diff --git a/src/qibotn/backends/qmatchatea.py b/src/qibotn/backends/qmatchatea.py
index 74e29a3..9353a34 100644
--- a/src/qibotn/backends/qmatchatea.py
+++ b/src/qibotn/backends/qmatchatea.py
@@ -76,7 +76,7 @@ class QMatchaTeaBackend(QibotnBackend):
         circuit,
         initial_state=None,
         nshots=None,
-        prob_type="U",
+        prob_type=None,
         return_array=False,
         **prob_kwargs,
     ):
@@ -123,6 +123,10 @@ class QMatchaTeaBackend(QibotnBackend):
                 f"Backend {self.name}-{self.platform} currently does not support initial state.",
             )
 
+        if prob_type == None:
+            prob_type = "U"
+            prob_kwargs = {"num_samples": 500}
+
         # To be sure the setup is correct and no modifications have been done
         self._setup_qmatchatea_backend()
 
diff --git a/src/qibotn/result.py b/src/qibotn/result.py
index 74c3f5a..b6d0b52 100644
--- a/src/qibotn/result.py
+++ b/src/qibotn/result.py
@@ -1,3 +1,4 @@
+from copy import deepcopy
 from dataclasses import dataclass
 from typing import Union
 
@@ -24,12 +25,13 @@ class TensorNetworkResult:
     def probabilities(self):
         """Return calculated probabilities according to the given method."""
         if self.prob_type == "U":
+            measured_probabilities = deepcopy(self.measured_probabilities)
             for bitstring in self.measured_probabilities[self.prob_type]:
-                self.measured_probabilities[self.prob_type][bitstring] = (
+                measured_probabilities[self.prob_type][bitstring] = (
                     self.measured_probabilities[self.prob_type][bitstring][1]
                     - self.measured_probabilities[self.prob_type][bitstring][0]
                 )
-            probabilities = self.measured_probabilities[self.prob_type]
+            probabilities = measured_probabilities[self.prob_type]
         else:
             probabilities = self.measured_probabilities[self.prob_type]
         return probabilities