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Comment by alpineidyll3 | Hacker News Reader

Note: that there is good physical reasons why the cost of QC may grow exponentially with qbits. Refrigeration is exponentially inefficient as T=>0 and the gap of a quantum system which sets the temperature you must cool to shrinks as you couple new degrees of freedom. This dynamic has been the basic reason for the sub exponential progress in the area (despite exponential expenditure)

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Not for photonic quantum computing. Only detectors require cooling, and it is possible to build adequately sized quantum computers with constant number of detectors using loop based architectures.

Even more realistic architectures are very very cost effective on the number of components https://quantumfrontiers.com/2023/06/21/what-is-the-logical-...

Yeah, it always struck me as odd that the main quantum computing research labs went for the "VLSI" approach straight away rather than building room-sized computers with qbits that are known to be more stable and don't require such aggressive refrigeration.

Between the costs of refrigeration/fabrication and the increased speed of decoherence, it's not hard to imagine that the approach of using superconducting qbits may be a dead end, despite quantum ECC.

There are very valid reasons for this. Historically, this is what happened.

* People did very basic qubit experiments with NMR in the late 90s. Very noisy experiments.

* Around 2000 they realized photonic quantum computers would be "easiest" because light experiences very little noise. The problem was that its insanely difficult to do non-linear interactions between photons (which is necessary to do any sort of non-trivial classical or quantum computing with photons). In 2001, somebody came up with a clever way of doing non-linear interactions by using fast detectors.

* Huge efforts started to try and build photonic quantum computers. Unfortunately, around 2004-05, people started to do estimates and it turned out that the number of sources and detectors needed with the clever way was humongous. Far more than we could hope to achieve, and there didn't seem to be any way of reducing it. People abandoned photonic quantum computing and started doing ion traps and superconducting.

* Interestingly around the same time in 2005, there emerged an alternate method of building photonic quantum computers, based on "cluster states". However, the method also had the same humongous resource problem, but it had an advantage: the framework could be modified and played with to improve it. Over the next two decades, very slowly people figured out improvement after improvement to this architecture to bring down the resource costs.

* At this point, this cluster-state photonic architecture has improved quite a bit and is starting to become very competitive with ion traps and superconducting qubits. PsiQuantum (whose article I shared above) is the leader in this right now. And they might win the race.

Yes, but all existing photonic platforms use post-selection which is even more clearly exponentially lossy. Although this could be solved with a deterministic single photo source if one can be found. Photonic quantum computing is an especially funny post-transistor paradigm because classical photonic computing is also quite attractive.

If you read the linked article, it explains how to avoid exponential loss on post-selection.

> Imagine you can toss coins, and you need to generate 20 coins showing Heads. If you repeatedly toss all 20 coins simultaneously until they all come up heads you’d typically have to do so millions of times before you succeed. This is even more true if each coin also has a 20% chance of rolling off the table (akin to photon loss). But if you can toss 20 coins, set aside (switch out!) the ones that came up heads and re-toss the others, then after only a small number of steps you will have 20 coins all showing heads. This large gap is fundamentally why the first whammy is not relevant: To generate a large photonic entangled state we begin by probabilistically attempting to generate a bunch of small ones. We then select out the success (multiplexing) and combine successes to (again, probabilistically) generate a slightly larger entangled state. We repeat a few steps of this. This possibility has been appreciated for more than twenty years, but hasn’t been done at scale yet because nobody has had a good enough optical switch until now.

Haha a self promotional blog post without any experimental data from a lab self-interested in the technology is somehow not persuasive to me.

There is no way to add entangled particles to an entangled photon state without new light matter interaction which may (and almost certainly) bring decoherence.

Ie to claim a good enough switch is possible is a claim demanding a ton of evidence, and there is none right now.

Ofc I would be thrilled to see it work! But explaining a scheme in back of the envelope fashion and measuring it are two vastly diff things.

The terms "realistic" and "cost effective" in the field of quantum mechanics do not have scalar definitions. The meaning changes depending on the problem you are working on and the deadline for funding opportunities.

This is not quite true. You only need to keep the qubits at a fixed temperature as you scale the system, so the resources required to add additional qubits grow only polynomially with the system size. Once you have many qubits with a sufficiently low (but constant) error rate, you can do quantum error correction which also only has polynomial overhead.

No you are missing the fact that if the qubits are coupled the systems fundamental gap has shrunk demanding a lower temperature for the same error rate.

You should think more about why your rosy scheme hasn't worked yet if you can't explain that empirically maybe you don't quite understand.

The qubits aren't all coupled all the time. The whole point of a 2-qubit gate is the coupling is controllable. Also -- why is the gap of the bare system important? The whole point of QEC is the creation of a decoherence free subspace. Your model is wrong and you don't understand quantum error correction.

The qubits aren't meant to be coupled, but in practice everything is coupled at some level. I find it plausible that having a system with tons of degrees of freedom on a limited energy scale might make it difficult to isolate all the degrees of freedom from each other.

Yeah but these are the most basic of basic problems in QC and people would be discarding approaches that didn’t have potential solutions to this issue. In superconducting QCs, neighboring qubits are not necessarily even resonant, and are in their own little metal boxes. With neutral atoms, you have extremely long lived internal states that barely couple to the environment, and the atoms are macroscopic distances from each other.

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