Pass to remove diagonal gates before measurement

[1ucian0]

This pass removes gates RZ and Z before measurement.

[jaygambetta]

why not T, S Tdg, Sdg, U1 etc. Can we make this remove diagonal gates.

[1ucian0]

T, S Tdg, Sdg and U1 added (and pass renamed). Is there any other diagonal gate?

[jaygambetta]

We could also remove control Z if the measurement is on both the qubits.

[1ucian0]

Updated to include CzGate. Are CrzGate and Cu1Gate animals of the same species?

[chriseclectic]

Yes, and also RZZGate.

[1ucian0]

CrzGate, Cu1Gate, and RZZGate added!

[ajavadia]

Consider this example:

from qiskit import QuantumRegister, QuantumCircuit, ClassicalRegister
from qiskit.transpiler import PassManager, transpile
from qiskit.transpiler.passes import RemoveDiagonalGatesBeforeMeasure, DAGFixedPoint

qr = QuantumRegister(2, 'qr')
cr = ClassicalRegister(1, 'cr')
circuit = QuantumCircuit(qr, cr)
circuit.rz(0.1, qr[1]).c_if(cr, 1)
circuit.barrier()
circuit.h(qr[0])
circuit.measure(qr[0], cr[0])

pm = PassManager()
pm.append(RemoveDiagonalGatesBeforeMeasure())
new_circuit = pm.run(circuit)

Here, this circuit:

                     ░ ┌───┐┌─┐
qr_0: |0>────────────░─┤ H ├┤M├
         ┌─────────┐ ░ └───┘└╥┘
qr_1: |0>┤ Rz(0.1) ├─░───────╫─
         └─┬──┴──┬─┘ ░       ║ 
 cr_0: 0 ══╡ = 1 ╞═══════════╩═
           └─────┘             

gets transformed into:

          ░ ┌───┐┌─┐
qr_0: |0>─░─┤ H ├┤M├
          ░ └───┘└╥┘
qr_1: |0>─░───────╫─
          ░       ║ 
 cr_0: 0 ═════════╩═
                    

which is incorrect. The reason, as I explained above, is that the crz gate is a predecessor of the measure, so it gets picked up by your algorithm. However, it is not a qubit predecessor, thus it should be ignored.

[1ucian0]

good catch about the classical predecessor situation!

[ajavadia]

Similarly when you check successors, you should only care about qubits. Consider this example:

from qiskit import QuantumRegister, QuantumCircuit, ClassicalRegister
from qiskit.transpiler import PassManager, transpile
from qiskit.transpiler.passes import RemoveDiagonalGatesBeforeMeasure, DAGFixedPoint

qr = QuantumRegister(3, 'qr')
cr = ClassicalRegister(2, 'cr')
circuit = QuantumCircuit(qr, cr)
circuit.crz(0.1, qr[0], qr[1]).c_if(cr, 1)
circuit.h(qr[2]).c_if(cr, 3)
circuit.measure(qr[0], cr[0])
circuit.measure(qr[1], cr[1])

pm = PassManager()
pm.append(RemoveDiagonalGatesBeforeMeasure())
new_circuit = pm.run(circuit)

This circuit:

Remains unchanged.

Note that the CRz gate can legitimately be removed, because it is followed immediately by measures.

The reason why it fails here is that the check for successors is showing the Ch gate as a successor -- but this is only due to the clbits.

[ajavadia]

In general I think going with DAG.nodes_on_wire() might be a better option here to only check the qubit wires.

[1ucian0]

Your second example can also be addressed with quantum_predecessors. I think is cleaner. Thoughts?

[ajavadia]

This looks good now! the quantum predecessors do the trick.