Through Two Doors at Once ·   
The Elegant Experiment That Captures the Enigma of Our Quantum Reality

An experiment: What would you expect to see on the screen?

“Depends on what one thinks is the nature of light ” — Light as particles

A bucket of sand as a metaphor for where the particles should land on the screen

The double slit experiment: Overlapping waves of light create an interference pattern

Discrete photons of light shot one-at-a-time also create an interference pattern!

With the discrete photons, the fringes appears random at first, but emerge over time

cover design: Daniel Lagin

An electromagnetic wave

The photoelectric effect

Figuring out which path the electron took — an apparatus imagined by Albert Einstein and Niels Bohr in a thought experiment from the 1920s

Alain Aspect and Philippe Grangier’s setup of a Mach-Zehnder interferometer (step-by-step)

Single photons are either reflected or transmitted through a beamsplitter

50% of the photons head to detector D1, the other remaining to detector D2

A second beamsplitter is added exactly at the point where the two paths cross

Alain Aspect’s experiment setup to test for Bell’s Inequality

Lucien Hardy experiment with two interferometers, based on Elitzur-Vaidman bomb paper

Dirk Bouwmeester experiment setup based on Roger Penrose interferometer design

Aephraim Steinberg’s experiment setup for weak measurements, to reconstruct particle trajectories through the double slit experiment

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