correctly updating global phase when removing -I gates from the circuit

[alexanderivrii]

Summary

This fixes a minor bug in the RemoveIdentityEquivalent transpiler pass (the pass was introduced in #12384), where unitary gates close to -I were correctly removed from the circuit but the global phase of the circuit was not updated. This includes, for instance, unitary gates with matrix close to [[-1, 0], [0, -1]] as well as RX(2 * pi) and similar gates.

Fixes #13778.

Thanks to @Cryoris and @ShellyGarion for joint debugging of the problem.

[qiskit-bot]

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

• @Qiskit/terra-core

[alexanderivrii]

Update: there is another part of the pass dealing with RX, RY, RZ and RXXGate, RYYGate, RZZGate, RZXGate gates that I should look at to see if a similar problem is possible that is similarly fixed in b7713a1.

[coveralls]

Pull Request Test Coverage Report for Build 13140017109

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This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 15 of 15 (100.0%) changed or added relevant lines in 1 file are covered.
• 17 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.009%) to 88.837%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	94.23%
crates/qasm2/src/lex.rs	4	91.73%
crates/qasm2/src/parse.rs	12	97.15%

Totals
Change from base Build 13135899673:	-0.009%
Covered Lines:	79742
Relevant Lines:	89762

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💛 - Coveralls

[Cryoris]

Thanks for tackling this!

I would expect this needs to cover the full spectrum of operations that are the identity up to a global phase, i.e. exp(i phi) * I 🙂 This could be done e.g. by just looking at the first entry of the matrix and extracting the phase (similar to how it is done in Operator.equiv).