Hacker News Reader

Comment by fxj | Hacker News Reader

The big unsolved problem in QC is the lifetime of the QC itself until decoherence. Imagine a computer where you can execute a maximum of 1800 commands (IBM Heron) and then it is broken. That is the status of QC at the moment. A QC might store TB of data and do searches with O(1), but there is no way (at the moment) to upload a TB database to the QC. What we need is a quantum processor that lives for hours or days in a coherent state, but what we have is milli seconds.

Just my 2 ct.

edit: IBMs Roadmap shows a QC with 1 Billion commands (gates) after 2033. With that machine they could (in principle) upload a 100MB database and do searches.

source: https://www.tomshardware.com/tech-industry/quantum-computing...

View on HN

The bigger question is how many qubits exactly you can keep arbitrarily entangled on your preferred time scale. Because a linear increase in the number of qubits gives you an exponential increase in capability (compared to a classical computer) for problems that are amenable to the quantum approach. So if adding qubits turns out to be exponentially difficult, then QC will not amount to much since you could've done the same thing in a classical simulation. If it can be done more or less arbitrarily with a non-exponential cost, it's a true asymptotic change for the kinds of problems QC can address.

It's not a big mystery how this will get accomplished however, which is why participants in the field seem hopeful (if modest and tempered). The theory of quantum error correction is rich and pretty well developed. It's just that engineering a system which has enough runway to be error corrected requires a lot of development and innovations in a lot of directions: qubit design and fabrication, quantum compilers, rapid experimental procedures, etc.

Edit to edit: Thinking of gate depths as permitting such-and-such megabyte databases as being "uploaded" isn't really a good or accurate metric in my opinion.

What puzzles me with IBMs roadmap is their scaling:

2025 156 qubits, 5000 gates

2028 156 qubits, 15K gates

2029 200 qubits, 100M gates

2033 2000 qubits, 1B gates

The jump from 15K to 100M gates looks fishy to me. Maybe I am wrong but I doubt that will work that way.

When will they (or anyone) be able to run Shor's algorithm to factor the number 35? There was a failed attempt in 2019.

https://en.wikipedia.org/wiki/Shor%27s_algorithm#Physical_im...

The key is to give an optimistic roadmap and then work on delivering it as fast as possible.

Maybe I am getting a bit cynical but it seems that is what all companies and workers are doing.

Lying enough to get people excited to fund you, but not quite enough to catch a fraud charge.

There's a huge difference between saying "we can do" and "we think we will be able to do".

The real difference is between "we think we can do it" and "we think we can get away with saying it".

It reads like wishful thinking, or an upper bound of what could happen in the best-case scenario.

Error correction can fix bit flips, but how can it help if qubits loose entanglement?

It's an unintuitive theorem in quantum computation that as long as you can protect against two types of errors (bit flips and sign flips) you are protected against almost all errors.

When qbits are not entangled, quantum computer doesn't exist anymore, this is not a kind of error that you can correct.

There isn't a good non-technical answer to your previous question but lemma 3.3 of this paper [1] says that if you can correct a finite set of errors for each qubit individually then you can also (essentially for free) correct all other errors. Specifically if you correct a set of errors which spans the set of possible errors (for example the Pauli errors on each qubit do this) then you correct all errors.

This is essentially the reason people are still interested in quantum computing, and why quantum error correction is viable at all. You can protect highly entangled crazy states from highly non-local complicated errors using only the resources needed to correct simple local errors.

[1] Key ideas in quantum error correction, Raussendorf 2012: https://royalsocietypublishing.org/doi/10.1098/rsta.2011.049...

Can you also compensate against decoherence? I thought this is the magic limit when the QC breaks.

Decoherence here pretty much just means "enough individual errors that you can't recover from them anymore". Theoretically yes, it is quite clear that you can protect against this. Practically doing so at the scale you need for a real, useful computer is very hard.

Quantum error correction corrects more than just bit flips. It corrects phase flips, accidental measurement, qubit erasure, small global rotations... lots of stuff.

I don't know exactly what you have in mind as "loss of entanglement"... But quantum error correction will very much preserve entanglement at the logical level against decay of entanglement at the physical level.

> do searches with O(1)

Nope. Grover's algorithm allows only O(sqrt(n)) search.

Quantum computers won't search unordered databases with fewer than ~ sqrt(N) queries. Proved in: https://arxiv.org/abs/quant-ph/9701001

I'm a bit biased, but non-superconducting modalities have considerably better coherence time properties. Neutral-atom, for example, has on the order of seconds. It has other constraining issues, but in theory the coherence times are better

Neutral atoms don’t fare much better for gate fidelity, error correction algorithms are the only way to compute useful problems then probably

Edit: quera is having its moment with the darpa announcement and the 48-logical qubit accomplishment in lab

> lives for hours or days in a coherent state

Or a few seconds, but with better error correction.

https://www.nature.com/articles/s41586-022-05434-1

Reply on news.ycombinator.com