Hands-On Quantum Uncertainty
This development of this workshop was supported by the Perimeter Institute of Theoretical Physics
Classical Diffraction of Light
A laser pointer is shone through a narrow slit formed by two pencils. What will you see?
Quantum Diffraction of Light
Draw the diffraction pattern above, and below that draw the pattern you would get if you used really, really, really, really faint light.
You spent a whole unit learning that light behaves as a wave – a spread out phenomena.
How do we know that it consists of many, many, many individual and localized photons?
- A short video from Brown University of photon-by-photon interference: http://www.physics.brown.edu/physics/demopages/Demo/modern/demo/7a5520.htm
- A short video from Brown University of photon-by-photon interference: http://www.physics.brown.edu/physics/demopages/Demo/modern/demo/7a5520.htm
http://phys.educ.ksu.edu/vqm/html/singleslit.html
Diffraction is wave phenomena that is demonstrated by photons.
This is an example of wave-particle duality.
The amount of diffraction is governed by Heisenberg’s
Uncertainty Principle.
The more certain you are of where a photon is, the less
certain you will be of where it is going.
= w/2 * x1 h / L
= w/2 * L/w * h/L
= h/2
The Heisenberg Uncertainty Principal restricts the product of these two uncertainties,
x * p
Put on the glasses, close one eye and then look at your
neighbor's eyes.
Try tilting your head.
How do you explain this?
If it gets through a vertical polarizer, then we are certain that it is vertically polarized.
It will go through a vertical filter but not a horizontal one.
However, we are uncertain about any other basis. We are
reduced to probabilities.
It has a 50% chance of going through a filter at 45 degrees.
Quantum Double-Slit Interference
Does a photon go through one slit or both?
What if the light is really, really low intensity?
An experiment was done in 2007 to test this with electrons.
electron beam
conducting plates
detection screen
top view
charged pin
If an electron passes near enough to the conducting plate it will induce a measurable current.
side view
Far from the plates - few electrons are detected, lots of interference
Near the plates - lots of electron detection, little interference.
If you are certain of which way it went, there will be no interference.
If the polarizer is at 45o, the pattern returns. Why?
After a photon passes through a 45o filter, we are uncertain whether it
was vertical or horizontal.
The polarizer is acting as a quantum eraser.
It erases our knowledge of which way the photon went around the pin.