Fix XEB error rate computation (quantumlib#6673)

* Fix XEB error rate computation

* Update tests

Author: eliottrosenberg

cirq/experiments/two_qubit_xeb.py

@@ -143,7 +143,9 @@ def _record(self, q0, q1) -> pd.Series:
 
     def xeb_fidelity(self, q0: 'cirq.GridQubit', q1: 'cirq.GridQubit') -> float:
         """Return the XEB fidelity of a qubit pair."""
-        return self._record(q0, q1).layer_fid
+        return noise_utils.decay_constant_to_xeb_fidelity(
+            self._record(q0, q1).layer_fid, num_qubits=2
+        )
 
     def xeb_error(self, q0: 'cirq.GridQubit', q1: 'cirq.GridQubit') -> float:
         """Return the XEB error of a qubit pair."""
@@ -177,8 +179,7 @@ def pauli_error(self) -> Dict[Tuple['cirq.GridQubit', 'cirq.GridQubit'], float]:
         """Return the Pauli error of all qubit pairs."""
         return {
             pair: noise_utils.decay_constant_to_pauli_error(
-                noise_utils.xeb_fidelity_to_decay_constant(self.xeb_fidelity(*pair), num_qubits=2),
-                num_qubits=2,
+                self._record(*pair).layer_fid, num_qubits=2
             )
             for pair in self.all_qubit_pairs
         }

cirq/experiments/two_qubit_xeb_test.py

@@ -148,10 +148,10 @@ def test_plotting(sampler, qubits, ax):
 @pytest.mark.parametrize(
     'q0,q1,pauli',
     [
-        (cirq.GridQubit(4, 4), cirq.GridQubit(5, 4), 1 / 8),
-        (cirq.GridQubit(5, 3), cirq.GridQubit(6, 3), 1 / 4),
-        (cirq.GridQubit(4, 3), cirq.GridQubit(5, 3), 0.8 + 3 / 40),
-        (cirq.GridQubit(6, 3), cirq.GridQubit(6, 4), 5 / 8),
+        (cirq.GridQubit(4, 4), cirq.GridQubit(5, 4), 0.09374999999999997),
+        (cirq.GridQubit(5, 3), cirq.GridQubit(6, 3), 0.18749999999999994),
+        (cirq.GridQubit(4, 3), cirq.GridQubit(5, 3), 0.65625),
+        (cirq.GridQubit(6, 3), cirq.GridQubit(6, 4), 0.46875),
     ],
 )
 def test_pauli_error(q0: cirq.GridQubit, q1: cirq.GridQubit, pauli: float):
@@ -174,10 +174,10 @@ def pauli_error(self) -> Dict[cirq.Qid, float]:
 @pytest.mark.parametrize(
     'q0,q1,pauli',
     [
-        (cirq.GridQubit(4, 4), cirq.GridQubit(5, 4), 1 / 8 - 0.03),
-        (cirq.GridQubit(5, 3), cirq.GridQubit(6, 3), 1 / 4 - 0.09),
-        (cirq.GridQubit(4, 3), cirq.GridQubit(5, 3), 0.8 + 3 / 40 - 0.08),
-        (cirq.GridQubit(6, 3), cirq.GridQubit(6, 4), 5 / 8 - 0.13),
+        (cirq.GridQubit(4, 4), cirq.GridQubit(5, 4), 0.09374999999999997 - 0.03),
+        (cirq.GridQubit(5, 3), cirq.GridQubit(6, 3), 0.18749999999999994 - 0.09),
+        (cirq.GridQubit(4, 3), cirq.GridQubit(5, 3), 0.65625 - 0.08),
+        (cirq.GridQubit(6, 3), cirq.GridQubit(6, 4), 0.46875 - 0.13),
     ],
 )
 def test_inferred_pauli_error(q0: cirq.GridQubit, q1: cirq.GridQubit, pauli: float):
@@ -191,10 +191,10 @@ def test_inferred_pauli_error(q0: cirq.GridQubit, q1: cirq.GridQubit, pauli: flo
 @pytest.mark.parametrize(
     'q0,q1,xeb',
     [
-        (cirq.GridQubit(4, 4), cirq.GridQubit(5, 4), 0.076),
-        (cirq.GridQubit(5, 3), cirq.GridQubit(6, 3), 0.128),
-        (cirq.GridQubit(4, 3), cirq.GridQubit(5, 3), 0.636),
-        (cirq.GridQubit(6, 3), cirq.GridQubit(6, 4), 0.396),
+        (cirq.GridQubit(4, 4), cirq.GridQubit(5, 4), 0.050999999999999934),
+        (cirq.GridQubit(5, 3), cirq.GridQubit(6, 3), 0.07799999999999996),
+        (cirq.GridQubit(4, 3), cirq.GridQubit(5, 3), 0.46099999999999997),
+        (cirq.GridQubit(6, 3), cirq.GridQubit(6, 4), 0.2709999999999999),
     ],
 )
 def test_inferred_xeb_error(q0: cirq.GridQubit, q1: cirq.GridQubit, xeb: float):
@@ -224,10 +224,10 @@ def test_inferred_single_qubit_pauli():
 @pytest.mark.parametrize(
     'q0,q1,pauli',
     [
-        (cirq.GridQubit(4, 4), cirq.GridQubit(5, 4), 1 / 8),
-        (cirq.GridQubit(5, 3), cirq.GridQubit(6, 3), 1 / 4),
-        (cirq.GridQubit(4, 3), cirq.GridQubit(5, 3), 0.8 + 3 / 40),
-        (cirq.GridQubit(6, 3), cirq.GridQubit(6, 4), 5 / 8),
+        (cirq.GridQubit(4, 4), cirq.GridQubit(5, 4), 0.09374999999999997),
+        (cirq.GridQubit(5, 3), cirq.GridQubit(6, 3), 0.18749999999999994),
+        (cirq.GridQubit(4, 3), cirq.GridQubit(5, 3), 0.65625),
+        (cirq.GridQubit(6, 3), cirq.GridQubit(6, 4), 0.46875),
     ],
 )
 def test_inferred_two_qubit_pauli(q0: cirq.GridQubit, q1: cirq.GridQubit, pauli: float):