The Feynmann processor: an introduction to quantum computation by Gerard Milburn. Like (I expect) most of us, I’ve never understood anything about quantum computation and have been vaguely suspicious that the whole project involves some kind of spurious informational free lunch. On the other hand, having read Feynmann’s excellent QED, I’m reasonably comfortable with the basic ideas of quantum electrodynamics (though I’ve never got on top of the nasty integrals required to actually work anything out). Feynmann’s discussion in terms of probabilty amplitudes steers clear of all that Heisenberg-style mysticism that seems to make the whole subject incomprehensible.
Anyway, this post by John Holbo at CT, and particularly this comment, led me to a Wikipedia article which made it clear how you quantum processing could yield impressive gains without any magical mumbo-jumbo, so I went on to look for more, and found this book in the library. It’s very easy going for a general reader, and makes things pretty clear, though I took a couple of readings to get the details straight.
As it happens, Gerard is at UQ and got a Federation Fellowship at the same time I did, so I’ll probably be pestering him for more info on all this.
John Quiggin 
Hard. The only link that led anywhere reasonable was the Wikipedia link and I didn’t understand the point. What’s the basic idea? How can you test propositions without (well) testing them? Suspicious but well aware of my own limited rationality. Maybe post something introductory by Prof Milburn?
There’s a more extensive discussion than the wikipedia article here.
This wiki entry on the Renninger negative-result experiment is easier to follow than the wiki link in JQ’s post.
But is this any different in principle from the Aharonov-Bohm effect? In both cases the experimental outcome is changed by something with which, classically, the particle under question does not interact.
If you do the math in these setups they are a lot less mysterious: the “counterfactuals” that never happen still affect the experimental setup and hence the equations. The problem comes with interpreting the state of the system before a measurement is made. Our mental apparatus automatically imposes classical notions on the system (or attempts to), but none fit; the state of the system prior to measurement is a superposition of classical states and no single classical state describes it.
On integrals in QED: check out the (out of print) “Quantum Mechanics and Path Integrals” – a very readable introduction. Although beware of the errors. What is particularly valuable about that book is that it gives a very intuitive treatment of the transition from “classical” quantum theory (Schroedinger equation) to quantum field theory via path integral formalism.
BTW, stochastic integrals (eg, as used in option pricing) are a form of path integral so that may be a good entry point for economists.
I enjoyed the Feyman Processor too and I agree that it is quite accessible for an interested layperson who is prepared to devote some time to understanding. I remember thinking that I thought there were a couple of regrettable mistakes in some of the maths in the nature of typos but which had the effect of confusing me. On the other had, the far more likely explanation is my lack of complete comprehension. I may fish out my copy and raise my queries if that is ok with you.
One of the most fascintating points for me was the discussion of the “Landauer Prinicple” which states that heat must be dissipated when information is erased in an irreversible process and therefore computers give off heat if they use such processes to store information. For me, a surprising application of the first law of thermodynamics.
I can also recommend Milburn’s earlier “Quantum Technology”, from the same press.
I also enjoyed Feynmann’s “QED”. I would alsorecommend his “The Character of Physical Law.”
The wikipedia article does not have any less mumbo jumbo than Mr Heisenbergs cat. If we had strapped a cat to the bomb in question then the experiment is not really so different to Heisenbergs original thought experiment.
What people need to accept it that quantum physics is weird. You can’t get away from the weirdness. And yet the entire information revolution is built on this weird stuff.
It was only after studying quantum physics at University that I gave up on fatalism. Which was helpful because fatalism is not so good for your mental health.
Oops, my reference shoud be to the “second law…”
Terje, philosophically, I don’t think you need quantum physics to refute fatalism. Sound arguments against on more general grounds.
Perhaps. However it was quantum physics that what worked for me.
Terje, I think you may be confusing Heisenberg and Schrodinger.
Oh bugger…. I was going to point out Schrodinger but I had a lecure 😦
Anyway, I had a question for you John;
When you said “Heisenberg-style mysticism“, were you implying that the mysticism was Heisenberg flavoured or that the uncertainty principal had a mystic flavour? I’ve always attributed the mysticism with the Copenhagen interpretation.
Cheers
Bruce
Arrggghhh… apoligies for the grammar (uncertainty principal has… mysticism to…)
I actually didn’t like the book that much, even though Gerard was one of my PhD supervisors and has had a huge infuence on my career (he, along with others inspired me to do a PhD on quantum computing, and now I’m a postdoc still studying the same stuff!).
John, I strongly recommend that you read Feynman’s original paper (actually, it’s a talk in a conference proceedings) on quantum computation. It’s easily accessible and gives some simple motivations for trying to build a quantum computer. I think that there is a reference given in Gerard’s book. Additionally I recommend that you read David Deustch’s original paper, it’s an excellent read though it requires a bit more math that Feynman’s paper.
I also recommend that you give Gerard a call! You might also want to try talking to Michael Nielsen. Anyway, he’s another UQ Fed Fellow and he literally wrote the text on quantum computing.
Another person that you might like to talk to is Jennifer Dodd, she’s a postdoc in the physics department who is very good at science communication, she is currently organising the BrisScience public lecture series (yes, that was a plug :-)).
Yes indeed I have.
http://en.wikipedia.org/wiki/Schrodinger's_cat
Mick, thank you very much for the references. I’ve had a quick peruse and they look excellent.
Will read Deutsch’s paper. Great that available on the Web. I’ve had his “Fabric of Reality” on the “to read” pile for ages.
This article in today’s Age is coincidentally about quantum computing. Obviously going to be fairly superficial.
The Age article contains a howler.
It spends a lot of time discussing the travelling salesman problem, but it’s highly unlikely that quantum computers of the type Prof. Simmons is working on can help with problems like the TSP.
The practical applications of the type of quantum computers the professor is working on are limited to three areas: breaking public-key cryptography, breaking secret-key cryptography, and doing simulations of quantum physics. Frankly, it’s not particularly good news for the world if public-key cryptgraphy is broken, because it may well mean we have to radically rethink how we keep our internet banking transactions secure. The second application is theoretically interesting but practically not very useful, as it’s trivial to guard against (just make the secret key twice as long). The third is the interesting one (and the one I know least about), but it appears to be very useful for the scientists in allowing them to simulate things they currently can’t really do.
The holy grail of quantum computing would be a quantum computer that could solve NP-complete problems like the TSP efficiently (to grossly oversimplify, an NP-complete problem is a problem for which it is easy to verify a solution if provided but appears to be very hard to find the solution in the first place). As I’ve mentioned before, if such a computer can be built it turns the world upside down overnight. But that’s not what the professor is working on.