# 257: Single-Photon Interference - Second Version

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## Using Dirac Notation to Analyze Single Particle Interference

The schematic diagram below shows a Mach-Zehnder interferometer for photons. When the experiment is run so that there is only one photon in the apparatus at any time, the photon is always detected at D_{2} and never at D_{1}. (1,2,3)

The quantum mechanical analysis of this striking phenomenon is outlined below. The photon leaves the source, S, and whether it takes the upper or lower path it interacts with a beam splitter, a mirror, and another beam splitter before reaching the detectors. At the beam splitters there is a 50% chance that the photon will be transmitted and a 50% chance that it will be reflected.

After the first beam splitter the photon is in an even linear superposition of being transmitted and reflected. Reflection involves a 90^{o} (π/2) phase change which is represented by exp(iπ/2) = i, where i = (-1)1/2. (See the appendix for a simple justification of the 90^{o} phase difference between transmission and reflection.) Thus the state after the first beam is given by equation 257.1.

\[ | \psi = \left( \frac{[ | T> + i|R ]}{2} \right) ^{ \frac{1}{2}}\]

Now |T> and |R> will be written in terms of |D_{1}> and |D_{2}> the states they evolve to at detection. |T> reaches |D_{1}> by transmission and |D_{2}> by reflection.

\[ |T> = \left( \frac{[|D1> + i|D2>]}{2} \right) ^{ \frac{1}{2}}\]

|R> reaches |D_{1}> by reflection and |D_{2}> by transmission.

\[ |R> = \left( \frac{[i|D1> + |D2>]}{2} \right) ^{ \frac{1}{2}}\]

Equations 257.2 and 257.3 are substituted into equation 257.1.

\[ | \psi > = \frac{[|D1> + i|D2> + i2|D1> + i|D2>]}{2} \]

It is clear (i^{2} = -1) that the first and third terms cancel (the amplitudes are 180^{o} out of phase), so that we end up with a final state given by equation 257.5.

\[ | \psi> = i|D_2> \] The probability of an event is the square of the absolute magnitude of the probability amplitude.

\[ P(D_2) = |i|^2 = 1 \]

Thus this analysis is in agreement with the experimental outcome that no photons are ever detected at D_{1}.

**Appendix**:

Suppose there is no phase difference between transmission and reflection. Then equations 257.1, 257.2, and 257.3 become

\[ | \psi = \left( \frac{[|T> + |R>]}{2} \right) ^{ \frac{1}{2}}\]

\[ | T> = \left( \frac{[|D1> + |D2>]}{2} \right) ^{ \frac{1}{2}}\]

\[ | \psi = \left( \frac{[|D_1> + |D_2> |D_1> + D_2>]}{2} \right) ^{ \frac{1}{2}}\]

Substitution of equations 257.8 and 257.9 into equation 257.7 yields

\[ | \psi > = |D_1> + |D_2> \]

Thus, the detection probabilities at the two detectors are:

\[ P(D_1) = 1 and P(D_2) = 1 \]

This result violates the principle of conservation of energy because the original photon has a probability of 1 of being detected at D_{1} and also a probability of 1 of being detected at D_{2}. In other words, the number of photons has doubled. Thus, there must be a phase difference between transmission and reflection, and a 90^{o} phase difference, as shown above, conserves energy.

**References**:

1. P. Grangier, G. Roger, and A. Aspect, "Experimental Evidence for Photon Anticorrelation Effects on a Beam Splitter: A New Light on Single Interferences," Europhys. Lett. 1, 173-179 (1986).

2. V. Scarani and A. Suarez, "Introducing Quantum Mechanics: One-particle Interferences," Am. J. Phys. 66, 718-721 (1998).

3. Kwiat, P, Weinfurter, H., and Zeilinger, A, "Quantum Seeing in the Dark, Sci. Amer. Nov. 1996, pp 72-78.