6 ms·

I don't think it's that the concepts are contrived (although the test cases certainly are), it's just that our quantum computers are really early in their devel

by tedivm 3y ago

I don't think it's that the concepts are contrived (although the test cases certainly are), it's just that our quantum computers are really early in their development and can't do the more complicated things yet. It's like if we had calculators that took an hour to do each arithmetic operation- the fact that people wouldn't use it doesn't mean arithmetic is contrived, just that it isn't as powerful as better alternatives. As the quote mentions, we'll need "many thousand qubits" to reach the point where society itself is getting new benefits out of things. These contrived cases help figure out how to scale the whole system.

upofadown 3y ago
It's more like if we had calculators that could not do any arithmetic operation at all... Someone someday might figure out how to make the calculator actually calculate, but not yet.
AbrahamParangi 3y ago
There's a fundamental question as to whether or not it's exponentially difficult to add additional qubits. If each marginal qubit is 5% more difficult to add as the previous one the task would be essentially impossible - and even if it wasn't impossible it would turn out to basically be cheating (in that you would be doing exponential work via either the quantum computing route or the conventional route).
- alpineidyll3 3y ago
  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)
  - abdullahkhalids 3y ago
    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-gate-speed-of-a-photonic-quantum-computer/ https://quantumfrontiers.com/2023/06/21/what-is-the-logical-...
    - pclmulqdq 3y ago
      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.
      - abdullahkhalids 3y ago
        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.
  - s1dev 3y ago
    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.
    - alpineidyll3 3y ago
      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.
      - gaze 3y ago
        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.
        fsh 3y ago
        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.
        gaze 3y ago
        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.
  - deleted 3y ago
    [deleted]
laxd 3y ago
> ... quantum computers are really early in their development and can't do the more complicated things yet. It's like if we had calculators that took an hour to do each arithmetic operation ... Reminds me of the story George Stibitz tells after developing one of the first devices that could be called a computer. Management at Bell Labs was not impressed that he had built a "20000 dollar calculator". https://youtu.be/qundvme1Tik?t=259 https://youtu.be/qundvme1Tik?t=259
- hcks 3y ago
  Well a “20000 dollar calculator” is something QC can only dream of

by tedivm 3y ago

upofadown 3y ago

It's more like if we had calculators that could not do any arithmetic operation at all... Someone someday might figure out how to make the calculator actually calculate, but not yet.

AbrahamParangi 3y ago

There's a fundamental question as to whether or not it's exponentially difficult to add additional qubits. If each marginal qubit is 5% more difficult to add as the previous one the task would be essentially impossible - and even if it wasn't impossible it would turn out to basically be cheating (in that you would be doing exponential work via either the quantum computing route or the conventional route).

alpineidyll3 3y ago
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)
- abdullahkhalids 3y ago
  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-gate-speed-of-a-photonic-quantum-computer/ https://quantumfrontiers.com/2023/06/21/what-is-the-logical-...
  - pclmulqdq 3y ago
    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.
    - abdullahkhalids 3y ago
      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.
- s1dev 3y ago
  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.
  - alpineidyll3 3y ago
    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.
    - gaze 3y ago
      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.
      - fsh 3y ago
        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.
        gaze 3y ago
        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.
- deleted 3y ago
  [deleted]

laxd 3y ago

> ... quantum computers are really early in their development and can't do the more complicated things yet. It's like if we had calculators that took an hour to do each arithmetic operation ... Reminds me of the story George Stibitz tells after developing one of the first devices that could be called a computer. Management at Bell Labs was not impressed that he had built a "20000 dollar calculator". https://youtu.be/qundvme1Tik?t=259 https://youtu.be/qundvme1Tik?t=259

hcks 3y ago
Well a “20000 dollar calculator” is something QC can only dream of