Quantum computer gets an undo button

Quantum mechanics is, mathematically, quite simple. But it has implications that require people to think differently about the world. One particularly hard-to-grasp idea is that, on the surface, some knowledge precludes obtaining other knowledge. This is a consequence of how we obtain it.

In an innovative experiment, researchers from Austria have demonstrated how to recover that lost information. Before you get the wrong impression, though, this is completely in agreement with the rules of quantum mechanics—it is simply a very clever way of playing with quantum states.

Quantum magic

Before looking at the experiment, note what makes this interesting. The keywords that turn up in these sorts of articles are superposition states and measurement. Imagine that we have 100 electrons, sitting in a magnetic field. Their individual magnetic fields are all, thanks to the applied field, pointing in the same direction. Now, we turn on a microwave for a specific period of time. Chosen correctly, all 100 electrons flip their fields so that they point in exactly the opposite direction. If we make a measurement, all electrons report the same spin. If we cut the time of the microwave pulse in half, however, something very strange happens: all the electrons end up with their fields pointing in both directions at once. This is called a superposition state. Once we make a measurement, though, we find half the electrons have their fields pointing in one direction and half have their fields pointing in the opposite direction—the superposition state vanishes. You might immediately think it was never there in the first place: we simply put in half the energy, so only half the electrons responded.