What equation describes the wavefunction of a single photon?

The Schrödinger equation describes the quantum mechanics of a single massive non-relativistic particle. The Dirac equation governs a single massive relativistic spin-½ particle. The photon is a massless, relativistic spin-1 particle.

What is the equivalent equation giving the quantum mechanics of a single photon?

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There is no quantum mechanics of a photon, only a quantum field theory of electromagnetic radiation. The reason is that photons are never non-relativistic and they can be freely emitted and absorbed, hence no photon number conservation. – Igor Ivanov Nov 9 '10 at 20:51
Igor, you should make your comment into an answer. It's at the moment the best one posted. – j.c. Nov 10 '10 at 18:09
OK, posted as an answer with an additional note. – Igor Ivanov Nov 10 '10 at 20:01

7 Answers

There is no quantum mechanics of a photon, only a quantum field theory of electromagnetic radiation. The reason is that photons are never non-relativistic and they can be freely emitted and absorbed, hence no photon number conservation.

Still, there exists a direction of research where people try to reinterpret certain quantities of electromagnetic field in terms of the photon wave function, see for example this paper.

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You can also say that the wavefunction of a photon is defined as long as the photon is not emitted or absorbed. The wavefunction of a single photons is used in single-photon interferometry, for example. In a sense, it is not much different from the electron, where the wave-function start to be problematic when electrons start to be created or annihilated... – Frédéric Grosshans Nov 17 '10 at 10:19

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I agree. For the electrons there is a possibility to slow them down to non-relativistic speeds, but there is no such possibility for photons. I would also add that there is an interesting discussion about photons and electrons in the Peierls's book "Surprises in theoretical physics". – Igor Ivanov Nov 18 '10 at 21:46

The maxwell equations, just like in classical electrodynamics. You'll need to use quantum field theory to work with them though.

http://en.wikipedia.org/wiki/Relativistic_wave_equations
http://en.wikipedia.org/wiki/Quantum_electrodynamics

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The general concept of quantum mechanics is that particles are waves. On of hand-waveing "derivations" of quantum mechanics is assumption that phase of particles behaves in the same way as phase of light $\exp( i \vec{k}\cdot \vec{x} - iE t / \hbar)$ (see Feynman Lectues on Physics, Volume 3, Chapter 7-2).

For light that is monochromatic (or almost monochromatic), just take Maxwell Equations plus add assumption that one photon can't be partially absorbed. Most of the time it suffices to use the paraxial approximation, or even - plane wave approximation. It works for standard quantum mechanics setups like Elitzur–Vaidman bomb-tester.

For nonmonochronatic light its much more complicated. More on nature of quantum mechanics of one photon: Iwo Bialynicki-Birula, On the Wave Function of the Photon, Acta Physica Polonica 86, 97-116 (1994).

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According to Wigner's analysis, the single photon Hilbert space is spanned by a basis parameterized by energy-momenta on the forward light cone boundary, and a helicity of $\pm 1$.

However, a manifestly Lorentz covariant description in position space has to include a fictitious longitudinal photon with a helicity of 0. This degree of freedom is pure gauge, and decouples. Interestingly enough, the state norm is now positive semidefinite, instead of positive definite, with the transverse modes having positive norm and the longitudinal ones having zero norm.

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you're on fire ! – user346 Dec 31 '10 at 7:49

There is a slight confusion in this question. In quantum field theory, the Dirac equation and the Schrödinger equation have very different roles. The Dirac equation is an equation for the field, which is not a particle. The time evolution of a particle, ie, a quantum state, is always given by the Schrödinger equation. The hamiltonian for this time evolution is written in terms of fields which obey a certain equation themselves. So, the proper answer is: Schrödinger equation with a hamiltonian given in terms of a massless vector field whose equation is nothing else but Maxwell's equation.

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Yup. Like this? http://www.nist.gov/pml/div684/fcdc/upload/preprint.pdf

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You should include more than a bare link. Especially a bare link to a PDF. Copying the abstract, for instance, would make this a much better answer. – dmckee♦ Jun 1 '11 at 4:46

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@kaonix , a +1 because I learned something, but dmckee is right that you should quote the abstract for a good answer. Also use the link tab on the editor to insert a link, or at least put the http:// in front so that one does not have to copy and paste to see the link. I will edit it for you. – anna v Jun 1 '11 at 4:54

A single photon is described quantum mechanically by the Maxwell equations, where the solutions are taken to be complex. The Maxwell equations can be written in the form of the matrix Dirac equation, where the Pauli two-component matrices, corresponding to spin 1/2 electrons, are replaced by analogous three-component matrices, corresponding to spin 1 photons. Since the Dirac equation and corresponding Maxwell equation are fully relativistic, there is no problem with the mass of the photon being zero, as there would be for a Schroedinger-like equation. See http://www.nist.gov/pml/div684/fcdc/upload/preprint.pdf.

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