# Q & A: Properties of Single Photons

Q:
What is white light? 1)Is it combination/sum of different energy type photons? And when they hit the object, only cartain energy type photons are absorbed, rest is reflected. 2)Or is it something different, like... superposition of different waveleghts, and the number of photons in every lightbeam is unknown.
- Lucas (age 31)
Poland
A:

Hi Lucas,

A classical beam of light is almost perfectly described by oscillating electric and magnetic fields, and is made up of many photons. Many properties of light seem simple as long as we are talking about classical beams of light. For example, white light is made of photons of many different colors, a laser beam is made of a collection of particles spread out slightly in space, and when the beam hits a beamsplitter, half of the photons go one way and half go the other way. This classical picture is intuitive, satisfying, and gives good predictions.

If you start trying to apply these ideas to single photons, your intution will quickly scream that something is wrong. Classically, you might guess that a single photon has a single color, a point-like position, and a definite path (even after a beamsplitter). However, quantum mechanics has told us that these guesses are false.

In fact, the true properties of single photons are usually almost the same as those of a classical beam of particles. For example, a photon can indeed contain a superposition of a range of frequencies (aka colors), so you can have a "white" single photon. Similarly, a photon generally has the same shape, or "mode," as the laser beam/source from which it came. This is allowed because photons, like all fundamental particles, are better described by probability clouds than by point-like objects. Finally, when you send a photon through a beamsplitter, half of its amplitude does indeed take each of the two paths.

Each of these topics are discussed in more detail on Ask the Van, but my main point is simply: quantum mechanics tells us that properties of single photons tend to be similar to properties of the beam from which they came. This is allowed because in quantum mechanics, properties can be in superpositions of different values (frequency, position, path).

Hope that makes sense,

David Schmid

(published on 11/06/2013)