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Why Future Supercomputing Requires Optics H commentary Why future supercomputing requires optics H. John Caulfi eld and Shlomi Dolev Could optical technology off er a solution to the heat generation and bandwidth limitations that the computing industry is starting to face? The benefi ts of energy-effi cient passive components, low crosstalk and parallel processing suggest that the answer may be yes. here are points in time at which a Matrix SLM well-known technology, which has been used and gradually improved Output vector T detector array for decades, reaches its physical limitations, forcing a shift towards dramatically diff erent technology. Automobiles, television, computers, photography and communication are all domains in Input vector which such shift s have occurred. Th e VCSEL array point at which these massive shift s in technology occur is oft en decades aft er the initial invention and oft en requires substantial investment. Th e time is fast approaching for such a shift in computer technology, as the frequency (bandwidth) limitations of silicon electronics and printed metallic tracks are reached. Th e computer industry has already acknowledged the situation by introducing multicore technology Lenses which implicitly recognizes the use of a distributed and parallel architecture to Lenses scale computer power. For decades, optics have proved to be the best means of conveying information from one point to another, as can clearly Figure 1 | The Stanford Vector Matrix Multiplier. The use of a parallel approach greatly improves be seen by the growth of the massive computing performance. VCSEL, vertical cavity surface emitting laser; SLM, spatial light modulator. Figure fi bre optics communications industry. reproduced with permission from ref. 11, © 2009 OSA. Th e fact that light beams can be easily transmitted in parallel in free space, and do not suff er from crosstalk, is of great electronic switching element — and that industrial-scale optical processing devices importance. Th e fabrication of multicore great progress has been made in all- have been partially successful6. In addition, chips requires fast buses to connect cores, optical switching and logic, there is an recent research activity in academia7–10, and new designs are considering the use opportunity to avoid light-to-electronics and the United States government of optical interconnects to aid reliable and and electronics-to-light conversions in the contract reviews from the Air Force and fast communication1. Indeed, the fi rst steps design of all-optical computers. from DARPA, are clear signs for great towards commercializing the use of optics It is important to note that the design excitement in the fi eld. for communication among cores were of optical computers does not necessarily Th e widely accepted Strong Church– recently reported by fi rms such as Intel and have to mimic the design of the electronic Turing thesis asserts that anything can be Avago, among others2–5. computer, just as the design of electronic computed by a digital computer. Accepting But optical technology also has the computers did not mimic the design of this suggests that optics off ers no new opportunity to go beyond being just mechanical computing devices. Optics computational capability. To be useful, the a convenient pipe for ultrafast data implies a new means for the application best that optics can do is to accomplish transmission, and actually perform data of parallelism, with many beams in space the same goals as digital electronics, processing. Given that the basic computing representing individual computations. but in a more effi cient or advantageous element of a computer is a transistor — an Recently, attempts to produce manner. Th e advantage could be in speed, NATURE PHOTONICS | VOL 4 | MAY 2010 | www.nature.com/naturephotonics 261 © 2010 Macmillan Publishers Limited. All rights reserved nnphoton_.2010.94_MAY10.inddphoton_.2010.94_MAY10.indd 226161 110.4.160.4.16 99:42:01:42:01 AAMM commentary energy consumption, heat generation or a 1 NAND the fundamental minimal operation another metric. A A’ required — the Boolean logic gates. Speed is a frequently suggested Laser Rolf Landauer12 showed that the energy advantage for optical computing. Ironically, needed to operate such a gate stems from the speed of optical processing is ultimately 0 0 the need to erase one of the output bits, as limited by the speed of electronic input B the Boolean logic gates have two binary and output. In essence, optical processing 0 AND inputs but only one binary output. Shortly is inextricably tied to Moore’s law for aft er that, Bennett, Landauer, Keyes and b electronic computing, and the maximum others realized that if the extra bit was benefi t that optics can off er today is to kept, there would be no minimum energy replace electronic functions that slow per gate operation. Aside from the energy system speeds. It is not the speed of needed to input data and readout results, components that counts; it is the speed the logic itself could be free. of the supercomputer as a whole. Optics Recently, the concept of logic systems can help in this respect. An algorithm’s that consume zero energy has been computational complexity is normally the proposed. Two independent ways to way algorithms are compared. Electronics Figure 2 | The concept of ‘zero-energy’ logic do this were recently discovered by has two ways to handle complexity: the use gates using optical passive elements. a, Powered Caulfi eld, Shamir, Hardy, Soref and of space (parallelism) and time. Optics has electronic elements to perform AND and NAND others13–15, accomplishing the theoretical a third way: fan-in and fan-out, in which boolean functions. A, A’ and B are the inputs of the thermodynamic ‘permission’ to do so. many independent beams may be modifi ed device. b, Rochon prisms. Optics, when compared with electronics, by a single pixel in an optical processor. has two important advantages — superior In principle, supercomputers based on fan-in and fan-out, and signal transmission optical technology may help to mitigate the Stanford multiplier. Th e input is a without the need to apply a voltage. the heat generation that plagues electronic vector of light sources that are projected Once generated, light propagates by itself computers. Th e heat problem of electronic in parallel by lenses on the rows of a slide according to Maxwell’s equations, and devices becomes much worse as speed or on a spatial light modulator, and the today we can manipulate the apparent increases and density decreases, both of output that encodes the multiplication speed of light in some striking ways, which are scaling rapidly according to product is obtained by gathering light without violating Maxwell’s equations Moore’s law (the two-year doubling in the from each row to a sensitive detector. or relativity. We also have at our number of transistors on a silicon chip). Loading data in parallel to the input vector disposal numerous passive elements for Early workers envisaged optical computers as and the spatial light modulator leads to a manipulating light that require no energy rivals of electronic computers. It now appears great improvement in computing speed. to be supplied and do not generate heat, for that they are allies in keeping Moore’s law Th e architecture supports principal DSP example waveguides, fi bres, mirrors, prisms alive. Faster operations require more power, primitives such as vector-by-matrix and beamsplitters. while smaller components provide less and matrix-by-matrix multiplications, Figure 2 presents two designs for optical area through which to remove the resulting convolution, correlation and discrete implementation of an energy-free logical heat from each element. Th e increased use Fourier transforms. Functioning as a gate. Both designs are zero-energy devices. of optical components that require little co-processor of a legacy processor, the Figure 2a is an example of using a new kind or, in the case of passive devices, no heat architecture can effi ciently process integers of logic (called directed logic) that gives dissipation may prove very important. Th ere as well as complex numbers and perform the same result as conventional Boolean are several approaches that use the special extended-precision arithmetic. Th ese logic but does so in a way that is optics- characteristics of light for information building blocks enable DSP-intensive friendly13. Th is device produces both A processing. Th ey are described in turn below. applications, and the fi nding of optimal AND B and also its complement A NAND solutions to several non-deterministic B. Note that NANDs can be combined Vector matrix multiplier polynomial-time complete problems using to implement any other kind of Boolean A vector matrix optical multiplier (Fig. 1) exponential space. Th e device operational logic gate. Figure 2b shows a generalized uses a lens to split the light received rate, of the order of at least 16 tera eight-bit optical logic element14,15. It is essentially from each entry of an input vector to integer operations per second, and power a physically embodied lookup table that the corresponding entries of the vectors rating, of more than 16,000 giga eight-bit computes A AND B, with NAND obtained of the matrix. When a beam traverses instructions per watt, show great promise by the use of additional Rochon prisms. the matrix, the intensity of the beam is for enhancing existing applications By choosing the beams to defl ect or not changed according to the value of the and introducing new optical computer to defl ect, we can implement any Boolean entry that the beam reached. Another applications such as video compression logic gate using only passive components lens is used to sum the resulting light or a three-dimensional geometry engine. (Rochon prisms of two diff erent intensity of each resulting vector.
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