Change photon_number_expectation

[dorakingx]

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Context:

Description of the Change:

Benefits:

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Related GitHub Issues:

[nquesada]

Is this related to this bug: #336 ?

[dorakingx]

No, it's not. When calculating the variance of third-body correlations, I needed to compute the expected value of polynomial products such as N_1^2N_2 . However, I was only able to handle cases where all exponents were either 1 or 2. Therefore, I modified the photon_number_expectation function to allow for the calculation of expected values with a wider range of exponents.

[nquesada]

Understood. You will need to add new tests in test_quantum.py and make sure to pass pylint and black on the PR. Finally, should you be interested in the theory behind photon_number_moments, we wrote a paper explaining all sort of useful properties: https://arxiv.org/abs/2212.06067 .