Isometry decomposition does not maintain gate width

The definition of an Isometry instruction does not always have the same number of qubits as the original instruction. e.g.

>>> qc = qk.QuantumCircuit(2)
>>> qc.iso(np.eye(4), [0, 1], [])
<qiskit.circuit.instructionset.InstructionSet object at 0x10b4bfe80>
>>> print(qc)
        ┌──────┐
q_0: |0>┤0     ├
        │  iso │
q_1: |0>┤1     ├
        └──────┘
>>> print(qc.decompose())
        ┌────┐
q_0: |0>┤ Id ├
        └────┘
q_1: |0>──────

This also occurs for a handful of the examples from https://github.com/Qiskit/qiskit-terra/blob/576b984/test/python/circuit/test_isometry.py#L36 . This confuses e.g. DAGCircuit.substitute_node which expects node replacements to keep the same shape.

[Cryoris]

I’m not quite convinced this is really a bug, is it? I would imagine that if you have a gate defined on a set of qubit but it doesn’t actually do anything then shouldn’t it decompose to no operation at all? 🙂

[kdk]

I don’t think this is an issue anymore (though I’m not exactly sure what’s changed to resolve it). If I try to reproduce the original issue:

>>> qc = qk.QuantumCircuit(2)
>>> qc.iso(np.eye(4), [0, 1], [])
>>> print(qc)
     ┌───────────┐
q_0: ┤0          ├
     │  Isometry │
q_1: ┤1          ├
     └───────────┘
>>> print(qc.decompose())
     ┌──────────────────┐
q_0: ┤0                 ├
     │  circuit-2630_dg │
q_1: ┤1                 ├
     └──────────────────┘
>>> print(qc.decompose().decompose())
     
q_0: 
     
q_1: 
     
>>> qk.transpile(qc, basis_gates=['u', 'cx'], translation_method='translator').draw()

q_0: 
     
q_1: 
     
>>> qk.transpile(qc, basis_gates=['u', 'cx'], translation_method='unroller').draw()
    
q_0: 
     
q_1: 
     

it looks like Isometry no longer generates Ids on some of the qubits (which was weird, but not necessarily a bug). If I try to reproduce the original substitute_node_with_dag issue:

>>> qc = qk.QuantumCircuit(2)
>>> qc.x(0)

>>> qc2 = qk.QuantumCircuit(2)
>>> qc2.append(qc.to_gate(), [0,1])
>>> qc2.draw()
     ┌───────────────┐
q_0: ┤0              ├
     │  circuit-2642 │
q_1: ┤1              ├
     └───────────────┘
>>> qc2.decompose().draw()
     ┌───┐
q_0: ┤ X ├
     └───┘
q_1: ─────
          
>>> qk.transpile(qc, basis_gates=['u', 'cx'], translation_method='translator').draw()
     ┌──────────┐
q_0: ┤ U(π,0,π) ├
     └──────────┘
q_1: ────────────
                 
>>> qk.transpile(qc, basis_gates=['u', 'cx'], translation_method='unroller').draw()
     ┌──────────┐
q_0: ┤ U(π,0,π) ├
     └──────────┘
q_1: ────────────
                 

things behave as expected, so I think this can be closed.