Add `cirq.state_vector_to_probabilities`

Is your feature request related to a use case or problem? Please describe. I need a function that takes in a state_vector and returns the probabilities of it’s amplitudes.

Describe the solution you’d like Add a def state_vector_to_probabilities() to https://github.com/quantumlib/Cirq/blob/master/cirq-core/cirq/value/probability.py

[optional] Describe alternatives/workarounds you’ve considered Defined my own:

from cirq.value.probability import validate_probability
def state_vector_to_probabilities(state_vector: np.ndarray) -> np.ndarray:
      probabilities = np.abs(state_vector) ** 2
      validate_probability(probabilities, "sum of the amplitudes of the state vector")
      return probabilities

What is the urgency from your perspective for this issue? Is it blocking important work? P3 - I’m not really blocked by it, it is an idea I’d like to discuss / suggestion based on principle

[mpharrigan]

Yes, I believe my comment https://github.com/quantumlib/Cirq/issues/4322#issuecomment-888539664 was me hastily transcribing notes from the Cirq Cync call that requested exactly that.

[vtomole]

@ishmum123 It seems like @Patil2099 hasn’t responded to my ping. Feel free to look at this issue.

@prathameshbhole Yeah, that’s right.