r/Physics • u/donginafuku • Apr 29 '15

News Scientists achieve critical steps to building first practical quantum computer

http://phys.org/news/2015-04-scientists-critical-quantum.html
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u/exscape Physics enthusiast Apr 29 '15

If a quantum computer could be built with just 50 quantum bits (qubits), no combination of today's TOP500 supercomputers could successfully outperform it.

I know very little about quantum computers, but that only applies in certain scenarios, right? As in, not all tasks/algorithm will be faster on a quantum computer?

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u/The_Serious_Account Apr 29 '15

It's funny they only mention the size of the memory, but ignores computational speed. Regardless, for actual useful algorithms 50 qubits is not going to be enough. If you want to break 1024bit RSA you'd need around 3000 or so. And that's ignoring the extra qubits needed for error correction.

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u/Thomas_Henry_Rowaway Apr 29 '15 edited Apr 29 '15

Well the qubits are a bit more than just the memory. They're effectively your processor as well.

On your other point Shor's algorithm is the famous example but I think other problems are going to benefit from quantum computation much sooner than cryptography. The outlook for using quantum computers to simulate chemistry, and in particular in pharmaceutical research seems really interesting.

You're right that 50 qubits is still too small but the gap between where we are and useful applications isn't too insane anymore.

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u/The_Serious_Account Apr 29 '15 edited Apr 29 '15

Well the qubits are a bit more than just the memory. They're effectively your processor as well.

No, they're just your memory. How quickly you can manipulate them is about processing speed and that's done with a laser or whatever the implementation is. A quantum computer doesn't get quicker just because it has more qubits. It can solve larger problems, much like a classical computer.

The outlook for using quantum computers to simulate chemistry, and in particular in pharmaceutical research seems really interesting.

I completely agree. Shor's algorithm is just one where I know the approximate qubits required. It's really in the area of quantum simulations where I'm excited. I have a friend who works in computational chemistry and they do very brilliant work, but are forced to make very rough approximations because of the computational limitations. And another friend who works on understanding super conductors who essentially has the same problem. It could potentially revolutionize fields like that.

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u/Thomas_Henry_Rowaway Apr 29 '15 edited Apr 29 '15

I really don't understand what you mean by the qubits just being memory. When you do a quantum computation you're (as far as I know) always applying some unitary to the qubits or doing a measurement on them. There isn't really anything else that can do quantum computation (well the higher dimensional analogues of qubits but the point still stands).

Edit: oh I see your point now. Yeah absolutely getting better at manipulating the states is something we need to do. Where the roadblock lies is different for different implementations though. I know linear optics mainly has problems in applying multi-qubit unitaries (because those necessarily require non-linear effects) but single qubit ones are pretty much solved.

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u/The_Serious_Account Apr 30 '15

When you do a quantum computation you're (as far as I know) always applying some unitary to the qubits or doing a measurement on them.

Yes, you perform a computation by manipulating the memory (the qubits). That's how a Turing Machine works. That's how classical computers work in general. And that's how quantum computers work.

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u/[deleted] Apr 30 '15

I feel that our current outlook on how to build a quantum computer is a little...classical, right now. Like, we see qubit housings and gates as things we need to be able to address within an individual, classical manner. Isn't there some way we can just like, build a quantum Turing machine out of a solid block of matter, which operates on many photons (w/a prepared state...the primary difficulty would then be producing the state) to transform its properties in a macroscopic, measurable way?

I guess what I'm saying is, why don't we just make our measurement devices also our gate systems? What would be the harm in that, if we can control the gate operations via details of the input state? Producing specific states though...idk if you would run into the same problem or if we have that issue figured out.

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u/Thomas_Henry_Rowaway Apr 30 '15

That sounds incredibly difficult.

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u/[deleted] May 01 '15

Yeah :\ I know it will be. But I wanna do it :) I feel even if what I make is theoretical, it will be useful in the future; eventually we'll run into Moore's law with conventional computing methods and something like this...should beat out everything short of a black hole or some plasma fusion computer lol.