
Stanford engineers' breakthrough heralds super-efficient light-based computers - Libertatea
http://engineering.stanford.edu/news/stanford-engineers-breakthrough-heralds-super-efficient-light-based-computers
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marcosdumay
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.

~~~
IshKebab
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.

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beloch
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.

~~~
marcosdumay
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.

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bostik
Does this sound like something out of Dirk Gently to anyone else?

> _Now the Stanford engineers believe they 've broken that bottleneck by
> inventing what they call an inverse design algorithm._

> _It works as the name suggests: the engineers specify what they want the
> optical circuit to do, and the software provides the details of how to
> fabricate a silicon structure to perform the task._

I'm quite sure the first novel, _Holistic Detective Agency_ described a
computer program that could do that. (In the novel it was of course sold to a
governmental body, maybe military.)

~~~
loarake
This is actually what we do in radiotherapy nowadays, it's even called
"inverse treatment planning". Basically a radio-oncologist or a dosimetrist
contours (manually, with a pen) organs on every slice of a CT scan and then we
just say "We want this much radiation in the tumour, and less than Y units in
surrounding healthy tissues".

Treatment planning software then simulates dose distributions from a bunch of
possible radiation beam collimations and adjusts the amount of radiation
coming out of each radiation "field" to minimize a cost function penalizing
overdosing healthy tissue and underdosing the tumour(s).

~~~
colinthompson
As a computer graphics professional, this fascinates me. The integrals you
describe are remarkably similar to a large variety of solutions we employ for
area estimation and integration in general. For example, in a path-tracing
paradigm, I would (naively probably) consider radiation beam collimation
similar to what we call the "cone-angle" of a particular integration method,
particularly WRT calculating illumination response for a given surface, etc.
Can you describe in more detail what kind of calculations your treatment
planning software does? It sounds fascinatingly similar to the advanced
physically based rendering algorithms that are in common use in computer
graphics these days. For example, I am guessing that radiation "fields" are
akin to "light sources"? If so, I would guess that your planning software is
doing all sorts of importance sampling of all these sources across a given
domain. Anyway, I was an art major so all my jargon is probably off, but
nevertheless I find your post fascinating.

~~~
loarake
I'm not super familiar with computer graphics, so you'll have to let me know
if my description fits what you guys do ;)

I found a youtube video
([https://www.youtube.com/watch?v=msX1ypCjkK4](https://www.youtube.com/watch?v=msX1ypCjkK4))
that should give you an idea of what I'm describing actually looks like.
Specifically it introduces the concept of a multi-leaf collimator which serves
as the main collimating device in modern radiotherapy. The other degree of
freedom is the angle of the gantry you see rotating around the patient.

Typically for every gantry angle, the treatment planning software would split
up an open field (no collimation) into a bunch of 1x1 cm^2 "beamlets" and
would simulate what kind of dose distribution you would get inside the patient
from each beamlet (you turn the patient CT into a big 3D grid of voxels to
simulate dose in).

You then throw all those dose distributions into an optimiser, and you do
what's called a fluence map optimisation which gives you the amount of
radiation you want to deliver out of each beamlet. This is the optimisation
step I described earlier where the cost function is basically a square
difference between the dose in each organ from a given set of beamlet weights
and what you want the dose to actually be. Healthy tissue is the limiting
factor so you give as much as you can to the tumour while making sure that
less than X% of the volume of a nearby organ gets more than Y units of
radiation. There's a final step at the end that turns the fluence maps into
actual deliverable apertures shaped by the multi leaf collimator.

There's a huge amount of work that goes into the simulation aspect. You can't
just model the radiation beam as pure light sources that attenuate in the body
via some exponential decay because the high energy photons scatter off
electrons which themselves scatter around while depositing energy (radiation
dose) away from the point of interaction. The gold standard is Monte Carlo
simulations (which is my area of research) since you can model the actual
physics of particle transport but in practice most clinics will use a faster
engine to generate dose distributions. The faster engines typically superpose
a primary component (a pure exponential decay) convolved with a kernel
representing the energy that gets deposited away from the point of
interaction.

That's probably way more information than you wanted ;)

~~~
undertow
Wow, that video is really cool! The sliding lock-tumbler mechanism offers a
really interesting amount of control over the aperture shaping the beam! Sort
of like a brush in photoshop!

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fferen
I uploaded the paper for anyone interested:
[https://drive.google.com/file/d/0B2R-ri3N25FnWHBWRzNlak01WEE...](https://drive.google.com/file/d/0B2R-ri3N25FnWHBWRzNlak01WEE/view)

~~~
dxbydt
Actually approachable math - senior undergrad math major or first year grad
can easily understand what's going on. ADMM is actually very old primal dual
setup solved via Lagrangian, goes back to 1980s I think. Good reference -
[https://web.stanford.edu/~boyd/papers/pdf/admm_slides.pdf](https://web.stanford.edu/~boyd/papers/pdf/admm_slides.pdf)

The article itself refers to Boyd's Convex Optimization textbook, which is
sort of a rite of passage for anybody doing anything math-related at Stanford
these days - even their MS in Management course requires a semester of convex
optimization.

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ses4j
The topic is fascinating, but the article doesn't talk to how this "reverse
design" algorithm works at all, which I think might be very relevant to this
audience. Does anyone have a link to more about the design algorithm and how
it works?

~~~
phkahler
TFA includes a link, but the paper is behind a paywall at Nature.

~~~
dxbydt
Stupid nature paywall honestly makes me furious. Why doesn't altman or some
other yc partner throw some money at nature & get all their articles up on the
web for free ? /endrant

~~~
kanzure
> Stupid nature paywall honestly makes me furious. Why doesn't altman or some
> other yc partner throw some money at nature & get all their articles up on
> the web for free ? /endrant

That's not ycombinator's job, that's Bill's job.

~~~
phkahler
Bill's too busy fucking up education.

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mstresh
I did my PhD work in silicon photonics (in a different lab group and not
associated with the authors in the paper) and thought I could chime in with
some extra background and why this result is interesting to the silicon
photonics community.

First off, silicon photonics has already made its way into several products,
mostly active optical cables (a device that directly converts an electrical
signal to an optical signal). See, for example, Luxtera/Molex, Acacia, and
Kotura/Mellanox. Additionally, many other companies have demoed interesting
things at trade shows (e.g. Cisco, Intel, Fujitsu, and others).

In general, the appeal of silicon photonics is that we can fabricate almost
all of the components of an optical link on a single chip using the same
fabrication tools as what you might find in a standard CMOS fab. Modulators,
detectors, switches, filters, and other devices have been demonstrated on a
single wafer. Many organizations (ePIXfab, IBM, IME A*STAR, Intel, Freescale,
and others) have fabrication processes that have all of these devices right
next to each other on a wafer and are capable of 25+ Gb/s data transmit and
receive.

Others in the comments have mentioned the lack of switches in the article.
Making optical switches in silicon has been demonstrated before, usually with
either a Mach-Zehnder interferometer or resonant structure. The phase of light
or resonance are most commonly adjusted through the thermo-optic or plasma
dispersion effect. I'm at work now, but I can dig up references if anyone is
interested later.

This result by Piggot, et. al., is most interesting because it is a unique
device geometry for performing a wavelength splitting function. The
performance of the device itself isn't particularly impressive relative to
other devices with similar functionality that have already been demonstrated
[1]. Additionally, the use of an MMI structure for wavelength multiplexing is
also not novel [2].

So how does this relate to "light-based computers?" The vision that places
like IBM research try to sell is that we will eventually integrate photonics
(either monolithically, or flipped in some form) onto our processors and
memory chips to enable high-throughput on- and off-chip I/O. This is still
likely 10 years away from commercial products. Near-term, look for silicon
photonics in your data centers and fiber-optic regional, metro, and long-haul
networks. (FTTx one day, but silicon photonics currently can't compete in
economics with a DML shoved into a TO can.)

[1] See
[http://www.nature.com/lsa/journal/v1/n3/full/lsa20121a.html](http://www.nature.com/lsa/journal/v1/n3/full/lsa20121a.html)
for a review article on silicon passive optical devices [2]
[http://dx.doi.org/10.1063/1.4812746](http://dx.doi.org/10.1063/1.4812746)

~~~
sounds
Could you clarify why splitting based on wavelength makes this useful in an
optical switch? I assume it's useful in making an interferometer?

~~~
mstresh
Generally speaking, being able to split based on wavelength lets you transmit
data on multiple wavelengths to increase your bandwidth. The flow is to
modulate each wavelength individually, mux them together, send them through a
single fiber or waveguide, and then demux them on the other side. In a switch,
you could imagine switching each wavelength individually and optionally
combining them into a single waveguide out of each port of the switch.

This particular device could not be used to make an interferometer. The device
has 1 input (call it port 1) and 2 outputs (call them ports 2 and 3). If you
input 1550 nm light to port 1, most of it goes to port 2. If you input 1310 nm
light to port 1, most of it goes to port 3. This also works backwards: if you
input 1550 nm light to port 2 most of it goes to port 1. If you input 1550 nm
light to port 3, 10% of it goes to port 1 and the other 90% gets radiated
outward as loss (crosstalk is -10 dB). So if you tried to input 1550 nm light
to both ports 2 and 3 there won't be much interference at port 1 unless there
is a large power imbalance between your two input beams.

~~~
sounds
Ok, poor question on my part, but a great answer. Thanks! Yes, wavelength-
division multiplexing can expand the capacity of a single strand.

I meant, packet switching.

Switching packets of photons would be done by transitioning from transparent
to opaque/inverted polarity/shifted wavelength/etc.

I'm way out of my depth here but an interferometer seems like one method that
current research is looking at to accomplish that. What do you think looks the
most promising?

~~~
mstresh
Yes, an interferometer is certainly a very common method to perform switching.
What ultimately gets used will depend on the technology/material system.
Silica-on-silicon and silicon photonics-based switches will likely use Mach-
Zehnder interferometers. MEMS switches currently use movable mirrors or
gratings.

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peter303
I heard similar "optical break" stories when I was at Stanford decades ago. i
hope this could the one ;-)

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tslug
If it's using infrared light for signaling, doesn't that mean waste heat could
more easily corrupt data?

~~~
marcosdumay
There are several different frequencies of infrared. Unless your chip is
getting too hot (in the 400°C or more), the best frequencies do not mix with
thermal radiation.

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smegel
Hmm zero cases of the word "transistor" appears in the article. My skeptics
hat is hovering above my head right now...

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PhantomGremlin
How does verbiage like this get written?

    
    
       The Stanford algorithm designs silicon structures
       so slender that more than 20 of them could sit
       side-by-side inside the diameter of a human hair.
    

Who is their target audience? Just how innumerate do you have to be to prefer
that text to some actual dimensions?

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anigbrowl
The target audience is TV and print media. Watch, this will be on local Bay
Area stations within 72 hours to fill 2.5 minutes of the evening broadcast.
Most news is targeted at people with a 6th grade understanding of the world.

