Nonclassical Light

A feature of quantum theory is that objects should have both particle and wave properties. Thus, things usually encountered as particles such as electrons or atoms show their quantum, or nonclassical, nature in the form of wavelike effects. Conversely, light, which can usually be described by a wave equation, shows its nonclassical side by acting like a particle.

In most optics experiments, even those involving lasers, the light produces only classical effects which can be described using 19th century electromagnetism. For example, a grocery scanner diode laser emits about 10¹⁵ photons per second. When such a stream encounters a half-silvered mirror, half of the light will be reflected, and half transmitted. With so many photons, the individual particle nature is hidden when the photons are detected at photodiodes sitting behind each exit port of the beamsplitter. If the original laser beam is replaced with a source of single photons, then the story is different: a lone photon might well have an equal chance of going towards either detector, but it will ultimately register in only one, a sure sign of quantum behavior.

One can probe these issues more deeply by using entangled photon pairs. Kevin Resch, Jeff Lundeen, and Aephraim Steinberg at the University of Toronto send ultraviolet (UV) light into a special crystal in which a single UV photon can produce two red photons in a process called down-conversion. One of the red photons is vertically polarized and the other is horizontally polarized, and therefore the photons can be time-delayed relative to one another by varying the thickness of birefringent material (which can swivel a light wave's orientation) traversed by the photon. By adjusting the delay between the photons, the researchers were able to change the number of photon pairs emerging from an interferometer without changing the intensity, or brightness, of the beam.

Owing to the intrinsic nonlinear response of the detectors this quantum interference effect then became apparent in the counting rate at a single detector (an effect never before observed) and not just in the coincidence rate between a pair of photodetectors. The researchers believe that the ability to observe such nonlinear responses in photodetection at the single photon level may be useful to the study of decoherence in photodetection and for providing an experimental basis for developing a more accurate theoretical description for photodetection. (Physical Review A, 1 February 2001; Kevin Resch contact: 416-946-3162, resch@physics.utoronto.ca; the article is not available electronically, but can be faxed to you.)