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.
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. Another method known as interaction-free imaging is used to locate an object without absorbing photons. 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. 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 Δ x = λ 2 {\displaystyle \Delta x={\frac {\lambda }{2}}} . 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. 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 or quantum systems to create units of measurement. By doing this, quantum metrology enhances the limits of accuracy beyond classical attempts.
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. 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.
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.
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