum gravimeters can detect risks early and assist in mitigating problems like sinkholes and mine shafts. Gravimeters can also be used to detect minerals and oils deep underground.
An accelerometer uses the same concept as a quantum gravimeter, for navigation. The ability to track minute changes in acceleration can provide information about the terrain and the environment. Quantum navigators do not rely on Global Positioning Systems (GPS) to steer towards a target.
Rydberg atoms are atoms that have absorbed energy to excite an electron to a higher, outer energy level. When the electron moves further from the nucleus of the atom, the strength of the atom’s polarization increases. This quality of Rydberg atoms makes them ideal quantum sensors for electric fields. Rydberg atoms have been successfully used as single microwave photon detectors. Rydberg atoms are also a popular candidate to simulate condensed matter systems due to their long-range interactions.
Atomic clocks use very insensitive electronic transitions in specific atoms to keep time with extreme accuracy. Optical clocks are used as the absolute frequency reference and have a significant impact in any application where timekeeping is essential. For example, in GPS, for high-speed broadband communications, and in the development of autonomous vehicles.
Trapped Ions
Electrical charge atomic ions trapped in eclectic or magnetic fields are also employed as quantum sensors. Laser-cooled motional states of trapped ions are extremely sensitive to electric fields and forces. Some advanced applications of trapped ions include ultrasensitive force microscopy, and detecting weak electric field noise above surfaces induced by absorbents. Trapped ions are also being explored as atomic clocks and as Rydberg ions.
Optomechanics
In the field of optomechanics, quantized mechanical vibrations coupled to light can detect weak forces. Apart from force measurements, optomechanical sensing applications include acceleration, magnetic fields, voltages, masses and spins.
Photons
Quantum sensing is also achieved with photons, which are fundamental particles of light. Squeezed light, which produces partially entangled photons with quantum fluctuations below the shot noise limit, is used for extremely sensitive sensing applications. For example, the Laser Interferometer Gravitational-Wave Observatory (LIGO), employs squeezed light to detect gravitational waves.
Nuclear magnetic resonance (NMR)
Nuclear magnetic resonance (NMR) uses intrinsic spin properties of atomic nuclei to detect weak magnetic fields. NMR is one of the earliest quantum sensors to be commercialized. They have broad applications in clinical magnetic resonance imaging (MRI), geological and archaeological surveys, and space missions. NMR devices are sturdy and easy to operate.
Defects in Diamond
Color centers in diamond is another magnetic quantum sensor that has gained a wide range of applicability over the last decade. Electronic defects, fabricated in diamond crystals can be operated at room temperature with low-cost laser sources. Defects can be synthesized by injecting nitrogen, silicon, germanium, and other atoms into the diamond lattice. Microscopic mapping of magnetic fields enabled by nitrogen-vacancy centers in diamond (NV center) has led to imaging of magnetic organelles in bacteria, microscopic responses in meteorites as well Covid-19 diagnosis devices.
Superconductors
The Superconducting Quantum Interference Device (SQUIDs) is a very sensitive magnetometer. Built with superconducting interferometers, SQUIDs are one of the oldest quantum sensors. SQUIDs have been successfully used for materials characterization and clinical magnetoencephalography.
Outlook
Quantum sensing has significantly advanced sensing technology in the last few years as highlighted in the examples above. With many government entities and private sectors accelerating quantum technology research and development, applications of quantum sensing will broaden and mature in the future. Other quantum mechanics-based device explorations in computing, simulation, and communications will have a profound impact on the growth of quantum sensing.
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References and Further Reading
C. L. Degen, F. Reinhard, and P. Cappellaro, Quantum sensing, Rev. Mod. Phys. 89, 035002 – Published 25 July 2017 DOI:https://doi.org/10.1103/RevModPhys.89.035002
Mahiro Abe et al, Matter-wave Atomic Gradiometer Interferometric Sensor (MAGIS-100), Quantum Sci. Technol. 6 044003, 2021 https://doi.org/10.1088/2058-9565/abf719
Barzanjeh, S., Xuereb, A., Gröblacher, S. et al. Optomechanics for quantum technologies.Nat. Phys. 18, 15–24 (2022). https://doi.org/10.1038/s41567-021-01402-0
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