After 50 years of effort, researchers made silicon emit light
wired.com
wired.com
Wavelength of the light emitted by these devices: ~4000nm
Latest generation commodity CPU transistor structure size: 7nm
Add to that that photons really don't like being trapped; you essentially need a delay line and optical amplifier to hold them indefinitely (that's essentially the core technology my whole PhD thesis centers around), it makes them a really impractical thing to store bits with. Things with a rest mass can be stored easily, though. Things like, say, electrons!
In this case, the pattern is:
1. Identify and refute common conception
2. Compare easily understood numbers
3. Assert basis of knowledge
4. Introduce a clear, tangible model for alternative thinking
5. Appropriate use of emotional resonance to capture and maintain attention
I don't see how the wavelength is comparable to transistor size because as you switch to the optical realm, the benefit of information propagation at speeds near c (or c, if you're pulling a vacuum) means physical size doesn't matter as much. At 4Ghz you can move information 7.5cm in one cycle, and that's a pretty large distance compared to any integrated circuit I've ever seen.
Why is storage necessary? If you can move bits to optical gates and get a result back it seems to me like you can work around the fact that, in an electrical system, capacitance and heat (due to density achieved in the quest for minimizing capacitance) start to limit the computation you can do.
Have you ever tried wiring any non-trivial logic without flip-flops? Say, a simple signal routing layer. Even the most basic bits of logic becomes much less efficient to downright impossible without storage.
If n=1.3 -> v = c/1.3 = 77% of c
Unfortunately something like 0.7c is about the fastest speed of EM wave propagation in an optical waveguide or along a copper waveguide. Another comment here gives a slightly faster example with n=1.3, which is maybe achievable in some kind of polymer. Or in highly purified water, for what it's worth.
You can get a mild speedup, 40% or something, by moving to free space. But that is an unbelievable can of worms, taking all the signals out of the waveguides and somehow still getting 1B signals going to the right place. The 40% speedup doesn't remotely pay for giving up solid state waveguides.
"Dispersive" fortunately doesn't mean a meaningful slowdown. It just means that a transmitted bit will travel at a range of slightly different speeds. If it goes very far, the shape of the pulse will get messed up. But that's a problem people are already pretty good at solving.
Where some are said to reach up to 99.x % the speed of light in a vacuum.
Most fascinating thing i've read years ago they'd be the prime candidate for manufacturing in space, because real vacuum.
Would be much appreciated.
https://www.computerhistory.org/storageengine/edsac-computer...
Not optical, but piezo electrical, usually with a crystal or air as the medium instead of mercury. Optical is much the same principle, a feedback loop incorporating the delay line, so the same bits get re-injected over and over again and can only be read out at specific points in time.
Like most things, the concept is much simpler than making one work — and I was hoping I could find some papers on the applied side of photon delay lines. (Since OP commented it was related to his/her PhD.)
Though as always with electrical based electronics - superconductors are room temperature are always heralded to be the big jump in many things. As always, soon, much like photonics or let alone the ability to easily design and implement asynchronous circuits, let alone CPU's.
Though I do wonder what other industries have the equivalent to moore's law driving them in both advancements and marketing?
I'm kinda drawing a blank of anything that has any progress metric defined. Though hopefully somebody else knows of something comparable in another form of production/business.
In the optics community we usually consider everything we can manipulate with refractive optics as "light" – and yes, I am fully aware that this goes down well into what's considered microwave radio.
My personal cutoff for where optics begins is, where I no longer can use an antenna that is part of a resonant _circuit_ to emit / receive the radiation, and have to resort to quantum mechanical state transitions.
It's a place that celebrates a history of making stuff glow.
Eindhoven likes light! And....making bridges out of beer crates: https://www.ed.nl/default/tu-e-studenten-vestigen-nieuw-reco...
>> cubic crystal lattice that allows electrons to move within the lattice under certain voltage conditions. But it doesn’t allow similar movement for photons, and that’s why light can’t move through silicon easily.
Uhhh.. not really. I’ll try to explain (forgive my ad-lib MatSci from 20 years ago). Efficient light generation is a matter of direct or indirect bandgap. A direct transition is one where the electron wave number is unchanged in dropping from the high to low energy state, so it can be completed with a single photon (light). An indirect transition fails conservation of energy and momentum with one photon, so it requires phonon (heat) interactions. Semiconductors have an energy gap between the highest few occupied state and the lowest few unoccupied states, and these are the only states that can exchange energy. Direct transitions generate mostly photons, so even if it gets absorbed, it will get re-emitted intact until it leaves the material. Indirect transitions means that phonons remove energy each time, so it all becomes heat. In normal conditions, Indirect materials are more transparent, although direct materials can become transparent by population inversion, which is when there are more electrons in the high-energy states then the low-energy states for the bandwidth of the photons being generated. Then any photon generated is more likely to generate more photons on its way out (stimulated emission) than to be absorbed. This is what you want. Okay I’ll stop now, but there are tricks that you can use to get this behavior in silicon, an indirect-bandgap material, which is the topic of the article.
>GigaIR: 512 Mbit/s – 1 Gbit/s, NRZI, 2-ASK, 4-ASK, 8b/10b
IrDA has its uses but it seems they don't deal with fiber comms.
40Gbps per wavelength, not per fiber. Many times 40Gbps over one fiber.
10/25/50/100 are the common rates in real-world Ethernet PHY standards.
(from memory) Current fibers have a dispersion minimum ~1300 nm and an attenuation minimum ~ 1550 nm
Imagine a 3D printer at atom scale. But because the scale is so small, the nozzle has to deposit a gas.
The magic is in making the individual gas molecules get to the right place.
Layer by layer, to what theory predicted would be a light emitting configuration.
Incredible achievement.
BTW It is worth noting from your link that the silicon is being grown on a GaAs substrate, so to be useful they would have to figure out how to grow the silicon wire on a silicon substrate. (GaAs already has many options for optical devices.)
Secondly, could using a "photonic" memory bus bring RAM access speeds close to cache speeds, or is the transmission distance/time not the main issue there?
Silicon is typically a really lousy photon emitter because it’s an indirect bandgap material. Turning an electron/hole pair into a photon requires an interaction with a phonon. It seems by getting the silicon to grow in a hexagonal orientation, it becomes a direct bandgap material leading to much higher emission efficiency.
It's the other way around: DRAM accesses are slow, that's why we need caches.
> translation (maybe with TLB miss)
In most architectures, the caches are physically addressed, so TLB lookups occur before even L1 cache access. Successful TLB lookups are extremely fast! And you can't skip the TLB, even if you don't have any data caches.
So to see if a memory location is contained in a cache line, a TLB lookup is needed to first get the physical address? I wouldn't have expected this, can you expand on why this is the case?
With physical addresses, you just have to clear your TLB cache.
You are totally right that if you can make the resultant communication speed faster you could theoretically do away with caches. However this approach wouldn’t solve that problem on its own. Also forget not that cache is expensive and DRAM is cheap!
Yes I’m aware that caches can be physically addressed and you could reorder the sequence I described. No you can’t skip the TLB, but a hit will be faster since you don’t have to perform translation.
No, it really can't. The distance between CPU core and DRAM chips is approximately 10 cm, so at a typical electrical propagation speed of around 2/3 c, the round-trip time is 1 ns. A full DRAM access, however, is on the order of 100 ns. So physical transmission speed only accounts for about 1% of DRAM access times.
On top of that is power requirements which are again orders of magnitude higher than on chip signals.
Optical has a chance to fix that for the same reasons it works so well for longer distance networking.
> On chip signals can be much faster because they don’t have the capacitive load.
You can routinely achieve > 30 Gb/s off-chip in copper cables over distances > 1 m using differential signaling [1]. Capacitive load is only a limiting factor if you directly drive the gate of a transistor.
[1] For example, high-end Xilinx FPGAs provide several of those transceivers. They really operate at > 30 GHz.
That’s why despite DRAM having pretty much constant latency for the last 20 years bus speeds and bank counts have been consistently increasing. Optical interconnects will help immensely.
We may also see things like off chip SRAM come back into vogue once its feasible to take advantage of their performance.
Journalists need to be educated: Transmission lines are photonic, so silicon already has connections carrying data around using photons. As you would expect, those photons are traveling at the speed of light in the material.
If I were king, I would demand that every optical-silicon publication explicitly describe why their optical photons are more desirable than microwave photons that are already in widespread use.
Sounds potentially like a big deal :)
How did you come up with that number? The speed of travel? That's not really the main bottleneck in current computer architectures, at least not yet.
Using photons would enable quite different architectures that we haven't even conceived of yet.
All you need to do is shoot it with a photon bean, and the when material can no longer absorb the photons you send at it, it will begin releasing them as reflection. But the one you are shooting in aren't the same ones that are coming out.
Who's the target audience for this analogy? If you understand what a brain cell is then you probably know what a transistor is too. I would bet that more people know what a transistor is than what a brain cell is.
> I would bet that more people know what a transistor is than what a brain cell is
I'm happy to take you up on that bet, but it depends on what bubble you ask. In San Francisco/Silicon Valley, that's probably true, but outside any high-tech bubbles, more people know that we have brain cells in our heads, than we have transistors in our computers, I'm fairly sure.
They _might_ know the transistor replaced vacuum tubes, but I doubt many would be able to tell what function either had, or be able to point out the transistors inside such a radio.
Here in the UK we had "the wireless", and I'm confident that my parents - late 70s - who were the generation of first domestic computer ownership in the UK would associate "transistor" primarily with computers.
(E.g. this song from 1982: https://learnsongs.ru/song/dinamik-na-plyazhe-pleshchet-voln...)
No chance. Everyone knows what a brain cell is, to some extent. Even people who have too few to rub together don't really feel like they know what a transistor is after reading the Wikipedia page for them, twice...
DOI = 'Digital Object Identifier'