Quantum Arcade

A hidden state is drawn. Steer yours (cyan) onto the ghost (dim) — fidelity ≥ 99% wins.

Set the odds with θ, then call the collapse. Long streaks get exponentially unlikely — that is the Born rule doing its job.

Drive the qubit with a rotation pulse. A perfect π pulse lands the whole amplitude on |1⟩ — this is exactly how real qubits get flipped.

Split one qubit onto two paths (H), twist the phase between them, recombine (H). The output probability swings from certain to impossible — with nothing removed, only phase.

One fixed state, two questions. Ask in Z: pure coin-flip. Ask in X: dead certain. The state didn't change — your question did.

Before and after. Which gate did it?

Turn one knob from “two separate qubits” to “one inseparable pair.” Watch qubit A's purity drop to ½ — alone, it becomes a pure coin flip, because the information lives in the pair.

Two dice, one wavefunction. Each roll is individually 50/50 random — and they always agree. Randomness and perfect correlation, at the same time.

Classical physics caps |S| at 2. A Bell pair reaches 2√2 ≈ 2.83. Collect rounds and watch your sampled S climb past what any local hidden-variable story allows.

A message qubit, an entangled pair, and two classical bits — the state moves without any qubit traveling. Step through the actual amplitudes.

Share a Bell pair in advance, and one qubit can carry two classical bits. Pick the bits; the decoder lands on the matching basis state with certainty.

CNOT is supposed to change the target. Put the target in |−⟩ and fire: the target shrugs — the control flips from |+⟩ to |−⟩. This backwards kick powers Deutsch, Grover, and phase estimation.

One button. It copies an unknown qubit. Try it.

Eight drawers, one prize. Classically you open ~4. Grover finds it in 2 iterations — and if you keep going past the sweet spot, the amplitude rotates away again. Try over-searching.

A mystery coin function: constant (same answer always) or balanced (half and half). Classically you must ask twice. The Deutsch circuit asks once — and is never wrong.

Hardware quantum RNGs sell exactly this: prepare |+⟩, measure, repeat. Sample away — entropy should hug 1 bit/bit.

This is the enemy every quantum computer fights. Turn up the noise on a crisp |+⟩ and watch its off-diagonal coherence — the part that interferes — bleed away into a classical coin.

Encode one bit into three qubits; majority vote fixes any single flip. Below ~50% noise the code beats the bare qubit — the same idea, scaled up, is how Willow-class processors reach below-threshold error correction.

Alice sends key bits in random bases; Bob measures in his own. Toggle an eavesdropper: her measurements must disturb the states (no-cloning, two cards up), and the error rate betrays her at ~25%.

Classically, a ball below the wall's height never crosses. Quantum mechanically the odds are small — but never zero. Thinner or lower walls help exponentially.