QuantumCircuit .clear() and .copy_empty_like() functionality

[pedrorrivero]

What should we add?

Sometimes it can be useful to erase all instructions in a QuantumCircuit. This request is for:

1. Clear functionality to erase all instructions from a QuantumCircuit → .clear().
   We not only need to clear all data, but (at least) also update the parameter table:

self.data.clear()
self._parameter_table.clear()

2. Copy empty functionality to get a cleared copy of a QuantumCircuit → .copy_empty_like().
   This functionality is already implemented for DAGCircuit, and should retain all relevant circuit properties other than the instructions and associated parameters (e.g. registers, bits, name, metadata). Notice that quantum_circuit.copy().clear() would give the desired outcome, however .copy() performs a deepcopy of the circuit, which we would then erase almost completely when deleting the instructions, so this method should provide a more efficient way to do this.

TODO:

• .clear()
• .copy_empty_like()

[pedrorrivero]

@mtreinish

[Cryoris]

Sounds like these could be useful, do you need this for e.g. copying attached calibrations or what's the usecase you have in mind? 🙂

[pedrorrivero]

@Cryoris my particular use case is building new circuits based on an original one. For instance to duplicate instructions:

def build_new_circuit(circuit: QuantumCircuit) -> QuantumCircuit:
    new_circuit = circuit.copy_empty_like()
    for instruction, qargs, cargs in circuit:
        new_circuit.append(instruction, qargs, cargs)
        new_circuit.append(instruction, qargs, cargs)
    return new_circuit

As you can see, it is important to retain the same quantum and classical registers, but also all relevant metadata as you mention. The easiest way forward is to create a cleared copy of the circuit.

I would therefore aim for qc.copy_empty_like() to be logically equivalent to qc.copy().clear(), but more efficient. I think this is the same behavior as in DAGCircuit.