The sample() method in DictStateFn does not handle complex amplitudes

[hay-k]

Information

• Qiskit Aqua version: 0.7.5
• Python version: 3.7.4
• Operating system: Windows 10

What is the current behavior?

Throws the following exception

  File "_generator.pyx", line 744, in numpy.random._generator.Generator.choice
TypeError: Cannot cast array data from dtype('complex128') to dtype('float64') according to the rule 'safe'

Steps to reproduce the problem

import qiskit.aqua.operators as op
op.DictStateFn({'0': 1+0j}).sample()

What is the expected behavior?

It is expected to return {'0': 1.0}

Suggested solutions

The line probs = np.array(list(self.primitive.values()))**2 in dict_state_fn.py is correct only in the case if all values in self.primitive are real. To handle complex values properly it should be modified as follows

value_arr = np.array(list(self.primitive.values()))
probs = (value_arr * value_arr.conj()).real