Wrap library circuits

[Cryoris]

Summary

Consistently wrap all library circuits into a gate, so that they appear as blocks when composing to another circuit.

Details and comments

Currently some library circuits are wrapped in a circuit block, some are not. This PR ensures that all are wrapped, so that

• circuit diagrams are clearer as you now directly see the library circuits as blocks
• we can defer the synthesis of the circuit, letting the compiler choose an optimal implementation
• closes compose should not unroll (by default) when using the circuit library #6147

Examples

qc = QuantumCircuit(3)
qc.initialize(np.eye(8)[:, 3], range(3))
qc.compose(QFT(3).inverse(), inplace=True)

     ┌──────────────────────────────┐┌─────────┐
q_0: ┤0                             ├┤0        ├
     │                              ││         │
q_1: ┤1 initialize(0,0,0,1,0,0,0,0) ├┤1 QFT_dg ├
     │                              ││         │
q_2: ┤2                             ├┤2        ├
     └──────────────────────────────┘└─────────┘

Grover:

oracle = QuantumCircuit(2, name='Oracle')
oracle.cz(0, 1)

grover = GroverOperator(oracle)

qc = QuantumCircuit(3)
qc.h(range(3))
qc.compose(grover, inplace=True)

     ┌───┐┌────┐
q_0: ┤ H ├┤0   ├
     ├───┤│  Q │
q_1: ┤ H ├┤1   ├
     ├───┤└────┘
q_2: ┤ H ├──────
     └───┘

Amplitude estimation:

state_in = QuantumCircuit(1)
state_in.ry(0.2, 0)

oracle = QuantumCircuit(1)
oracle.z(0)

grover = GroverOperator(oracle, state_in)

ae = PhaseEstimation(3, grover)
ae.compose(state_in, [3], front=True, inplace=True)

                   ┌──────┐
eval_0: ───────────┤0     ├
                   │      │
eval_1: ───────────┤1     ├
                   │  QPE │
eval_2: ───────────┤2     ├
        ┌─────────┐│      │
   q_0: ┤ RY(0.2) ├┤3     ├
        └─────────┘└──────┘

and decomposed once:

            ┌───┐                             ┌──────┐
eval_0: ────┤ H ├────────■────────────────────┤2     ├
            ├───┤        │                    │      │
eval_1: ────┤ H ├────────┼───────■────────────┤1 QFT ├
            ├───┤        │       │            │      │
eval_2: ────┤ H ├────────┼───────┼───────■────┤0     ├
        ┌───┴───┴────┐┌──┴───┐┌──┴───┐┌──┴───┐└──────┘
   q_0: ┤ R(0.2,π/2) ├┤ Q**1 ├┤ Q**2 ├┤ Q**4 ├────────
        └────────────┘└──────┘└──────┘└──────┘

Todo

• Wrap the blueprint circuits
• Wrap newcomers to the circuit library (like arithmetic circuits)