From f35bac8212c5b491b6c252050eaf7a0f94f9d9e6 Mon Sep 17 00:00:00 2001
From: TimWeaving <weaving.timothy@gmail.com>
Date: Tue, 28 Nov 2023 11:35:42 +0000
Subject: [PATCH] Expectation value is a real quantity

---
 symmer/operators/base.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/symmer/operators/base.py b/symmer/operators/base.py
index 39ea7780..80c0c54f 100644
--- a/symmer/operators/base.py
+++ b/symmer/operators/base.py
@@ -830,7 +830,7 @@ def expval(self, psi: "QuantumState") -> complex:
             complex: The expectation value.
         """
         if self.n_terms > psi.n_terms:
-            return psi.dagger * self * psi
+            return (psi.dagger * self * psi).real
         else:
             if self.n_terms > 1:
                 @process.parallelize
@@ -841,7 +841,7 @@ def f(P, 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],