[HN Gopher] TSMC bets on unorthodox optical tech
___________________________________________________________________
TSMC bets on unorthodox optical tech
Author : Rohitcss
Score : 224 points
Date : 2025-05-26 17:15 UTC (1 days ago)
(HTM) web link (spectrum.ieee.org)
(TXT) w3m dump (spectrum.ieee.org)
| Liftyee wrote:
| As I understand it (from designing high-speed electronics), the
| major limitations to data/clock rates in copper are signal
| integrity issues. Unwanted electromagnetic interactions all
| degrade your signal. Optics is definitely a way around this, but
| I wonder if/when it will ever hit similar limits.
| moelf wrote:
| luckily photons are boson (if we ever pushes things to this
| level of extreme)
| Taek wrote:
| This comment appears insightful but I have no idea what it
| means. Can someone elaborate?
| scheme271 wrote:
| Electrons are fermions which means that two electrons can't
| occupy the same quantum state (Pauli exclusion principle).
| Bosons don't have the limit so I believe that implies that
| you can have stronger signals at the low end since you can
| have multiple photons conveying or storing the same
| information.
| xeonmc wrote:
| Also less chance for external interference.
| cycomanic wrote:
| What the previous poster is implying is that electrons
| interact much more strongly than photons. Hence electrons
| are very good for processing (e.g. building a transistor),
| while photons are very good for information transfer. This
| is also a reason why much of the traditional "optical
| computer" research was fundamentally flawed, just from
| first principles one could estimate that power requirements
| are prohibitive.
| nullc wrote:
| > This is also a reason why much of the traditional
| "optical computer" research was fundamentally flawed
|
| presumably also because photons at wavelengths we can
| work with are BIG
| xeonmc wrote:
| Fermions can "hit each other" whereas bosons "pass through
| each other".
|
| (Strong emphasis on the looseness of the scare quotes.)
| EA-3167 wrote:
| The energy densities required for photon-photon interactions
| are so far beyond anything we need to worry about that it's a
| non-issue. Photons also aren't going to just ignore local
| potential barriers and tunnel at the energy levels and scales
| involved in foreseeable chip designs either.
| wyager wrote:
| P-p interactions are not an issue but we do have high enough
| field intensities in high bandwidth fibers to run into p-f
| nonlinearity issues.
| notepad0x90 wrote:
| isn't attenuation also an issue with copper? maybe with small
| electronics it is negligible given the right amps? in other
| words, with with no interference, electrons will face impedance
| and start losing information.
| to11mtm wrote:
| Attenuation is going to be an issue for any signal, but in my
| experience Fiber can go for many miles without a repeater
| whereas something like Coax you're going one to two orders of
| magnitude less. [0]
|
| [0] - Mind you, some of that for Coax is due to other issues
| around CTB and/or the challenge that in Coax, you've got many
| frequencies running through alongside each frequency having
| different attenuation per 100 foot...
| spwa4 wrote:
| > Coax is due to other issues around CTB and/or the
| challenge that in Coax, you've got many frequencies running
| through alongside each frequency having different
| attenuation per 100 foot
|
| Actually this is true for fibers as well. In DWDM (all
| internet links are DWDM, including fiber-to-the-home in
| most places) you have many frequencies running alongside
| and each frequency has differences in attenuation (though
| generally measured per kilometer, not 100 foot)
|
| Optical light are standing electromagnetic waves. Which
| means they don't disrupt each other. Electrical signals
| aren't standing waves. They affect each other.
|
| The difference can be put like this: how many X (electrical
| waves, but essentially everything, protons, ...) fit on the
| tip of a needle? (or in a cable)
|
| 1) electrical waves? Some finite number. Can be large of
| course, but ...
|
| 2) photons (ie. fiber signals)? ALL OF THEM. Literally
| every photon that exists in the entire universe would
| happily join every other photon on the tip of a needle
| nothing would interfere with anything else
| bgnn wrote:
| This is the main mechanism of interference anyhow, called
| inter-symbol-interference.
| lo0dot0 wrote:
| Optics also have signal integrity issues. In practice OSNR and
| SNR limit optics. Cutting the fiber still breaks it. Small
| vibrations also affect the signal's phase.
| cycomanic wrote:
| Phase variations will not introduce any issues here, they
| most certainly are talking about intensity modulation. You
| can't really (easily) do coherent modulation using incoherent
| light sources like leds.
|
| SNR is obviously an issue for any communication system,
| however fiber attenuation is orders of magnitude lower than
| coax.
|
| The bigger issues in this case would be mode-dispersion,
| considering that they are going through "imaging" fibres,
| i.e. different spatial components of the light walking off to
| each other causing temporal spread of the pulses until they
| overlap and you can't distinguish 1's and 0's.
| abdullahkhalids wrote:
| Mode dispersion is frequency dependent phase changes.
| cycomanic wrote:
| That's chromatic dispersion, mode dispersion is spatial
| "path" dependent phase changes. Vibration is actually
| somewhat more relevant because if it wasn't for that we
| could theoretically undo mode dispersion (we would need
| phase information though).
|
| That said all of that is irrelevant to what the previous
| speaker said, vibration induced phase variation as an
| impairment. Thats just not an issue, vibrations are way
| too slow to impair optical comms signals.
| mycall wrote:
| How do the gravity wave optical paths solve the vibration
| issues? Couldn't TSMC do something similar?
| BardiaPezeshki wrote:
| aha! that is true with lasers that are coherent. But not with
| LEDs. We don't care about modes, polarization, or phase.
| Also, no worry about feedback into the lasers, so no
| isolators. LEDs are way easier!
| wyager wrote:
| We already regularly run into optical nonlinearity issues in
| submarine cables. The instantaneous EM fields generated in high
| bandwidth fiber are sufficiently strong to cause nonlinear
| interactions with the fiber medium that we have to correct for.
| Salgat wrote:
| I don't believe this is a factor for the distances that
| inter-chip transmission has. From what I can find, this is at
| most an issue for communication spanning tens to hundreds of
| meters in a datacenter.
| m3kw9 wrote:
| If each cable is 10gb/s and uses 1 pixel to convert into
| electrical signals, would that mean they need a 10 giga frame per
| second sensor?
| cpldcpu wrote:
| I think that's just a simplifying example. They would most
| likely not use an image sensor, but a photodetector with a
| broadband amplifier.
| lo0dot0 wrote:
| No, not necessarily. If you can distinguish different amplitude
| levels you can do better. For example four amplitude modulation
| (4AM) carries two bits per symbol. There is also the option to
| use coherent optics, which can detect phase, and carry
| additional information in the phase.
| amelius wrote:
| > The transmitter acts like a miniature display screen and the
| detector like a camera.
|
| So if I'm streaming a movie, it could be that the video is
| actually literally visible inside the datacenter?
| tehjoker wrote:
| maybe a low res bit packed binary motion picture thats
| uncompressed
| tails4e wrote:
| No, this is just an a analogy. The reality is the data is
| heavily modulated, and also the video is encoded so at no point
| would something that visually looks like an image be visible in
| the fibre.
| lo0dot0 wrote:
| Obviously this is not how video compression and packets work
| but for the sake of the argument consider the following. The
| article speaks of a 300 fiber cable. A one bit per pixel square
| image with approx. 300 pixels is 17x17 in size. Not your
| typical video resolution.
| fecal_henge wrote:
| Not your typical frame rate either.
| mystified5016 wrote:
| Not any more than if you blinked an LED each time a bit comes
| across your network connection.
| topspin wrote:
| They story states there are 300 optical lines in the
| "fiberbundle." Let's assume this is arranged as 20x15 and the
| wavelength of the led is visible and bright enough to perceive.
| So if your unencoded, monochrome 20x15 movie was aligned on
| every frame and rendered at 10E9 FPS, then yes, your movie
| would be visible at the end of one of these cables through a
| magnifying glass.
| speedbird wrote:
| Headline seems misleading. They're buildings detectors for
| someone, not 'betting' on it.
| ggm wrote:
| The word is mostly used for finance investment promotion. I'd
| guess that it's written to attract those bloggers/casters, and
| make money for somebody.
| walthamstow wrote:
| The 'bet' is investing time and money into something that may
| not yield results. It's pretty common business language.
| ls612 wrote:
| Forgive the noob question but what stops us from making optical
| transistors?
| qwezxcrty wrote:
| I think the most fundamental reason is that there is no
| efficient enough nonlinearity at optical frequencies. So two
| beams(or frequencies in some implementation) tends not to
| affect each other in common materials, unless you have a very
| strong source (>1 W) so the current demonstrations for all-
| optical switching are mostly using pulsed sources.
| ls612 wrote:
| I wonder if considerably more engineering and research effort
| will be applied here when we reach the limit of what silicon
| and electrons can do.
| cycomanic wrote:
| No this is not an engineering issue, it's a problem of
| fundamental physics. Photons don't interact easily. That
| doesn't mean there are not specialised applications where
| optical processing can make sense, e.g. a matrix
| multiplication is really just a more complex lens so it's
| become very popular to make ML accelerators based on this.
| momoschili wrote:
| Contrary to the prior commenter, there is definitely
| significant engineering going toward this, but it's not
| clear or likely that photonic computing will supplant
| electronic computing (at least not anytime soon), but
| rather most seem to think of it as an accelerator for
| highly parallel tasks. Two major ways people are thinking
| of achieving this are using lithium niobate devices which
| mediate nonlinear optical effects via light-matter
| interaction, and silicon photonic devices with electrically
| tunable elements. In the past there was a lot of work with
| III-V semiconductors (GaAs/InAs/GaN/AlN etc) but that seems
| to have leveled off in favor of lithium niobate.
|
| Photonics has definitely proved itself in communications
| and linear computing, but still has a way to in terms of
| general (nonlinear) compute.
| ls612 wrote:
| Yeah that was sorta what I was thinking is there a clever
| way to exploit light interacting with matter to turn the
| gate on/off.
| perlgeek wrote:
| As somebody who tried to do a PHD in optical communications,
| this is 100% correct.
|
| I wonder if meta material might provide such nonlinearities
| in the future.
| elorant wrote:
| Photons are way more difficult to control than electrons. They
| don't interact with each other.
| dartharva wrote:
| Electric signals can be made to operate and switch currents on-
| demand in CMOS transistors to make logic gates and eventually
| CPUs. Light signals, however, operate linearly - photons just
| pass through each other and don't interact.
| qwezxcrty wrote:
| Not an expert in communications. Would the SerDes be the new
| bottleneck in the approach? I imagine there is a reason for
| serial interfaces dominating over the parallel ones, maybe timing
| skew between lanes, how can this be addressed in this massive
| parallel optical parallel interface?
| to11mtm wrote:
| > timing skew between lanes
|
| That's a big part of it. I remember in the Early Pentium 4
| days, starting to see a lot more visible 'squiggles' on PCB
| traces on motherboards; the squiggles essentially being a case
| of 'these lines need more length to be about as long as the
| other lines and not skew timing'
|
| In the case of what the article is describing, I'm imagining a
| sort of 'harness cable' that has a connector on each end for
| all the fibers, and the fibers in the cable itself are all the
| same length, there wouldn't be a skew timing issue. (Instead,
| you worry about bend radius limitations.)
|
| > Would the SerDes be the new bottleneck in the approach
|
| I'd think yes, but at the same time in my head I can't really
| decide whether it's a harder problem than normal mux/demux.
| fecal_henge wrote:
| SerDes is already frequently parallelised. The difference is
| you never expect the edges or even the entire bits to arrive at
| the same time. You design your systems to recover timing per
| link so the skew doesnt become the constraint on the line rate.
| bgnn wrote:
| one can implement SerDes at any point of the electro-optical
| boundary. For example, if we have 1 Tbps incoming NRZ data
| from the fiber, and the CMOS technology at hand only allows
| 10 GHz clock speed for the slicers, one can have 100x
| receivers (photodiode, TIA, slicer), or 1x photodiode, 100x
| TIA + slicer, or 1x photodiode + TIA and 100x slicers. The
| most common id the last one, and it spits out 100x parallel
| data.
|
| Things get interesting if the losses are high and there needs
| to be a DFE. This limits speed a lot, but then copper
| solutions moved to sending multi-bit symbols (PAM 3,
| 4,5,6,8,16.. ) which can also be done in optical domain. One
| can even send multiple wavelengths in optical domain, so
| there are ways to boost the baud rate without requiring high
| clock frequencies.
| waterheater wrote:
| >serial interfaces dominating over the parallel ones
|
| Semi-accurate. For example, PCIe remains dominant in computing.
| PCIe is technically a serial protocol, as new versions of PCIe
| (7.0 is releasing soon) increase the serial transmission rate.
| However, PCIe is also parallel-wise scalable based on
| performance needs through "lanes", where one lane is a total of
| four wires, arranged as two differential pairs, with one pair
| for receiving (RX) and one for transmitting (TX).
|
| PCIe scales up to 16 lanes, so a PCIe x16 interface will have
| 64 wires forming 32 differential pairs. When routing PCIe
| traces, the length of all differential pairs must be within
| <100 mils of each other (I believe; it's been about 10 years
| since I last read the spec). That's to address the "timing skew
| between lanes" you mention, and DRCs in the PCB design software
| will ensure the trace length skew requirement is respected.
|
| >how can this be addressed in this massive parallel optical
| parallel interface?
|
| From a hardware perspective, reserve a few "pixels" of the
| story's MicroLED transmitter array for link control, not for
| data transfer. Examples might be a clock or a data frame
| synchronization signal. From the software side, design a
| communication protocol which negotiates a stable connection
| between the endpoints and incorporates checksums.
|
| Abstractly, the serial vs. parallel dynamic shifts as
| technology advances. Raising clock rates to shove more data
| down the line faster (serial improvement) works to a point, but
| you'll eventually hit the limits of your current technology.
| Still need more bandwidth? Just add more lines to meet your
| needs (parallel improvement). Eventually the technology
| improves, and the dynamic continues. A perfect example of that
| is PCIe.
| cycomanic wrote:
| They are doing 10 Gb/s over each fibre, to get to 10 Gb/s you
| have already undergone a parallel -> serial conversion in
| electronics (clock rates of your asics/fpgas are much lower),
| to increase the serial rate is in fact the bottleneck. Where
| the actual optimum serial rate is highly depends on the cost of
| each transceiver, e.g. long haul optical links operate at up to
| 1 Tb/s serial rates while datacenter interconnects are 10-25G
| serial AFAIK.
| cycomanic wrote:
| That article is really low on details and mixes up a lot of
| things. It compares microleds to traditional WDM fiber
| transmission systems with edge emitting DFB lasers and ECLs, but
| in datacentre interconnects there's plenty of optical links
| already and they use VCSELs (vertical cavity surface emitting
| lasers), which are much cheaper to manufacture. People also have
| been putting these into arrays and coupling to multi-core fiber.
| The difficulty here is almost always packaging, i.e. coupling the
| laser. I'm not sure why microleds would be better.
|
| Also transmitting 10 Gb/s with a led seems challenging. The
| bandwidth of an incoherent led is large, so are they doing
| significant DSP (which costs money and energy and introduces
| latency) or are they restricting themselves to very short (10s of
| m) links?
| tehjoker wrote:
| short links it's in the article
| cycomanic wrote:
| Ah I missed the 10m reference there. I'm not sure it makes
| more sense though. Typical intra-datacenter connections are
| 10s-100s of meters and use VCSELs, so introducing microleds
| just for the very short links instead of just parallelising
| the VCSEL connections (which is being done already)? If they
| could actually replace the VCSEL I would sort of see the
| point.
| jauntywundrkind wrote:
| There's been a constant drum-beat that even intra-rack is
| trying to make its way to optical as fast as it can, that
| copper is more and more complex and expensive to scale
| faster. If we have a relatively affordable short range
| optical system that doesn't require heavy computational
| work to do, that sounds like a godsend, like a way to
| increase bits per joule while reducing expensive cabling
| cost.
|
| Sure yes, optical might use expensive longer range optical
| today! But using that framing to assess new technologies &
| what help the could be may be folly.
| qwezxcrty wrote:
| I guess they are doing direct modulated IMDD for each link so
| the DSP burden is not related to the coherence of diodes? Also
| indeed very short reach in the article.
| cycomanic wrote:
| The problem with both leds and imaging fibres is that modal
| dispersion is massive and completely destroys your signal
| after only a few meters of propagation. So unless you do MMSE
| (which I assume would be cost prohibitive), you really can
| only go a few meters. IMDD doesn't really make a difference
| here.
| adgjlsfhk1 wrote:
| I think this is intended for short distances (e.g. a few
| cm). cpu to GPU and network card to network card still will
| be lasers, the question is whether you can do core to core
| or CPU to ram with optics
| cycomanic wrote:
| But why are they talking about multicore fibres then? I
| would have expected ribbons. You might be right though.
| topspin wrote:
| > I would have expected ribbons.
|
| The cable is just 2D parallel optical bus. With a bundle
| like this, you can wrap it with a nice, thick PVC (or
| whatever) jacket and employ a small, square connector
| that matches the physical scheme of the 2D planar
| microled array.
|
| It's a brute force, simple minded approach enabled by
| high speed, low cost microled arrays. Pretty cool I
| think.
|
| The ribbon concept could be applicable to PCBs though.
| cycomanic wrote:
| You might be right and they are talking about fibre
| bundles, but that that's something different to a
| multicore fibre (and much larger as well, which could
| pose significant problems especially if we are talking cm
| links). What isn't addressed is that leds are quite
| spatially incoherent and beam divergence is strong, so
| the fibres they must use are pretty large, coupling via
| just a connector might not be easy especially if we want
| to avoid crosstalk.
|
| What I'm getting at is, that I don't see any advantage
| over vcsel arrays. I'm not convinced that the price point
| is that different.
| topspin wrote:
| > You might be right and they are talking about fibre
| bundles
|
| The caption of the image of the cable and connector
| reads: "CMOS ASIC with microLEDs sending data with blue
| light into a _fiberbundle_. " So yes, fibre bundles.
|
| > I don't see any advantage over vcsel arrays
|
| They claim the following advantages: 1.
| Low energy use 2. Low "computational overhead"
| 3. Scalability
|
| All of these at least pass the smell test. LEDs are
| indeed quite efficient relative to lasers. They cite
| about an order of magnitude "pJ/bit" advantage for the
| system over laser based optics, and I presume they're
| privy to vcsels. When you're trying to wheedle nuclear
| reactor restarts to run your enormous AI clusters, saving
| power is nice. The system has a parallel "conductor"
| design that likely employs high speed parallel CMOS
| latches, so the "computational overhead" claim could make
| sense: all you're doing is latching bits to/from PCB
| traces or IC pins so all the SerDes and multiplexing cost
| is gone. They claim that it can easily be scaled to more
| pixels/lines. Sure, I guess: low power makes that easier.
|
| There you are. All pretty simple.
|
| I think there is use case for this outside data centers.
| We're at the point where copper transmission lines are a
| real problem for consumers. Fiber can solve the signal
| integrity problem for such use cases, however--despite
| several famous runs at it (Thunderbolt, Firewire)--the
| cost has always precluded widespread adoption outside
| niche, professional, or high-end applications. Maybe LED
| based optics can make fiber cost competitive with copper
| for such applications: one imagines a very small, very
| low power microLED based transceiver costing only
| slightly more than a USB connector on each end of such a
| cable with maybe 4-8 parallel fibers. Just spit-balling
| here
| nsteel wrote:
| Aren't they also claiming this is more reliable? I'm told
| laser reliability is a hurdle for CPO.
|
| And given the talk about this as a CPO alternative, I was
| assuming this was for back plane and connections of a few
| metres, not components on the same PCB.
| topspin wrote:
| > Aren't they also claiming this is more reliable?
|
| Indeed they do. I overlooked that.
|
| I know little about microLED arrays and their
| reliability, so I won't guess about how credible this is:
| LED reliability has a lot of factors. The cables involved
| will probably be less reliable than conventional laser
| fiber optics due to the much larger number of fibers that
| have to be precision assembled. Likely to be more fragile
| as well.
|
| On-site fabricating or repairing such cables likely isn't
| feasible.
| nsteel wrote:
| I understand that CPO reliability concerns are
| specifically with the laser drivers. It's very expensive
| to replace your whole chip when one fails. Even if the
| cables are a concern (I've no idea), having more reliable
| drivers would still be preferable to less reliable
| cables, given how much cheaper/easier replacing cables
| would be (up to a point, of course).
| topspin wrote:
| > I understand that CPO reliability concerns are
| specifically with the laser drivers.
|
| Yes. I've replaced my share of dead transceivers, and I
| suspect the laser drivers were the failure mode of most
| of them.
|
| That doesn't fill in the blank for me though: how
| reliable are high speed, dense microLEDs?
| nsteel wrote:
| And are they going to work out any better than Linear
| Drive Optics, the more obvious alternative?
| topspin wrote:
| LDO is just integration. It certainly has value:
| integration almost always does. So it's clearly the
| obvious _optimization_ of conventional serial optical
| communication.
|
| This new TSMC work with parallel incoherent optics is
| altogether distinct. No DSP. No SerDes. Apples and
| oranges.
| nsteel wrote:
| Ok, but I'm just after solutions to problems I have
| talking to other chips. I don't mind what's novel and
| what's optimisation. Whatever is adopted, in either case
| it's a step-change from the past 20 years of essentially
| just copper and regular serdes in this space.
|
| And I'm not sure how much of this is actually TSMC's
| work, the title is misleading.
|
| Edit: actually, they are working on the detector side.
| BardiaPezeshki wrote:
| we use borosilicate fibers that are used for illumination
| applications. You might have seen a bundle in a
| microscope light for example. And they are incredibly
| robust compared to single mode fibers. Note the very
| tight bend angle in the picture - that's a 3mm bend
| radius. Imagine doing that with a single mode fiber!
| topspin wrote:
| > And they are incredibly robust
|
| See my other comment about non-datacenter applications.
| There is a serious opportunity here for fixing signal
| integrity problems with contemporary high bandwidth
| peripherals. Copper USB et al. are no good and in
| desperate need of a better medium.
| BardiaPezeshki wrote:
| The fiber cables we use are basically 2D arrays of 50um
| thick fibers that match the LED and detector arrays.
| We've made connectors and demonstrated very low crosstalk
| between the fibers. Advantage over VCSELs is much lower
| power consumption overall, much lower cost (LEDs are dirt
| cheap and extremely high yield), because we are blue
| light, the detector arrays are much easier and can be
| modified camera technology, and most importantly, much
| better reliability. VCSELs are notorious for bad rel.
| rajnathani wrote:
| The article is about chip interconnects. Think like replacing
| PCIe, NVLink, or HBM/DDR RAM buses, with optical communication.
| BardiaPezeshki wrote:
| In datacenters people use optics for longer distances (10m to
| 2km). Within a rack it is almost always copper. The reason is
| that for short distances lasers are too expensive, unreliable,
| and consume too much power. We think microLED based links might
| replace copper at short distances (sub 10m). MicroLEDs into
| relatively thick fiber cores (50um) are much easier to package
| than standard single mode laser based optics.
|
| on the distance - exactly right. The real bottleneck now in AI
| clusters is the interconnect within a rack or sub 10m. So that
| is the market we are addressing.
|
| On your second point - exactly! Normally people think LEDs are
| slow and suck. That is the real innovation. At Avicena, we've
| figured out how to make LEDs blink on and off at 10Gb/s. This
| is really surprising and amazing! So with simple on-off
| modulation, there is no DSP or excess energy use. The article
| says TSMC is developing arrays of detectors, based on their
| camera process, that also receive signals at 10Gb/s. Turns out
| this is pretty easy for a camera with a small number of pixels
| (~1000). We use blue light, which is easily absorbed in
| silicon. BTW, feel free to reach out to Avicena, and happy to
| answer questions.
| throwaway48476 wrote:
| So, toslink?
| adrian_b wrote:
| The article mentions lengths of up to 10 m, so this technology
| is restricted to links inside a cabinet or between closely
| located cabinets.
|
| The claimed advantage is a very high aggregate throughput and
| much less energy per bit than with either copper links or
| traditional laser-based optical links.
|
| For greater distances, lasers cannot be replaced by anything
| else.
| albertzeyer wrote:
| There is also optical neuromorphic computing, as an alternative
| to electronic neuromorphic computing like memristors. It's an
| fascinating field, where you use optical signals to perform
| analog computing. For example:
|
| https://www.nature.com/articles/s41566-020-00754-y
|
| https://www.nature.com/articles/s44172-022-00024-5
|
| As far as I understood, you can only compute quite small neural
| networks until the noise signal gets too large, and also only a
| very limited set of computations works well in photonics.
| cycomanic wrote:
| The issue with optical neuromorphic computing is that the field
| has been doing the easy part, i.e. the matrix multiplication.
| We have known for decades that imaging/interference networks
| can do matrix operations in a massively parallel fashion. The
| problem is the nonlinear activation function between your
| layers. People have largely been ignoring this, or just
| converted back to electrical (now you are limited again by the
| cost/bandwidth of the electronics).
| seventytwo wrote:
| Seems hard to imagine there's not some non-linear optical
| property they could take advantage of
| cycomanic wrote:
| The problem is intensity/power, as discussed previously
| photon-photon interactions are weak, so you need very high
| intensities to get a reasonable nonlinear response. The
| issue is, that optical matrix operations work by spreading
| out the light over many parallel paths, i.e. reducing the
| intensity in each path. There might be some clever ways to
| overcome this, but so far everyone has avoided that
| problem. They said we did "optical deep learning" what they
| really did was an optical matrix multiplication, but saying
| that would not have resulted in a Nature publication.
| programjames wrote:
| There is, and people have trained purely optical neural
| networks:
|
| https://arxiv.org/abs/2208.01623
|
| The _real_ issue is trying to backpropagate those nonlinear
| optics. You need a second nonlinear optical component that
| matches the derivative of the first nonlinear optical
| component. In the paper above, they approximate the
| derivative by slightly changing the parameters, but that
| means the training time scales linearly with the number of
| parameters in each layer.
|
| Note: the authors claim it takes O(sqrt N) time, but
| they're forgetting that the learning rate mu = o(1/sqrt N)
| if you want to converge to a minimum:
| Loss(theta + dtheta) = Loss(theta) + dtheta * dLoss(theta)
| + O(dtheta^2) = Loss(theta) +
| mu * sqrtN * C (assuming Lipschitz continuous) ==>
| min(Loss) = mu * sqrtN * C/2
| smj-edison wrote:
| With this design, how do they route enough pins from the chip to
| the optical transceiver? Would it take chiplets to get enough
| lanes?
| nsteel wrote:
| There's a link to their press release in the article, it probably
| answers some questions here: https://avicena.tech/avicena-
| announces-modular-lightbundle-o...
| rajnathani wrote:
| Minor: It would be nice if the company TSMC is collaborating with
| on this, Avicena, is mentioned in the HN title.
| stingraycharles wrote:
| It's against HN policy to editorialize the titles
| rajnathani wrote:
| But it is also against HN rules to keep sensationalistic
| titles, dang (moderator 1) routinely modifies titles wherever
| necessary.
| sunray2 wrote:
| Somewhat related: there's a relatively big push for optical
| interconnects and integrated optics in quantum computing. Maybe
| this article yields insight onto what may happen in future.
|
| With quantum computing, one is forced to use lasers. Basically,
| we can't transmit quantum information with the classical light
| from LEDs (handwaving-ly: LEDs emit a distribution of possible
| photon numbers, not single photons, so you lose control at the
| quantum level). Moreover, we often also need the narrow linewidth
| of lasers, so that we can interact with atoms in the way we want
| them to. That is, not to excite unwanted atomic energy levels. So
| you see in trapped ion quantum computing people tripping over
| themselves to realise integration of laser optics, through fancy
| engineering that i don't fully understand like diffraction
| gratings within the chip that diffract light onto the ions. It's
| an absolutely crucial challenge to overcome if you want to make
| trapped ion quantum computers with more than several tens of
| ions.
|
| Networking multiple computers via said optical interconnects is
| an alternative, and also similarly difficult.
|
| What insight do i gleam from this IEEE article, then? I believe
| if this approach with the LEDs works out for this use case, then
| I'd see it as a partial admission of failure for laser-integrated
| optics at scale. It is, after all, the claim in the article that
| integrating lasers is too difficult. And then I'd expect to see
| quantum computing struggle severely to overcome this problem.
| It's still research at this stage, so let's see if Nature's cards
| fall fortuitously.
| avsteele wrote:
| TSMC's approach here sounds sensible but I don't think it
| speaks much to QC. It is a pretty different problem domain. The
| trapped-ion QCs can use much more expensive / less practical
| lasers and optics and still be useful.
| mmmBacon wrote:
| Quantum computing is still a technology of the future. When we
| are still talking about 12 qubits as a breakthrough, there's a
| long way to go. Optical interconnects are the least of quantum
| computing's problems.
|
| However, it's not correct to say lasers are unreliable. It's
| fundamentally false and it's not supported by field data from
| today's pluggable modules. 10's of millions of lasers are
| deployed in data centers today in pluggable modules.
|
| It's also useful to remember that an LED is essentially the
| gain region of a laser without the reflectors. When lasers fail
| in the field, they fail for the same reasons an LED will fail;
| moisture or contamination penetration of the semiconductor
| material.
|
| An LED is not useful for quantum computing. To create a Bell
| pair (2qubits) you need a coherent light source to create
| correlated photons. The photons produced by an incoherent light
| source like an LED are fundamentally uncorrelated.
| ziofill wrote:
| Actually optical interconnects are the _biggest_ of
| (photonic) quantum computing problems. If we had good enough
| optical interconnects (i.e. with low enough optical loss) we
| would already have a fault-tolerant quantum computer. See
| https://www.nature.com/articles/s41586-024-08406-9 (also note
| that Aurora produces 12 physical qubit modes _at each clock
| cycle_ )
| dgfl wrote:
| Trapped-ion and neutral-atom QC require lasers because the
| light signal needs to be coherent. That's the main feature of a
| classical laser, really. The explanation with the number of
| photons doesn't really cut it, because even a perfect laser
| does not have a definite photon number: coherent states are
| inherently uncertain in both photon number and phase. But LEDs
| are even worse, because the light signal is truly incoherent.
| It's not even a good quantum state, it's a classical
| superposition of incoherent photons that you can't really use
| for any quantum control.
|
| But even more than that, this seems to me like a purely on-chip
| solution. For trapped ions and neutral atoms you really need to
| translate to free-space optics at some point.
| sunray2 wrote:
| Indeed, it is nuanced, as you point out. For example, you
| can't just attenuate a laser and use that as a single photon
| source (instead you'd get a coherent state). To realise a
| true single photon source you need an additional (quantum)
| process, like controlled stimulated emission from single
| atoms, or driving some nonlinear crystal to generate photon
| pairs (that's using spontaneous parametric down conversion, i
| think). And that's where the coherence properties of the
| laser are essential.
|
| As for fully integrated optics, it's where quantum computers
| eventually want to be, and there's no physical limitations
| currently. But perhaps it's too early to say whether we would
| absolutely require free space optics because it's impossible
| to do some optics thing another way.
| dartharva wrote:
| I wonder if I will ever see a photonic CPU in my lifetime.
| Probably not, you'll have to invent a completely new material
| never seen before that somehow enables nonlinear interactions
| with light signals. It'd be nothing short of magic.
| cubefox wrote:
| This article is misleading. TSMC doesn't "bet" on the tech by
| Avicena (the startup in question). Instead, Avicena appears to
| simply pay TSMC to help them with manufacturing. Here is the
| linked press release by Avicena:
|
| https://www.businesswire.com/news/home/20250422988144/en/Avi...
|
| Noting also that there have been multiple articles on IEEE
| Spectrum about this startup in the past, I really hope the
| journalists don't own the stock or are otherwise biased.
| BardiaPezeshki wrote:
| TSMC is developing custom detectors for Avicena on their own
| dime. They almost never do anything like for start-ups. That is
| why it is a bet on Avicena. They are nurturing this technology
| because they think it has real potential. See the TSMC quotes
| in the article.
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