Update Split2QUnitaries to handle SWAP unitary gates #13531
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| def test_2q_swap_with_1_qubit_gates(self): | ||
| """Test that a 2q unitary matching a swap gate with 1-qubit gates before and after is correctly processed.""" | ||
| qc = QuantumCircuit(2) | ||
| qc.h(0) | ||
| qc.x(1) | ||
| qc.swap(0,1) | ||
| qc.sx(0) | ||
| qc.sdg(1) | ||
| qc.global_phase += 1.2345 | ||
| qc_split = QuantumCircuit(2) | ||
| qc_split.append(UnitaryGate(Operator(qc)), [0, 1]) | ||
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| pm = PassManager() | ||
| pm.append(Collect2qBlocks()) | ||
| pm.append(ConsolidateBlocks()) | ||
| pm.append(Split2QUnitaries()) | ||
| res = pm.run(qc_split) | ||
| res_op = Operator.from_circuit(res, final_layout=res.layout.final_layout) |
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Can you add a test that has this in it as part of a larger circuit that runs through the full transpilation pipeline? I want to make sure that we validating that we're tracking and composing the final permutations correctly.
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Done. Also added a smaller test which does non-trivial swapping (with swap gates removed using the elide permutation pass) before the unitary test which surfaced a possible bug in the layout composition phase. We need to discuss whether in this phase we should compose on the existing layout or (as was the case before my change) on its inverse.
| self.assertTrue(expected_op.equiv(res_op)) | ||
| self.assertTrue( | ||
| matrix_equal(expected_op.data, res_op.data, ignore_phase=False) | ||
| ) |
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Can we add an assertion here that swap has been removed from the output?
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I've added self.assertNotIn("swap", res.count_ops()) but I don't think it's enough - there's also the possibility that we didn't split the unitary at all (note that it's also not being checked in the tests relating to the original split unitary code).
Any idea how to easily verify that the unitary was removed? Note that just using count_ops is tricky since we have a unitary both before (as the 2-qubit unitary) and after (as two individual 1-qubit unitaries). I can compare the count itself (1 before vs 2 after) but it feel less robust than it should.
| self.assertTrue(expected_op.equiv(res_op)) | ||
| self.assertTrue( | ||
| matrix_equal(expected_op.data, res_op.data, ignore_phase=False) | ||
| ) |
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I would add an assertion that we've removed the swap gate from the circuit. This test could still pass if there was no transformation on the input circuit.
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If I recall correctly, we don't run the |
Summary
Updates the
Split2QUnitariestranspiler pass to handle 2-qubit unitary gates which can be decomposed as aswapgate and 1-qubit gates.Closes #13476
Details and comments
This PR extends the
Split2QUnitariesin the following manner:Split2QUnitariesperforms, it checks whether any unitary encountered is a SWAP gate.Split2QUnitaries) and instead of adding SWAP gates, performs a virtual swap by dynamically changing the qubit layout (affecting subsequent gates and the final layout) similar to what theElidePermutationsdoes.Since step 2 requires generating a new DAG (as opposed to what
Split2QUnitariesdoes to identity-equivalent unitary gates), it is not performed during the initial pass, only if SWAP gates were detected.