google module of cirq has been moved to a separate module

[dariavh]

Describe the bug
When trying to use the sycamore device for simulating my wavefunction, I get the error AttributeError: module 'cirq' has no attribute 'google' because the cirq.google module has been moved into its own module cirq_google.

To Reproduce
Steps to reproduce the behavior:

import tequila as tq

H = tq.paulis.X(0)

wfn = tq.simulate(H, samples=100, device="sycamore", backend="cirq")

Computer (please complete the following information):

• OS: Linux Darwin Kernel Version 22.3.0
• Version: Python 3.7.12, tequila 1.8.9

Additional context
I think the bugfix would consist of renaming all cirq.google references to cirq_google (need to check thoroughly) and adding the cirq_google package to the optional backends list in the requirements.txt. I can provide this in a new PR if you want?

[kottmanj]

That would be amazing!

What would be great to add a small test alongside (in tests/test_simulator_backends.py). E.g.

import importlib
HAS_GOOGLE = importlib.util.find_spec('cirq_google')
@pytest.mark.skipif(condition=HAS_GOOGLE, reason="cirq_google not installed")
def test_cirq_google_devices():
    some tests

and in the same way, including the HAS_GOOGLE in src/tequila/simulators/simulator_cirq.py in order to throw a meaningful error message if someone tries to use the devides that need the cirq_google package installed.

[dariavh]

Hi Jakob,

It seems that a lot has changed in the cirq package over the last years. The Foxtail and Bristlecone devices are deprecated, see 0.14.0 release for the full list. The optimized_for_sycamore() method is deprecated as well and is replaced by optimize_for_target_gateset() that can e.g. optimize an abstract circuit in cirq to gates that are compatible on the Google Sycamore devices.

I am currently working out the details, but I would refactor the build_device_circuit method in simulator_cirq.py as:

if isinstance(device, cirq.Device):
       if device in [cirq_google.Sycamore, cirq_google.Sycamore23]:
             try:
                  circuit = cirq.optimize_for_target_gateset(circuit=c, gateset=cirq_google.SycamoreTargetGateset())
             except ValueError:
                  raise TequilaCirqException('could not optimize for device={}'.format(device))
       else:
             ### under construction (potentially on other branches)
             raise TequilaException('Only Sycamore and Sycamore23 devices currently functional. Sorry!')

[kottmanj]

Looks good to me!

would suggest the following error handling, then the original Exception from cirq is also displayed (often useful)

             except ValueError as E:
                  original_message = str(E)
                  raise TequilaCirqException('original message:\n{}\n\ncould not optimize for device={}'.format(original_message,device))