feat: using int indices in place of letters

src/qibo/quantum_info/basis.py

@@ -1,5 +1,4 @@
 from itertools import product
-from string import ascii_letters
 from typing import Optional
 
 import numpy as np
@@ -93,15 +92,18 @@ def pauli_basis(
 
     pauli_labels = {"I": matrices.I, "X": matrices.X, "Y": matrices.Y, "Z": matrices.Z}
     basis_single = backend.cast([pauli_labels[label] for label in pauli_order])
+    if backend.name == "tensorflow":
+        einsum = np.einsum
+    else:
+        einsum = backend.np.einsum
 
     if nqubits > 1:
         dim = 2**nqubits
-        letters = [ascii_letters[3 * i : 3 * (i + 1)] for i in range(nqubits)]
-        lhs = ",".join(letters)
-        rhs = "".join([letter for group in zip(*letters) for letter in group])
-        basis_full = backend.np.einsum(
-            f"{lhs}->{rhs}", *[basis_single for _ in range(nqubits)]
-        ).reshape(4**nqubits, dim, dim)
+        input_indices = [range(3 * i, 3 * (i + 1)) for i in range(nqubits)]
+        output_indices = (i for indices in zip(*input_indices) for i in indices)
+        operands = [basis_single for _ in range(nqubits)]
+        inputs = [item for pair in zip(operands, input_indices) for item in pair]
+        basis_full = einsum(*inputs, output_indices).reshape(4**nqubits, dim, dim)
     else:
         basis_full = basis_single