# Quantum imaging

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, quantum lithography, sub-shot-noise imaging[3], 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.[4]

## 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.[5] Another method known as interaction-free imaging is used to locate an object without absorbing photons.[6] 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.[7]

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 ${\displaystyle \Delta x={\frac {\lambda }{2}}}$.[8] 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.[9] Quantum lithography has potential applications in communications and computing.

Another type of quantum imaging is called quantum metrology, or quantum sensing. This process essentially is method that achieves higher levels of accuracy than classical optics. It takes advantage of quanta (individual packets of energy) to create units of measurement. By doing this, quantum metrology enhances the limits of accuracy beyond classical attempts.[10]

## 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 doesn’t view the subject object while the second, a single-pixel (bucket) detector, views the object.[11] 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.[12] 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[13] 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 and 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.[14]

## Current uses

Quantum imaging has a lot of potential to expand. If further researched, it could be used to store patterns of data in quantum computers and allow communication through highly encrypted information. Furthermore, better quantum imaging can allow improvement in detection of faint objects, amplified images, and accurate position of lasers. Today, quantum imaging (mostly ghost imaging) is used in areas of military and medical use. The military is able 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 allows 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 allows 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. Similar to the military, it is used to look at objects that cannot be seen with the human eye such as bones and organs.[15]