Merge pull request #173 from UCL-CCS/expval

When state has few terms compared with operator, faster to do direct…
UCL-CCS · Nov 28, 2023 · 7c96dd6 · 7c96dd6
2 parents f182e2c + f35bac8
commit 7c96dd6
Showing 1 changed file with 11 additions and 8 deletions.
diff --git a/symmer/operators/base.py b/symmer/operators/base.py
@@ -829,16 +829,19 @@ def expval(self, psi: "QuantumState") -> complex:
         Returns:
             complex: The expectation value.
         """
-        if self.n_terms > 1:
-            @process.parallelize
-            def f(P, psi):
-                return single_term_expval(P, psi)
-
-            expvals = np.array(f(self, psi))
+        if self.n_terms > psi.n_terms:
+            return (psi.dagger * self * psi).real
         else:
-            expvals = np.array(single_term_expval(self, psi))
+            if self.n_terms > 1:
+                @process.parallelize
+                def f(P, psi):
+                    return single_term_expval(P, psi)
+
+                expvals = np.array(f(self, psi))
+            else:
+                expvals = np.array(single_term_expval(self, psi))
 
-        return np.sum(expvals * self.coeff_vec)
+            return np.sum(expvals * self.coeff_vec).real
 
     def __mul__(self, 
             mul_obj: Union["PauliwordOp", "QuantumState", complex],