Comment by bradleyland | Hacker News Reader

Comment by bradleyland | Hacker News Reader

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This is a good example of asking the wrong question.

How fast does an electron move? 2200 km/s.

How fast does an electrical signal propagate from one end of a circuit to the other? This is a much more relevant, but also more difficult, question. The unit used to measure this performance is called Velocity Factor [1]. Unfortunately, I don't know what the VF would be within a modern microprocessor, but in a wire, it varies from 50% to 99% of the speed of light in a vacuum.

As is typical for mass media news, this story has been summarized in a way that glosses over a lot of the detail.

Photonic computers have a very, very long way to go. Even this tiny beam beam splitter is thousands of times larger than current transistors. Intel uses a 14 nanometer process for Broadwell. The University of Utah beam splitter is 2400 nanometers square. Let's not ignore the fact that a beam splitter and a transistor aren't even close cousins. A beam splitter can't perform any logic.

All of this technology is still very much in the realm of basic science. A lot of the assumptions about the speed improvements are based on napkin math involving theoretical numbers. If anyone ought to know that the difference between theory and reality is often muddied by the trip through the physical world, it would be Photonics researchers. Fiber optic (probably the most widespread application of Photonics research) performance is significantly impacted by real-world factors. Far more so than electromagnetic mediums.

[1]: http://en.wikipedia.org/wiki/Velocity_factor#Typical_velocit...

> Photonic computers have a very, very long way to go. Even this tiny beam beam splitter is thousands of times larger than current transistors. Intel uses a 14 nanometer process for Broadwell.

The POC is a major milestone. Miniaturization will come. Don't forget that at one time, we had to use flatbed trucks to move around 5MB of RAM.

Absolutely. A 2.4 micron beam splitter is a hell of an achievement. I don't know what the next closest is in size, but the vast majority of beam splitters I'm familiar with are measured in millimeters. I'm sure there are others doing work on miniaturization, but 2.4 micron would seem like genuine landmark progress.

What we need next is a practical photonic transistor at these scales!

The main benefit optical interconnects aim to provide is not about the speed with which photons propagate, which as you point out is not all that much faster than electrons in copper, but it is about bandwidth.

Propagation speed of course affects latency, which as you point out is already running up against relativistic limits, but the physics of photonics vs. electronics affects the maximum bandwidth achievable in a modulated digital signal. At a high level, one issue is that the capacitance and inductance present in copper limit the ability of discrete pulses to remain discrete voltage levels that still are distinguishable between logical high and logical low when modulated at a very high frequency due to the capacitance of the wire which effectively blurs the pulses together as they propagate. The other fundamental issue with electronic interconnects is the issue of shot noise, which for information theoretic reasons can further limit the theoretical maximum bandwidth logical high/low voltages in copper. Photonics does not have these limitations, but as you say, is still a very long way off from practical application. It is important to point out, though, that the motivation (unlike the article's slightly clickbaity title) is not speed but bandwidth.

It's worth clarifying that I do believe photonic computers are incredibly exciting. I was just a little annoyed at how the news report appeared to focus on the wrong attributes. I have little doubt that the researcher interviewed spoke at length about the subject, but there's an incredible challenge involved in boiling this kind of information down to a 60 second news spot.

I guess that's what it takes to keep people excited about science though. Not everyone is going to be interested in the nitty gritty.

Excellent clarification, thank you.

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