qiskit/quantum_info/operators/channel/kraus.py
def _evolve(self, state, qargs=None):
        """Evolve a quantum state by the QuantumChannel.

        Args:
            state (QuantumState): The input statevector or density matrix.
            qargs (list): a list of QuantumState subsystem positions to apply
                           the operator on.

        Returns:
            QuantumState: the output quantum state.

        Raises:
            QiskitError: if the operator dimension does not match the
            specified QuantumState subsystem dimensions.
        """
        # If subsystem evolution we use the SuperOp representation
        if qargs is not None:
            return SuperOp(self)._evolve(state, qargs)

        # Otherwise we compute full evolution directly
        state = self._format_state(state)
        if state.shape[0] != self._input_dim:
            raise QiskitError(
                "QuantumChannel input dimension is not equal to state dimension."
            )
        if state.ndim == 1 and self._data[1] is None and len(
                self._data[0]) == 1:
            # If we only have a single Kraus operator we can implement unitary-type
            # evolution of a state vector psi -> K[0].psi
            return np.dot(self._data[0][0], state)
        # Otherwise we always return a density matrix
        state = self._format_state(state, density_matrix=True)
        kraus_l, kraus_r = self._data
        if kraus_r is None:
            kraus_r = kraus_l
        return np.einsum('AiB,BC,AjC->ij', kraus_l, state,
                         np.conjugate(kraus_r))
def _evolve(self, state, qargs=None):
        """Evolve a quantum state by the QuantumChannel.

        Args:
            state (QuantumState): The input statevector or density matrix.
            qargs (list): a list of QuantumState subsystem positions to apply
                           the operator on.

        Returns:
            QuantumState: the output quantum state.

        Raises:
            QiskitError: if the operator dimension does not match the
            specified QuantumState subsystem dimensions.
        """
        # If subsystem evolution we use the SuperOp representation
        if qargs is not None:
            return SuperOp(self)._evolve(state, qargs)

        # Otherwise we compute full evolution directly
        state = self._format_state(state)
        if state.shape[0] != self._input_dim:
            raise QiskitError(
                "QuantumChannel input dimension is not equal to state dimension."
            )
        if state.ndim == 1 and self._data[1] is None and len(
                self._data[0]) == 1:
            # If we only have a single Kraus operator we can implement unitary-type
            # evolution of a state vector psi -> K[0].psi
            return np.dot(self._data[0][0], state)
        # Otherwise we always return a density matrix
        state = self._format_state(state, density_matrix=True)
        kraus_l, kraus_r = self._data
        if kraus_r is None:
            kraus_r = kraus_l
        return np.einsum('AiB,BC,AjC->ij', kraus_l, state,
                         np.conjugate(kraus_r))
Evolve a quantum state by the QuantumChannel.

        Args:
            state (QuantumState): The input statevector or density matrix.
            qargs (list): a list of QuantumState subsystem positions to apply
                           the operator on.

        Returns:
            QuantumState: the output quantum state.

        Raises:
            QiskitError: if the operator dimension does not match the
            specified QuantumState subsystem dimensions.