diff --git a/src/qibo/models/circuit.py b/src/qibo/models/circuit.py
index b909bb8e56..4aff6fd184 100644
--- a/src/qibo/models/circuit.py
+++ b/src/qibo/models/circuit.py
@@ -43,9 +43,6 @@ class _Queue(list):
     def __init__(self, nqubits):
         super().__init__(self)
         self.nqubits = nqubits
-        self.moments = [nqubits * [None]]
-        self.moment_index = nqubits * [0]
-        self.nmeasurements = 0
 
     def to_fused(self):
         """Transform all gates in queue to :class:`qibo.gates.FusedGate`."""
@@ -88,24 +85,30 @@ def from_fused(self):
                 queue.append(gate.gates[0])
         return queue
 
-    def append(self, gate: gates.Gate):
-        super().append(gate)
-        if gate.qubits:
-            qubits = gate.qubits
-        else:  # special gate acting on all qubits
-            qubits = tuple(range(self.nqubits))
-
-        if isinstance(gate, gates.M):
-            self.nmeasurements += 1
-
-        # calculate moment index for this gate
-        idx = max(self.moment_index[q] for q in qubits)
-        for q in qubits:
-            if idx >= len(self.moments):
-                # Add a moment
-                self.moments.append(len(self.moments[-1]) * [None])
-            self.moments[idx][q] = gate
-            self.moment_index[q] = idx + 1
+    @property
+    def nmeasurements(self):
+        return len(list(filter(lambda gate: isinstance(gate, gates.M), self)))
+
+    @property
+    def moments(self):
+        moments = [self.nqubits * [None]]
+        moment_index = self.nqubits * [0]
+        for gate in self:
+            qubits = (
+                gate.qubits
+                if not isinstance(gate, gates.CallbackGate)
+                else tuple(range(self.nqubits))  # special gate acting on all qubits
+            )
+
+            # calculate moment index for this gate
+            idx = max(moment_index[q] for q in qubits)
+            for q in qubits:
+                if idx >= len(moments):
+                    # Add a moment
+                    moments.append(len(moments[-1]) * [None])
+                moments[idx][q] = gate
+                moment_index[q] = idx + 1
+        return moments
 
 
 class Circuit:
diff --git a/tests/test_models_circuit.py b/tests/test_models_circuit.py
index 8d83b9d83e..5181f57a61 100644
--- a/tests/test_models_circuit.py
+++ b/tests/test_models_circuit.py
@@ -19,8 +19,10 @@ def test_parametrizedgates_class():
 
 
 def test_queue_class():
+    from qibo.callbacks import EntanglementEntropy
     from qibo.models.circuit import _Queue
 
+    entropy = EntanglementEntropy([0])
     queue = _Queue(4)
     gatelist = [
         gates.H(0),
@@ -29,12 +31,14 @@ def test_queue_class():
         gates.H(2),
         gates.CNOT(1, 2),
         gates.Y(3),
+        gates.CallbackGate(entropy),
     ]
     for g in gatelist:
         queue.append(g)
     assert queue.moments == [
         [gatelist[0], gatelist[1], gatelist[3], gatelist[5]],
         [gatelist[2], gatelist[4], gatelist[4], None],
+        [gatelist[6] for _ in range(4)],
     ]