qiskit/quantum_info/operators/channel/choi.py
def compose(self, other, qargs=None, front=False):
        """Return the composition channel self∘other.

        Args:
            other (QuantumChannel): a quantum channel.
            qargs (list): a list of subsystem positions to compose other on.
            front (bool): If False compose in standard order other(self(input))
                          otherwise compose in reverse order self(other(input))
                          [default: False]

        Returns:
            Choi: The composition channel as a Choi object.

        Raises:
            QiskitError: if other cannot be converted to a channel or
            has incompatible dimensions.
        """
        if qargs is not None:
            return Choi(
                SuperOp(self).compose(other, qargs=qargs, front=front))

        # Convert to Choi matrix
        if not isinstance(other, Choi):
            other = Choi(other)
        # Check dimensions match up
        if front and self._input_dim != other._output_dim:
            raise QiskitError(
                'input_dim of self must match output_dim of other')
        if not front and self._output_dim != other._input_dim:
            raise QiskitError(
                'input_dim of other must match output_dim of self')

        if front:
            first = np.reshape(other._data, other._bipartite_shape)
            second = np.reshape(self._data, self._bipartite_shape)
            input_dim = other._input_dim
            input_dims = other.input_dims()
            output_dim = self._output_dim
            output_dims = self.output_dims()
        else:
            first = np.reshape(self._data, self._bipartite_shape)
            second = np.reshape(other._data, other._bipartite_shape)
            input_dim = self._input_dim
            input_dims = self.input_dims()
            output_dim = other._output_dim
            output_dims = other.output_dims()

        # Contract Choi matrices for composition
        data = np.reshape(
            np.einsum('iAjB,AkBl->ikjl', first, second),
            (input_dim * output_dim, input_dim * output_dim))
        return Choi(data, input_dims, output_dims)
def compose(self, other, qargs=None, front=False):
        """Return the composition channel self∘other.

        Args:
            other (QuantumChannel): a quantum channel.
            qargs (list): a list of subsystem positions to compose other on.
            front (bool): If False compose in standard order other(self(input))
                          otherwise compose in reverse order self(other(input))
                          [default: False]

        Returns:
            Choi: The composition channel as a Choi object.

        Raises:
            QiskitError: if other cannot be converted to a channel or
            has incompatible dimensions.
        """
        if qargs is not None:
            return Choi(
                SuperOp(self).compose(other, qargs=qargs, front=front))

        # Convert to Choi matrix
        if not isinstance(other, Choi):
            other = Choi(other)
        # Check dimensions match up
        if front and self._input_dim != other._output_dim:
            raise QiskitError(
                'input_dim of self must match output_dim of other')
        if not front and self._output_dim != other._input_dim:
            raise QiskitError(
                'input_dim of other must match output_dim of self')

        if front:
            first = np.reshape(other._data, other._bipartite_shape)
            second = np.reshape(self._data, self._bipartite_shape)
            input_dim = other._input_dim
            input_dims = other.input_dims()
            output_dim = self._output_dim
            output_dims = self.output_dims()
        else:
            first = np.reshape(self._data, self._bipartite_shape)
            second = np.reshape(other._data, other._bipartite_shape)
            input_dim = self._input_dim
            input_dims = self.input_dims()
            output_dim = other._output_dim
            output_dims = other.output_dims()

        # Contract Choi matrices for composition
        data = np.reshape(
            np.einsum('iAjB,AkBl->ikjl', first, second),
            (input_dim * output_dim, input_dim * output_dim))
        return Choi(data, input_dims, output_dims)
Return the composition channel self∘other.

        Args:
            other (QuantumChannel): a quantum channel.
            qargs (list): a list of subsystem positions to compose other on.
            front (bool): If False compose in standard order other(self(input))
                          otherwise compose in reverse order self(other(input))
                          [default: False]

        Returns:
            Choi: The composition channel as a Choi object.

        Raises:
            QiskitError: if other cannot be converted to a channel or
            has incompatible dimensions.