Fix UnitarySynthesis for 3+ qubits when compiled for a backend

[Cryoris]

Summary

When encountering a 3+ qubit UnitaryGate, UnitarySynthesis currently replaces the whole dag with the definition of the block instead of appending it, see

qiskit/crates/accelerate/src/unitary_synthesis.rs

Line 375 in 079f69f

out_dag = synth_dag;

Details and comments

This also leads to #13586 because the qubit/clbit interners are overwritten, but maybe more importantly this can lead to issues when multiple 3-qubit blocks are present in the circuit. E.g. running

import numpy as np
from qiskit.quantum_info import Operator
from qiskit.circuit.library import UnitaryGate
from qiskit import QuantumCircuit, transpile
from qiskit_ibm_runtime.fake_provider import FakeSherbrooke

backend = FakeSherbrooke()

x = QuantumCircuit(3)
x.x(2)
x_mat = Operator(x)

qc = QuantumCircuit(3, 1)
qc.unitary(x_mat, range(3))
qc.unitary(np.eye(2 ** 3), range(3))

tqc = transpile(qc, backend, optimization_level=0)
print(tqc.count_ops())

produces an empty circuit, since the last block implements the identity.

Some additional tests, such as the precise case of #13586 and a series of 3q UnitaryGates, should be added.

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 12434705540

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 23 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.01%) to 88.941%

---

Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	5	91.73%
crates/qasm2/src/parse.rs	18	96.23%

Totals
Change from base Build 12420636821:	-0.01%
Covered Lines:	79419
Relevant Lines:	89294

---

💛 - Coveralls

[alexanderivrii]

Suggested change

	Fixed a bug in the :class:`.UnitarySynthesis` transpiler pass, where blocks of
	:class:`.UnitaryGate`\s of 3 qubits or more were not correctly synthesized.
	This lead e.g. to the circuit being overwritten with the last processed block or
	to internal panics when encountering measurements after a such a block.
	Fixed `#13586 <https://github.com/Qiskit/qiskit/issues/13586>`__.
	Fixed a bug in the :class:`.UnitarySynthesis` transpiler pass, where blocks of
	:class:`.UnitaryGate`\s on 3 qubits or more were not correctly synthesized.
	This led, e.g., to the circuit being overwritten with the last processed block or
	to internal panics when encountering measurements after such a block.
	Fixed `#13586 <https://github.com/Qiskit/qiskit/issues/13586>`__.

[alexanderivrii]

Thanks for the fix, @Cryoris!

Looks good! (modulo some minor spell-checking nitpicks about the release notes)

Some additional tests, such as the precise case of #13586 and a series of 3q UnitaryGates, should be added.

Are you planning to add a test for that (like the one you have in the code snippet in the description)?