Add gates.CNOT as a native gate in gate decomposition

[csookim]

This covers issue #1421.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[csookim]

Example

circ = Circuit(2)
circ.add(gates.H(0))
circ.add(gates.CNOT(0, 1))
circ.add(gates.SWAP(0, 1))
circ.add(gates.CZ(0, 1))
circ.add(gates.M(0, 1))

gate_list = [gates.GPI2, gates.RZ, gates.Z, gates.M, gates.CNOT]
native_gates = NativeGates(0).from_gatelist(gate_list)

custom_pipeline = Passes([Unroller(native_gates=native_gates)])
unrolled_circuit, _ = custom_pipeline(circ)

print(unrolled_circuit.draw(line_wrap=100))

Output

q0: ─Z─GPI2─o─o─X─o────────o────────M─
q1: ────────X─X─o─X─Z─GPI2─X─Z─GPI2─M─

[alecandido]

As said in the issue, the unroller should be entirely reviewed, since there are many more combinations that now are not accounted for.

In any case, they may also be unrealistic, and support is not needed in the short term.

For the time being, just fix the docstrings, and it should be ready to go.

[alecandido]

@csookim, you didn't have to ask again for a review: I already approved, and the modifications I asked were so minor that I trusted you on how to implement them :)

[alecandido]

Btw, why are you deleting Codecov comments? Just out of curiosity
(the checks in the workflows are still there; so it's just a different in appearance, but no substantial change, so it's even fine)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.94%. Comparing base (8fb9af9) to head (3281871).
Report is 24 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1422   +/-   ##
=======================================
  Coverage   99.94%   99.94%           
=======================================
  Files          78       78           
  Lines       11297    11304    +7     
=======================================
+ Hits        11291    11298    +7     
  Misses          6        6           

Flag	Coverage Δ
unittests	99.94% <100.00%> (+<0.01%)	⬆️

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[csookim]

Btw, why are you deleting Codecov comments? Just out of curiosity (the checks in the workflows are still there; so it's just a different in appearance, but no substantial change, so it's even fine)

I deleted it to avoid confusion, as the removed one was the coverage of the previous code. Will it be updated automatically?

[alecandido]

I deleted it to avoid confusion, as the removed one was the coverage of the previous code. Will it be updated automatically?

Indeed, if you leave it there it will edit the same comment, instead of adding a new one :)

[Simone-Bordoni]

You can make this decomposition work for every gate by first decomposing into CZ and then decomposing CZ into CNOT. This is how it works for iSWAP decomposition (line 238-250).

[csookim]

Yes, but it adds two H gates next to the CNOT and it requires optimization (fusion) of 1q gates. Since Paul will use simple circuits for testing, I only added basic decompositions. As @alecandido mentioned, it would be better to entirely review the unroller.

[Simone-Bordoni]

It should be a realy easy check to remove two successive H gates, however if you need it only for simple applications it is fine