Circuit to OpenQASM conversion breaks when using ECR gates

[judealnas]

Fixes #608

Added "Gecres" conversion to "ecr".
Since "Gecres" is the key associated with the ECR unitary and is most likely used in existing processor specs, this seemed like the fix least likely to break existing code.

[rileyjmurray]

Thanks for this, @judealnas!

If it's alright with @coreyostrove, I'm inclined to add 'Gecr' as a pyGSTi standard gatename as well. This would involve adding the following lines to pygsti/tools/internalgates.py:

• after line 198, add * 'Gecr' : alternative name for the echoed cross-resonance gate, matching OpenQASM / IBM conventions. as line 199.
• after line 305, add std_unitaries['Gecr'] = std_unitaries['Gecres'] # alias as line 306.
• after line 406, add gate_dict['Gecr'] = gate_dict['Gecres'] as line 407.

[coreyostrove]

Thank you for these fixes, @judealnas. Before we can accept a PR from an external contributor our organization requires that we have a contributor license agreement (CLA) on file. This is fairly low-effort though and only needs to be done once. If you send us an email at the sandia.gov address located in the contributor guidelines here we can get you a copy.

I appreciate that this is a bit of a hassle for just a couple lines of code which I apologize for, so if you'd prefer not to then we can incorporate these patches on our end and get those merged in (while properly recognizing you for your contributions, of course).

@rileyjmurray: In full agreement about adding those gate aliases.

[judealnas]

@coreyostrove I'd be happy to sign the CLA.

[coreyostrove]

Excellent, thank you. Shoot us an email at pygsti [at] sandia.gov at your convenience and we'll send you a copy.

[rileyjmurray]

Chiming in to report that we've received the signed CLA.

@judealnas can you make the changes I proposed in this comment: #609 (comment) ?