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Add option to not always normalise #227

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Merged
merged 1 commit into from
Aug 28, 2025
Merged

Add option to not always normalise #227

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10 changes: 8 additions & 2 deletions tensorcircuit/quantum.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1645,6 +1645,7 @@ def reduced_density_matrix(
state: Union[Tensor, QuOperator],
cut: Union[int, List[int]],
p: Optional[Tensor] = None,
normalize: bool = True,
) -> Union[Tensor, QuOperator]:
"""
Compute the reduced density matrix from the quantum state ``state``.
Expand All @@ -1658,6 +1659,7 @@ def reduced_density_matrix(
:type p: Optional[Tensor]
:return: The reduced density matrix.
:rtype: Union[Tensor, QuOperator]
:normalize: if True, returns a trace 1 density matrix. Otherwise does not normalize.
"""
if isinstance(cut, list) or isinstance(cut, tuple) or isinstance(cut, set):
traceout = list(cut)
Expand Down Expand Up @@ -1700,7 +1702,9 @@ def reduced_density_matrix(
rho = backend.reshape(
rho, [2 ** (freedom - len(traceout)), 2 ** (freedom - len(traceout))]
)
rho /= backend.trace(rho)
if normalize:
rho /= backend.trace(rho)


else:
w = state / backend.norm(state)
Expand All @@ -1715,7 +1719,9 @@ def reduced_density_matrix(
rho = w @ backend.adjoint(w)
else:
rho = w @ backend.diagflat(p) @ backend.adjoint(w)
rho /= backend.trace(rho)
if normalize:
rho /= backend.trace(rho)

return rho


Expand Down