Example added BasicSim docs that shows default shots

[isaacdevlugt]

In Qiskit, when you use this device / backend you’re hard-corded at shots=1024. I assume Qiskit users are aware of this. However, when I used this device in PennyLane, I ran this circuit not expecting it to work because I didn’t specify shots.

pl_qfunc = qml.from_qiskit(qc, measurements=[qml.classical_shadow(wires=range(n))])
pl_circuit = qml.QNode(pl_qfunc, device=qml.device("qiskit.basicsim", wires=n))
pl_circuit()

I had to go digging into the source code to figure out what was going on. As a PennyLane user, I found this confusing (does that matter? lol). Anyway, a simple addition to the docs seems fine to me.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.45%. Comparing base (eb865f7) to head (df21d54).

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@@             Coverage Diff             @@
##            master     #549      +/-   ##
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- Coverage   100.00%   99.45%   -0.55%     
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  Files            7        7              
  Lines          557      550       -7     
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- Hits           557      547      -10     
- Misses           0        3       +3     

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[austingmhuang]

Hi @isaacdevlugt thanks for the PR! Just to clarify, when you say "hardcoded" do you mean that even if you specify e.g. shots=10000 on device initialization the device still runs with 1024? Or do you mean that the device has a default value of running with 1024 shots?

[isaacdevlugt]

Hey @austingmhuang! I mean the latter. 1024 is hardcoded as the default number (can be overridden).

• the plugin:

  pennylane-qiskit/pennylane_qiskit/basic_aer.py

  Line 99 in eb865f7

  def __init__(self, wires, shots=1024, backend="basic_simulator", **kwargs):

• qiskit: https://github.com/Qiskit/qiskit/blob/df379876ba10d6f490a96723b6dbbf723ec45d7a/qiskit/providers/basic_provider/basic_simulator.py#L260

[trbromley]

Why don't we update BasicSim to make shots a required argument?

[austingmhuang]

@trbromley I think the idea is that when a User doesn't pass in a # of shots, there is supposed to be a warning that lets users know that analytic mode is not supported on the basicsim device and inform them that the circuit was ran with 1024 shots. Right now I think this warning is bugged so it isn't showing up but this is the way it's done when using the new QiskitDevice. Obviously we can just raise an error instead but I think this solution is a bit more consistent with how devices in pennylane generally work.