Quantum imaging

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Quantum imaging [1] [2] 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, [3] quantum lithography, [4] imaging with undetected photons, [5] sub-shot-noise imaging, [6] [7] 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. [8]

Contents

Quantum imaging methods

Quantum imaging can be done in different methods. One method uses scattered light from a free-electron laser. This method converts the light to quasi-monochromatic pseudo-thermal light. [9] Another method known as interaction-free imaging is used to locate an object without absorbing photons. [10] One more method of quantum imaging is known as ghost imaging. This process uses a photon pair to define an image. The image is created by correlations between the two photons, the stronger the correlations the greater the resolution. [11]

Quantum lithography is a type of quantum imaging that focuses on aspects of photons to surpass the limits of classical lithography. Using entangled light, the effective resolution becomes a factor of N lesser than the Rayleigh limit of . [12] Another study determines that waves created by Raman pulses have narrower peaks and have a width that is four times smaller than the diffraction limit in classical lithography. [13] Quantum lithography has potential applications in communications and computing.

Another type of quantum imaging is called quantum metrology, or quantum sensing. The goal of these processes is to achieve higher levels of accuracy than equivalent measurements from classical optics. They take advantage of quantum properties of individual particles [14] or quantum systems [15] to create units of measurement. By doing this, quantum metrology enhances the limits of accuracy beyond classical attempts. [16]

Photonics

In photonics and quantum optics, quantum sensors are often built on continuous 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 using optical states of light which have squeezing or two-mode entanglement. These states are particularly sensitive to record physical transformations that are finally detected by interferometric measurements.

In practice

Absolute photon sources

Many of the procedures for executing quantum metrology require certainty in the measurement of light. An absolute photon source is knowing the origin of the photon which helps determine which measurements relate for the sample being imaged. The best methods for approaching an absolute photon source is through spontaneous parametric down-conversion (SPDC). Coincidence measurements are a key component for reducing noise from the environment by factoring in the amount of the incident photons registered with respect to the photon number. [17] However, this is not a perfected system as error can still exist through inaccurate detection of the photons.

Types of quantum metrology

Quantum ellipsometry

Classical ellipsometry is a thin film material characterization methodology used to determine reflectivity, phase shift, and thickness resulting from light shining on a material. Though, it can only be effectively used if the properties are well known for the user to reference and calibrate. Quantum ellipsometry has the distinct advantage of not requiring the properties of the material to be well-defined for calibration. This is because any detected photons will already have a relative phase relation with another detected photon assuring the measured light is from the material being studied. [18]

Quantum optical coherence tomography (QOCT)

Optical coherence tomography uses Michelson interferometry with a distance adjustable mirror. Coherent light passes through a beam splitter where one path hits the mirror then the detector and the other hits a sample then reflects into the detector. The quantum analogue uses the same premise with entangle photons and a Hong–Ou–Mandel interferometer. Coincidence counting of the detected photons permits more recognizable interference leading to less noise and higher resolution.

Real-world applications

As research in quantum imaging continues, more and more real-world methods arise. Two important ones are ghost imaging and quantum illumination. Ghost imaging takes advantage of two light detectors to create an image of an object that is not directly visible to the naked eye. The first detector is a multi-pixel detector that does not view the subject object while the second, a single-pixel (bucket) detector, views the object. [18] The performance is measured through the resolution and signal-to-noise ratio (SNR). SNRs are important to determine how well an image looks as a result of ghost imaging. On the other hand, resolution and the attention to detail is determined by the number of “specks” in the image. [19] Ghost imaging is important as it allows an image to be produced when a traditional camera is not sufficient.

Quantum Illumination was first introduced by Seth Lloyd and collaborators at MIT in 2008 [20] and takes advantage of quantum states of light. The basic setup is through target detection in which a sender prepares two entangled system, signal and idler. The idler is kept in place while the signal is sent to check out an object with a low-reflective rate and high noise background. A reflection of the object is sent back and then the idler and reflected signal combined to create a joint measurement to tell the sender one of two possibilities: an object is present or an object is absent. A key feature of quantum illumination is entanglement between the idler and reflected signal is lost completely. Therefore, it is heavily reliant on the presence of entanglement in the initial idler-signal system. [21]

Current uses

Quantum imaging is expected to have a lot of potential to expand. In the future, it could be used to store patterns of data in quantum computers and allow communication through highly encrypted information [ citation needed ]. Quantum imaging techniques can allow improvement in detection of faint objects, amplified images, and accurate position of lasers. Today, quantum imaging (mostly ghost imaging) is studied and tested in areas of military and medical use. The military aims to use ghost imaging to detect enemies and objects in situations where the naked eye and traditional cameras fail. For example, if an enemy or object is hidden in a cloud of smoke or dust, ghost imaging can help an individual to know where a person is located and if they are an ally or foe. In the medical field, imaging is used to increase the accuracy and lessen the amount of radiation exposed to a patient during x-rays. Ghost imaging could allow doctors to look at a part of the human body without having direct contact with it, therefore, lowering the amount of direct radiation to the patient [ citation needed ]. Similar to the military, it is used to look at objects that cannot be seen with the human eye such as bones and organs with a light with beneficial properties. [22]

Related Research Articles

<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.

Quantum key distribution (QKD) is a secure communication method that implements a cryptographic protocol involving components of quantum mechanics. It enables two parties to produce a shared random secret key known only to them, which then can be used to encrypt and decrypt messages. The process of quantum key distribution is not to be confused with quantum cryptography, as it is the best-known example of a quantum-cryptographic task.

<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">Spontaneous parametric down-conversion</span> Optical process

Spontaneous parametric down-conversion is a nonlinear instant optical process that converts one photon of higher energy into a pair of photons of lower energy, in accordance with the law of conservation of energy and law of conservation of momentum. It is an important process in quantum optics, for the generation of entangled photon pairs, and of single photons.

<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.

In quantum mechanics, a quantum eraser experiment is an interferometer experiment that demonstrates several fundamental aspects of quantum mechanics, including quantum entanglement and complementarity. The quantum eraser experiment is a variation of Thomas Young's classic double-slit experiment. It establishes that when action is taken to determine which of 2 slits a photon has passed through, the photon cannot interfere with itself. When a stream of photons is marked in this way, then the interference fringes characteristic of the Young experiment will not be seen. The experiment also creates situations in which a photon that has been "marked" to reveal through which slit it has passed can later be "unmarked." A photon that has been "unmarked" will interfere with itself once again, restoring the fringes characteristic of Young's experiment.

A delayed-choice quantum eraser experiment, first performed by Yoon-Ho Kim, R. Yu, S. P. Kulik, Y. H. Shih and Marlan O. Scully, and reported in early 1998, is an elaboration on the quantum eraser experiment that incorporates concepts considered in John Archibald Wheeler's delayed-choice experiment. The experiment was designed to investigate peculiar consequences of the well-known double-slit experiment in quantum mechanics, as well as the consequences of quantum entanglement.

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 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 related to the fields of quantum imaging, quantum metrology, and quantum sensing. The effect exploits the quantum mechanical state of light called the NOON state. Quantum lithography was invented at Jonathan P. Dowling's group at JPL, and has been studied by a number of groups.

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. The field of quantum sensing deals with the design and engineering of quantum sources 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.

Ghost imaging is a technique that produces an image of an object by combining information from two light detectors: a conventional, multi-pixel detector that does not view the object, and a single-pixel (bucket) detector that does view the object. Two techniques have been demonstrated. A quantum method uses a source of pairs of entangled photons, each pair shared between the two detectors, while a classical method uses a pair of correlated coherent beams without exploiting entanglement. Both approaches may be understood within the framework of a single theory.

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.

Integrated quantum photonics, uses photonic integrated circuits to control photonic quantum states for applications in quantum technologies. As such, integrated quantum photonics provides a promising approach to the miniaturisation and scaling up of optical quantum circuits. The major application of integrated quantum photonics is Quantum technology:, for example quantum computing, quantum communication, quantum simulation, quantum walks and quantum metrology.

<span class="mw-page-title-main">Cavity optomechanics</span> Branch of physics

Cavity optomechanics is a branch of physics which focuses on the interaction between light and mechanical objects on low-energy scales. It is a cross field of optics, quantum optics, solid-state physics and materials science. The motivation for research on cavity optomechanics comes from fundamental effects of quantum theory and gravity, as well as technological applications.

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.

Quantum optical coherence tomography (Q-OCT) is an imaging technique that uses nonclassical (quantum) light sources to generate high-resolution images based on the Hong-Ou-Mandel effect (HOM). Q-OCT is similar to conventional OCT but uses a fourth-order interferometer that incorporates two photodetectors rather than a second-order interferometer with a single photodetector. The primary advantage of Q-OCT over OCT is insensitivity to even-order dispersion in multi-layered and scattering media.

References

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  15. ↑ Hanson, R.; Gywat, O.; Awschalom, D. D. (2006-10-26). "Room-temperature manipulation and decoherence of a single spin in diamond". Physical Review B. 74 (16): 161203. arXiv: quant-ph/0608233 . Bibcode:2006PhRvB..74p1203H. doi:10.1103/PhysRevB.74.161203. S2CID   5055366.
  16. ↑ "Quantum metrology - Latest research and news | Nature". www.nature.com. Retrieved 2018-12-08.
  17. ↑ Recent advances in metrology and fundamental constants : Varenna on Lake Como, Villa Monastero, 25 July-4 August 2000. Quinn, T. J. (Terry J.), Leschiutta, Sigfrido., Tavella, P. (Patrizia), Società italiana di fisica., IOS Press. Amsterdam: IOS Press. 2001. ISBN   978-1-61499-002-4. OCLC   784969866.{{cite book}}: CS1 maint: others (link)
  18. 1 2 Simon, David S.; Jaeger, Gregg; Sergienko, Alexander V. (2017). Quantum Metrology, Imaging, and Communication. Quantum Science and Technology. Springer International Publishing. ISBN   9783319465494.
  19. ↑ Genovese, Marco (2016-07-01). "Real applications of quantum imaging". Journal of Optics. 18 (7): 073002. arXiv: 1601.06066 . Bibcode:2016JOpt...18g3002G. doi:10.1088/2040-8978/18/7/073002. ISSN   2040-8978. S2CID   118514937.
  20. ↑ Lloyd, Seth (2008-09-12). "Enhanced Sensitivity of Photodetection via Quantum Illumination". Science. 321 (5895): 1463–1465. Bibcode:2008Sci...321.1463L. CiteSeerX   10.1.1.1015.347 . doi:10.1126/science.1160627. ISSN   1095-9203. PMID   18787162. S2CID   30596567.
  21. ↑ Shapiro, Jeffrey H.; Pirandola, Stefano; Maccone, Lorenzo; Lloyd, Seth; Guha, Saikat; Giovannetti, Vittorio; Erkmen, Baris I.; Tan, Si-Hui (2008-10-02). "Quantum Illumination with Gaussian States". Physical Review Letters. 101 (25): 253601. arXiv: 0810.0534 . Bibcode:2008PhRvL.101y3601T. doi:10.1103/PhysRevLett.101.253601. PMID   19113706. S2CID   26890855.
  22. ↑ "Defense.gov News Article: Army Develops 'Ghost' Imaging to Aid on Battlefield". archive.defense.gov. Retrieved 2018-12-05.

Bibliography

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