From b3c29a11d3c82285fb464e730f04c10d9107549b Mon Sep 17 00:00:00 2001
From: Dan Mills <52407433+daniel-mills-cqc@users.noreply.github.com>
Date: Tue, 10 Dec 2024 15:35:31 +0000
Subject: [PATCH] Remove commute_coeff

---
 qermit/noise_model/noise_model.py  | 18 ++++++++++-----
 qermit/noise_model/qermit_pauli.py | 30 +------------------------
 tests/noise_model_test.py          | 36 ------------------------------
 3 files changed, 14 insertions(+), 70 deletions(-)

diff --git a/qermit/noise_model/noise_model.py b/qermit/noise_model/noise_model.py
index 5c465706..fd182e5f 100644
--- a/qermit/noise_model/noise_model.py
+++ b/qermit/noise_model/noise_model.py
@@ -115,10 +115,14 @@ def to_ptm(self) -> Tuple[NDArray, Dict[Tuple[Pauli, ...], int]]:
                     pauli_iterable=error,
                     qubit_list=[Qubit(i) for i in range(self.n_qubits)],
                 )
-
-                ptm[index][index] += error_rate * QermitPauli.commute_coeff(
-                    pauli_one=pauli, pauli_two=error_pauli
+                commute_coeff = (
+                    1
+                    if pauli.qubit_pauli_tensor.commutes_with(
+                        error_pauli.qubit_pauli_tensor
+                    )
+                    else -1
                 )
+                ptm[index][index] += error_rate * commute_coeff
 
         # Some checks that the form of the PTM is correct.
         identity = tuple(Pauli.I for _ in range(self.n_qubits))
@@ -190,8 +194,12 @@ def from_ptm(
                     pauli_iterable=pauli_two_tuple,
                     qubit_list=[Qubit(i) for i in range(len(pauli_two_tuple))],
                 )
-                commutation_matrix[index_one][index_two] = QermitPauli.commute_coeff(
-                    pauli_one=pauli_one, pauli_two=pauli_two
+                commutation_matrix[index_one][index_two] = (
+                    1
+                    if pauli_one.qubit_pauli_tensor.commutes_with(
+                        pauli_two.qubit_pauli_tensor
+                    )
+                    else -1
                 )
 
         error_rate_list = np.matmul(ptm.diagonal(), np.linalg.inv(commutation_matrix))
diff --git a/qermit/noise_model/qermit_pauli.py b/qermit/noise_model/qermit_pauli.py
index a05fdd9d..98f9ec45 100644
--- a/qermit/noise_model/qermit_pauli.py
+++ b/qermit/noise_model/qermit_pauli.py
@@ -48,34 +48,6 @@ def __init__(
         self.phase = phase
         self.qubit_list = qubit_list
 
-    @staticmethod
-    def commute_coeff(pauli_one: QermitPauli, pauli_two: QermitPauli) -> int:
-        """Calculate the coefficient which result from commuting pauli_one
-        past pauli_two. That is to say P_2 P_1 = c P_1 P_2 where c is the
-        coefficient returned by this function.
-
-        :param pauli_one: First Pauli
-        :param pauli_two: Second Pauli
-        :raises Exception: Raised if the Paulis do not act
-            on matching qubits.
-        :return: Coefficient resulting from commuting the two Paulis.
-        """
-        if not pauli_one.qubit_list == pauli_two.qubit_list:
-            raise Exception(
-                "The given Paulis must act on the same qubits. "
-                + f"In this case the qubits acted on by pauli_one {pauli_one.qubit_list} "
-                + f"differ from those of pauli_two {pauli_two.qubit_list}."
-            )
-        power = sum(
-            pauli_one.X_list[qubit] * pauli_two.Z_list[qubit]
-            for qubit in pauli_one.qubit_list
-        )
-        power += sum(
-            pauli_one.Z_list[qubit] * pauli_two.X_list[qubit]
-            for qubit in pauli_one.qubit_list
-        )
-        return (-1) ** power
-
     def is_measureable(self, qubit_list: List[Qubit]) -> bool:
         """Checks if this Pauli would be measurable on the given qubits in the
         computational bases. That is to say if at least one  Pauli on the given
@@ -208,7 +180,7 @@ def __hash__(self):
         return hash(key)
 
     def __str__(self) -> str:
-        return str(self.qubit_pauli_tensor)
+        return str(self.qubit_pauli_tensor.string) + str(self.qubit_pauli_tensor.coeff)
 
     def __eq__(self, other: object) -> bool:
         """Checks for equality by checking all qubits match, and that all
diff --git a/tests/noise_model_test.py b/tests/noise_model_test.py
index 60fb942d..26a5a6d4 100644
--- a/tests/noise_model_test.py
+++ b/tests/noise_model_test.py
@@ -112,42 +112,6 @@ def test_qermit_pauli_from_iterable() -> None:
     pauli.qubit_pauli_tensor.coeff == 1 + 0j
 
 
-def test_qermit_pauli_commute_coeff() -> None:
-    # This tests a few commutation coefficients
-    # which have been verified by hand.
-    verified_list = [
-        # Single qubit Paulis
-        ((([0], [1]), ([1], [0])), -1),
-        ((([1], [1]), ([1], [1])), 1),
-        ((([1], [0]), ([0], [1])), -1),
-        ((([0], [0]), ([0], [1])), 1),
-        ((([1], [0]), ([0], [0])), 1),
-        ((([1], [1]), ([0], [1])), -1),
-        # Two qubit Paulis
-        ((([0, 1], [1, 0]), ([1, 0], [0, 1])), 1),
-        ((([0, 1], [1, 0]), ([0, 0], [1, 1])), -1),
-        ((([0, 0], [0, 0]), ([0, 0], [1, 1])), 1),
-    ]
-
-    for verified in verified_list:
-        n_qubits = len(verified[0][0][0])
-
-        pauli_one = QermitPauli(
-            Z_list=verified[0][0][0],
-            X_list=verified[0][0][1],
-            qubit_list=[Qubit(i) for i in range(n_qubits)],
-        )
-        pauli_two = QermitPauli(
-            Z_list=verified[0][1][0],
-            X_list=verified[0][1][1],
-            qubit_list=[Qubit(i) for i in range(n_qubits)],
-        )
-        assert (
-            QermitPauli.commute_coeff(pauli_one=pauli_one, pauli_two=pauli_two)
-            == verified[1]
-        )
-
-
 def test_noise_model_logical_error_propagation() -> None:
     pytket_ciruit = Circuit(2).H(0).CX(0, 1).measure_all()