States of an electromagnetic field are called classical when they can be expressed as probabilistic sums of coherent states. Such states describe almost all sources of radiation in everyday technology. By contrast, no such appropriate sum of coherent states can be found for a nonclassical or quantum state. Investigating such states is the business of modern quantum optics laboratories.
Writing in Physical Review Letters, Stefano Pirandola of the University of York, UK, proposes an idealized quantum readout mechanism that takes advantage of the quantum nature of nonclassical states. He describes a digital memory device reminiscent of optical storage media like a DVD.
In his model, digital information is stored in cells with two different reflectivities, representing the two values of a bit. Each cell is then irradiated by light subsequently measured by detectors. When the mean number of photons for each cell is fixed at a low number, the author’s calculations indicate that a nonclassical source of light retrieves significantly more information than a classical one.
Better data transfer at the “few photon” level should give us optical disks that spin faster. This work may also lead to the safe readout of photodegradable memory devices, such as dye-based optical disks or photosensitive microfilms, using faint quantum light to retrieve the data safely, where classical light would be destructive. – Sami Mitra