move SU2RotationGate to basic_gates/

[anurudhp]

#669 (comment)

• Move SU2RotationGate to basic_gates/ directory
• Add a builder method .from_matrix so you can derive the parameters of the gate given an arbitrary 2-qubit unitary.
• override wire_symbol
• test that bloq_autotester works.
• Add a docstring (maybe just move the docstring from rotation_matrix method to the class)
• Add a BloqExample
• Update and run dev_tools/autogenerate-bloqs-notebooks.py to generate a doc
• Make sure theta, phi and lambd can be symbolics.

Skipped (due to global phase mismatch)

• override as_cirq_op to return a PhasedXZGate
• override CIRQ_GATE_TO_BLOQ_MAP in _cirq_gate_to_bloq to convert a cirq.PhasedXZGate to SU2RotationGate
• test that assert_decompose_is_consistent_with_t_complexity works
• Override build_call_graph

[anurudhp]

@tanujkhattar one question: cirq.PhasedXZGate and SU2RotationGate differ by a global phase, i.e. $SU_2(\theta, \phi, \lambda) = e^{i(\phi+\lambda-\theta)} \text{PhasedXZ}(x=\frac{2\theta}{\pi}, z=1-\frac{\lambda+\phi}\pi, a=\frac{\lambda}\pi - 0.5)$. (this only matters when using controlled GSQP inside a larger algorithm).

Should we still map them ignoring global phase? Another idea could be to just used PhasedXZ in GQSP rotations, and apply a single global phase correction at the end.

[mpharrigan]

What was the conclusion from the meeting? Were we going to remove the decomposition and leave this as an "atomic" bloq? And add its tensor. Also: re: global phase: we said that it needs to match, so if cirq.PhasedXZ isn't up to the task we can leave that out.

[anurudhp]

@mpharrigan I dropped the PhasedXZ conversion.

About the decomposition, I am fine with either case. Perhaps its still useful to have a decomposition in terms of Pauli rotations in case someone wants to generate circuits?

[anurudhp]

@tanujkhattar @mpharrigan could one of you review this PR? thanks!

I also have two questions:

• Should I make the bloq atomic?
• Is there anything specific to be done to support symbolic params?

[tanujkhattar]

Answered all your questions and requested a few changes. Looking great overall.

Should I make the bloq atomic?

No

Is there anything specific to be done to support symbolic params?

Suggested minor improvements

[tanujkhattar]

LGTM % one final suggestion to update the _cirq_gate_to_bloq to return GlobalPhase bloq when it sees a cirq.GlobalPhaseGate.

Feel free to merge after that.

[anurudhp]

_cirq_gate_to_bloq already converts to GlobalPhase (merged with #828)

Qualtran/qualtran/cirq_interop/_cirq_to_bloq.py

Lines 347 to 348 in 4dffc97

	if isinstance(gate, cirq.GlobalPhaseGate):
	return GlobalPhase(coefficient=gate.coefficient)