Optical logic?

David Miller has researched both the possibilities of optical logic and the challenges in exploiting it. His early discovery of optical bistability in semiconductors [1] was one of the triggers, together with simultaneous work by others at Bell Labs, for a serious examination of optical logic.

Optical logic has many serious challenges, which he has pointed out in two key publications, one relatively early in this field [2], and another more recently [3]. First, it is difficult for any optical logic device to achieve the necessary qualitative criteria for use in a logic system. Second, the energy of nearly all proposed optical logic gates is larger than that of modern electronic gates; this is a serious problem because arguably energy is one of the major limitations on continuing to scale information processing to higher capacities. So proposing a logic technology that takes more energy than the current dominant and successful technology – silicon CMOS – it not likely to succeed. Third, if it were to attempt to displace silicon CMOS, it would face daunting cost targets and exponentially difficult challenges in fabrication at the astonishing scales possible in CMOS.

He did, however, invent optoelectronic logic devices with relatively low energies [4] (see also a later review [5]), and, with colleagues at Bell Labs, was able to extend these ideas [6] to satisfy the necessary qualitative criteria for logic systems. These devices allowed serious experiments in highly parallel optical logic computers and telecommunications switching systems. Among the achievements stimulated by these devices was work by his colleagues at Bell Labs demonstrating that working optical logic systems with over 60,000 light beams were quite feasible [7], showing a capability for parallelism in optics that remains one of its strengths.

Out of this body of work, it became increasingly clear that, if there was to be a future for such approaches, it lay largely in solving problems of interconnections inside systems. That remains a major opportunity for optics. Any such approach requires very close integration of optoelectonic and electronic devices, a point he realized early on [8] (see also the critical review [2] and a later review of optoelectronic logic [9]) , but the physical possibilities and advantages are clear.

See also what may be the only other optical gate – one based on solitons retiming in fibers – that satisfies the necessary qualitative requirements for optical logic [10], though the energy requirements and physical dimensions here also are likely too large for large scale use.

Whether optical logic has any role is still an open question. Despite David Miller’s arguments against most of the ideas of “optical transistors” [3], many researchers continue to pursue this. One apparent feature that drives some of this work may be a belief that optics offers much faster logic than transistors. This may, however, be based on a misunderstanding of the speeds of electronics and the reasons why we do not typically run electronic systems at higher clock rates. Electronic transistors internally can run at picosecond times. We do not run them at such speeds for at least two reasons: one reason is the practical difficulty of running any electrical wire interconnect at very high speeds, and one could argue that optics could avoid those difficulties (which is a genuine argument). However, possibly the main reason we do not run large logic systems at such speeds is that the chip would melt. Even with the very low energies in electronic logic, just clocking all the logic gates on a large chip at high clock rates would generate too large a power dissipation to allow us to remove the heat by reasonable means. Hence, unless optical logic could offer substantially lower energies than CMOS gates (and currently there does not appear to be any optical logic device that does), we would not be able to run the optical logic faster either in any large system.

Certainly as we move to consider quantum computers and quantum information processing, optical devices that could work at the single photon level would be very attractive [3], and any serious possibility for that remains an exciting challenge for optics.

[1] D. A. B. Miller, S. D. Smith and A. Johnston, “Optical Bistability and Signal Amplification in a Semiconductor Crystal. Application of New Low-Power Nonlinear Effects in InSb,” Appl. Phys. Lett. 35, 658‑660 (1979).

[2] D. A. B. Miller, “Device requirements for digital optical processing” in “Digital Optical Computing,” ed. R. A. Athale, SPIE Critical Reviews of Optical Science and Technology, CR35,68‑76, (1990).

[3] D. A. B. Miller, “Are optical transistors the next logical step?” Nature Photonics 4, 3 – 5 (2010) https://doi.org/10.1038/nphoton.2009.240

[4] D. A. B. Miller, D. S. Chemla, T. C. Damen, A. C. Gossard, W. Wiegmann, T. H. Wood and C. A. Burrus, “Novel Hybrid Optically Bistable Switch: The Quantum Well Self Electro‑Optic Effect Device,” Appl. Phys. Lett. 45, 13‑15 (1984).

[5] D. A. B. Miller, “Quantum-well self-electro-optic effect devices,” Optical and Quantum Electronics 22, S61‑S98, (1990).

[6] A. L. Lentine, H. S. Hinton, D. A. B. Miller, J. E. Henry, J. E. Cunningham, and L. M. F. Chirovsky, “Symmetric self-electro-optic effect device: Optical set-reset latch,” Appl. Phys. Lett. 52, 1419‑1421 (1988)

[7] F. B. McCormick, T. J. Cloonan, F. A. P. Tooley, A. L. Lentine, J. M. Sasian, J. L. Brubaker, R. L. Morrison, S. L. Walker, R. J. Crisci, R. A. Novotny, S. J. Hinterlong, H. S. Hinton, and E. Kerbis, “Six-stage digital free-space optical switching network using symmetric self-electro-optic-effect devices,” Appl. Opt. 32, 5153-5171 (1993) https://doi.org/10.1364/AO.32.005153

See also H. S. Hinton, D. A. B. Miller, “Free-Space Photonics in Switching” AT&T Technical Journal, Jan/Feb, 84‑92, (1992).

[8] D. A. B. Miller, M. D. Feuer, T. Y. Chang, S. C. Shunk, J. E. Henry, D. J. Burrows, and D. S. Chemla, “Field-effect transistor self-electrooptic effect device: integrated photodiode, quantum well modulator and transistor,” IEEE Photonics Tech. Lett. 1, 61‑64 (1989).

[9] A. L. Lentine and D. A. B. Miller, “Evolution of the SEED technology: bistable logic gates to optoelectronic smart pixels,” IEEE J. of Quantum Electronics 29, 655‑669 (1993).

[10] M. N. Islam, C. E. Soccolich, and D. A. B. Miller, “Low-Energy ultrafast fiber soliton logic gates,” Optics Lett. 90, 909‑911 (1990).