The Double-Slit Experiment
Originally performed by Thomas Young in 1801 to demonstrate the wave nature of light, the modern quantum version of the double-slit experiment demonstrates the fundamental concept of wave-particle duality.
When particles such as photons or electrons are fired at a barrier with two slits, they form an interference pattern on a screen behind the barrier—characteristic of waves. However, when observed or measured at the slits to determine which path the particle takes, the interference pattern disappears, and they behave as discrete particles.
This experiment powerfully illustrates that the act of measurement itself affects the system, a cornerstone principle of quantum mechanics.
Bell's Inequality
Formulated by John Stewart Bell in 1964, Bell's theorem proves that quantum mechanics is incompatible with local hidden-variable theories.
Experiments testing Bell's inequality measure the properties (like spin or polarization) of entangled particle pairs. The correlations between these measurements systematically violate the mathematical limits (Bell inequalities) that would apply if the particles possessed pre-existing, independent properties (hidden variables) before being measured.
The experimental confirmation of this violation demonstrates that "spooky action at a distance" is a real feature of our universe.
Quantum Entanglement Demos
Modern laboratories routinely create and utilize entangled photons to test the bounds of quantum theory and develop new technologies like quantum cryptography.
By utilizing spontaneous parametric down-conversion, a single high-energy photon is split in a non-linear crystal into two lower-energy entangled photons. When the polarization of one is measured, the other's state is instantaneously determined, regardless of the distance separating them.