Skip to main content

Conditionals

QuantumJS supports classical conditionals — gates that execute only if a classical bit holds a specific value. This is used in mid-circuit measurement and quantum teleportation protocols.

_if() with Callback​

Pass a callback to conditionally execute a block of gates:

circuit({ qubits: 2 }, Q => {
  Q.bit(0).h();
  Q.bit(1).measure();

  // Apply X to q[0] only if c[0] == 1
  Q.bit(0)._if(Q.cbit(0), q => {
    q.x();
  });
});

_if() on Previous Gate​

Without a callback, _if() conditions the immediately preceding gate:

circuit({ qubits: 2 }, Q => {
  Q.bit(0).h();
  Q.bit(1).x()._if(Q.cbit(0)); // x on q[1] only if c[0] == 1
});

Classical Bit Reference​

Q.cbit(0)   // c[0] — specific classical bit
Q.cbit()    // c   — whole classical register

By default the condition checks == 1. You can modify it:

const cb = Q.cbit(0);
cb.isTrue();   // c[0] == 1 (default)
cb.isFalse();  // c[0] == 0

QASM Output​

OpenQASM 3.0:

if (c[0] == 1) {
  x q[1];
}

OpenQASM 2.0:

if(c[0]==1) x q[1];