4 ms·

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 Hero

by fxj 3y ago

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/ibm-demonstrates-useful-quantum-computing-within-133-qubit-heron-announces-entry-into-quantum-centric-supercomputing-era https://www.tomshardware.com/tech-industry/quantum-computing...

red75prime 3y ago
> do searches with O(1) Nope. Grover's algorithm allows only O(sqrt(n)) search.
reikonomusha 3y ago
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.
- fxj 3y ago
  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.
  - mettamage 3y ago
    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.
    - pclmulqdq 3y ago
      Lying enough to get people excited to fund you, but not quite enough to catch a fraud charge.
      - echelon 3y ago
        There's a huge difference between saying "we can do" and "we think we will be able to do".
        oldgradstudent 3y ago
        The real difference is between "we think we can do it" and "we think we can get away with saying it".
  - layer8 3y ago
    It reads like wishful thinking, or an upper bound of what could happen in the best-case scenario.
  - sampo 3y ago
    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_implementation https://en.wikipedia.org/wiki/Shor%27s_algorithm#Physical_im...
- sesm 3y ago
  Error correction can fix bit flips, but how can it help if qubits loose entanglement?
  - sebzim4500 3y ago
    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.
    - sesm 3y ago
      When qbits are not entangled, quantum computer doesn't exist anymore, this is not a kind of error that you can correct.
      - eigenket 3y ago
        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.2011.0494 https://royalsocietypublishing.org/doi/10.1098/rsta.2011.049...
        fxj 3y ago
        Can you also compensate against decoherence? I thought this is the magic limit when the QC breaks. See also: https://blogs.scientificamerican.com/observations/decoherence-is-a-problem-for-quantum-computing-but/ https://blogs.scientificamerican.com/observations/decoherenc...
packetlost 3y ago
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
- qmaybe 3y ago
  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
- deleted 3y ago
  [deleted]
zozbot234 3y ago
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.
panarky 3y ago
> 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 https://www.nature.com/articles/s41586-022-05434-1
michael_nielsen 3y ago
Quantum computers won't search unordered databases with fewer than ~ sqrt(N) queries. Proved in: https://arxiv.org/abs/quant-ph/9701001 https://arxiv.org/abs/quant-ph/9701001
deleted 3y ago
[deleted]

by fxj 3y ago

red75prime 3y ago

> do searches with O(1) Nope. Grover's algorithm allows only O(sqrt(n)) search.

reikonomusha 3y ago

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.

fxj 3y ago
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.
- mettamage 3y ago
  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.
  - pclmulqdq 3y ago
    Lying enough to get people excited to fund you, but not quite enough to catch a fraud charge.
    - echelon 3y ago
      There's a huge difference between saying "we can do" and "we think we will be able to do".
      - oldgradstudent 3y ago
        The real difference is between "we think we can do it" and "we think we can get away with saying it".
- layer8 3y ago
  It reads like wishful thinking, or an upper bound of what could happen in the best-case scenario.
- sampo 3y ago
  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_implementation https://en.wikipedia.org/wiki/Shor%27s_algorithm#Physical_im...
sesm 3y ago
Error correction can fix bit flips, but how can it help if qubits loose entanglement?
- sebzim4500 3y ago
  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.
  - sesm 3y ago
    When qbits are not entangled, quantum computer doesn't exist anymore, this is not a kind of error that you can correct.
    - eigenket 3y ago
      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.2011.0494 https://royalsocietypublishing.org/doi/10.1098/rsta.2011.049...
      - fxj 3y ago
        Can you also compensate against decoherence? I thought this is the magic limit when the QC breaks. See also: https://blogs.scientificamerican.com/observations/decoherence-is-a-problem-for-quantum-computing-but/ https://blogs.scientificamerican.com/observations/decoherenc...

packetlost 3y ago

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

qmaybe 3y ago
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
deleted 3y ago
[deleted]

zozbot234 3y ago

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.

panarky 3y ago

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

michael_nielsen 3y ago

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

deleted 3y ago

[deleted]