Photon dynamics in the double-slit experiment

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The Dynamics of photons in the double-slit experiment describes the relationship between classical electromagnetic waves and photons, the quantum counterpart of classical electromagnetic waves, in the context of the double-slit experiment. The dynamics of a photon can be completely described by the classical Maxwell's equations with only a reinterpretation of the classical field as a probability amplitude for the photon.

1 Classical description of the double-slit experiment
2 Quantum description of the double-slit experiment
- 2.1 Energy and momentum of photons
  - 2.1.1 Energy
  - 2.1.2 Momentum
- 2.2 The nature of probability in quantum mechanics
  - 2.2.1 Probability for a single photon
  - 2.2.2 Probability amplitudes
3 See also
4 References

[edit] Classical description of the double-slit experiment

[edit] Electromagnetic wave equations

Main article: Electromagnetic wave equation

The electromagnetic wave equations are a simplified version of Maxwell's equations which describe the propagation of electromagnetic waves through a medium or in a vacuum. The homogeneous form of the equation, written in terms of either the electric field E or the magnetic field B, takes the form:

$\nabla^2 \mathbf{E} \ - \ { 1 \over c^2 } {\partial^2 \mathbf{E} \over \partial t^2} \ \ = \ \ 0$

$\nabla^2 \mathbf{B} \ - \ { 1 \over c^2 } {\partial^2 \mathbf{B} \over \partial t^2} \ \ = \ \ 0$

where c is the speed of light in the medium. In a vacuum, c = 2.998 x 10⁸ meters per second, which is the speed of light in free space.

The magnetic field is related to the electric field through Faraday's law

$\nabla \times \mathbf{B} = {1 \over c} \frac{ \partial \mathbf{E}} {\partial t}$ .

[edit] Plane wave solution of the electromagnetic wave equation

Main article: Sinusoidal plane-wave solutions of the electromagnetic wave equation

The plane sinusoidal solution for an electromagnetic wave traveling in the z direction is (cgs units and SI units)

$\mathbf{E} ( \mathbf{r} , t ) = \begin{pmatrix} E_x^0 \cos \left ( kz-\omega t + \alpha_x \right ) \\ E_y^0 \cos \left ( kz-\omega t + \alpha_y \right ) \\ 0 \end{pmatrix} = E_x^0 \cos \left ( kz-\omega t + \alpha_x \right ) \hat {\mathbf{x}} \; + \; E_y^0 \cos \left ( kz-\omega t + \alpha_y \right ) \hat {\mathbf{y}}$

for the electric field and

$\mathbf{B} ( \mathbf{r} , t ) = \hat { \mathbf{z} } \times \mathbf{E} ( \mathbf{r} , t ) = \begin{pmatrix} -E_y^0 \cos \left ( kz-\omega t + \alpha_x \right ) \\ E_x^0 \cos \left ( kz-\omega t + \alpha_y \right ) \\ 0 \end{pmatrix} = -E_y^0 \cos \left ( kz-\omega t + \alpha_x \right ) \hat {\mathbf{x}} \; + \; E_x^0 \cos \left ( kz-\omega t + \alpha_y \right ) \hat {\mathbf{y}}$

for the magnetic field, where k is the wavenumber,

$\omega_{ }^{ } = c k$

is the angular frequency of the wave, and $c$ is the speed of light. The hats on the vectors indicate unit vectors in the x, y, and z directions.

Electromagnetic radiation can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This diagram shows a plane linearly polarised wave propagating from left to right.

The plane wave is parameterized by the amplitudes

$E_x^0 = \mid \mathbf{E} \mid \cos \theta$

$E_y^0 = \mid \mathbf{E} \mid \sin \theta$

and phases

$\alpha_x^{ } , \alpha_y$

where

$\theta \ \stackrel{\mathrm{def}}{=}\ \tan^{-1} \left ( { E_y^0 \over E_x^0 } \right )$ .

and

$\mid \mathbf{E} \mid^2 \ \stackrel{\mathrm{def}}{=}\ \left ( E_x^0 \right )^2 + \left ( E_y^0 \right )^2$ .

The solution can be written concisely as

$\mathbf{E} ( \mathbf{r} , t ) = \mid \mathbf{E} \mid \mathrm{Re} \left \{ |\psi\rangle \exp \left [ i \left ( kz-\omega t \right ) \right ] \right \}$

where

$|\psi\rangle \ \stackrel{\mathrm{def}}{=}\ \begin{pmatrix} \psi_x \\ \psi_y \end{pmatrix} = \begin{pmatrix} \cos\theta \exp \left ( i \alpha_x \right ) \\ \sin\theta \exp \left ( i \alpha_y \right ) \end{pmatrix}$

is the Jones vector in the x-y plane. The notation for this vector is the bra-ket notation of Dirac, which is normally used in a quantum context. The quantum notation is used here in anticipation of the interpretation of the Jones vector as a quantum state vector.

[edit] Spherical and cylindrical wave solutions of the electromagnetic wave equation

[edit] Spherical waves

Main article: Wave equation

The solution for spherical waves emanating from the origin is

$\mathbf{E} ( \mathbf{r} , t ) = \mid \mathbf{E} ( \mathbf{r_0} , t ) \mid \left ( { r_0 \over r} \right ) \mathrm{Re} \left \{ |\psi\rangle \exp \left [ i \left ( kr-\omega t \right ) \right ] \right \}$

where $r$ is the distance from the origin and $r 0$ is some distance from the origin at which the electric field $\mathbf{E} ( \mathbf{r_0} , t )$ is measured.

Again, the magnetic field is related to the electric field by

$\mathbf{B} ( \mathbf{r} , t ) = \hat { \mathbf{r} } \times \mathbf{E} ( \mathbf{r} , t )$

where the unit vector is in the radial direction.

[edit] Cylindrical waves

The cylindrical solutions of the wave equation for waves emanating from an infinitely long line are Bessel functions. For large distances from the line, the solution reduces to

$\mathbf{E} ( \mathbf{r} , t ) = \mid \mathbf{E} ( \mathbf{r_0} , t ) \mid \left ( { r_0 \over r} \right )^{1/2} \mathrm{Re} \left \{ |\psi\rangle \exp \left [ i \left ( kr-\omega t \right ) \right ] \right \}$

$\mathbf{B} ( \mathbf{r} , t ) = \hat { \mathbf{r} } \times \mathbf{E} ( \mathbf{r} , t )$

where $r$ is now the distance from the line. This solution falls off as the square root of distance while the spherical solution falls off as the distance.

[edit] Huygens' principle

Main article: Huygens–Fresnel principle

$Wave Diffraction in the manner of Huygens.$

Wave Diffraction in the manner of Huygens.

Huygen's principle states that each point of an advancing wave front is in fact the center of a fresh disturbance and the source of a new train of waves; and that the advancing wave as a whole may be regarded as the sum of all the secondary waves arising from points in the medium already traversed.

This means that a plane wave impinging on two nearby slits in a barrier can be thought of as two coherent sources of light emanating from each of the slits. If the slits are very long compared with the distance at which the waves are observed, then the waves are cylindrical waves. If the slits are very short compared with the distance they are observed, then the waves are spherical waves. In either case the electric field for the wave emanating from each slit is proportional to

$\mathrm{Re} \left \{ |\psi\rangle \exp \left [ i \left ( kr-\omega t \right ) \right ] \right \} \ \stackrel{\mathrm{def}}{=}\ \mathrm{Re} \left \{ |\phi\rangle \right \}$ .

[edit] Interference

Main article: Interference

Consider two slits separated by a distance d. Place a screen a distance L from the slits. The distance from slit 1 to a point x on the screen is

$r_1 = \sqrt{ L^2 + x^2 }$

and the distance from slit 2 to the point x on the screen is

$r_2 = \sqrt{ L^2 + (x-d)^2 }$ .

For large L and small x compared with L, the difference between the two distances is approximately

$\Delta r \approx {xd \over r_1} \approx {xd \over L}$ .

The electric field at point x is given by the superposition of the states of the waves from each of the slits and is proportional to the real part of

$|\phi_1\rangle + |\phi_2\rangle = |\psi\rangle \left \{ \exp \left [ i \left ( kr_1 -\omega t \right ) \right ] + \exp \left [ i \left ( kr_2 -\omega t \right ) \right ] \right \} = |\phi_1\rangle \exp \left [ i \left ( k\Delta r -\omega t \right ) \right ]$ .

$Thomas Young's sketch of two-slit diffraction of light.$

Thomas Young's sketch of two-slit diffraction of light.

The total electromagnetic energy striking the screen at point x is proportional to the square of the electric field and is therefore proportional to

$\cos^2 \left ( k \Delta r \right ) \approx \cos^2 \left ( 2 \pi {xd \over \lambda } \right )$

where $λ$ is the wavelength of the light. The fields from the two slits constructively interfere and form antinodes when the phase is equal to multiples of $π$

$2 \pi {xd \over \lambda } = n \pi \quad n=0,1,2,\cdots$

$x_n = { n \lambda \over 2 d } \quad n=0,1,2,\cdots$ .

The waves destructively interfere and form nodes halfway in between the antinodes.

[edit] Quantum description of the double-slit experiment

Main article: Photon

The treatment to this point has been classical. It is a testament, however, to the generality of Maxwell's equations for electrodynamics that the treatment can be made quantum mechanical with only a reinterpretation of classical quantities. The reinperpretation is that the state vectors

$\mid \phi \rangle$

in the classical description of the double-slit experiment become quantum state vectors in the description of photons.

[edit] Energy and momentum of photons

The reinterpretation is based on the experiments of Max Planck and the interpretation of those experiments by Albert Einstein.

The important conclusion from these early experiments is that electromagnetic radiation is composed of irreducible packets of energy, known as photons.

[edit] Energy

Max Planck presents Albert Einstein with the Max-Planck medal, Berlin June 28, 1929

The energy of each packet is related to the angular frequency of the wave by the relation

$\epsilon = \hbar \omega$

where $\hbar$ is an experimentally determined quantity known as Planck's constant. If there are $N$ photons in a box of volume $V$ , the energy in the electromagnetic field is

$N \hbar \omega$

and the energy density is

${N \hbar \omega \over V}$

The energy of a photon can be related to classical fields through the correspondence principle which states that for a large number of photons, the quantum and classical treatments must agree. Thus, for very large $N$ , the quantum energy density must be the same as the classical energy density

${N \hbar \omega \over V} = \mathcal{E}_c = \frac{\mid \mathbf{E} \mid^2}{8\pi}$ .

The number of photons in the box is then

$N = \frac{V }{8\pi \hbar \omega}\mid \mathbf{E} \mid^2$ .

[edit] Momentum

Double-slit experiment when performed with electrons. The results are similar for photons. The figures show the buildup over time of electron collisions with the screen.

The correspondence principle also determines the momentum of the photon. The momentum density is

$\mathcal{P}_c = {N \hbar \omega \over cV} = {N \hbar k \over V}$

which implies that the momentum of a photon is

$\hbar k$ .

[edit] The nature of probability in quantum mechanics

[edit] Probability for a single photon

There are two ways in which probability can be applied to the behavior of photons; probability can be used to calculate the probable number of photons in a particular state, or probability can be used to calculate the likelihood of a single photon to be in a particular state. The former interpretation violates energy conservation. The latter interpretation is the viable, if nonintuitive, option. Dirac explains this in the context of the double-slit experiment:

“	Some time before the discovery of quantum mechanics people realized that the connexion between light waves and photons must be of a statistical character. What they did not clearly realize, however, was that the wave function gives information about the probability of one photon being in a particular place and not the probable number of photons in that place. The importance of the distinction can be made clear in the following way. Suppose we have a beam of light consisting of a large number of photons split up into two components of equal intensity. On the assumption that the beam is connected with the probable number of photons in it, we should have half the total number going into each component. If the two components are now made to interfere, we should require a photon in one component to be able to interfere with one in the other. Sometimes these two photons would have to annihilate one another and other times they would have to produce four photons. This would contradict the conservation of energy. The new theory, which connects the wave function with probabilities for one photon gets over the difficulty by making each photon go partly into each of the two components. Each photon then interferes only with itself. Interference between two different photons never occurs.	”
—Paul Dirac, The Principles of Quantum Mechanics, Fourth Edition, Chapter 1

[edit] Probability amplitudes

The probability for a photon to be in a particular polarization state depends on the fields as calculated by the classical Maxwell's equations. The state of the photon is proportional to the field. The probability itself is quadratic in the fields and consequently is also quadratic in the quantum state of the photon. In quantum mechanics, therefore, the state or probability amplitude contains the basic probability information. In general, the rules for combining probability amplitudes look very much like the classical rules for composition of probabilities:[The following quote is from Baym, Chapter 1]

The probability amplitude for two successive probabilities is the product of amplitudes for the individual possibilities. ...
The amplitude for a process that can take place in place in one of several indistinguishable ways is the sum of amplitudes for each of the individual ways. ...
The total probability for the process to occur is the absolute value squared of the total amplitude calculated by 1 and 2.

[edit] See also

[edit] References

Jackson, John D. (1998). Classical Electrodynamics (3rd ed.). Wiley. ISBN 0-471-30932-X.
Baym, Gordon (1969). Lectures on Quantum Mechanics. W. A. Benjamin. ISBN 0-8053-0667-6.
Dirac, P. A. M. (1958). The Principles of Quantum Mechanics, Fourth Edition. Oxford. ISBN 0-19-851208-2.