Add classical_renyi_relative_entropy and renyi_relative_entropy to quantum_info.entropies

doc/source/api-reference/qibo.rst

@@ -1271,7 +1271,7 @@ Callbacks
 Callbacks provide a way to calculate quantities on the state vector as it
 propagates through the circuit. Example of such quantity is the entanglement
 entropy, which is currently the only callback implemented in
-:class:`qibo.callbacks.EntanglementEntropy`.
+:class:`qibo.callbacks.Entanglemententropy`.
 The user can create custom callbacks by inheriting the
 :class:`qibo.callbacks.Callback` class. The point each callback is
 calculated inside the circuit is defined by adding a :class:`qibo.gates.CallbackGate`.
@@ -1284,7 +1284,7 @@ This can be added similarly to a standard gate and does not affect the state vec
 Entanglement entropy
 ^^^^^^^^^^^^^^^^^^^^
 
-.. autoclass:: qibo.callbacks.EntanglementEntropy
+.. autoclass:: qibo.callbacks.Entanglemententropy
    :members:
    :member-order: bysource
 
@@ -1537,18 +1537,24 @@ Shannon entropy
 .. autofunction:: qibo.quantum_info.shannon_entropy
 
 
-Classical Relative Entropy
+Classical relative entropy
 """"""""""""""""""""""""""
 
 .. autofunction:: qibo.quantum_info.classical_relative_entropy
 
 
-Classical Rényi Entropy
+Classical Rényi entropy
 """""""""""""""""""""""
 
 .. autofunction:: qibo.quantum_info.classical_renyi_entropy
 
 
+Classical Rényi relative entropy
+""""""""""""""""""""""""""""""""
+
+.. autofunction:: qibo.quantum_info.classical_relative_renyi_entropy
+
+
 Entropy
 """""""
 
@@ -1562,7 +1568,7 @@ Entropy
     and ``state`` is non-Hermitian, an error will be raised when using `cupy` backend.
 
 
-Relative Entropy
+Relative entropy
 """"""""""""""""
 
 .. autofunction:: qibo.quantum_info.relative_entropy
@@ -1576,13 +1582,19 @@ Relative Entropy
     an error will be raised when using `cupy` backend.
 
 
-Rényi Entropy
+Rényi entropy
 """""""""""""
 
 .. autofunction:: qibo.quantum_info.renyi_entropy
 
 
-Entanglement Entropy
+Rényi relative entropy
+""""""""""""""""""""""
+
+.. autofunction:: qibo.quantum_info.renyi_relative_entropy
+
+
+Entanglement entropy
 """"""""""""""""""""
 
 .. autofunction:: qibo.quantum_info.entanglement_entropy

src/qibo/quantum_info/entropies.py

@@ -170,7 +170,7 @@
     Another special case is the limit :math:`\\alpha \\to 0`, where the function is
     reduced to :math:`\\log\\left(|\\mathbf{p}|\\right)`, with :math:`|\\mathbf{p}|`
     being the support of :math:`\\mathbf{p}`.
-    This is knows as the `Hartley entropy <https://en.wikipedia.org/wiki/Hartley_function>`
+    This is knows as the `Hartley entropy <https://en.wikipedia.org/wiki/Hartley_function>`_
     (also known as *Hartley function* or *max-entropy*).
 
     In the limit :math:`\\alpha \\to \\infty`, the function reduces to
@@ -238,6 +238,106 @@
     return renyi_ent
 
 
+def classical_relative_renyi_entropy(
+    prob_dist_p, prob_dist_q, alpha: Union[float, int], base: float = 2, backend=None
+):
+    """Calculates the classical relative Rényi entropy between two discrete probability distributions.
+
+    This function is also known as
+    `Rényi divergence <https://en.wikipedia.org/wiki/R%C3%A9nyi_entropy#R%C3%A9nyi_divergence>`_.
+
+    For :math:`\\alpha \\in (0, \\, 1) \\cup (1, \\, \\infty)` and probability distributions
+    :math:`\\mathbf{p}` and :math:`\\mathbf{q}`, the classical relative Rényi entropy is defined as
+
+    .. math::
+        H_{\\alpha}(\\mathbf{p} \\, \\| \\, \\mathbf{q}) = \\frac{1}{\\alpha - 1} \\,
+            \\log\\left( \\sum_{x} \\, \\frac{\\mathbf{p}^{\\alpha}(x)}
+            {\\mathbf{q}^{\\alpha - 1}(x)} \\right) \\, .
+
+    A special case is the limit :math:`\\alpha \\to 1`, in which the classical Rényi divergence
+    coincides with the :func:`qibo.quantum_info.entropies.classical_relative_entropy`.
+
+    Another special case is the limit :math:`\\alpha \\to 1/2`, where the function is
+    reduced to :math:`-2 \\log\\left(\\sum_{x} \\, \\sqrt{\\mathbf{p}(x) \\, \\mathbf{q}(x)} \\right)`.
+    The sum inside the :math:`\\log` is known as the
+    `Bhattacharyya coefficient <https://en.wikipedia.org/wiki/Bhattacharyya_distance>`_.
+
+    In the limit :math:`\\alpha \\to \\infty`, the function reduces to
+    :math:`\\log(\\max_{x}(\\mathbf{p}(x) \\, \\mathbf{q}(x))`.
+
+    Args:
+        prob_dist_p (ndarray or list): discrete probability distribution :math:`p`.
+        prob_dist_q (ndarray or list): discrete probability distribution :math:`q`.
+        alpha (float or int): order of the Rényi entropy.
+        base (float): the base of the log. Defaults to  :math:`2`.
+        backend (:class:`qibo.backends.abstract.Backend`, optional): backend to be
+            used in the execution. If ``None``, it uses
+            :class:`qibo.backends.GlobalBackend`. Defaults to ``None``.
+
+    Returns:
+        float: Classical relative Rényi entropy :math:`H_{\\alpha}(\\mathbf{p} \\, \\| \\, \\mathbf{q})`.
+    """
+    if backend is None:  # pragma: no cover
+        backend = GlobalBackend()
+
+    if isinstance(prob_dist_p, list):
+        # np.float64 is necessary instead of native float because of tensorflow
+        prob_dist_p = backend.cast(prob_dist_p, dtype=np.float64)
+    if isinstance(prob_dist_q, list):
+        # np.float64 is necessary instead of native float because of tensorflow
+        prob_dist_q = backend.cast(prob_dist_q, dtype=np.float64)
+
+    if (len(prob_dist_p.shape) != 1) or (len(prob_dist_q.shape) != 1):
+        raise_error(
+            TypeError,
+            "Probability arrays must have dims (k,) but have "
+            + f"dims {prob_dist_p.shape} and {prob_dist_q.shape}.",
+        )
+
+    if (len(prob_dist_p) == 0) or (len(prob_dist_q) == 0):
+        raise_error(TypeError, "At least one of the arrays is empty.")
+
+    if not isinstance(alpha, (float, int)):
+        raise_error(
+            TypeError, f"alpha must be type float, but it is type {type(alpha)}."
+        )
+
+    if alpha < 0.0:
+        raise_error(ValueError, "alpha must a non-negative float.")
+
+    if base <= 0:
+        raise_error(ValueError, "log base must be non-negative.")
+
+    if (any(prob_dist_p < 0) or any(prob_dist_p > 1.0)) or (
+        any(prob_dist_q < 0) or any(prob_dist_q > 1.0)
+    ):
+        raise_error(
+            ValueError,
+            "All elements of the probability array must be between 0. and 1..",
+        )
+    if np.abs(np.sum(prob_dist_p) - 1.0) > PRECISION_TOL:
+        raise_error(ValueError, "First probability array must sum to 1.")
+
+    if np.abs(np.sum(prob_dist_q) - 1.0) > PRECISION_TOL:
+        raise_error(ValueError, "Second probability array must sum to 1.")
+
+    if alpha == 0.5:
+        return -2 * np.log2(np.sum(np.sqrt(prob_dist_p * prob_dist_q))) / np.log2(base)
+
+    if alpha == 1.0:
+        return classical_relative_entropy(
+            prob_dist_p, prob_dist_q, base=base, backend=backend
+        )
+
+    if alpha == np.inf:
+        return np.log2(max(prob_dist_p / prob_dist_q)) / np.log2(base)
+
+    prob_p = prob_dist_p**alpha
+    prob_q = prob_dist_q ** (1 - alpha)
+
+    return (1 / (alpha - 1)) * np.log2(np.sum(prob_p * prob_q)) / np.log2(base)
+
+
 def entropy(
     state,
     base: float = 2,
@@ -503,6 +603,101 @@
     return (1 / (1 - alpha)) * log / np.log2(base)
 
 
+def relative_renyi_entropy(
+    state, target, alpha: Union[float, int], base: float = 2, backend=None
+):
+    if backend is None:  # pragma: no cover
+        backend = GlobalBackend()
+
+    if (
+        (len(state.shape) >= 3)
+        or (len(state) == 0)
+        or (len(state.shape) == 2 and state.shape[0] != state.shape[1])
+    ):
+        raise_error(
+            TypeError,
+            f"state must have dims either (k,) or (k,k), but have dims {state.shape}.",
+        )
+
+    if (
+        (len(target.shape) >= 3)
+        or (len(target) == 0)
+        or (len(target.shape) == 2 and target.shape[0] != target.shape[1])
+    ):
+        raise_error(
+            TypeError,
+            f"target must have dims either (k,) or (k,k), but have dims {target.shape}.",
+        )
+
+    if not isinstance(alpha, (float, int)):
+        raise_error(
+            TypeError, f"alpha must be type float, but it is type {type(alpha)}."
+        )
+
+    if alpha < 0.0:
+        raise_error(ValueError, "alpha must a non-negative float.")
+
+    if base <= 0.0:
+        raise_error(ValueError, "log base must be non-negative.")
+
+    if (
+        abs(purity(state) - 1.0) < PRECISION_TOL
+        and abs(purity(target) - 1) < PRECISION_TOL
+    ):
+        return 0.0
+
+    if alpha == 1.0:
+        return relative_entropy(state, target, base, backend=backend)
+
+    if alpha == np.inf:
+        if backend.__class__.__name__ in ["CupyBackend", "CuQuantumBackend"]:
+            eigenvalues_state, eigenvectors_state = np.linalg.eigh(state)
+            new_state = np.zeros_like(state, dtype=complex)
+            new_state = backend.cast(new_state, dtype=new_state.dtype)
+            for eigenvalue, eigenstate in zip(
+                eigenvalues_state, np.transpose(eigenvectors_state)
+            ):
+                new_state += np.sqrt(eigenvalue) * np.outer(
+                    eigenstate, np.conj(eigenstate)
+                )
+
+            eigenvalues_target, eigenvectors_target = np.linalg.eigh(target)
+            new_target = np.zeros_like(target, dtype=complex)
+            new_target = backend.cast(new_target, dtype=new_target.dtype)
+            for eigenvalue, eigenstate in zip(
+                eigenvalues_target, np.transpose(eigenvectors_target)
+            ):
+                new_target += np.sqrt(eigenstate) * np.outer(
+                    eigenstate, np.conj(eigenstate)
+                )
+        else:
+            from scipy.linalg import sqrtm  # pylint: disable=C0415
+
+            # astype method needed because of tensorflow
+            new_state = sqrtm(state).astype("complex128")
+            new_target = sqrtm(target).astype("complex128")
+            new_state = backend.cast(new_state, dtype=new_state.dtype)
+            new_target = backend.cast(new_target, dtype=new_target.dtype)
+
+        log = np.log2(
+            backend.calculate_norm_density_matrix(new_state @ new_target, order=1)
+        )
+
+        return -2 * log / np.log2(base)
+
+    if len(state.shape) == 1:
+        state = np.outer(state, np.conj(state))
+
+    if len(target.shape) == 1:
+        target = np.outer(target, np.conj(target))
+
+    log = np.linalg.matrix_power(state, alpha)
+    log = log @ np.linalg.matrix_power(target, 1 - alpha)
+    log = np.log2(np.trace(log))
+
+    return (1 / (alpha - 1)) * log / np.log2(base)
+
+
 def entanglement_entropy(
     state,
     bipartition,