Programming the Universe: A Quantum Computer Scientist Takes on the Cosmos

by Seth Lloyd

Recall the double-slit experiment. In that model, the electron does two things at once: it goes through both slits simultaneously. When you take a measurement to determine which slit the electron has gone through, it will show up at one slit or the other at random. Similarly, when you take a quantum computer that is doing two things at once and measure to see what it’s doing, you will find it doing one or the other of those things at random. If you want to see the interference pattern in the double-slit experiment, you must wait until the electron has hit the screen, so that the two waves—one from one slit, one from the other—can interfere with each other. The interference pattern comes from the “duet” of the two waves. In a quantum computation, if you wish to get the full benefit of the computation, you must not look at the computation while it is occurring. To get the full symphonic effect of a quantum computation, you must let the all the waves in the computation interfere with one another. You must let the “voices” of the computation blend together on their own.