Qubits & Selection

All gate operations start with selecting one or more qubits. QuantumJS provides several fluent selectors on the circuit object Q.

Selectors

Q.bit(0)          // Single qubit by index
Q.bits([0, 2])    // Multiple qubits by index array
Q.all()           // All qubits in the active scope
Q.first()         // Qubit at index 0
Q.last()          // Qubit at the highest index

These return a QBitProxy, which is where you chain gate calls.

Chaining Gates

Selectors return a proxy that lets you chain any number of gates:

Q.bit(0).h().x().y().z();   // Apply H, X, Y, Z to qubit 0 in sequence
Q.all().measure();            // Measure every qubit

Multi-Qubit Operations

When selecting multiple qubits with .bits() or .all(), single-qubit gates are broadcast to each qubit in the selection:

// Applies H to q[0], q[1], and q[2]
Q.bits([0, 1, 2]).h();

For controlled gates (like cx), indices are paired one-to-one between control and target selections.

Scoped Selectors

Inside staircase loops (see Staircase Loops), first(), last(), and all() resolve relative to the current sub-circuit scope:

Q.shrinkUp(q => {
  q.first().cx(q.last()); // first/last within the shrinking scope
});

Classical Bits

Use Q.cbit() to reference the classical register for conditional operations:

Q.cbit(0)        // Classical bit c[0]
Q.cbit()         // Whole classical register c

See Conditionals for usage with _if().