**There has been a significant emergence of quantum-based applications in recent years, which has mainly been brought about by scientists realizing how the quantum properties of nanoscale materials can be manipulated and utilized. One such subset of quantum technology is quantum optics, and in this article, we give a brief overview of the many different areas that fall into the quantum optics classification.**

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Quantum optics is primarily an area of physics which uses a combination of semi-classical physics and quantum mechanics principles to investigate and manipulate how photons of light interact with matter, and the phenomena which can be produced, at the subatomic level. This is how you would explain quantum optics in its broadest sense. However, whilst some of the most prominent applications is lasers and quantum computing, there has been a lot of research into the fundamental principles of how photons behave at this level, and this has helped to realize many different subsets and phenomenon within quantum optics which contribute heavily to the realization of the physical applications.

## Coincidence Correlation

Coincidence correlation is an area of quantum optics that is used to see if someone is observing a single quantum system. This is done by assuming that a single system can only emit one photon at a time and observes (via a photodetector) the quantum system as a single photon emitter. If it is found that more than detector observes the source, then the likelihood is that it is not a one photon system and is unlikely to be a single quantum system. It is a fundamental process that enables someone to determine the presence of a single quantum system, i.e. it is a test rather than an application, but it can be used in conjunction with other quantum optic application areas.

One example is with quantum entanglement (detailed more below). Coincidence correlation can be used to prove or disprove the correlations with a quantumly entangled network and will employ a combination of optical polarizers and photodetectors to filter quantum states and determine if there is correspondence at both ends of the entangled pair.

## Quantum Entanglement

Quantum entanglement is a phenomenon that occurs between quantum systems, where the components of each quantum system become one and indescribable from each other, i.e. instead of two separate quantum states, the whole system becomes one quantum network state. The types of components which can experience this phenomenon include electrons, photon, atoms and molecules. This extends to long-range distances, and the measurement of one part of the quantum system enables the properties of the corresponding particle in the quantum system to be revealed.

The different properties that can be revealed at different ends of an entangled network include position, momentum, spin and polarization. In many cases, one of the quantum particles is described as a superimposition with an indefinite value for the entangled particle. However, if one of these particles is measured, it can provide definite value for the corresponding pair. Quantum entanglement is often utilized in quantum computing applications (which is described in more detail below).

## Quantum Teleportation

Quantum teleportation is another phenomenon that has a lot of use in quantum computing, as well as quantum communications, and is closely related to quantum entanglement. Quantum teleportation is the process by which the information held within a qubit can be transported from one location to another, without the qubit itself being transported.

For those who don’t know, a qubit, otherwise known as a quantum bit, is the building block of many quantum networks, especially in quantum information processing applications and can adopt a 0 value, a 1 value or a superimposed 0 or 1 value. This means that qubits can perform quantum operations in more than one value simultaneously.

## Quantum Information Processing

Quantum information processing, i.e quantum computing, is a computing process (and memory storage) that relies on qubits as opposed to binary bits. The ability to of qubits to superimpose, compared to binary bits that adopt a 0 or 1 value only, enables simultaneous operations to occur (several quantum systems can be operated in parallel), and in turn allows quantum computers to be much faster than their classical counterparts. The qubits within a quantum computer stores information quantum mechanically by utilizing the ½ spin state of electrons (up and down) and the polarization of photons (horizontal and vertical) within the quantum network. This correlates to a positional arrangement that can be identified when the qubits are entangled, and so long as the computer can control the spin operation and the interactions between electron spins, the readout can measure the single spin states and bulk spin resonance of each quantum network to determine the information contained within.

## Quantum Communication

Quantum communication is an area which is closely related to quantum information processing but is more to do with quantum cryptography than it is computing—such as quantum key distribution. Quantum key distribution uses quantum mechanics to perform cryptographic tasks or break encrypted systems. Quantum key distribution works when two people use a communication system that utilizes single photons, which are randomly polarized, to transmit a series of random number sequences. The randomness of the polarization is generated by using quantum optics. These sequences act as the keys in the cryptographic system and the system uses both a classical channel and a quantum channel to connect the communication points.

The qubits are sent over the quantum channel and the classical channel performs classical operations and can be used to see if anyone is trying to hack the system. Because the information is transmitted via the quantum network, and not the classical channel, the classical channel can be hacked but no information will be obtained. However, because the signals under normal conditions are correlated, any correlation imperfections (due to a hack) between the classical network and the quantum network will be detected by the receiver and can be used to determine when a hack has been attempted.

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