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, which have optimized precision and beat current limits in sensor technology. [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]

Contents

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. [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 that, which has quantized energy levels, uses quantum coherence to measure a physical quantity, or uses entanglement to improve measurements beyond what can be done with classical sensors. [4] There are 4 criteria for solid-state quantum sensors: [4]

  1. The system has to have discrete, resolvable energy levels.
  2. You can initialize the sensor and you can perform readout (turn on and get answer).
  3. You can coherently manipulate the sensor.
  4. 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. [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]

Related Research Articles

<span class="mw-page-title-main">Quantum teleportation</span> Physical phenomenon

Quantum teleportation is a technique for transferring quantum information from a sender at one location to a receiver some distance away. While teleportation is commonly portrayed in science fiction as a means to transfer physical objects from one location to the next, quantum teleportation only transfers quantum information. The sender does not have to know the particular quantum state being transferred. Moreover, the location of the recipient can be unknown, but to complete the quantum teleportation, classical information needs to be sent from sender to receiver. Because classical information needs to be sent, quantum teleportation cannot occur faster than the speed of light.

<span class="mw-page-title-main">Quantum entanglement</span> Correlation between quantum systems

Quantum entanglement is the phenomenon of a group of particles being generated, interacting, or sharing spatial proximity in such a way that the quantum state of each particle of the group cannot be described independently of the state of the others, including when the particles are separated by a large distance. The topic of quantum entanglement is at the heart of the disparity between classical and quantum physics: entanglement is a primary feature of quantum mechanics not present in classical mechanics.

<span class="mw-page-title-main">Squeezed coherent state</span> Type of quantum state

In physics, a squeezed coherent state is a quantum state that is usually described by two non-commuting observables having continuous spectra of eigenvalues. Examples are position and momentum of a particle, and the (dimension-less) electric field in the amplitude and in the mode of a light wave. The product of the standard deviations of two such operators obeys the uncertainty principle:

<span class="mw-page-title-main">Optical parametric oscillator</span>

An optical parametric oscillator (OPO) is a parametric oscillator that oscillates at optical frequencies. It converts an input laser wave with frequency into two output waves of lower frequency by means of second-order nonlinear optical interaction. The sum of the output waves' frequencies is equal to the input wave frequency: . For historical reasons, the two output waves are called "signal" and "idler", where the output wave with higher frequency is the "signal". A special case is the degenerate OPO, when the output frequency is one-half the pump frequency, , which can result in half-harmonic generation when signal and idler have the same polarization.

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 field promises to develop measurement techniques that give better precision than the same measurement performed in a classical framework. Together with quantum hypothesis testing, it represents an important theoretical model at the basis of quantum sensing.

In quantum optics, a NOON state or N00N state is a quantum-mechanical many-body entangled state:

Quantum radar is a speculative 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 microwave range, which exploits quantum features, from the point of view of the radiation source and/or the output detection, and is able to outperform a classical counterpart. One approach is based on the use of input quantum correlations combined with a suitable interferometric quantum detection at the receiver.

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 in classical optics. Examples of quantum imaging are quantum ghost imaging, quantum lithography, imaging with undetected photons, sub-shot-noise imaging, and quantum sensing. Quantum imaging may someday be useful for storing patterns of data in quantum computers and transmitting large amounts of highly secure encrypted information. Quantum mechanics has shown that light has inherent "uncertainties" in its features, manifested as moment-to-moment fluctuations in its properties. Controlling these fluctuations—which represent a sort of "noise"—can improve detection of faint objects, produce better amplified images, and allow workers to more accurately position laser beams.

<span class="mw-page-title-main">Yoshihisa Yamamoto (scientist)</span> Japanese applied physicist (born 1950)

Yoshihisa Yamamoto is the director of Physics & Informatics Laboratories, NTT Research, Inc. He is also Professor (Emeritus) at Stanford University and National Institute of Informatics (Tokyo).

In quantum information theory, quantum discord is a measure of nonclassical correlations between two subsystems of a quantum system. It includes correlations that are due to quantum physical effects but do not necessarily involve quantum entanglement.

In quantum mechanics, the cat state, named after Schrödinger's cat, refers to a quantum state composed of a superposition of two other states of flagrantly contradictory aspects. Generalizing Schrödinger's thought experiment, any other quantum superposition of two macroscopically distinct states is also referred to as a cat state. A cat state could be of one or more modes or particles, therefore it is not necessarily an entangled state. Such cat states have been experimentally realized in various ways and at various scales.

Photonic molecules are a form of matter in which photons bind together to form "molecules". They were first predicted in 2007. Photonic molecules are formed when individual (massless) photons "interact with each other so strongly that they act as though they have mass". In an alternative definition, photons confined to two or more coupled optical cavities also reproduce the physics of interacting atomic energy levels, and have been termed as photonic molecules.

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 region of space, and it is either lost or reflected, depending on whether a target is absent or present, respectively. In principle, quantum illumination can be beneficial even if the original entanglement is completely destroyed by a lossy and noisy environment.

Quantum microscopy allows microscopic properties of matter and quantum particles to be measured and imaged. Various types of microscopy use quantum principles. The first microscope to do so was the scanning tunneling microscope, which paved the way for development of the photoionization microscope and the quantum entanglement microscope.

<span class="mw-page-title-main">Quantum feedback</span>

Quantum feedback or quantum feedback control is a class of methods to prepare and manipulate a quantum system in which that system's quantum state or trajectory is used to evolve the system towards some desired outcome. Just as in the classical case, feedback occurs when outputs from the system are used as inputs that control the dynamics. The feedback signal is typically filtered or processed in a classical way, which is often described as measurement based feedback. However, quantum feedback also allows the possibility of maintaining the quantum coherence of the output as the signal is processed, which has no classical analogue.

Continuous-variable (CV) quantum information is the area of quantum information science that makes use of physical observables, like the strength of an electromagnetic field, whose numerical values belong to continuous intervals. One primary application is quantum computing. In a sense, continuous-variable quantum computation is "analog", while quantum computation using qubits is "digital." In more technical terms, the former makes use of Hilbert spaces that are infinite-dimensional, while the Hilbert spaces for systems comprising collections of qubits are finite-dimensional. One motivation for studying continuous-variable quantum computation is to understand what resources are necessary to make quantum computers more powerful than classical ones.

Spin squeezing is a quantum process that decreases the variance of one of the angular momentum components in an ensemble of particles with a spin. The quantum states obtained are called spin squeezed states. Such states have been proposed for quantum metrology, to allow a better precision for estimating a rotation angle than classical interferometers. However a wide body of work contradicts this analysis. In particular, these works show that the estimation precision obtainable for any quantum state can be expressed solely in terms of the state response to the signal. As squeezing does not increase the state response to the signal, it cannot fundamentally improve the measurement precision.

<span class="mw-page-title-main">Giacomo Mauro D'Ariano</span> Italian quantum physicist

Giacomo Mauro D'Ariano is an Italian quantum physicist. He is a professor of theoretical physics at the University of Pavia, where he is the leader of the QUIT group. He is a member of the Center of Photonic Communication and Computing at Northwestern University; a member of the Istituto Lombardo Accademia di Scienze e Lettere; and a member of the Foundational Questions Institute (FQXi).

Levitation based inertial sensing is a new and rapidly growing technique for measuring linear acceleration, rotation and orientation of a body. Based on this technique, inertial sensors such as accelerometers and gyroscopes, enables ultra-sensitive inertial sensing. For example, the world's best accelerometer used in the LISA Pathfinder in-flight experiment is based on a levitation system which reaches a sensitivity of and noise of .

In quantum physics, entanglement depth characterizes the strength of multiparticle entanglement. An entanglement depth means that the quantum state of a particle ensemble cannot be described under the assumption that particles interacted with each other only in groups having fewer than particles. It has been used to characterize the quantum states created in experiments with cold gases.

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