The Double-Slit Experiment
Send particles through two slits one at a time and each lands as a single dot, yet thousands of dots build up bright and dark stripes, the mark of waves that add and cancel. If anything records which slit each particle used, the stripes vanish. The pattern needs the two paths to stay unrecorded so they can interfere.
Why do physicists call this the heart of quantum physics?
The physicist Richard Feynman said the double-slit experiment holds "the only mystery" of quantum physics. Every quantum computer uses the same effect. Its gates steer chances so that wrong answers cancel out and right answers add up.
So if you understand this one experiment well, you understand the engine of quantum computing. Let's build it up one step at a time.
- How can two waves cancel each other out?
- Worked example: where are the dark stripes for green light?
- What happens when particles go through one at a time?
- Do heavy things have a wavelength too? Compute it.
- Why do the stripes vanish when you check which slit?
- Run it: two mirrors in a rowINTERACTIVE
- Try this: make one path half a wave longerINTERACTIVE
- Try this: leave a which-path recordINTERACTIVE
- What should you take away from the double slit?
You’ve read the opening of chapter 3. Pro unlocks the other 9 sections — plus every chapter of every course, with circuits you can run right on the page. That’s $11.99 a month, about the price of a coffee, or $99.99 a year (save 30%). The first chapter of every course, and the whole math course, stay free.