Quantum Computer Works Better Shut Off

waimate writes "A New Scientist article relates how its possible to get answers from a quantum computer even when your program isn't running." From the article: "With the right set-up, the theory suggested, the computer would sometimes get an answer out of the computer even though the program did not run. And now researchers from the University of Illinois at Urbana-Champaign have improved on the original design and built a non-running quantum computer that really works."

Comments

Misleading

[karvind]

Running in quantum computers is doing "unitary transformations" and doing measurements on them. So as the article claimed, it is not that you are not doing anything. The only way not to run "it" is by putting it in eigenstate of the system (as well completely isolate it from any external perturbations). If you put it in a mixed state - yes it will evolve with time and then when you do the measurement it will give you "a" eigen state with certain probability. So yes in the end you are still doing measurement which is equally important and consitutes "running" the computer.

Does anyone know what is new here ?

Re:Black Magic

[MindStalker]

If you read the description its not actually as wrong headed as it sounds, the whole "its not running" thing is really inacurate. Basically they have a "program" that does a database search, this "program" is actually physical hardware that is run by optics. They "ran" it by shooting a photon at it, but then blocking the photon before it entered the program thus it "not running". The trick is that properties of the photon continued into the program and it worked. This really isn't new news for anyone who has every dealt with the slit experiment or any number of experiments that show that photons can be in multiple places at once.

Nature

[Anonymous Coward]

From an article in Nature:

http://www.nature.com/news/2006/060220/full/060220 -10.html#B1

http://www.nature.com/nature/journal/v439/n7079/ab s/nature04523.html

IANAP, but I'll try to explain...

[SirBruce]

I am not a physicist, but I did spend a couple years in college studying it with an intent to become one, and I still maintain a layman's interest in the subject. Unfortunately, the math is beyond me. In any case, a lot of people are confused how this "works", and so I thought I'd try to help.

Someone else already posted an useful background URL with is a good place to start:
http://en.wikipedia.org/wiki/Elitzur-Vaidman_bomb- testing_problem

Basically, what you have here is something called "interaction-free measurement". Because of the quantum mechanics work, a particle's wavefunction evolves in a certain way over time, which then "collapses" when you measure it to something specific. How it evolves is not deterministic, but probabillistic. Because of this, you can set up a quantum system whereby when you place a certain object in it at a certain place, you can change the whole system given the nature of what you add to it.

In this case, you have a quantum computer composed of mirrors that runs on photons. The mirrors are pre-set in a certain configuration to run a certain "program". No electricity is needed to "run" the program; you just inject photons into it and it spits out results when you measure it.

What they've done here is then place that computer in a certain location in an existing quantum mechanical system, the one which the photon comes from. This photon is associated with its own set of mirrors and detectors, and because of where and how the computer is placed into it, it effects the larger system.

Thanks to QM, you can then tweak the exterior system so that the chances of a photon ever actually getting to the quantum computer are infinitessimally small. But because there's still a small chance, the very nature of the computer in that location allows you to determine the results it would generate, even though a photon never actually gets into it to "run" it.

So, it's not to misleading to say the program never actually "runs". And you could say the computer isn't "on", but since it's just a mechanical-optical construct it's always "on". More importantly, though, is that exactly where the "computer" is becomes blurred; while it's true that it's particular programming is self-contained, by hooking it up to the external quantum system, you're sort-of making it a part of the computer as well. The "work" is being done by the photons outside the computer; remove them and you don't get anything.

Wow, reading the above, I didn't really do a good job of explaining this at all. But basically, even though the quantum program never actually executes, you still need to create it, and you still need to put it in a certain spot so that its quantum effect on the world around it can still be measured, and from that, you can infer what the program would actually do.

Bruce