Should ConstantQubitNoiseModel apply noise first rather than second?

[kevinsung]

Cirq/cirq/devices/noise_model.py

Lines 204 to 209 in e904305

	def noisy_moment(self, moment: 'cirq.Moment',
	system_qubits: Sequence['cirq.Qid']):
	return [
	moment,
	ops.Moment([self.qubit_noise_gate(q) for q in system_qubits])
	]

As you can see, the original moment gets applied, then the noisy moment. This means that the measurement results of measurement gates are not affected by the noise, which may be surprising. For instance, a circuit consisting only of measurements will be sampled as if it is noiseless.

[dabacon]

Yeah generally this is probably not what is desired. I would suggest that we start by adding the ability to toggle between before and after and add deprecation warning about the default for this being switched.

[mpharrigan]

Also see in cirq.contrib.noise_models how measurement is special-cased

[95-martin-orion]

@dkafri Which of these (noise before or after the affected gate) is more consistent with the error-to-noise pipeline? I think it's feasible to make this change, but if noise-after-gate is a better match for what we measure from hardware then it's not worth the switch.

[95-martin-orion]

Discussed offline with Dvir: shifting noise to before the affected gate shouldn't affect consistency with HW noise. (A more accurate model would actually split the noise half before, half after, but the improvement this provides is negligible)

[dabacon]

In general when I've had to do simulations for error corrections, I need both before and after. After for after preparation steps, before for gates and measurements. I think if we have to chose one, before is better, but just wanted to point out that this makes the preparation errors hard to use.

[dkafri]

Dave makes a good point. What I've done in the past is just specify a flag for whether to prepend vs append errors.

[tanujkhattar]

From cirq sync:

Defaulting to before will cause a breaking change. Let's provide an optional parameter to users to prepend instead of append.

From hardware team's perspective, it before/after shouldn't matter much.