Add ZerosSampler

[fedimser]

A dummy sampler which implements cirq.Sampler and returns zeroes for all measurements.

Fixes #2818

[mpharrigan]

This looks good.

1. Could you add the async sampler methods as well? not blocking but I personally use these methods in my workflows
2. Right now this can be used for a simple check that there are no glaring errors in one's experimental workflow. This could be hugely more useful if it could also validate that you're using supported gates and extant qubits. Unfortunately, right now the gateset validation logic is trapped inside the Quantum Engine serialization routines, but I encourage you to think about how to incorporate gateset validation into ZerosSampler

[mpharrigan]

👍

[fedimser]

Hi Matthew,

1. ZeroSampler inherits from Sampler, which has default implementation of run_async which calls run and of run_sweep_async which calls run_sweep. So now async methods of ZerosSampler will work as expected, and I don't need to implement them. Am I right?
2. I didn't work with gate sets in Cirq before, but here is what I can suggest. We can pass SerializableGateSet object to ZeroSampler constructor. Then in run_sweep we will check for all operations whether it is_supported_operation. And if there is no gate set, don't do this validation. Does it make sense? Do you think it's better to do this in a follow-up PR?
   Also, what are "extant qubits"?

[mpharrigan]

1: ok good
2: that sounds entirely reasonable to me
2b, re: follow up PR. up to you. If it's not too drastic of a change, I'd say just put it into this because it will likely involve changing the API of ZerosSampler slightly
2c: re: extant qubits. real devices only have certain qubits and a common failure mode is accidentally using e.g. a LineQubit when you should use GridQubits or using GridQubit(0,0) when the physical numbering scheme starts somewhere else

[fedimser]

Hi @mpharrigan,
I added gateset validation, PTAL.

Regarding extand qubits: there is a class cirq.devices.Device, which has method qubit_set. So we could pass the device to a constructor (like I did with gate set) and in run_sweep() get all qubits referenced by a device, and check that they are all in this qubit_set.
However, this can be simplified: we could just pass qubit_set. Or, if we are passing device, we can just call Device.validate_circuit.
What do you prefer?
Also, does Device.validate_circuit validates gateset? If so we don't need SerializableGetSet in ZerosSampler (which I just added).

[dabacon]

Question to @mpharrigan : shouldn't the device and gateset validation be handled by setting an appropriate device on the circuit? What is the primary use case for this class, is it to do this validation? Then maybe it should be called something slightly different?

[dabacon]

A couple of drive by comments.

[fedimser]

Can we merge this?
@mpharrigan @dabacon

[balopat]

LGTM

[CirqBot]

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Missing statuses: ['cla/google']

[CirqBot]

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Missing statuses: ['cla/google']

[balopat]

@googlebot please rescan