Quantum — Noise and Error: Why Some Uncertainty Can’t Be Removed

Every physical experiment is subject to some uncertainty. Repeated measurements on physical systems prepared in the same way necessarily do not give identical outcomes. The results fluctuate.. This is noise. If the measurements are designed to verify that the preparations are identical, fluctuations will ensure that some give YES, but a few will give NO. These are errors. If the systems in questions are quantum systems, fluctuations in the measurement results is called quantum noise.

If all noise refers to fluctuations of measurement results why should we distinguish quantum noise from classical noise? The difference is subtle. In the classical case ( the physics of Newton, Maxwell, Boltzmann…), we assume that, with enough effort, we can make noise and error arbitrarily small, for example by lowering the temperature. In the quantum case this is impossible. The quantum world is a source of irreducible uncertainty. Fortunately quantum theory shows us how to manipulate the odds to our advantage. We have many more levers available than simply lowering the temperature. Quantum technology is the business of engineering those levers to control noise by directly intervening in the quantum world. The discovery of quantum mechanics is the discovery of new ways to intervene in the physical world.

Learning is impossible without noise and error. It should come as no surprise that quantum noise enables new kinds of learning machines. I will return to this in the next post.