Instruction should include qubits it acts on

What is the expected enhancement?

Qiskit currently lacks a class to represent the concept “a gate together with the qubits it acts on.” This concept is useful because it is the basic unit a quantum circuit is composed of. For example, it can be the type of object appended to a circuit, so that QuantumCircuit.append can accept a single argument instead of multiple. It can also be used to simplify circuit construction in other ways; for example, circuit = QuantumCircuit(instructions) where instructions is an iterable of instructions (in fact it could be an “iterable tree” of instructions). After some discussion with @ajavadia we agree that the Instruction class could be updated to serve this purpose.

[kevinsung]

Thanks @jakelishman, it’s good to know there is work being done on this front. In circuit = QuantumCircuit(instructions) I meant that instructions could have type Iterable[Instruction] where Instruction is the (proposed) class that represents “gate + qubits” which does not actually exist in Qiskit currently. By “iterable tree” I meant that it could in fact be a recursive type InstructionTree = Union[Instruction, Iterable[InstructionTree]] because this can be flattened into a list of instructions. The qubit ordering wouldn’t be an issue if the user refers to the qubits directly rather than by index.

[jakelishman]

Closing this as it’s not the direction that’s been taken for the data model, because we’re separating the concept of “operation” (Instruction, in this case) from its runtime operands (qubits, clbits, some parameters, etc). This is to keep better composibility throughout the library, which will (hopefully) be helping our internal memory usage and avoid a lot of spurious copying in the library.