cancel_zero_controls#

cancel_zero_controls(qc: QuantumCircuit) → QuantumCircuit[source]#

Cancel controlled gates whose control qubit is guaranteed to be |0⟩.

Every qubit starts in |0⟩ (and re-enters |0⟩ after qb_alloc). A controlled gate conditioned on |1⟩ acting on such a qubit is a no-op. For symmetric controlled-phase gates (CP, CZ) the gate is a no-op if either qubit is |0⟩.

This is particularly effective on QFT-style circuits, where many controlled-phase gates act on qubits that have not yet been touched.

Parameters:

qcQuantumCircuit: The input circuit.

Returns:

QuantumCircuit: A new circuit with redundant controlled gates removed.

Examples

Cancel a CX whose control qubit is still in |0⟩:

>>> from qrisp import QuantumCircuit, PassManager
>>> from qrisp import cancel_zero_controls
>>> qc = QuantumCircuit(2)
>>> qc.cx(0, 1)  # Qubit 0 starts in \|0⟩ — the CX is a no-op
>>> qc.h(1)       # Now qubit 1 is marked as used
>>> print(qc)
qb_58: ──■───────
       ┌─┴─┐┌───┐
qb_59: ┤ X ├┤ H ├
       └───┘└───┘
>>> pm = PassManager()
>>> pm += cancel_zero_controls
>>> optimized_qc = pm.run(qc)
>>> print(optimized_qc)

qb_58: ─────
       ┌───┐
qb_59: ┤ H ├
       └───┘

Symmetric controlled-phase gates (CZ, CP) cancel when either qubit is |0⟩:

>>> qc = QuantumCircuit(2)
>>> qc.cz(0, 1)   # both qubits start in \|0⟩ — CZ is a no-op
>>> pm = PassManager()
>>> pm += cancel_zero_controls
>>> optimized = pm.run(qc)
>>> print(optimized)

qb_60:

qb_61: