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Friday, September 07, 2007

Quantum Computing and Quantum Phy.

I have been told quite often that
You don't have to understand Quantum Mechanics to work in Quantum Computing.
Thats a good thing since I've also been told
Nobody really understands Quantum Mechanics.
I've also been told
You don't have to have studied Quantum Mechanics to work in Quantum Computing.
I am skeptical of that. However, I was wondering about the other end- if you do have a background in Physics does it help? So I asked Fred Green (of Green's Conjecture) about this since he has a PhD in Physics, works in a computer science department, and works on Quantum Computing. Here is what he said.
Learning quantum computing helped me understand quantum mechanics better. As a physicist I never thought about measurement theory or entanglement, which were foundational issues, irrelevant to what I was doing. In quantum computing, we reason directly about these things all the time.
He didn't quite answer my question, but he raised a more interesting question. Should quantum physicists learn quantum computing?

In an earlier post I noted that Jerry Seinfeld said Comedians should do lots of proofs. Not for their actual routines, but to better practice their craft. Perhaps its also good advice for people who want to be quantum mechanics (like auto mechanics, but on smaller cars) to learn some Quantum Computing. Not for their actual research, but to better practice their craft.

10 comments:

  1. that was a funky post

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  2. When people say "You don't have to have studied Quantum Mechanics to work in Quantum Computing," they mean that you don't need to know much of what is taught in standard quantum mechanics courses. Full knowledge of quantum mechanics is not needed to work on the algorithmic side of quantum computing in much the same way that computer scientists working on (classical) algorithms don't need to learn the physics used to make physical computers work. In both cases an abstract mathematical model of underlying units of computation (bits or qubits) and their operations suffices. Standard quantum mechanics books do not cover this model, but most introductions to quantum computing not only describe the model, but teach some quantum physics in order to motivate the model.

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  3. We need a nature math structure to embed the quantum computing. If it is proved from physical principal that practise quantum computer cannot be made, then what is the goal and furture of quantum computing? We need a nature structure in quantum computing.

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  4. While a grad student at Stanford, I took a quantum mechanics class from Stephen Shenker (a string theorist) who took the time to learn some quantum computing in order to teach it to us in class. He covered Shor's factoring algorithm and Grover's search algorithm, and also covered some basic entanglement theory like Bell's theorem. As one with a background in quantum computing and information, it made the class more enjoyable, but I must also admit that I learned quantum mechanics from this perspective.

    I think the claim that you don't need to know quantum mechanics to do quantum computing is right in some respect - there are no infinite dimensional Hilbert spaces, spherical harmonics, representations of SU(2), Hermite polynomials, energy levels, etc. needed to understand the idea of quantum computing, nor the earliest discovered algorithms like Shor's or Grover's. Nonetheless, there are more recent advances that draw from more advanced topics in quantum mechanics, such as the recent string of works on evaluating NAND trees which originated in ideas from scattering theory. To say that you "don't need to know X in order to do Y" is a misleading statement since imho, the more you know the better. With more tools and concepts at your disposal, the better off you are for having more ideas towards proving theorems, or otherwise for finding new ideas in general.

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  5. Should quantum physicists learn quantum computing?

    Yes.

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  6. There are two ways of doing research in general:

    (a) Determine what subjects S_1, ..., S_17 you need to learn. Learn them all before starting with research.

    (b) Pick a research question and try to solve it. Learn things as you need them.

    Approach (b) has worked quite well for me. And, in that sense, you don't need quantum mechanics to do quantum computing.

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  7. It depends on what "quantum computing" means. If you mean quantum complexity theory, then the only quantum mechanics you need is a 30-minute introduction to the basic framework. (Similarly, you need almost no electrical engineering to do classical complexity theory.) If you mean quantum information theory, then you need a little more of the abstract mathematical results and framework, but still not very much. If you actually want to build a quantum computer, then you need to know a lot about real-world physics.

    Someday, if quantum computers are built, then a large fraction of the people working in the field will need to know serious physics. (There's no way a real quantum computer will look remotely like a quantum Turing machine, and even quantum circuits probably won't be at all realistic. Actually getting results from real machines will require understanding realistic models.) There will still be a role for quantum complexity theory, but not a very large one, the same way only a tiny fraction of the world's computer scientists are complexity theorists.

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  8. "Perhaps its also good advice for people who want to be quantum mechanics (like auto mechanics, but on smaller cars) to learn some Quantum Computing. Not for their actual research, but to better practice their craft."

    I'm not sure about this. But I think basic quantum computing is a good way of better understanding quantum mechanics. Most quantum physics classes start with a particle in a box, and move on to spherical harmonics, etc. But by looking at *digital* quantum mechanics -- that is, at qubits -- I think the concepts become much more clear, and quantum mechanics becomes much less mysterious.

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  9. "There are two ways of doing research in general:

    (a) Determine what subjects S_1, ..., S_17 you need to learn. Learn them all before starting with research.

    (b) Pick a research question and try to solve it. Learn things as you need them.

    Approach (b) has worked quite well for me. And, in that sense, you don't need quantum mechanics to do quantum computing."

    Bill what do you think of this? Does this deserve a separate post?

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  10. "Should quantum physicists learn quantum computing?"

    IMHO, they can profitably learn model order reduction theory too.

    Because these days, everyone is "climbing the same mountain."

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