arrange_swaps#

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

Flip SWAP qubit order so that unused qubits come first.

A SWAP gate decomposes into three CX gates. When a SWAP involves a qubit that has not yet been touched by any prior operation, that qubit is in the |0⟩ state, and the first CX in the decomposition is controlled on |0⟩ — a no-op.

This pass reorders the qubits of each SWAP so that any untouched qubit appears first. This exposes the redundant CX to downstream passes such as cancel_zero_controls(), which can then remove it.

After reordering, SWAPs that involve two untouched qubits are dropped entirely (a SWAP between |0⟩ states does nothing). SWAPs where both qubits have already been used are left in their original order.

Parameters:

qcQuantumCircuit: Input quantum circuit.

Returns:

QuantumCircuit: Circuit with SWAP qubit order flipped where beneficial.

Examples

A SWAP(0, 1) where qubit 1 hasn’t been touched yet.

Setup:

>>> from qrisp import QuantumCircuit, PassManager, decompose
>>> from qrisp import arrange_swaps, cancel_zero_controls
>>>
>>> qc = QuantumCircuit(2)
>>> qc.x(0)
>>> qc.swap(0, 1)          # qubit 1 is untouched

Without arrange_swaps, the SWAP decomposes into three CX gates. The first CX targets the unused qubit (qubit 1), but since that qubit is still in |0⟩ the CX is a no-op — it remains in the circuit anyway:

>>> pm_raw = PassManager()
>>> pm_raw += decompose()
>>> print(pm_raw.run(qc))
      ┌───┐     ┌───┐
qb_0: ┤ X ├──■──┤ X ├──■──
      └───┘┌─┴─┐└─┬─┘┌─┴─┐
qb_1: ─────┤ X ├──■──┤ X ├
           └───┘     └───┘

With arrange_swaps the qubit order is flipped so the unused qubit comes first. cancel_zero_controls then removes the first CX (controlled on |0⟩). After decomposition, one CX is gone:

>>> pm = PassManager()
>>> pm += arrange_swaps
>>> pm += decompose()
>>> pm += cancel_zero_controls
>>> print(pm.run(qc))
      ┌───┐     ┌───┐
qb_0: ┤ X ├──■──┤ X ├
      └───┘┌─┴─┐└─┬─┘
qb_1: ─────┤ X ├──■──
           └───┘

A SWAP between two untouched qubits is dropped entirely (a SWAP between |0⟩ states is the identity):

>>> qc = QuantumCircuit(2)
>>> qc.swap(0, 1)
>>> result = pm.run(qc)
>>> print(pm.run(qc))

qb_0:

qb_1: