Merge pull request #996 from qiboteam/stinespring

Include `stinespring` representation in `random_quantum_channel`

src/qibo/quantum_info/random_ensembles.py

@@ -4,8 +4,6 @@
 from typing import Optional, Union
 
 import numpy as np
-from scipy import stats
-from scipy.linalg import expm
 
 from qibo import Circuit, gates
 from qibo.backends import GlobalBackend, NumpyBackend
@@ -16,6 +14,7 @@
     choi_to_kraus,
     choi_to_liouville,
     choi_to_pauli,
+    choi_to_stinespring,
     vectorization,
 )
 
@@ -192,6 +191,8 @@ def random_unitary(dims: int, measure: Optional[str] = None, seed=None, backend=
         R = np.diag(D)
         unitary = np.dot(Q, R)
     elif measure is None:
+        from scipy.linalg import expm
+
         H = random_hermitian(dims, seed=seed, backend=NumpyBackend())
         unitary = expm(-1.0j * H / 2)
         unitary = backend.cast(unitary, dtype=unitary.dtype)
@@ -206,6 +207,9 @@ def random_quantum_channel(
     order: str = "row",
     normalize: bool = False,
     precision_tol: Optional[float] = None,
+    validate_cp: bool = True,
+    nqubits: Optional[int] = None,
+    initial_state_env=None,
     seed=None,
     backend=None,
 ):
@@ -220,7 +224,8 @@ def random_quantum_channel(
             returns Liouville representation. If ``"pauli"``, returns Pauli-Liouville
             representation. If "pauli-<pauli_order>" or "chi-<pauli_order>", (e.g. "pauli-IZXY"),
             returns it in the Pauli basis with the corresponding order of single-qubit Pauli elements
-            (see :func:`qibo.quantum_info.pauli_basis`). Defaults to ``"liouville"``.
+            (see :func:`qibo.quantum_info.pauli_basis`). If ``"stinespring"``,
+            returns random channel in the Stinespring representation. Defaults to ``"liouville"``.
         measure (str, optional): probability measure in which to sample the unitary
             from. If ``None``, functions returns :math:`\\exp{(-i \\, H)}`, where
             :math:`H` is a Hermitian operator. If ``"haar"``, returns an Unitary
@@ -236,6 +241,21 @@ def random_quantum_channel(
             problem. Any eigenvalue :math:`\\lambda <` ``precision_tol`` is set
             to 0 (zero). If ``None``, ``precision_tol`` defaults to
             ``qibo.config.PRECISION_TOL=1e-8``. Defaults to ``None``.
+        validate_cp (bool, optional): used when ``representation="stinespring"``.
+            If ``True``, checks if the Choi representation of superoperator
+            used as intermediate step is a completely positive map.
+            If ``False``, it assumes that it is completely positive (and Hermitian).
+            Defaults to ``True``.
+        nqubits (int, optional): used when ``representation="stinespring"``.
+            Total number of qubits in the system that is interacting with
+            the environment. Must be equal or greater than the number of
+            qubits that Kraus representation of the system superoperator acts on.
+            If ``None``, defaults to the number of qubits in the Kraus operators.
+            Defauts to ``None``.
+        initial_state_env (ndarray, optional): used when ``representation="stinespring"``.
+            Statevector representing the initial state of the enviroment.
+            If ``None``, it assumes the environment in its ground state.
+            Defaults to ``None``.
         seed (int or :class:`numpy.random.Generator`, optional): Either a generator of
             random numbers or a fixed seed to initialize a generator. If ``None``,
             initializes a generator with a random seed. Defaults to ``None``.
@@ -252,7 +272,14 @@ def random_quantum_channel(
             f"representation must be type str, but it is type {type(representation)}",
         )
 
-    if representation not in ["chi", "choi", "kraus", "liouville", "pauli"]:
+    if representation not in [
+        "chi",
+        "choi",
+        "kraus",
+        "liouville",
+        "pauli",
+        "stinespring",
+    ]:
         if (
             ("chi-" not in representation and "pauli-" not in representation)
             or len(representation.split("-")) != 2
@@ -296,6 +323,16 @@ def random_quantum_channel(
             pauli_order=pauli_order,
             backend=backend,
         )
+    elif representation == "stinespring":
+        super_op = choi_to_stinespring(
+            super_op,
+            precision_tol=precision_tol,
+            order=order,
+            validate_cp=validate_cp,
+            nqubits=nqubits,
+            initial_state_env=initial_state_env,
+            backend=backend,
+        )
 
     return super_op
 

tests/test_quantum_info_random.py

@@ -148,7 +148,16 @@ def test_random_unitary(backend):
 @pytest.mark.parametrize("measure", [None, "haar"])
 @pytest.mark.parametrize(
     "representation",
-    ["chi", "chi-IZXY", "choi", "kraus", "liouville", "pauli", "pauli-IZXY"],
+    [
+        "chi",
+        "chi-IZXY",
+        "choi",
+        "kraus",
+        "liouville",
+        "pauli",
+        "pauli-IZXY",
+        "stinespring",
+    ],
 )
 def test_random_quantum_channel(backend, representation, measure):
     with pytest.raises(TypeError):