Go further · Chapter 4 · 4a · Real-world case
How do scientists do the double-slit experiment with single particles?
The short answer (tap to open)
To send particles through two slits one at a time, physicists make the source so weak that only one particle is in the apparatus at once, and use detectors that record each arrival as a single dot. Over minutes the dots build the familiar stripes. Teams have done it with light, electrons, atoms and large molecules.
How do you get "one at a time"?
Dim the source. For light, filters cut a beam down until photons arrive thousands of times a second but each crosses the apparatus alone. For electrons, a weak source sends roughly one at a time, with metres of empty space between them. There's never a second particle to interfere with. Each one interferes only with its own possibilities.
What do the slits look like?
Real slits are tiny. Young used a card with a thin divider. Electron experiments often skip slits and use an electron biprism: a wire thinner than a hair, charged so that electrons passing either side bend towards each other and overlap, just like two slits. Bologna's team used one in 1974, and Akira Tonomura's team at Hitachi used one in 1989 to film the stripes building up dot by dot. In 2013 a team in Nebraska made true double slits small enough for electrons and could open and close each slit.
8 dots: scattered, with no pattern.
Simulated, using the frame counts from Hitachi's 1989 film.
Each frame, in words
- 8 dots: scattered, with no pattern.
- 270 dots: still mostly scattered.
- 2,000 dots: stripes start to show.
- 160,000 dots: clear bright and dark stripes.
- One slit closed: a single smooth hump, with no stripes.
Simulated, using the frame counts from Hitachi's 1989 film.
How big can the particles get?
Neutrons, helium atoms, then molecules. In 1999 a Vienna team sent football-shaped molecules of 60 carbon atoms through a grating. In 2019 they used molecules of about 2,000 atoms, heavier than some proteins. Each still built stripes one by one. The harder part is keeping them isolated: a single bump from a stray gas molecule records the path and kills the pattern (chapter 3).
What if you try to see which slit it went through?
Put a detector at one slit and the stripes vanish, every time. As chapter 3 explains, the record of the path does it, not a person looking.
Check
Two quick ones
Collect
Your aha card
One particle, two paths, one dot.
Thousands of dots, one at a time, build the stripes.