Optical computing: Difference between revisions

Revision as of 01:41, 30 June 2022

Revision as of 12:01, 12 July 2022

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{{short description|Computer that uses photons or light waves}}

'''Optical computing''' or '''photonic computing''' uses light waves produced by [[laserslaser]]s or incoherent sources for data processing, data storage or data communication for computing. For decades, photons have shown promise to enable a higher [[Bandwidth (signal processing)|bandwidth]] than the [[electronselectron]]s used in conventional computers (see [[optical fiber]]s).

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 consume 30% of their energy converting electronic energy into photons and back; this conversion also slows the transmission of messages. All-optical computers eliminate the need for optical-electrical-optical (OEO) conversions, thus reducing electrical power consumption.<ref>{{cite book |first=D.D. |last=Nolte |title=Mind at Light Speed: A New Kind of Intelligence |url=https://books.google.com/books?id=Q9lB-REWP5EC&pg=PA34 |date=2001 |publisher=Simon and Schuster |isbn=978-0-7432-0501-6 |page=34}}</ref>

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| invent1 = K. Jain

| invent2 = G.W. Pratt, Jr.

}}</ref> can be used to create optical [[logic gatesgate]]s,<ref name=jainprattpatent /> which in turn are assembled into the higher level components of the computer's [[central processing unit]] (CPU). These will be nonlinear optical crystals used to manipulate light beams into controlling other light beams.

Like any computing system, an optical computing system needs three things to function well:

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A significant challenge to optical computing is that computation is a [[nonlinear]] process in which multiple signals must interact. Light, which is an [[electromagnetic wave]], can only interact with another electromagnetic wave in the presence of electrons in a material,<ref>{{cite book|isbn=978-0387946597 |author=Philip R. Wallace|title= Paradox Lost: Images of the Quantum|date=1996}}</ref> and the strength of this interaction is much weaker for electromagnetic waves, such as light, than for the electronic signals in a conventional computer. This may result in the processing elements for an optical computer requiring more power and larger dimensions than those for a conventional electronic computer using transistors.{{Citation needed|date=December 2008}}

A further misconception{{by whom|date=May 2019}} is that since light can travel much faster than the [[drift velocity]] of electrons, and at frequencies measured in [[Terahertz (unit)|THz]], optical transistors should be capable of extremely high frequencies. However, any electromagnetic wave must obey the [[Bandwidth-limited pulse|transform limit]], and therefore the rate at which an optical transistor can respond to a signal is still limited by its [[spectral bandwidth]]. In [[fiber-optic communication]]s, practical limits such as [[dispersion (optics)|dispersion]] often constrain [[Wavelength-division multiplexing|channelschannel]]s to bandwidths of 10s of GHz, only slightly better than many silicon transistors. Obtaining dramatically faster operation than electronic transistors would therefore require practical methods of transmitting [[ultrashort pulse]]s down highly dispersive waveguides.

==Photonic logic==

[[File:optical-NOT-gate-int.svg|thumb|right|Realization of a photonic controlled-NOT gate for use in quantum computing]]

Photonic logic is the use of photons ([[light]]) in [[logic gate]]s (NOT, AND, OR, NAND, NOR, XOR, XNOR). Switching is obtained using [[nonlinear optics|nonlinear optical effectseffect]]s when two or more signals are combined.<ref name=jainprattpatent />

[[Optical cavity|Resonator]]s are especially useful in photonic logic, since they allow a build-up of energy from [[constructive interference]], thus enhancing optical nonlinear effects.

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===Time delays optical computing===

The basic idea is to delay light (or any other signal) in order to perform useful computations.<ref name="oltean_hamiltonian">{{cite conference|last=Oltean|first=Mihai|title= A light-based device for solving the Hamiltonian path problem |conference=Unconventional Computing| pages= 217–227| publisher= Springer LNCS 4135|doi=10.1007/11839132_18|date=2006|arxiv=0708.1496}}</ref> Of interest would be to solve [[NP-completeness|NP-complete problemsproblem]]s as those are difficult problems for the conventional computers.

There are 2 basic properties of light that are actually used in this approach:

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===Optical Fourier co-processors===

Many computations, particularly in scientific applications, require frequent use of the 2D [[discrete Fourier transform]] (DFT) – for example in solving differential equations describing propagation of waves or transfer of heat. Though modern GPU technologies typically enable high-speed computation of large 2D DFTs, techniques have been developed that can perform continuous Fourier transform optically by utilising the natural [[Fourier optics#Fourier transforming property of lenses|Fourier transforming property of lenseslens]]es. The input is encoded using a [[liquid crystal]] [[spatial light modulator]] and the result is measured using a conventional CMOS or CCD image sensor. Such optical architectures can offer superior scaling of computational complexity due to the inherently highly interconnected nature of optical propagation, and have been used to solve 2D heat equations.<ref>{{cite journal| author= A. J. Macfaden, G. S. D. Gordon, T. D. Wilkinson |title=An optical Fourier transform coprocessor with direct phase determination|journal= Scientific Reports | volume = 7 |issue=1|pages=13667|date=2017|doi=10.1038/s41598-017-13733-1|pmid=29057903|pmc=5651838|bibcode=2017NatSR...713667M}}</ref>

=== Ising machines ===