Fix Statevector.evolve modifying input state in-place (Fixes 15750) (backport #15905)

[mergify]

Summary

Statevector.evolve(gate) should leave the input state untouched. However, the input state is currently modified with DiagonalGate as an argument (see #15750). This is caused by a shallow copy and an in-place-array modification when gate has a global phase. This PR adds failing tests, a fix for the failing tests, and a reno release note for the bug fix.

Details and comments

Proof and cause of bug

Statevector.evolve calls Statevector._evolve_instruction where the following code is run (lines 1029-1030)

        if obj.definition.global_phase:
            statevec._data *= np.exp(1j * float(obj.definition.global_phase))

This is an in-place modification of statevec._data. However, when Statevector.evolve is called, a shallow copy is made (line 461). This means that a gate with a global phase modifies the original input state. This is verified with the following short script:

from qiskit.quantum_info import Statevector
from qiskit.circuit.library import DiagonalGate
import numpy as np

gate = DiagonalGate([1, -1])
print(f"Global Phase of instruction is {gate.definition.global_phase:0.3f} rad")

sv = Statevector.from_label("1")
sv_original = sv.copy()
_ = sv.evolve(gate)

np.testing.assert_allclose(sv_original.data, sv.data)

Possible fixes and implemented fix

1. (Implemented fix) Reassign statevec._data when applying the global phase, instead of an in-place multiplication. This comes at the cost of copying the _data array but only when the instruction has a global phase.
2. Deep copy instead of a shallow copy. Instead of _copy.copy (shallow copy) at the beginning of Statevector.evolve, we could call self.copy(). However, this causes test.python.primitives.test_statevector_estimator.TestStatevectorEstimator.test_reset to fail. I tracked this down to the internal rng being copied and the behaviour for resets changing. Instead of fixing the deep copy of the rng, the first fix was chosen.

Implemented Tests

I implemented four tests which verify that five different instructions do not modify the input state to Statevector.evolve. Only the one with DiagonalGate failed. With the fix it passes.

1. XGate: Passes.
2. CXGate: Passes.
3. Operator(XGate()): Passes.
4. QuantumCircuit (Bell-state preparation): Passes.
5. DiagonalGate([1.0, -1.0, -1.0, 1.0]): Failed but now passes.

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   This is an automatic backport of pull request Fix Statevector.evolve modifying input state in-place (Fixes 15750) #15905 done by Mergify.

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[coveralls]

Coverage Report for CI Build 26231217034

Coverage increased (+1.1%) to 88.822%

Details

• Coverage increased (+1.1%) from the base build.
• Patch coverage: 1 of 1 lines across 1 file are fully covered (100%).
• 18 coverage regressions across 4 files.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

18 previously-covered lines in 4 files lost coverage.

File	Lines Losing Coverage	Coverage
crates/qasm2/src/lex.rs	7	91.26%
crates/qasm2/src/parse.rs	6	97.15%
crates/circuit/src/parameter/symbol_expr.rs	4	74.38%
crates/circuit/src/parameter/parameter_expression.rs	1	90.53%

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Coverage Stats

Relevant Lines:	87973
Covered Lines:	78139
Line Coverage:	88.82%
Coverage Strength:	1365084.92 hits per line

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💛 - Coveralls