Fix density matrix trace drift after measurement collapse#8171
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anshjaiswal12 wants to merge 3 commits into
Open
Fix density matrix trace drift after measurement collapse#8171anshjaiswal12 wants to merge 3 commits into
anshjaiswal12 wants to merge 3 commits into
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Post-measurement normalization by outcome probability can leave the density matrix with trace slightly off 1; reusing it as initial_state compounds the error. Renormalize by trace after collapse and add regression tests that check trace at each simulation step. Fixes quantumlib#5916
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Pull post-measurement trace correction into _renormalize_density_matrix_trace, use np.isclose for the zero-trace guard, and deduplicate the quantumlibgh-5916 regression circuit in tests.
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## main #8171 +/- ##
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- Coverage 99.60% 99.60% -0.01%
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Files 1118 1118
Lines 101356 101409 +53
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+ Hits 100956 101004 +48
- Misses 400 405 +5 ☔ View full report in Codecov by Harness. 🚀 New features to boost your workflow:
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Collapse _assert_density_matrix_trace_one signature to one line to satisfy the incremental format check on CI.
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Summary
After a measurement collapse,
measure_density_matrixdivides by the outcome probability. Floating-point error can leave the density matrix with trace slightly ≠ 1. Reusingfinal_density_matrixasinitial_statecompounds that drift (gh-5916).Renormalize by trace after collapse and add regression tests.
Fixes #5916
Tests
test_density_matrix_trace_after_each_moment_gh5916test_density_matrix_trace_stable_under_repeated_simulationtest_density_matrix_trace_stable_across_circuit_familiestest_density_matrix_trace_repeated_simulation_performance_smokeAI disclosure: Cursor IDE was used only to read/search the codebase and help debug the issue. All code in this PR was written and reviewed by me.