Add Bell state |Φ+ circuit model

[joe-ucp]

Adds 2-qubit Bell state circuit for observable estimation submissions

• Created bell_state_phi_plus model directory with QASM circuit file

• Added README.md with circuit description, observables, and references

• Updated circuit-models.json with bell_state_phi_plus_2qubit instance

• Updated issue template to include new circuit instance in dropdown

[minhctran]

Hi. Thanks so much for the submission. While Bell state preparation is certainly a fundamental demonstration of quantum advantage in terms of correlations, this tracker is specifically focused on computations that are hard to simulate classically. Because this submission doesn't quite fit that specific criteria, I'd suggest passing on this one.

[joe-ucp]

Thanks for the review, Minh.

Below is a short clarification on why I added the Bell |Φ+⟩ circuit and what I’m proposing.

This is a minimal, reproducible calibration circuit for validating a new measurement-reduction method I submitted, Quantum Eye. The Bell state is the correct scientific baseline for that validation: trivial to reproduce, trivial to verify, and it removes ambiguity before attempting anything that scales up.

To give a bit more context: I have several larger-scale tests (including multi-qubit GHZ, reconstruction-based VQE observables, and long-range correlation recovery) that I would like to submit in the future. Those only make scientific sense after the underlying measurement-reduction method is validated in the simplest possible setting. Establishing correctness on the Bell state is therefore the root step.

Once this baseline is confirmed, the larger submissions become interpretable, reproducible, and meaningful within the Tracker framework.

Please let me know what I can adjust so this can move forward.