## Schroedingerâ€™s machines

The next in the popular series of BrisScience lectures will be on Monday 31 March. As the title, Schroedingerâ€™s machines indicates, it’s on the fascinating topic of quantum computing. More over page.

SCHROEDINGER’S MACHINES – Professor Gerard Milburn

Monday 31 March 2008

6:30 pm to 7:30 pm (Doors open at 6 pm)

Ithaca Auditorium, Brisbane City Hall

Free, no booking required

There will be refreshments following the talk, and Gerard will be available to answer any questions.

For further information or to subscribe to the mailing list, visit www.BrisScience.org or contact Joel Gilmore ([email protected], 0411 267 044) or Nelle Ross ([email protected]).

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Schroedingerâ€™s machines – Professor Gerard Milburn

The quantum world is strange: particles act like waves, waves act like particles and a single particle can be in two places at once. While all this stayed in the remote realm of atomic physics, there was little cause for concern. However recent progress in laser physics, nanotechnology and superconductivity is bringing the quantum world to your doorstep, or at least your internet port.

In this talk I will describe how quantum strangeness is being harnessed to develop new technologies, such as using the quantum nature of light to encrypt internet connections and using clouds of atoms, forced into a quantum wave-like state, to detect the motion of molten rocks from an orbiting satellite. Quantum mechanics may be strange but it is how the world works and increasingly it is being put to work in useful, real world technologies.

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Gerard Milburn obtained a PhD in theoretical physics in 1982 and has since become a world expert in quantum information theory, currently working at the University of Queensland. He is a member of a number of international advisory committees including the Perimeter Institute for Theoretical Physics in Canada. He has authored over 200 papers, and four books – including two non-technical books on quantum technology.

Date: Monday 31 March 2008

Time: 6:30 pm to 7:30 pm (Doors open at 6 pm)

Venue: Ithaca Auditorium, Brisbane City Hall

THIS IS A FREE EVENT, NO BOOKINGS REQUIRED

FWIW, quantum computers have always struck me as a technology which has attracted far more hype than is justified by their potential usefulness.

Warning: technical content ahead…Aside from breaking public-key cryptography (and, as I understand it, it’ll always be fairly easy to stay ahead of the cryptanalyist’s quantum computers by just extending key length), the only vaguely useful thing they can do is a quadratic speedup for brute force search. That’s moderately interesting, but hardly earth-shattering.

But I’d be happy to be disabused of my notions, if I were in Brisbane attend the talk

From #1:

Aside from breaking public-key cryptography (and, as I understand it, itâ€™ll always be fairly easy to stay ahead of the cryptanalyistâ€™s quantum computers by just extending key length), the only vaguely useful thing they can do is a quadratic speedup for brute force search. Thatâ€™s moderately interesting, but hardly earth-shattering.This is what

wikipedia says too. Just increasing key-length

is enough to secure cryptography to quantum computer attack.

I guess they may well come up with other algorithms for other things though.. so give them time.

However… all of this (even the quadratic search speedup) is still just theoretical for problems of non-trivial size, as getting even a single qbit is pretty hard. I’m guessing that there may be practical or (as yet unforseen) theoretical problems in building quantum computers that make it impossible to scale them up to more than a handful of qbits. Given that I know little about it though, I’d be interested to hear how sure these quantum computing people are that there are no fundamental difficulties in scaling quantum computers up to more than a small number of qbits. Is it possible, for example, that the difficulty in maintaining a large quantum system in a quantum state (i.e. not having it ‘collapse’ to a particular state) puts a fundamental limit on the size of computer that can be built?

Perhaps Gerard will talk about such issues in his talk…. and perhaps someone who can make the talk can post a quick summary here (provided JQ is willing)?

And JQ: Thanks for advertising these things.

RM and PR:

Michael Brooks recently on The Science Show said:

that quantum entanglement had been used “to lock down the security of the Swiss national elections” and since:

wmmbb: that’s a slightly different thing – quantum encryption.

From what I’ve read, professionals in the area are fairly dismissive of the idea, because it solves a non-problem.

Essentially, what quantum encryption does is solve the problem of securely transmitting information between two points, where a separate secure channel for sharing the “key” already exists. But there are already a number of very good methods for doing this; in practice, these existing methods are far more secure than every other link in the information security chain. For the really paranoid, there are “one-time pad” systems, which are

completelyunbreakable assuming the one-time pad remains secure (though the system is impractical for most uses).Of course when it comes down to quantum processes, my knowledge is closer to zero than to one, and in fact within increasing miniaturization that electronic paradigm of computing becomes uncertain.

Quantum computing as a fundamental change in processing data creates new problems. It turns out that encryption is a critical issue because quantum computers would be able to figure out the prime factors of very large numbers used by “electronic computers” to secure data.

Since I understand nothing about these matters a google search did come up with some accessible information. Alan Boyle, Science Editor for MSNBC clearly explains some of these issues.

I’m more interested to see if quantum entanglement can be used to create instant communications. That is, communications that are not limited by the speed of light but are instantaneous over any distance. Anyone know if this is thought to be a possibility?

Imagine! An internet with no lag! Or relatively little lag. Perhaps quantum entanglement communication stations could be set up in each city around the world. Thus Seattle to Brisbane: instant communication. Then from the quantum entanglement station Bris. to my place at the speed of light. Sounds good to me!

Quantum computing is likely to be useful in giving us different ideas with which to model our world than with practical applications but I don’t care if it was never “useful” and secretly hope it won’t because I want our society to have a place for thinkers of non useful ideas.

We want more people thinking the impossible. The trend to commercialise research and only to put a value on things that might have economic benefit is not good. We want more dreamers and investigators of ideas for their own sake. There are enough of us out here looking to make a dollar. I find it sad that so many researchers and thinkers have to try to justify their existence by showing how their ideas can be made useful.

The most promising application of quantum computing (QC), and the main application foreseen for it by pioneering thinkers such as the legendary Richard Feynman, is for quantum mechanical simulation.

Simulating quantum mechanical systems tends to be a problem that is in an NP complexity class – that is, like factorising extremely large numbers, it’s intractably difficult for conventional computers. But it is practical with quantum computers.

Why simulate quantum mechanical systems? Well, think about this – all chemistry, all molecules, all chemical bonds, and essentially everything there is to know about chemistry can be understood and described completely by applying the Schrodinger Equation and quantum mechanics to the electrons and ions and atoms in the molecules.

All chemical bonding, and interaction between different molecules, different chemicals or different receptors doesn’t need to be studied empirically – you just calculate it.

If you need to find a particular molecule that can interact in a certain way with, say, a particular enzyme, or the surface of a flu virus, or a cancer cell, there is no need for painstaking experiment – you just calculate the quantum mechanics, and you have the answer.

Ikonoclast —

No. You can’t get instantaneous communication from entangled states. It’s a basic fact about entanglement that it *doesn’t* actually convey any information on its own.

Example: say you and I have the canonical entangled quantum state of two particles with opposite spin. Then I get the same probability distribution from measuring my particle _no matter what_ you do with yours. I can’t tell what you’ve done, or whether you’ve done anything at all, by looking at my particle. We only notice the weird quantum when we exchange notes, and see that our measurements are always complementary and more highly-correlated than would be possible classically. Obviously this note-comparing process is limited by light-speed.

This result is necessary, because if you could send information faster-than-light then we’d have a contradiction between quantum theory and special relativity.

Kevin Cox said, “I want our society to have a place for thinkers of non useful ideas.”

It does KC. There’s the field of metaphysics.

Ali says, “This result is necessary, because if you could send information faster-than-light then weâ€™d have a contradiction between quantum theory and special relativity.”

I would ask, can we be so prescriptive at this point? I didn’t think the TOE (Theory of Everything) was complete. Until it’s complete (if ever) how can we know?

On reflection, I retract that last post. That’s the silliest thing I’ve ever written.

Thanks Luke. I’d forgotten about that, and, yes, that could indeed be very useful.

The other day I saw a bloke wearing a tee-shirt inscribed ‘Schroedinger’s cat is dead’. Is this true?

No not dead…. just half alife.