Photons – particles of light – are not like the particles we encounter in everyday life, like say, a marble, which is localized in space. Instead, photons are distributed across space. The way in which a photon is distributed, which we can loosely think of as its *shape*, is characterized by its so-called *wave-packet* or *wave-function*. There are many different ways in which photons can be produced experimentally, and the wave-packets of the produced photons varies enormously between these different schemes.

In my previous paper “Quantum gate characterization in an extended Hilbert space” (discussed in my previous post), we examined the effects of mode-matching upon the operation of linear optics quantum gates. During this research, one observation we made was that the significance of mode-mismatch in a circuit depends very heavily on the wave-packets of the photons used in the experiment. Naturally, every experimentalist wants to minimize the effects of mode-mismatch since it destroys the operation of their gates. This motivates the question “what type of wave-packets minimize the effects of mode-mismatch?”. In this paper – joint work with Tim Ralph and Michael Nielsen – (pre-print available at quant-ph/0505139) we address this question and establish two criteria which minimize such effects:

- Photons should be Gaussian in shape (
*i.e.*they should look like a Bell-curve) - Photons should be as
*broad*as possible (*i.e.*the Gaussian curve should be*stetched out*as much as possible)

From a practical perspective this means that experimentalists should try and employ photon sources which produce photons of this type. In principle this should improve the operation of experimental quantum gates.