Which Measurement

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Each screen names some states to distinguish, or a circuit to identify.
Pick the one measurement that does the job in a single shot, with no error.
Where a screen says keep, the measurement must not break that property.
8 questions and three lives. Right answers in a row score double, triple, then four times. Keys 1 to 4 answer, and Enter moves on.

Manual Measurement quiz

Most hardware reads qubits in the Z basis, so every other measurement is a rotation followed by a Z reading.

To read	Apply, then read Z
Z basis	nothing
X basis	H
Y basis	S†, then H
Bell basis	CNOT, then H on the control

A parity check reads one bit about a pair through an ancilla. It projects onto the +1 or −1 eigenspace, so a state already inside one, such as a Bell state, is left untouched while other states change. That is why parity checks can tell Bell states apart without breaking them, and why error correction is built from parity checks.