Quantum computing for dummies

Heard people talking about quantum computing, but not really sure you understand what they mean? Well, you’re far from alone (as the late great Richard Feynman once said, “anyone who claims to understand quantum physics doesn’t understand quantum physics”), but why let that stop you from trying to get a layman’s grasp of the basic ideas?

That, one assumes, is the spirit in which this brief introduction to quantum computing at Silicon.com has been written [via SlashDot]… though I’m in no position to comment on how accurate or useful it is. Input from passing physicists is, as always, more than welcome. 🙂

Hang on, what’s quantum entanglement when it’s at home?

I was afraid you were going to ask. Quantum entanglement is the point where scientists typically abandon all hope of being understood because the thing being described really does defy the classical logic we’re used to.

An object is said to become quantumly entangled when its state cannot be described without also referring to the state of another object or objects, because they have become intrinsically linked, or correlated.

No physical link is required however – entanglement can occur between objects that are separated in space, even miles apart – prompting Albert Einstein to famously dub it “spooky action at a distance”.

The correlation between entangled objects might mean that if the spin state of two electrons is entangled, their spin states will be opposites – one will be up, one down. Entangled photons could also share opposing polarisation of their waveforms – one being horizontal, the other vertical, say. This shared state means that a change applied to one entangled object is instantly reflected by its correlated fellows – hence the massive parallel potential of a quantum computer.

Accuracy aside, what’s interesting to me is seeing this sort of bluffer’s guide in a venue like Silicon.com, which is more of a business organ than a tech one. Prepping the Valley VCs for upcoming investment decisions, perhaps?

3 thoughts on “Quantum computing for dummies”

  1. I’ve read discussions on this a lot, but I still can’t understand it:

    Why can’t quantum entanglement be used for FTL communication?

    Is it because the spin/waveform is a probability?

  2. Doug, you are correct that quantum entanglement cannot be used for FTL communication. The confusion arises because a measurement made on one particle of a quantum-entangled pair will “instantaneously” determine the outcome of a similar measurement on its partner in a distant location. But, and this is very important, you *can’t predetermine* what the measurement at the original location will yield, so you can’t use it to send a message. Consider, for example, two standard dice, perfectly entangled (not easy) so that if you measured (e.g. rolled) one of them, then measuring (rolling) the other would always yield the opposite face, such that the sum of the numbers displayed would always be seven. If I rolled and found a 1 on die #1, that would guarantee a 6 on die #2, far away. If I found a 2 on die #1, then that would guarantee a 5 on die #2, etc. But when I rolled die #1, I could still not have any idea what number, in advance, that I would get on it. As such, I could not use die #1 to send a message to a distant person rolling die #2. All I could ever do is predict accurately that the distant person rolling die #2 would get 7 minus whatever I got on die #1. Loosely speaking, it would seem at first glance as if the dice communicated, but the people using them can’t. But unfortunately, thinking of it as the dice communicating is not quite the right picture either. The only accurate view that I know is from the combined perspective of quantum mechanics and the special theory of relativity, so that’s why it is difficult to explain this to non-physicists. For me, the even more amazing thing about quantum entanglement is that it can be used for computing! Sure, it may ultimately follow from the math and physics, but it isn’t obvious at all. After all, if I recall correctly, the possibility of quantum computing wasn’t mentioned in any of my physics textbooks ~30 years ago, despite the fact that the underlying physics principles were more than half a century old, even then.

  3. On the contrary. One of the famous communications conundrums is co-ordinating an action over an unreliable link. (wiki/Two Generals’ Problem). Extending your analogy, a leader General could use the die to choose a random hour; the entangled die would communicate specific information to the second General: the time of an attack. Surely this is FTL communication of information?

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