A quantum sensor utilizes properties of quantum mechanics, such as
quantum entanglement,
quantum interference, and
quantum state
In quantum physics, a quantum state is a mathematical entity that provides a probability distribution for the outcomes of each possible measurement on a system. Knowledge of the quantum state together with the rules for the system's evolution in ...
squeezing, which have optimized precision and beat current limits in
sensor technology.
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 with
photonic systems
or
solid state systems.
Characteristics
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.
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 Quantum illumination is a paradigm for target detection that employs quantum entanglement between a signal electromagnetic mode and an idler electromagnetic mode, as well as joint measurement of these modes. The signal mode is propagated toward a re ...
of targets, which have been used to improve detection of weak signals by the use of quantum correlation.
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.
These states are sensitive to physical transformations that are detected by interferometric measurements.
Quantum sensing can also be utilized in non-photonic areas such as
spin qubits,
trapped ions,
flux qubit
In quantum computing, more specifically in superconducting quantum computing, flux qubits (also known as persistent current qubits) are micrometer sized loops of superconducting metal that is interrupted by a number of Josephson junctions. These d ...
s,
and
nanoparticles. 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.
In solid-state physics, a quantum sensor is a quantum device that responds to a stimulus. Usually this refers to a sensor that, which has
quantized energy levels, uses
quantum coherence
In physics, two wave sources are coherent if their frequency and waveform are identical. Coherence is an ideal property of waves that enables stationary (i.e., temporally or spatially constant) interference. It contains several distinct concepts, ...
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:
- The system has to have discrete, resolvable energy levels.
- You can initialize the sensor and you can perform readout (turn on
and get answer).
- You can coherently manipulate the sensor.
- The sensor interacts with a physical quantity and has some response to that
quantity.
Research and applications
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
Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...
.
Many measurement devices utilize quantum properties in order to probe measurements such as
atomic clock
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...
s,
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.
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 tube
Photomultiplier tubes (photomultipliers or PMTs for short) are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. They are members of the class of vacuum tubes, more specif ...
s (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
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
diodes.
The
Defense Advanced Research Projects Agency
The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military.
Originally known as the Adv ...
(DARPA) launched a research program in optical quantum sensors that seeks to exploit ideas from
quantum metrology
Quantum metrology is the study of making high-resolution and highly sensitive measurements of physical parameters using quantum theory to describe the physical systems, particularly exploiting quantum entanglement and quantum squeezing. This fie ...
and
quantum imaging Quantum imaging is a new sub-field of quantum optics that exploits quantum correlations such as quantum entanglement of the electromagnetic field in order to image objects with a resolution or other imaging criteria that is beyond what is possible ...
, such as
quantum lithography
Quantum lithography is a type of photolithography, which exploits non-classical properties of the photons, such as quantum entanglement, in order to achieve superior performance over ordinary classical lithography. Quantum lithography is closely ...
and the
NOON state
In quantum optics, a NOON state or N00N state is a quantum-mechanical many-body Quantum entanglement, entangled state:
: , \psi_\text \rangle = \frac, \,
which represents a superposition of ''N'' particles in mode ''a'' with zero particles in ...
, in order to achieve these goals with optical sensor systems such as
lidar
Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
. The
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
judges quantum sensing to be the most mature of quantum technologies for military use, theoretically replacing
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
in areas without coverage or possibly acting with
ISR
ISR may refer to:
Organizations
* Institute for Strategy and Reconciliation, a think tank, relief and development organization
* Institutional and Scientific Relations, a Directorate of the European Commission
* International Star Registry, a com ...
capabilities or detecting submarine or subterranean structures or vehicles, as well as
nuclear material
Nuclear material refers to the metals uranium, plutonium, and thorium, in any form, according to the IAEA. This is differentiated further into "source material", consisting of natural and depleted uranium, and "special fissionable material", con ...
.
;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.
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
In quantum physics, light is in a ''squeezed state'' if its electric field strength ''Ԑ'' for some phases \vartheta has a quantum uncertainty smaller than that of a coherent state. The term ''squeezing'' thus refers to a reduced quantum unce ...
to measure signals below the
standard quantum limit.
Squeezed light
In quantum physics, light is in a ''squeezed state'' if its electric field strength ''Ԑ'' for some phases \vartheta has a quantum uncertainty smaller than that of a coherent state. The term ''squeezing'' thus refers to a reduced quantum unce ...
has also been used to detect signals below the
standard quantum limit in
plasmonic sensors and
atomic force microscopy
Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very-high-resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the op ...
.
;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. The diminishing projection noise has direct applications in communication protocols and nano-Nuclear Magnetic Resonance.
;Other uses of entanglement
Entanglement can be used to improve upon existing
atomic clock
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...
s or create more sensitive
magnetometer
A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, o ...
s.
;Quantum radars
Quantum radar Quantum radar is a speculative Remote sensing, remote-sensing technology based on quantum-mechanical effects, such as the uncertainty principle or quantum entanglement. Broadly speaking, a quantum radar can be seen as a device working in the microwa ...
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. 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.
;Neuroimaging
In
neuroimaging
Neuroimaging is the use of quantitative (computational) techniques to study the structure and function of the central nervous system, developed as an objective way of scientifically studying the healthy human brain in a non-invasive manner. Incre ...
, the first quantum brain scanner uses magnetic imaging and could become a novel whole-brain scanning approach.
;Gravity cartography of subterraneans
Quantum gravity-gradiometers that could be used to and investigate subterraneans are also in development.
References
{{emerging technologies, quantum=yes, other=yes
Quantum information science
Sensors