code: def photon_number(cov, mu, params, hbar=2.0):
      r"""Calculates the mean photon number for a given one-mode state.
  
      Args:
          cov (array): :math:`2\times 2` covariance matrix
          mu (array): length-2 vector of means
          params (None): no parameters are used for this expectation value
          hbar (float): (default 2) the value of :math:`\hbar` in the commutation
              relation :math:`[\x,\p]=i\hbar`
  
      Returns:
          tuple: contains the photon number expectation and variance
      """
      # pylint: disable=unused-argument
      ex = (np.trace(cov) + mu.T @ mu) / (2 * hbar) - 1 / 2
      var = (np.trace(cov @ cov) + 2 * mu.T @ cov @ mu) / (
          2 * hbar**2
      ) - 1 / 4
      return ex, var
  
imports: import math
  import cmath
  import numpy
  from scipy.special import factorial
  import pennylane
  from pennylane.ops import Identity
  from pennylane import Device
  from .._version import __version__
label_desc: Use image node_id 20 for calling a global function with example usage: photon_number(cov, mu, params, hbar) and returns: ex,  var