So what is it that I do? – Part 1: Quantum Mechanics

My work in physics focusses on quantum computing, one of those hot topics that the media love to talk about, but nobody actually seems to know what it is. So providing a very layman’s explanation of quantum computing, that even I can understand, seems like a reasonable place to start. Over the next few posts I hope to provide something along the lines of Quantum Computing for Complete Twits, or perhaps even more rudimentary than that. I’ll start by trying to explain some of the basic ideas of quantum mechanics, the study of things which are very very small, which is necessary if we’re going to understand the idea behind quantum computing.

In the world around us (not the TV show) we’re all used to seeing objects with precisely defined states. By states I mean things like position, velocity, whatever. Any old property will do. As it turns out, when we delve into the world of quantum mechanics this is no longer the case. For example, if you try and look at things like individual atoms, their states are no longer very well defined. Instead things are sort of fuzzy. If you try and measure the position of an atom, you just can’t get a precise answer as to where it is. This doesn’t present much of conceptual problem to us, right?. “Surely the fuzziness is just experimental error and noise?” we might say. Strangely this isn’t the case at all. In fact the fuzziness is an instrinsic property of physics. That is to say, things actually are fuzzy, and no matter how high-tech our measurement devices are, we will never be able to overcome it. From here, I’m afraid, things only begin to become more strange. The idea of states not being well defined, or fuzzy, leads on to the next bizarre phenomena of quantum mechanics, that of superpositions.

We’ve all grown up used to things being in one place or another. In quantum mechanics the rules are a bit looser than this and in fact things can be in one place and another, called a superposition of the different states. This should be taken quite literally as meaning that something can simultaneously be in multiple places at once. How do we know this? Hopefully we all remember our high-school optics experiments where we shine a laser beam through a double slit and observe interference effects on the other side. Well we can do this with single photons too. The problem is that the photon is a fundamental, indivisible unit, unlike a bright beam of laser light. When a single photon hits a double slit, it doesn’t become two photons. There’s still only one photon. Nonetheless, we can oberserve exactly the same kind of interference effect at the back end of the double slit as we do with laser light. This seems like a bit of a contradiction. If the individual photons are indivisble and always go one way or another at the double slit then cleary there would be no interference pattern. We’d just see two bright spots, corresponding to the two slits. However we do see interference patterns. The explanation is that the single photon enters a superposition of the two possible routes and self-interference occurs at the other side of the double slit. In other words, even though there is only one photon, it simultaneously takes both routes. Strange but true!

3 thoughts on “So what is it that I do? – Part 1: Quantum Mechanics”

  1. This is absolutely fascinating, thank you! This is the most coherent explanation of quantum mechanics that I’ve read.

  2. I’ve just seem the film “what the bleep ” and it is all about quantum mechanics.I am now addicted to this weird world of endless potential and hard to grasp facts that at the same time feel true and correct.

  3. My usual view about QM is along this line: the objects has a definite state just like CM, however, when one tries to get classical properties out of a QM state (by ‘collapsing’ the wave function with an experiment), the answer includes the intrinsic noise. A mystery to me is the actual wave equation. This seems to depend on how we choose to see the object: The wave equation of an electron in the particle-in-a-box and one around a proton is different. What causes the construction of the wave function? The boundary conditions? The experimental results?

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