Fixing inconsist usage of DirMPS when describing canonical form

pytket/extensions/cutensornet/structured_state/mps.py

@@ -38,7 +38,12 @@
 
 
 class DirMPS(Enum):
-    """An enum to refer to relative directions within the MPS."""
+    """An enum to refer to relative directions within the MPS.
+
+    When used to refer to the canonical form of a tensor, LEFT means that its conjugate
+    transpose is its inverse when connected to its left bond and physical bond.
+    Similarly for RIGHT.
+    """
 
     LEFT = 0
     RIGHT = 1

pytket/extensions/cutensornet/structured_state/mps_gate.py

@@ -284,8 +284,8 @@ def _apply_2q_unitary_nonadjacent(
             optimize={"path": [(0, 1)]},
         )
 
-        # The site tensor is now in canonical form (since S is contracted to the right)
-        self.canonical_form[l_pos] = DirMPS.RIGHT  # type: ignore
+        # The site tensor is now in canonical form
+        self.canonical_form[l_pos] = DirMPS.LEFT  # type: ignore
 
         # Next, "push" the `msg_tensor` through all site tensors between `l_pos`
         # and `r_pos`. Once again, this is just contract_decompose on each.
@@ -306,7 +306,7 @@ def _apply_2q_unitary_nonadjacent(
             )
 
             # The site tensor is now in canonical form
-            self.canonical_form[pos] = DirMPS.RIGHT  # type: ignore
+            self.canonical_form[pos] = DirMPS.LEFT  # type: ignore
 
         # Finally, contract the `msg_tensor` with the site tensor in `r_pos` and the
         # `r_gate_tensor` from the decomposition of `gate_tensor`
@@ -402,7 +402,7 @@ def _apply_2q_unitary_nonadjacent(
             # Since we are contracting S to the "left" in `svd_method`, the site tensor
             # at `pos+1` is canonicalised, whereas the site tensor at `pos` is the one
             # where S has been contracted to and, hence, is not in canonical form
-            self.canonical_form[pos + 1] = DirMPS.LEFT  # type: ignore
+            self.canonical_form[pos + 1] = DirMPS.RIGHT  # type: ignore
             self.canonical_form[pos] = None
             # Update fidelity lower bound
             this_fidelity = 1.0 - info.svd_info.discarded_weight