Following are some basic Terminology from the Quantum Optical Domain, borrowed from Wikipedia Online mainly to keep it readable to a broad distribution, rather than too technical.

Quantum Computing -- is computing using quantum-mechanical phenomena, such as superposition and entanglement. A quantum computer is a device that performs quantum computing. Such a computer is different from binary digital electronic computers based on transistors. Whereas common digital computing requires that the data be encoded into binary digits (bits), each of which is always in one of two definite states (0 or 1), quantum computation uses quantum bits or qubits, which can be in superpositions of states. A quantum Turing machine is a theoretical model of such a computer and is also known as the universal quantum computer.

Quantum superposition is a fundamental principle of quantum mechanics. It states that, much like waves in classical physics, any two (or more) quantum states can be added together ("superposed") and the result will be another valid quantum state; and conversely, that every quantum state can be represented as a sum of two or more other distinct states. Mathematically, it refers to a property of solutions to the Schrödinger equation; since the Schrödinger equation is linear, any linear combination of solutions will also be a solution.

Quantum entanglement is a physical phenomenon which occurs when pairs or groups of particles are generated, interact, or share spatial proximity in ways such that the quantum state of each particle cannot be described independently of the state of the other(s), even when the particles are separated by a large distance—instead, a quantum state must be described for the system as a whole.

A quantum Turing machine (QTM), also a universal quantum computer, is an abstract machine used to model the effect of a quantum computer. It provides a very simple model which captures all of the power of quantum computation. Any quantum algorithm can be expressed formally as a particular quantum Turing machine. Such Turing machines were first proposed in a 1985 article written by Oxford University physicist David Deutsch suggesting quantum gates could function in a similar fashion to traditional digital computing binary logic gates.

Quantum Circuit -- In quantum information theory, a quantum circuit is a model for quantum computation in which a computation is a sequence of quantum gates, which are reversible transformations on a quantum mechanical analog of an n-bit register. This analogous structure is referred to as an n-qubit register.

Quantum Logic Gate -- In quantum computing and specifically the quantum circuit model of computation, a quantum logic gate (or simply quantum gate) is a basic quantum circuit operating on a small number of qubits. They are the building blocks of quantum circuits, like classical logic gates are for conventional digital circuits.

Quantum computing models

There are a number of quantum computing models, distinguished by the basic elements in which the computation is decomposed. The four main models of practical importance are:

Optical Computing -- Optical or photonic computing uses photons produced by lasers or diodes for computation. Photons promise to allow a higher bandwidth than the electrons used in conventional computers. Most research projects focus on replacing current computer components with optical equivalents, resulting in an optical digital computer system processing binary data. This approach appears to offer the best short-term prospects for commercial optical computing, since optical components could be integrated into traditional computers to produce an optical-electronic hybrid. However, optoelectronic devices lose 30% of their energy converting electrons into photons and back. This also slows down transmission of messages. All-optical computers eliminate the need for optical-electrical-optical (OEO) conversions.

Optical Transistors -- An optical transistor is a device that switches or amplifies optical signals. Light incident on an optical transistor’s input changes the intensity of light emitted from the transistor’s output. Output power is supplied by an additional optical source. Since the input signal intensity may be weaker than that of the source, an optical transistor amplifies the optical signal. The device is the optical analog of the electronic transistor that forms the basis of modern electronic devices. Optical transistors provide a means to control light using only light and has applications in optical computing and fiber-optic communication networks. Such technology has the potential to exceed the speed of electronics, while consuming less power.

Optical Interconnects -- Optical interconnect is a way of communication by optical cables. Compared to traditional cables, optical fibers are capable of a much higher bandwidth, from 10 Gbit/s up to 100 Gbit/s. The technology is currently being introduced as a way to link computers to mobile devices, as well as on motherboards and devices within computers.

IBM has created a prototype optical interconnect using wavelength-division multiplexing (WDM). They suggest that if successful, this technology could lead to the first computer capable of exascale computing (a computer that can perform a billion billion computations per second). A waveguide emits eight different colored beams into several different ports of a modulator, which allows eight signals to be transferred concurrently. This multi-wavelength beam travels through the chip, with optical switches controlling the direction.

Optical Comparator (Manufacturing) -- An optical comparator (often called just a comparator in context) is a device that applies the principles of optics to the inspection of manufactured parts. In a comparator, the magnified silhouette of a part is projected upon the screen, and the dimensions and geometry of the part are measured against prescribed limits. The measuring happens in any of several ways. The simplest way is that graduations on the screen, being superimposed over the silhouette, allow the viewer to measure, as if a clear ruler were laid over the image. Another way is that various points on the silhouette are lined up with the reticle at the centerpoint of the screen, one after another, by moving the stage on which the part sits, and a digital read out reports how far the stage moved to reach those points. Finally, the most technologically advanced methods involve software that analyzes the image and reports measurements. The first two methods are the most common; the third is newer and not as widespread, but its adoption is ongoing in the digital era.

A photonic integrated circuit (PIC) or integrated optical circuit is a device that integrates multiple (at least two) photonic functions and as such is analogous to an electronic integrated circuit. The major difference between the two is that a photonic integrated circuit provides functionality for information signals imposed on optical wavelengths typically in the visible spectrum or near infrared 850 nm-1650 nm. The most commercially utilized material platform for photonic integrated circuits is indium phosphide, which allows for the integration of various optically active and passive functions on the same chip. Initial examples of photonic integrated circuits were simple 2 section distributed Bragg reflector lasers, consisting of two independently controlled device sections - a gain section and a DBR mirror section. Consequently, all modern monolithic tunable lasers, widely tunable lasers, externally modulated lasers and transmitters, integrated receivers, etc. are examples of photonic integrated circuits. Current state-of-the-art devices integrate hundreds of functions onto single chip.[1] Pioneering work in this arena was performed at Bell Laboratories. Most notable academic centers of excellence of photonic integrated circuits in InP are the University of California at Santa Barbara, USA, and the Eindhoven University of Technology in the Netherlands.


Comparison to Electronic Integration Unlike electronic integration where silicon is the dominant material, system photonic integrated circuits have been fabricated from a variety of material systems, including electro-optic crystals such as lithium niobate, silica on silicon, Silicon on insulator, various polymers and semiconductor materials which are used to make semiconductor lasers such as GaAs and InP. The different material systems are used because they each provide different advantages and limitations depending on the function to be integrated. For instance, silica (silicon dioxide) based PICs have very desirable properties for passive photonic circuits such as AWGs (see below) due to their comparatively low losses and low thermal sensitivity, GaAs or InP based PICs allow the direct integration of light sources and Silicon PICs enable co-integration of the photonics with transistor based electronics. The fabrication techniques are similar to those used in electronic integrated circuits in which photolithography is used to pattern wafers for etching and material deposition. Unlike electronics where the primary device is the transistor, there is no single dominant device. The range of devices required on a chip includes low loss interconnect waveguides, power splitters, optical amplifiers, optical modulators, filters, lasers and detectors. These devices require a variety of different materials and fabrication techniques making it difficult to realize all of them on a single chip. Newer techniques using resonant photonic interferometry is making way for UV LEDs to be used for optical computing requirements with much cheaper costs leading the way to PHz consumer electronics.

Advantages of Photonic Integrated Circuits Photonic integrated circuits can allow optical systems to be made more compact and higher performance than with discrete optical components. They also offer the possibility of integration with electronic circuits to provide increased functionality. One challenge to achieving this level of integration is the size discrepancy between electronic and photonic components.The emerging field of nanoplasmonics is focused on creating ultracompact components for realizing truly nanoscale photonic devices to match their electronic counterparts. An example of the new breed of components is a recently proposed novel type of bandpass plasmonic filter that uses a response similar to electromagnetically induced transparency to achieve multichannel filtering. This allows easy control over the filtering wavelengths and bandwidths for applications in wavelength multiplexing systems for optical computing and communications in highly integrated all-optical circuits. Photonic integrated circuits should also be immune to the hazards of functionality losses associated with electromagnetic pulse (EMP), though may not be immune to high neutron flux.


Examples of Photonic Integrated Circuits -- The primary application for photonic integrated circuits has been in the area of fiber-optic communication though applications in other fields such as biomedical and photonic computing are also possible.

The arrayed waveguide grating (AWG) which are commonly used as optical (de)multiplexers in wavelength division multiplexed (WDM) fiber-optic communication systems are an example of a photonic integrated circuit which has replaced previous multiplexing schemes which utilized multiple discrete filter elements. Since separating optical modes is a need for quantum computing, this technology may be helpful to miniaturize quantum computers (see linear optical quantum computing). Another example of a photonic integrated chip in wide use today in fiber-optic communication systems is the externally modulated laser (EML) which combines a distributed feedback laser diode with an electro-absorption modulator on a single InP based chip.