fix: Correct conjugate transpose in apply_unitary_density_bmm#316
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yurekami wants to merge 1 commit into
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fix: Correct conjugate transpose in apply_unitary_density_bmm#316yurekami wants to merge 1 commit into
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Fixes mit-han-lab#254 The apply_unitary_density_bmm function was computing only the conjugate of the matrix (U*) instead of the conjugate transpose (U†) when applying ρ → U ρ U†. For the Hermitian adjoint U†, we need both: - Element-wise complex conjugation - Matrix transposition Before: matdag = torch.conj(mat) # Only conjugate After: matdag = torch.conj(mat.permute(...)) # Conjugate + transpose This fix corrects the density matrix evolution for all gates. Files fixed: - torchquantum/functional/gate_wrapper.py (line 301) - torchquantum/density/density_func.py (line 210) The einsum implementation was already correct; this aligns the bmm implementation with it. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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Summary
Fixes #254
This PR fixes a critical bug in the
apply_unitary_density_bmmfunction where the density matrix evolution ρ → U ρ U† was computed incorrectly.The Bug
The code was computing only the conjugate of the matrix instead of the conjugate transpose (Hermitian adjoint):
For U† (Hermitian adjoint), we need both:
Example
As reported in the issue, applying Y gate to |00⟩⟨00| was giving wrong results:
Files Changed
torchquantum/functional/gate_wrapper.py- line 301torchquantum/density/density_func.py- line 210Note
The einsum implementation (
apply_unitary_density_einsum) was already correct. This fix aligns the bmm implementation with it.🤖 Generated with Claude Code