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Comment by marcosdumay | Hacker News Reader

The article talks about switching in a single place, as if by accident. The abstract has no mention of switches at all. Yet, "light-based computers" is something completely dependent on switches, and not much of anything else.

If somebody has access to the actual paper, is there any chance that can lead to switches? Or just multiplexing / demultiplexing?

Update: Thanks fferen. It did demuxing. No work related to switches, except for a "in the future, somebody may use a similar technique to make a switch" on the end (yeah, maybe, using some other dozen techniques not yet discovered, and this one). Demuxing is very important, just that it alone does not lead to practical photonics.

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Yeah I was going to say... They've done something which is cool, but really we already knew it was possible, and is not the hard bit of optical computing.

Essentially they made a fancy prism. We need an optical transistor.

It's very difficult to get photons to strongly interact with each other. An optical transistor would revolutionize both classical optical computing and quantum computing.

There are optical switches around. They work by making the material transparent or not depending on an impulse.

The problem is that they are big, slow, monochromatic, hard to build and actually consume more energy than transistors to operate. That's why a breakthrough on that would be interesting.

Vanadnium Dioxide: http://phys.org/news/2013-11-vanadium-dioxide-smart-glass-bl...

The actual paper is about realization of an algorithmically-designed optical device rather than optical switching. The device is "just" a computer-designed multiplexer.

Could such a device alter the relative phase of two separate beams of light? This optical device would take as input two beams of light with some arbitrary phase difference, and output the same two beams but with their phases changed to some fixed offset relative to each other.

With such a device an actual switch becomes easy: Make the device change the two inputs to be antiphase and combine them. If one of them is missing, the other passes through unphased (pun intended), but if both are present they cancel each other out; aka you have an xor switch!

First, waves still pass through each other when destructively interfering. They temporarily cancel out each others' amplitudes, but not each others' change-in-amplitude, so they continue to propagate.

Second, XOR is not a universal gate (even when combined with NOT). XOR can only sum things together (mod 2). For universal computation you need to be able to multiply (e.g. with an AND gate or a Toffoli gate).

Yes, a conventional 2-port MMI will generally give you a pi/2 phase shift of one port relative to another. If you combine the 2 ports again with another MMI, you have built a Mach-Zehnder interferometer with ideally pi phase difference (if you've perfectly matched the length of the 2 waveguides) between the top and bottom signal paths. If you insert a phase shifter in one or both arms you can control the light at the output of the MZ by varying the voltage applied to the phase shifter. You can then modulate the voltage and produce an AM or PM signal at the output of the MZM. This is currently how some commercially available photonic communication IC's send data over the network.

There are limitations on how good the extinction (cancellation) can be based on how well the losses are matched in the respective waveguides.

In this case of this paper, I imagine that the phase relationship will be much more complex and it will highly wavelength dependent.

More specifically, their algorithm seems to design linear optical systems (multiplexing,demultiplexing,phase shifting,etc are among them: actually any LT given by a complex matrix of phasors wich is at most unitary can be realized). This matrix can be generalized to the continuous case with a self-adjoint operator but I suppose you get size restrictions to reproduce precisely a complicated operator.

Switching of course is nonlinear.

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