RobustPhaseEstimation vs QuantumPhaseEstimation

[tcNickolas]

Is your feature request related to a problem? Please describe.
RobustPhaseEstimation and QuantumPhaseEstimation return quite different values as "phases", which makes switching between the two non-trivial.

• QPE returns (after measurement) an integer which, when divided by 2^(bits of precision), produces a floating-point number between 0 and 1, which needs to be multiplied by 2π to get a phase e^{iϕ}. The API documentation does not describe the return, and the only range in the docs is ϕ∈[0,2π) which seems to suggest that no multiplication by 2π is necessary.
• Robust PE returns a floating point phase between -π and π which corresponds to e^{iϕ} directly.

Describe the solution you'd like
It would be great to unify the outputs of these operations or at least to document them better, with specific output descriptions and examples.

[msoeken]

The different in implementation can be observed in this sample:

https://github.com/microsoft/Quantum/blob/783f4a7ba71e6f2f9d4db0861473ebfa2c3cf1f2/samples/algorithms/integer-factorization/Shor.qs#L243--275

[tcNickolas]

Ideally the documentation would be sufficient to use the library operation without looking at the samples repo and the source code of the operation... I just got burned by this again, since the docs for QuantumPhaseEstimation still hint that the result is ϕ∈[0,2π).

[msoeken]

@tcNickolas: I agree that this issue leads to problems and needs to be addressed. My comment about the sample was only meant as additional information to the problem itself.