3 ms·

I'm not a physicist, but isn't this kind of like building a fridge and saying it's reversing time because it makes it look like the usual process of heat dissip

by panic 8y ago

I'm not a physicist, but isn't this kind of like building a fridge and saying it's reversing time because it makes it look like the usual process of heat dissipation is running in reverse? Or is there something more fundamental going on here?

lisper 8y ago
That's not a bad analogy. The difference is that the fridge requires energy input, while quantum "time reversal" does not. I'm putting "time reversal" in scare quotes because it's not really reversing time, it's "simply" (again in scare quotes because actually doing it is not so simple) reversing the normal direction of the evolution of a quantum process.
- rightbyte 8y ago
  How do we know that there are no work done by the computer on the states? They didn't test for eg. 010 back and forth. There might be a 000 bias, etc.
  - lisper 8y ago
    I don't know what you mean by "010 back and forth" or "000 bias", but I'm talking about theory, not practice. Even under ideal conditions a refrigerator requires energy input in order to work. A quantum computer does not. I have no idea whether this particular implementation was dissipative or not.
    - rightbyte 8y ago
      The autors write that the registers went from 000 to something and back approx. 50% of the time when the random chance of it was really small. An ideal 386 computer could be thought off as not requiring energy too, right? A looping pseudo random number generator would then be time reversal. It's not saying anything really.
      - lisper 8y ago
        > An ideal 386 computer could be thought off as not requiring energy too, right? No. Classical computing is necessarily dissipative: https://en.wikipedia.org/wiki/Landauer%27s_principle https://en.wikipedia.org/wiki/Landauer%27s_principle
- ZeroFries 8y ago
  What is time-reversal if not evolution in reverse?
  - lisper 8y ago
    That's all it is.
rightbyte 8y ago
I'm not a physicist but that seems accurate. Click bait paper. I mean, non casual filter (time travel filters!) are nothing more fancy than a post measurement analysis where we know y_n+1 at y_n, for example.
- diminoten 8y ago
  Don't knock it, building the first fridge was a pretty big deal!
- gus_massa 8y ago
  > Click bait paper. The "time reversal operator" and "time reversal symmetry" are usual technical terms in Quantum Physics, the authors didn't invent them. In an advanced quantum physics course you get 2 or 3 weeks about them, and the profesor will spend a long time explaining what they mean exactly. The problem is that the name is too catchy and it generate a lot of misunderstanding when they are used in the press coverage of the technical articles.
nickbauman 8y ago
Considering there are reputable physicists out there that are extremely skeptical a useful quantum computer can ever be made in the first place...
- deusofnull 8y ago
  Do you have any references I / others could take a look at? Ive heard such skepticism but maybe not from the most reputable sources. Thanks!
  - xutopia 8y ago
    I've heard a documentary about some physicists saying that quantum hasn't provided anything substantial yet and is hard to prove and often explainable without quantum theory. That said here is a starting point: https://en.wikipedia.org/wiki/Quantum_supremacy#Skepticism https://en.wikipedia.org/wiki/Quantum_supremacy#Skepticism
    - isolli 8y ago
      > quantum hasn't provided anything substantial yet I believe you meant "quantum computing hasn't provided anything substantial yet". I first read it as "quantum theory hasn't provided anything substantial yet", and I was a bit shocked!
  - nickbauman 8y ago
    Oded Goldreich, Stephen Wolfram, Leonid A. Levin and Gil Kalai are skeptics I know about.
    - throwawaymath 8y ago
      While I'm similarly bearish on quantum computing, in fairness Oded Goldreich is actually a complexity theorist and cryptographer, not a physicist. He works on fundamental things like theory of computing, so his view is applicable. But Gil Kalai is a much stronger reference (and frankly the only one you really need to cite, since if you follow his arguments you've followed all of them more or less).

by panic 8y ago

lisper 8y ago

That's not a bad analogy. The difference is that the fridge requires energy input, while quantum "time reversal" does not. I'm putting "time reversal" in scare quotes because it's not really reversing time, it's "simply" (again in scare quotes because actually doing it is not so simple) reversing the normal direction of the evolution of a quantum process.

rightbyte 8y ago
How do we know that there are no work done by the computer on the states? They didn't test for eg. 010 back and forth. There might be a 000 bias, etc.
- lisper 8y ago
  I don't know what you mean by "010 back and forth" or "000 bias", but I'm talking about theory, not practice. Even under ideal conditions a refrigerator requires energy input in order to work. A quantum computer does not. I have no idea whether this particular implementation was dissipative or not.
  - rightbyte 8y ago
    The autors write that the registers went from 000 to something and back approx. 50% of the time when the random chance of it was really small. An ideal 386 computer could be thought off as not requiring energy too, right? A looping pseudo random number generator would then be time reversal. It's not saying anything really.
    - lisper 8y ago
      > An ideal 386 computer could be thought off as not requiring energy too, right? No. Classical computing is necessarily dissipative: https://en.wikipedia.org/wiki/Landauer%27s_principle https://en.wikipedia.org/wiki/Landauer%27s_principle
ZeroFries 8y ago
What is time-reversal if not evolution in reverse?
- lisper 8y ago
  That's all it is.

rightbyte 8y ago

I'm not a physicist but that seems accurate. Click bait paper. I mean, non casual filter (time travel filters!) are nothing more fancy than a post measurement analysis where we know y_n+1 at y_n, for example.

diminoten 8y ago
Don't knock it, building the first fridge was a pretty big deal!
gus_massa 8y ago
> Click bait paper. The "time reversal operator" and "time reversal symmetry" are usual technical terms in Quantum Physics, the authors didn't invent them. In an advanced quantum physics course you get 2 or 3 weeks about them, and the profesor will spend a long time explaining what they mean exactly. The problem is that the name is too catchy and it generate a lot of misunderstanding when they are used in the press coverage of the technical articles.

nickbauman 8y ago

Considering there are reputable physicists out there that are extremely skeptical a useful quantum computer can ever be made in the first place...

deusofnull 8y ago
Do you have any references I / others could take a look at? Ive heard such skepticism but maybe not from the most reputable sources. Thanks!
- xutopia 8y ago
  I've heard a documentary about some physicists saying that quantum hasn't provided anything substantial yet and is hard to prove and often explainable without quantum theory. That said here is a starting point: https://en.wikipedia.org/wiki/Quantum_supremacy#Skepticism https://en.wikipedia.org/wiki/Quantum_supremacy#Skepticism
  - isolli 8y ago
    > quantum hasn't provided anything substantial yet I believe you meant "quantum computing hasn't provided anything substantial yet". I first read it as "quantum theory hasn't provided anything substantial yet", and I was a bit shocked!
- nickbauman 8y ago
  Oded Goldreich, Stephen Wolfram, Leonid A. Levin and Gil Kalai are skeptics I know about.
  - throwawaymath 8y ago
    While I'm similarly bearish on quantum computing, in fairness Oded Goldreich is actually a complexity theorist and cryptographer, not a physicist. He works on fundamental things like theory of computing, so his view is applicable. But Gil Kalai is a much stronger reference (and frankly the only one you really need to cite, since if you follow his arguments you've followed all of them more or less).