How do you make a one-photon-thick beam of light?
Category: Physics Published: July 23, 2014
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University
There is no such thing as a one-photon-thick beam of light. Photons are not solid little balls that can be lined up in a perfectly straight beam that is one photon wide. Instead, photons are quantum objects. As such, photons act somewhat like waves and somewhat like particles at the same time. When traveling through free space, photons act mostly like waves. Waves can take on a variety of beam widths. But they cannot be infinitely narrow since waves are, by definition, extended objects. The more you try to narrow down the beam width of a wave, the more it will tend to spread out as it travels due to diffraction. This is true of water waves, sound waves, and light waves. The degree to which a light beam diffracts and diverges depends on the wavelength of the light. Light beams with larger wavelengths diverge more strongly than light beams with smaller wavelengths, all else being equal. As a result, smaller-wavelength beams can be made much narrower than larger-wavelength beams. The narrowness of a light beam therefore is ultimately limited by wave diffraction, which depends on wavelength, and not by a physical width of photon particles. The way to get the narrowest beam of light possible is by using the smallest wavelength available to you and focusing the beam, and not by lining up photons (which doesn't really make sense in the first place).
Furthermore, photons are bosons, meaning that many photons can overlap in the exact same quantum state. Millions of photons can all exist at the same location in space, going the same direction, with the same polarization, the same frequency, etc. In this additional way, the notion of a "one-photon-thick" beam of light does not really make any sense. Coherent beams such as laser beams and radar beams are composed of many photons all in the same state. The number of photons in a light beam is more an indication of the beam's brightness than of the beam's width. It does make sense to talk about a beam with a brightness of 1 photon per second. This statement means that a sensor receives one photon of energy from the light beam every second (which is a very faint beam of light, but is encountered in astronomy). Furthermore, we could construct a light source that only emits one photon of light every second. But we discover that as soon as the photon travels out into free space, the single photon spreads out into a wave that has a non-zero width and acts just like a coherent beam containing trillions of photons. Therefore, even if a beam has a brightness of only one photon per second, it still travels and spreads out through space like any other light beam.