“The noise is the signal” was a saying of Rolf Landauer, one of the founding fathers of mesoscopic physics. What he meant was that fluctuations in time of a measurement can be a source of information that is not present in the time-averaged value. As figure 1 reminds us, some types of noise are more interesting than others. A physicist with access to sensitive ways of distinguishing the granularity in a signal may delight in the noise.
Noise plays a uniquely informative role in the particle–wave duality. In 1909, Albert Einstein realized that electromagnetic fluctuations vary depending on whether the energy is carried by waves or by particles. The magnitude of energy fluctuations scales linearly with the mean energy for classical waves, but it scales with the square root of the mean energy for classical particles. Since a photon is neither a classical wave nor a classical particle, the...
