Performance improvement of _DiagonalEstimator (#9229)

* performance improvement of _DiagonalEstimator * optimize Co-authored-by: Jake Lishman <[email protected]> * add reno Co-authored-by: Jake Lishman <[email protected]> Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com> (cherry picked from commit 0df0d29)
Qiskit · Dec 5, 2022 · 214b028 · 214b028
1 parent 778f840
commit 214b028
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diff --git a/qiskit/algorithms/minimum_eigensolvers/diagonal_estimator.py b/qiskit/algorithms/minimum_eigensolvers/diagonal_estimator.py
@@ -181,19 +181,14 @@ def aggregate(measurements):
     return aggregate
 
 
+_PARITY = np.array([-1 if bin(i).count("1") % 2 else 1 for i in range(256)], dtype=np.complex128)
+
+
 def _evaluate_sparsepauli(state: int, observable: SparsePauliOp) -> complex:
-    return sum(
-        coeff * _evaluate_bitstring(state, paulistring)
-        for paulistring, coeff in observable.label_iter()
-    )
-
-
-def _evaluate_bitstring(state: int, paulistring: str) -> float:
-    """Evaluate a bitstring on a Pauli label."""
-    n = len(paulistring) - 1
-    return np.prod(
-        [-1 if state & (1 << (n - i)) else 1 for i, pauli in enumerate(paulistring) if pauli == "Z"]
-    )
+    packed_uint8 = np.packbits(observable.paulis.z, axis=1, bitorder="little")
+    state_bytes = np.frombuffer(state.to_bytes(packed_uint8.shape[1], "little"), dtype=np.uint8)
+    reduced = np.bitwise_xor.reduce(packed_uint8 & state_bytes, axis=1)
+    return np.sum(observable.coeffs * _PARITY[reduced])
 
 
 def _check_observable_is_diagonal(observable: SparsePauliOp) -> bool:

diff --git a/releasenotes/notes/fix-sampling-vqe-performance-b5bfe92c2d3e10ab.yaml b/releasenotes/notes/fix-sampling-vqe-performance-b5bfe92c2d3e10ab.yaml
@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixed a performance bug that :class:`.SamplingVQE` evaluated the energies of eigenstates
+    in a slow way.