Which Gate

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|0⟩ north
|1⟩ south
|+⟩ +x
|−⟩ −x
|+i⟩ +y
|−i⟩ −y

Twenty questions on H, X, Y, Z and S. Some show the Bloch vector before and after one gate, and others give the gate's action in kets. Three lives, and right answers in a row score double, triple, then four times. Keys H, X, Y, Z and S answer, and Enter moves on.

Manual Gate quiz

The sphere

The Bloch sphere draws a pure qubit as a unit vector.

a|0⟩ + b|1⟩ → (2 Re a^*b, 2 Im a^*b, |a|² − |b|²)

State	Points along
\|0⟩ and \|1⟩	+z and −z, north and south
\|+⟩ and \|−⟩	+x and −x
\|+i⟩ and \|−i⟩	+y and −y

A global phase does not move the vector, so X|−⟩ = −|−⟩ is drawn where |−⟩ is. Questions that ask for an exact state say so, as with X|0⟩ = |1⟩ against Y|0⟩ = i|1⟩.

The gates

Each gate turns the sphere about an axis.

Gate	Turn
X	half turn about x
Y	half turn about y
Z	half turn about z
H	half turn about (x + z)/√2, halfway between x and z
S	quarter turn about z, taking \|+⟩ to \|+i⟩

A wrong answer costs a life, and each answer shows the gate and what it does. The same sphere is drawn larger, with gates you can press, on the qubit page.