[sunray2]

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.

[dgfl]

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]

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.

[mmmBacon]

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]

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)

[avsteele]

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.