ElidePermutation undoes routing

[ajavadia]

Environment

• Qiskit version: 1.3 dev
• Python version: 3.10
• Operating system: mac

What is happening?

Circuits that are mapped and routed can become unmapped unintentionally due to ElidePermutation treating inserted SWAPs as unwanted.

How can we reproduce the issue?

Suppose i ran some routing passes and got the following circuit on a line of 4 qubits. Then i just want to bring it down to some basis gates. Optimization level 2 and 3 (due to ElidePermutation) will undo the mapping.

circuit_routed = QuantumCircuit(4)
circuit_routed.cx(0, 1)
circuit_routed.h(1)
circuit_routed.swap(1, 2)
circuit_routed.cx(2, 3)
print(circuit_routed)

pm = generate_preset_pass_manager(basis_gates=['cx', 'sx', 'rz'],  routing_method='none', optimization_level=2)
circuit_basis = pm.run(circuit_routed)

circuit_basis = pm.run(circuit_routed)
print(circuit_basis)

gives

q_0: ──■───────────────
     ┌─┴─┐┌───┐        
q_1: ┤ X ├┤ H ├─X──────
     └───┘└───┘ │      
q_2: ───────────X───■──
                  ┌─┴─┐
q_3: ─────────────┤ X ├
                  └───┘
global phase: π/4
                                                                             
Qubit(QuantumRegister(4, 'q'), 0) -> 0 ──■───────────────────────────────────
                                       ┌─┴─┐┌─────────┐┌────┐┌─────────┐     
Qubit(QuantumRegister(4, 'q'), 1) -> 1 ┤ X ├┤ Rz(π/2) ├┤ √X ├┤ Rz(π/2) ├──■──
                                       └───┘└─────────┘└────┘└─────────┘  │  
Qubit(QuantumRegister(4, 'q'), 2) -> 2 ───────────────────────────────────┼──
                                                                        ┌─┴─┐
Qubit(QuantumRegister(4, 'q'), 3) -> 3 ─────────────────────────────────┤ X ├
                                                                        └───┘

What should happen?

routing_method='none' should disable ElidePermutation (or anything that wants to remove swaps)

Any suggestions?

No response