- H
Beam splitter: a branch that hits it splits in two. Half the probability bounces back, with its phase flipped (+ → −), and half passes through.
- ▒
Barrier: steel bounces the ball, but 15% of each branch tunnels through.
- Z
Phase plate: anything passing through gets its phase flipped. Cyan branches are |+⟩ and magenta ones are |−⟩.
- ✕
Interference: when two branches meet, they recombine. With the same phase, the merged ball keeps going their way. With opposite phases, it comes out the other way.
- p
Expected damage: a branch with probability p deals p damage, so splitting covers more bricks with weaker hits.
- 〰
Entangled bricks come in pairs. Break one and its partner shatters too.
- 👁
The paddle is a detector. A branch reaching the bottom is measured: with probability p, it's the real ball. Real and caught: it bounces. Real and missed: you lose a life. Not real: it vanishes, and its probability moves to the other branches.
- ⌛
Decoherence: a superposition lasts 12 s, then it collapses on its own. Space collapses it right away.