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

by Seth Lloyd

To see Landauer’s principle in action, look at how bits are erased in computers. As noted in chapter 2, in a contemporary electronic computer a bit is stored on a capacitor. A capacitor is a bucket for electrons. When you charge up the capacitor, you put electrons in the bucket; when you discharge it, you dump the electrons out of the bucket. In a computer, an uncharged capacitor registers a 0 and a charged capacitor registers a 1. To erase a bit in an electronic computer, just empty the bucket: close a switch and let the electrons on the capacitor flow out. When the capacitor has been discharged, the bucket is empty: the bit has been restored to 0. But the microscopic state of the electrons “remembers” whether the capacitor was charged or not; that is, as they flow out of the capacitor, they heat up. This temperature change remains as evidence of the initial state of the capacitor. Its bit of information has been transferred to the microscopic motion of the electrons.