Single Photon Interference

Jeff, Jacob, Bryce, Edward, and Julie

Young’s Double Slit Experiment

YAY!

What is it?

• First conducted in 1801 by Thomas Young
• Light passed through a barrier with two slits (before it usually passed through a single slit to make the light coherent)
• The light diffracted through each slit
• Created interference pattern
• In order to interfere the light that passes through the slit must be of the same frequency and polarization and it must be coherent

Particles vs. Waves

Particle

• If light acts as a particle, only two slits will appear on the screen

Wave

• The light will diffract and interfere, making many fringes

Why is this important?

• This proves light acts not only as a particle, but also as a wave!

Equations for the Fringes

Many Photon Interference

Acquisition Time: .1 sec

Attenuation: 3 optical depths

Amplification: x1

Single Photon

• When a single photon is used, wave interference is still present
• It is impossible for a photon to act as a wave as it is can only go through one slit
• If one slit is blocked, there is no interference

Energy of a Single Photon

• Power of 1 photon per meter
• P=9.421∗10⁻¹¹W
• The power of our laser is
• P=9.1∗10⁻⁷W
• So we use 3 optical depths to attenuate the beam to single photon energy levels.

Single Photon Interference

Acquisition Time: .1 sec

Attenuation: 3 optical depths

Amplification: x100

Single Photon Interference

Acquisition Time: .1 sec

Attenuation: 3 optical depths

Amplification: x200

Single Photon Interference

Acquisition Time: 1 sec

Attenuation: 3 optical depths

Amplification: x100

Interferometry

Mach-Zehnder Interferometer

• Power of 1 photon per meter
• P=9.421∗10⁻¹¹W
• The power of our laser is
• P=6.17∗10⁻⁵W
• So we use 5 optical depths to attenuate the beam to single photon energy levels.

Strong field

Acquisition Time: .1sec

Attenuation: 5 optical depths

Amplification: x100

Single Photon

Acquisition Time: .1 sec

Attenuation: 5 optical depths

Amplification: x100

Single Photon – Horizontal Polarization Blocked

Acquisition Time: .1 sec

Attenuation: 5 optical depths

Amplification: x100

Single Photon – Vertical Polarization Blocked

Acquisition Time: .1 sec

Attenuation: 5 optical depths

Amplification: x100

Important Observations

• When path of photon is unknown, fringes are observed
• When path of photon is known, fringes are not observed

Summary

• Young’s Double Slit Experiment
• Fringes were observed under high intensity, as well as with only a single photon.
• Interferometer
• Fringes were observed under high intensity as well as with only a single photon.
• When either path of the interferometer was blocked, fringes were not observed.

What Problems Did We Encounter?

Single Photon Interference

Acquisition Time: .1 sec

Attenuation: 5 optical depths

Amplification: x100

Conclusion

• Single photons behaved the same way as the high intensity beam.
• In the double slit experiment, the single photon appeared to interfere with itself – going through both slit simultaneously.
• In the interferometer the single photon appeared to take both paths simultaneously – blocking a path removed interference fringes.

Sources

• http://www.studyphysics.ca/newnotes/20/unit04_light/chp1719_light/lesson58.htm
• http://dev.physicslab.org/asp/applets/doubleslit/default.asp
• http://physics.about.com/od/lightoptics/a/doubleslit.htm
• http://www.physics.brown.edu/physics/demopages/Demo/modern/demo/7a5520.htm
• The book – Seeing the Light