5 ms·

Acknowledging my very primitive understanding of quantum computing, would it be possible to simulate quantum computing?

by anon1m0us 7y ago

jcims 7y ago
Yes. Slowly.
dshields1 7y ago
Yes. Right now simulators tend to be faster, cheaper, and more accurate than real quantum computers.
- jnevelson 7y ago
  How can a simulation be more accurate than what it's simulating?
  - LegitShady 7y ago
    I think he means returns correct results to known problems more reliably.
  - jeeceebees 7y ago
    As far as I understand, it's because what it's simulating is a logical qubit which is different from the very noisy, almost instantaneously collapsing physical qubits present in current quantum computers. Software simulates what's supposed to happen while the hardware only approaches it through many repeated trials.
  - deleted 7y ago
    [deleted]
  - dshields1 7y ago
    Current quantum computers are noisy. Gates aren’t perfectly implemented. Qubits are prone to dephasing and decoherence.
  - Jtsummers 7y ago
    Physical quantum computers have noise. Let's take a (simplified) scenario. You've set up your quantum circuit with one qubit, and put it in a position where it will measure as either 0 or 1 with equal chance. In the simulation it'll come out as 0 or 1 with actual equal chance. In the real world, other factors will create a bias one way or the other (this may not be consistent, either) so that it comes out more like 60% 0 to 40% 1, even over 1000s of trials. If you set up a circuit where you've entangled two qubits so that they should come out as the same value (00 or 11) and the configuration says they should come out with 50% chance of either, the simulation will show that. The outputs of 01 and 10 will never show up in the simulation. But in the real world, there's still a chance that you get those. You'll likely (on IBM's quantum computers) get something like 1-5% 01, 1-5% 10, 45-50% 11, 45-50% 00 (again, over thousands of runs). If you want to see how this plays out with simulations and real quantum computers, IBM [0] has free access (constrained by credits when you want to run on real quantum computers, they reset each day). [0] https://quantum-computing.ibm.com/ https://quantum-computing.ibm.com/
  - gowld 7y ago
    Quantum computers are used to simulate digital computers, so a digital computer simulating a quantum computer simulating a digital computer can cheat.
Strilanc 7y ago
Yes. Here's an online drag and drop simulator: https://algassert.com/quirk https://algassert.com/quirk Until recently [1], classical simulation was faster/cheaper/more-accurate than any existing quantum hardware. But the hardware has been improving and all classical simulation of quantum computation takes exponential time with respect to some important property such as the number of qubits, the depth of the computation, the number of non-trivial gates, or etc. [1]: https://ai.googleblog.com/2019/10/quantum-supremacy-using-programmable.html https://ai.googleblog.com/2019/10/quantum-supremacy-using-pr...
- jhallenworld 7y ago
  I wonder how optimizing the existing simulators are. I suppose any program with constant input could be reduced to no work, but with potentially exponentially long compile time. But there must be simpler optimizations... Before we declare quantum supremacy, we should make sure the simulator we are comparing with is a good one, not a straw-man one. I've seen this problem time and again with hardware accelerators. The accelerator is faster than some crappy software, but with a little work with a profiler, the software beats the hardware. Of course optimizing will not make an fundamentally exponential problem polynomial, but it can help a lot.
  - Strilanc 7y ago
    > Before we declare quantum supremacy, we should make sure the simulator we are comparing with is a good one, not a straw-man one. A big part of writing the supremacy paper was optimizing the simulators. We did three different styles of simulation: 1) A state vector simulator with hand rolled SIMD assembly (called qSim; not yet publicaly released). This required too much space at 53 qubits. (Well, unless you're going to use the majority of all disk space on summit, which brings its own obstacles. IBM says they can run it that way in a few days, but we'll see.) 2) Treating the quantum circuit as a tensor network and doing optimized contraction to avoid the space blowup (called qFlex https://github.com/ngnrsaa/qflex https://github.com/ngnrsaa/qflex). This required too much time at 53 qubits. 3) Custom code written to run on a supercomputer instead of distributed computers. There's only so much effort you can put in before you have to call it. I think it's more likely for an algorithmic break to save a factor of 10 than for optimization to save a factor of 10 at this point.. although someone should probably try using GPUs or FPGAs. I also take the view that if it takes a month to produce a new optimized implementation of a classical simulator that beats the quantum hardware, then the quantum hardware is still outperforming classical hardware for that month. The theoretical bounds are important, but in the day-to-day context of a race they aren't directly relevant.
fuklief 7y ago
Currently, yes. Eventually, no.
randomsearch 7y ago
Yeah, up to a threshold which you could roughly say is somewhere in the 40-100 qubit range depending on what type of (fairly useful) computation you’re doing. It’s linear algebra with exponentially large matrices, but there are lots of tricks you can do to optimise simulation - up to a point.
tcgv 7y ago
Yes it is! But incredibly slow and resource intensive. A few days ago I shared a blog post here on HN about a simple quantum computing simulador I built as a weekend project, for learning purposes: - https://thomasvilhena.com/2019/11/quantum-computing-for-programmers https://thomasvilhena.com/2019/11/quantum-computing-for-prog... Basically it is just linear algebra ;)

Acknowledging my very primitive understanding of quantum computing, would it be possible to simulate quantum computing?

jcims 7y ago

Yes. Slowly.

dshields1 7y ago

Yes. Right now simulators tend to be faster, cheaper, and more accurate than real quantum computers.

jnevelson 7y ago
How can a simulation be more accurate than what it's simulating?
- LegitShady 7y ago
  I think he means returns correct results to known problems more reliably.
- jeeceebees 7y ago
  As far as I understand, it's because what it's simulating is a logical qubit which is different from the very noisy, almost instantaneously collapsing physical qubits present in current quantum computers. Software simulates what's supposed to happen while the hardware only approaches it through many repeated trials.
- deleted 7y ago
  [deleted]
- dshields1 7y ago
  Current quantum computers are noisy. Gates aren’t perfectly implemented. Qubits are prone to dephasing and decoherence.
- Jtsummers 7y ago
  Physical quantum computers have noise. Let's take a (simplified) scenario. You've set up your quantum circuit with one qubit, and put it in a position where it will measure as either 0 or 1 with equal chance. In the simulation it'll come out as 0 or 1 with actual equal chance. In the real world, other factors will create a bias one way or the other (this may not be consistent, either) so that it comes out more like 60% 0 to 40% 1, even over 1000s of trials. If you set up a circuit where you've entangled two qubits so that they should come out as the same value (00 or 11) and the configuration says they should come out with 50% chance of either, the simulation will show that. The outputs of 01 and 10 will never show up in the simulation. But in the real world, there's still a chance that you get those. You'll likely (on IBM's quantum computers) get something like 1-5% 01, 1-5% 10, 45-50% 11, 45-50% 00 (again, over thousands of runs). If you want to see how this plays out with simulations and real quantum computers, IBM [0] has free access (constrained by credits when you want to run on real quantum computers, they reset each day). [0] https://quantum-computing.ibm.com/ https://quantum-computing.ibm.com/
- gowld 7y ago
  Quantum computers are used to simulate digital computers, so a digital computer simulating a quantum computer simulating a digital computer can cheat.

Strilanc 7y ago

Yes. Here's an online drag and drop simulator: https://algassert.com/quirk https://algassert.com/quirk Until recently [1], classical simulation was faster/cheaper/more-accurate than any existing quantum hardware. But the hardware has been improving and all classical simulation of quantum computation takes exponential time with respect to some important property such as the number of qubits, the depth of the computation, the number of non-trivial gates, or etc. [1]: https://ai.googleblog.com/2019/10/quantum-supremacy-using-programmable.html https://ai.googleblog.com/2019/10/quantum-supremacy-using-pr...

jhallenworld 7y ago
I wonder how optimizing the existing simulators are. I suppose any program with constant input could be reduced to no work, but with potentially exponentially long compile time. But there must be simpler optimizations... Before we declare quantum supremacy, we should make sure the simulator we are comparing with is a good one, not a straw-man one. I've seen this problem time and again with hardware accelerators. The accelerator is faster than some crappy software, but with a little work with a profiler, the software beats the hardware. Of course optimizing will not make an fundamentally exponential problem polynomial, but it can help a lot.
- Strilanc 7y ago
  > Before we declare quantum supremacy, we should make sure the simulator we are comparing with is a good one, not a straw-man one. A big part of writing the supremacy paper was optimizing the simulators. We did three different styles of simulation: 1) A state vector simulator with hand rolled SIMD assembly (called qSim; not yet publicaly released). This required too much space at 53 qubits. (Well, unless you're going to use the majority of all disk space on summit, which brings its own obstacles. IBM says they can run it that way in a few days, but we'll see.) 2) Treating the quantum circuit as a tensor network and doing optimized contraction to avoid the space blowup (called qFlex https://github.com/ngnrsaa/qflex https://github.com/ngnrsaa/qflex). This required too much time at 53 qubits. 3) Custom code written to run on a supercomputer instead of distributed computers. There's only so much effort you can put in before you have to call it. I think it's more likely for an algorithmic break to save a factor of 10 than for optimization to save a factor of 10 at this point.. although someone should probably try using GPUs or FPGAs. I also take the view that if it takes a month to produce a new optimized implementation of a classical simulator that beats the quantum hardware, then the quantum hardware is still outperforming classical hardware for that month. The theoretical bounds are important, but in the day-to-day context of a race they aren't directly relevant.

fuklief 7y ago

Currently, yes. Eventually, no.

randomsearch 7y ago

Yeah, up to a threshold which you could roughly say is somewhere in the 40-100 qubit range depending on what type of (fairly useful) computation you’re doing. It’s linear algebra with exponentially large matrices, but there are lots of tricks you can do to optimise simulation - up to a point.

tcgv 7y ago

Yes it is! But incredibly slow and resource intensive. A few days ago I shared a blog post here on HN about a simple quantum computing simulador I built as a weekend project, for learning purposes: - https://thomasvilhena.com/2019/11/quantum-computing-for-programmers https://thomasvilhena.com/2019/11/quantum-computing-for-prog... Basically it is just linear algebra ;)