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Measurement

Measurement collapses qubits to classical bits and is required to read results from a circuit.

Basic Measurement​

Q.bit(0).measure();     // Measures q[0] → c[0]
Q.all().measure();      // Measures all qubits (one-to-one mapping)
Q.bit(1).measureTo(0);  // Measures q[1] → c[0] explicitly

When measuring a single qubit with .measure(), it maps to the classical bit at the same index (c[0] for q[0], etc). The classical register grows automatically to accommodate.

Basis Measurement (Bloch Tomography)​

To measure in a basis other than the computational (Z) basis, QuantumJS applies the appropriate basis-change gates before measuring:

Q.bit(0).measureX(); // H → measure         (X basis)
Q.bit(0).measureY(); // S† → H → measure    (Y basis)
Q.bit(0).measureZ(); // measure              (Z basis, default)
Q.bit(0).measureW(); // S → H → T → H → measure  (W basis)
Q.bit(0).measureV(); // S → H → T† → H → measure (V basis)

These are useful for quantum state tomography experiments.

Measuring to a Specific Classical Bit​

Q.bit(2).measureTo(1); // q[2] → c[1]

You can also pass a CBitProxy:

Q.bit(2).measureTo(Q.cbit(1)); // equivalent

Whole Register Measurement​

Q.all().measure(); // q → c (full register, one-to-one)

This emits a single bulk measure statement in OpenQASM 3.0:

c = measure q;

Or in OpenQASM 2.0:

measure q -> c;