text: """
  Module for measuring quantum objects.
  """
  
  import numpy as np
  from qutip.qobj import Qobj
  from qutip import identity
  from qutip.qip.operations.gates import expand_operator
  
  
  def _verify_input(op, state):
      if not isinstance(op, Qobj):
          raise TypeError("op must be a Qobj")
      if not op.isoper:
          raise ValueError("op must be all operators or all kets")
      if not isinstance(state, Qobj):
          raise TypeError("state must be a Qobj")
      if state.isket:
          if op.dims[-1] != state.dims[0]:
              raise ValueError(
                  "op and state dims should be compatible when state is a ket")
      elif state.isoper:
          if op.dims != state.dims:
              raise ValueError(
                  "op and state dims should match"
                  " when state is a density matrix")
      else:
          raise ValueError("state must be a ket or a density matrix")
  
  
  def _measurement_statistics_povm_ket(state, ops):
      r"""
      Returns measurement statistics (resultant states and probabilities)
      for a measurements specified by a set of positive operator valued
      measurements on a specified ket.
  
      Parameters
      ----------
      state : :class:`.Qobj` (ket)
              The ket specifying the state to measure.
  
      ops : list of :class:`.Qobj`
        List of measurement operators :math:`M_i` (specifying a POVM such that
        :math:`E_i = M_i^\dagger M_i`).
  
  
      Returns
      -------
      collapsed_states : list of :class:`.Qobj` (kets)
          The collapsed states (kets) obtained after measuring the qubits and
          obtaining the qubit specified by the target in the state specified by
          the index.
  
      probabilities : list of floats
          The probability of measuring a state in a the state specified by the
          index.
      """
      probabilities = []
      collapsed_states = []
  
      for i, op in enumerate(ops):
          p = np.absolute((state.dag() * op.dag() * op * state)[0][0][0])
          probabilities.append(p)
          if p != 0:
              collapsed_states.append((op * state) / np.sqrt(p))
          else:
              collapsed_states.append(None)
  
      return collapsed_states, probabilities
  
  
  def _measurement_statistics_povm_dm(density_mat, ops):
      r"""
      Returns measurement statistics (resultant states and probabilities)
      for a measurements specified by a set of positive operator valued
      measurements on a specified ket or density matrix.
  
      Parameters
      ----------
      state : :class:`.Qobj` (density matrix)
          The ket or density matrix specifying the state to measure.
  
      ops : list of :class:`.Qobj`
          List of measurement operators :math:`M_i` (specifying a POVM s.t.
          :mathm:`E_i = M_i^\dagger M_i`)
  
      Returns
      -------
      collapsed_states : list of :class:`.Qobj`
          The collapsed states (density matrices) obtained after measuring the
          qubits and obtaining the qubit specified by the target in the state
          specified by the index.
  
      probabilities : list of float
          The probability of measuring a state in a the state specified by the
          index.
      """
      probabilities = []
      collapsed_states = []
  
      for i, op in enumerate(ops):
          st = op * density_mat * op.dag()
          p = st.tr()
          probabilities.append(p)
          if p != 0:
              collapsed_states.append(st/p)
          else:
              collapsed_states.append(None)
  
      return collapsed_states, probabilities
  
  
  def measurement_statistics_povm(state, ops, targets=None):
      r"""
      Returns measurement statistics (resultant states and probabilities) for a
      measurement specified by a set of positive operator valued measurements on
      a specified ket or density matrix.
  
      Parameters
      ----------
      state : :class:`.Qobj`
          The ket or density matrix specifying the state to measure.
  
      ops : list of :class:`.Qobj`
          List of measurement operators :math:`M_i` or kets.  Either:
  
          1. specifying a POVM s.t. :math:`E_i = M_i^\dagger M_i`
          2. projection operators if ops correspond to
             projectors (s.t. :math:`E_i = M_i^\dagger = M_i`)
          3. kets (transformed to projectors)
  
      targets : list of ints, optional
                Specifies a list of target "qubit" indices on which to apply
                the measurement using qutip.qip.gates.expand_operator to expand
                ops into full dimension.
  
  
      Returns
      -------
      collapsed_states : list of :class:`.Qobj`
          The collapsed states obtained after measuring the qubits and obtaining
          the qubit specified by the target in the state specified by the index.
  
      probabilities : list of floats
          The probability of measuring a state in a the state specified by the
          index.
      """
      if all(map(lambda x: x.isket, ops)):
          ops = [op * op.dag() for op in ops]
  
      if targets:
          N = int(np.log2(state.shape[0]))
          ops = [expand_operator(op, N=N, targets=targets) for op in ops]
  
      for op in ops:
          _verify_input(op, state)
  
      E = [op.dag() * op for op in ops]
  
      is_ID = sum(E)
      if not is_ID == identity(is_ID.dims[0]):
          raise ValueError("measurement operators must sum to identity")
  
      if state.isket:
          return _measurement_statistics_povm_ket(state, ops)
      else:
          return _measurement_statistics_povm_dm(state, ops)
  
  
  def measurement_statistics_observable(state, op, targets=None):
      """
      Return the measurement eigenvalues, eigenstates (or projectors) and
      measurement probabilities for the given state and measurement operator.
  
      Parameters
      ----------
      state : :class:`.Qobj`
          The ket or density matrix specifying the state to measure.
  
      op : :class:`.Qobj`
          The measurement operator.
  
      targets : list of ints, optional
          Specifies a list of targets "qubit" indices on which to apply the
          measurement using :func:`qutip.qip.gates.expand_operator` to expand op
          into full dimension.
  
      Returns
      -------
      eigenvalues: list of float
          The list of eigenvalues of the measurement operator.
  
      eigenstates_or_projectors: list of :class:`.Qobj`
          If the state was a ket, return the eigenstates of the measurement
          operator. Otherwise return the projectors onto the eigenstates.
  
      probabilities: list of float
          The probability of measuring the state as being in the corresponding
          eigenstate (and the measurement result being the corresponding
          eigenvalue).
      """
      if targets:
          N = int(np.log2(state.shape[0]))
          op = expand_operator(op, N=N, targets=targets)
  
      _verify_input(op, state)
  
      eigenvalues, eigenstates = op.eigenstates()
      if state.isket:
          probabilities = [(e.dag() * state).norm() ** 2 for e in eigenstates]
          return eigenvalues, eigenstates, probabilities
      else:
          projectors = [v * v.dag() for v in eigenstates]
          probabilities = [(p * state).tr() for p in projectors]
          return eigenvalues, projectors, probabilities
  
  
  def measure_observable(state, op, targets=None):
      """
      Perform a measurement specified by an operator on the given state.
  
      This function simulates the classic quantum measurement described in many
      introductory texts on quantum mechanics. The measurement collapses the
      state to one of the eigenstates of the given operator and the result of the
      measurement is the corresponding eigenvalue.
  
      Parameters
      ----------
      state : :class:`.Qobj`
          The ket or density matrix specifying the state to measure.
  
      op : :class:`.Qobj`
          The measurement operator.
  
      targets : list of ints, optional
          Specifies a list of target "qubit" indices on which to apply the
          measurement using :func:`qutip.qip.gates.expand_operator` to expand op
          into full dimension.
  
      Returns
      -------
      measured_value : float
          The result of the measurement (one of the eigenvalues of op).
  
      state : :class:`.Qobj`
          The new state (a ket if a ket was given, otherwise a density matrix).
  
      Examples
      --------
  
      Measure the z-component of the spin of the spin-up basis state:
  
      >>> measure_observable(basis(2, 0), sigmaz())
      (1.0, Quantum object: dims = [[2], [1]], shape = (2, 1), type = ket
      Qobj data =
      [[-1.]
       [ 0.]])
  
      Since the spin-up basis is an eigenstate of sigmaz, this measurement always
      returns 1 as the measurement result (the eigenvalue of the spin-up basis)
      and the original state (up to a global phase).
  
      Measure the x-component of the spin of the spin-down basis state:
  
      >>> measure_observable(basis(2, 1), sigmax())
      (-1.0, Quantum object: dims = [[2], [1]], shape = (2, 1), type = ket
      Qobj data =
      [[-0.70710678]
       [ 0.70710678]])
  
      This measurement returns 1 fifty percent of the time and -1 the other fifty
      percent of the time. The new state returned is the corresponding eigenstate
      of sigmax.
  
      One may also perform a measurement on a density matrix. Below we perform
      the same measurement as above, but on the density matrix representing the
      pure spin-down state:
  
      >>> measure_observable(ket2dm(basis(2, 1)), sigmax())
      (-1.0, Quantum object: dims = [[2], [2]], shape = (2, 2), type = oper
      Qobj data =
      [[ 0.5 -0.5]
       [-0.5  0.5]])
  
      The measurement result is the same, but the new state is returned as a
      density matrix.
      """
      eigenvalues, eigenstates_or_projectors, probabilities = (
          measurement_statistics_observable(state, op, targets))
      i = np.random.choice(range(len(eigenvalues)), p=probabilities)
      if state.isket:
          eigenstates = eigenstates_or_projectors
          state = eigenstates[i]
      else:
          projectors = eigenstates_or_projectors
          state = (projectors[i] * state * projectors[i]) / probabilities[i]
      return eigenvalues[i], state
  
  
  def measure_povm(state, ops, targets=None):
      r"""
      Perform a measurement specified by list of POVMs.
  
      This function simulates a POVM measurement. The measurement collapses the
      state to one of the resultant states of the measurement and returns the
      index of the operator corresponding to the collapsed state as well as the
      collapsed state.
  
      Parameters
      ----------
      state : :class:`.Qobj`
          The ket or density matrix specifying the state to measure.
  
      ops : list of :class:`.Qobj`
          List of measurement operators :math:`M_i` or kets.  Either:
  
          1. specifying a POVM s.t. :math:`E_i = M_i^\dagger M_i`
          2. projection operators if ops correspond to projectors (s.t.
             :math:`E_i = M_i^\dagger = M_i`)
          3. kets (transformed to projectors)
  
      targets : list of ints, optional
          Specifies a list of target "qubit" indices on which to apply
          the measurement using :func:`qutip.qip.gates.expand_operator`
          to expand ``ops`` into full dimension.
  
      Returns
      -------
      index : float
          The resultant index of the measurement.
  
      state : :class:`.Qobj`
          The new state (a ket if a ket was given, otherwise a density matrix).
      """
      collapsed_states, probabilities = measurement_statistics_povm(state,
                                                                    ops, targets)
      index = np.random.choice(range(len(collapsed_states)), p=probabilities)
      state = collapsed_states[index]
      return index, state
  
  
  def measurement_statistics(state, ops, targets=None):
      r"""
      A dispatch method that provides measurement statistics handling both
      observable style measurements and projector style measurements(POVMs and
      PVMs).
  
      For return signatures, please check:
  
      - :func:`~measurement_statistics_observable` for observable measurements.
      - :func:`~measurement_statistics_povm` for POVM measurements.
  
      Parameters
      ----------
      state : :class:`.Qobj`
          The ket or density matrix specifying the state to measure.
  
      ops : :class:`.Qobj` or list of :class:`.Qobj`
          - measurement observable (:class:.Qobj); or
          - list of measurement operators :math:`M_i` or kets (list of
            :class:`.Qobj`) Either:
  
            1. specifying a POVM s.t. :math:`E_i = M_i^\dagger * M_i`
            2. projection operators if ops correspond to projectors (s.t.
               :math:`E_i = M_i^\dagger = M_i`)
            3. kets (transformed to projectors)
  
      targets : list of ints, optional
          Specifies a list of target "qubit" indices on which to apply the
          measurement using :func:`qutip.qip.gates.expand_operator` to expand ops
          into full dimension.
      """
      if isinstance(ops, list):
          return measurement_statistics_povm(state, ops, targets)
      else:
          return measurement_statistics_observable(state, ops, targets)
  
  
  def measure(state, ops, targets=None):
      r"""
      A dispatch method that provides measurement results handling both
      observable style measurements and projector style measurements (POVMs and
      PVMs).
  
      For return signatures, please check:
  
      - :func:`~measure_observable` for observable measurements.
      - :func:`~measure_povm` for POVM measurements.
  
      Parameters
      ----------
      state : :class:`.Qobj`
          The ket or density matrix specifying the state to measure.
  
      ops : :class:`.Qobj` or list of :class:`.Qobj`
          - measurement observable (:class:`.Qobj`); or
          - list of measurement operators :math:`M_i` or kets (list of
            :class:`.Qobj`) Either:
  
            1. specifying a POVM s.t. :math:`E_i = M_i^\dagger M_i`
            2. projection operators if ops correspond to projectors (s.t.
               :math:`E_i = M_i^\dagger = M_i`)
            3. kets (transformed to projectors)
  
      targets : list of ints, optional
          Specifies a list of target "qubit" indices on which to apply the
          measurement using :func:`qutip.qip.gates.expand_operator` to expand ops
          into full dimension.
      """
      if isinstance(ops, list):
          return measure_povm(state, ops, targets)
      else:
          return measure_observable(state, ops, targets)
  