Merge pull request #872 from qiboteam/stinespring

`Stinespring` superoperator representation

doc/source/api-reference/qibo.rst

@@ -1469,6 +1469,12 @@ Choi to Chi-matrix
 .. autofunction:: qibo.quantum_info.choi_to_chi
 
 
+Choi to Stinespring
+"""""""""""""""""""
+
+.. autofunction:: qibo.quantum_info.choi_to_stinespring
+
+
 Kraus to Choi
 """""""""""""
 
@@ -1493,6 +1499,12 @@ Kraus to Chi-matrix
 .. autofunction:: qibo.quantum_info.kraus_to_chi
 
 
+Kraus to Stinespring
+""""""""""""""""""""
+
+.. autofunction:: qibo.quantum_info.kraus_to_stinespring
+
+
 Liouville to Choi
 """""""""""""""""
 
@@ -1528,6 +1540,12 @@ Liouville to Chi-matrix
 .. autofunction:: qibo.quantum_info.liouville_to_chi
 
 
+Liouville to Stinespring
+""""""""""""""""""""""""
+
+.. autofunction:: qibo.quantum_info.liouville_to_stinespring
+
+
 Pauli-Liouville to Liouville
 """"""""""""""""""""""""""""
 
@@ -1553,6 +1571,12 @@ Pauli-Liouville to Chi-matrix
 .. autofunction:: qibo.quantum_info.pauli_to_chi
 
 
+Pauli-Liouville to Stinespring
+""""""""""""""""""""""""""""""
+
+.. autofunction:: qibo.quantum_info.pauli_to_stinespring
+
+
 Chi-matrix to Choi
 """"""""""""""""""
 
@@ -1595,6 +1619,42 @@ Chi-matrix to Kraus
     it in its default setting (``validate_cp=True``).
 
 
+Chi-matrix to Stinespring
+"""""""""""""""""""""""""
+
+.. autofunction:: qibo.quantum_info.chi_to_stinespring
+
+
+Stinespring to Choi
+"""""""""""""""""""
+
+.. autofunction:: qibo.quantum_info.stinespring_to_choi
+
+
+Stinespring to Liouville
+""""""""""""""""""""""""
+
+.. autofunction:: qibo.quantum_info.stinespring_to_liouville
+
+
+Stinespring to Pauli-Liouville
+""""""""""""""""""""""""""""""
+
+.. autofunction:: qibo.quantum_info.stinespring_to_pauli
+
+
+Stinespring to Kraus
+""""""""""""""""""""
+
+.. autofunction:: qibo.quantum_info.stinespring_to_kraus
+
+
+Stinespring to Chi-matrix
+"""""""""""""""""""""""""
+
+.. autofunction:: qibo.quantum_info.stinespring_to_chi
+
+
 Kraus operators as probabilistic sum of unitaries
 """""""""""""""""""""""""""""""""""""""""""""""""
 

src/qibo/gates/channels.py

@@ -1,7 +1,7 @@
 """Define quantum channels."""
 from itertools import product
 from math import exp, sqrt
-from typing import Tuple
+from typing import Optional, Tuple
 
 import numpy as np
 
@@ -39,7 +39,7 @@ def apply(self, backend, state, nqubits):  # pragma: no cover
     def apply_density_matrix(self, backend, state, nqubits):
         return backend.apply_channel_density_matrix(self, state, nqubits)
 
-    def to_choi(self, nqubits: int = None, order: str = "row", backend=None):
+    def to_choi(self, nqubits: Optional[int] = None, order: str = "row", backend=None):
         """Returns the Choi representation :math:`\\mathcal{E}`
         of the Kraus channel :math:`\\{K_{\\alpha}\\}_{\\alpha}`.