EntanglementEntropy use circuits with fixed nqubits

[scarrazza]

This PR raises an error if the user allocates multiple circuits using different number of qubits but sharing the same EntanglementEntropy object. @igres26 could you please check if this PR raises a proper error in your example?

[stavros11]

Thanks for implementing this. I think it works if one uses the entropy as part of circuit (@igres26 can confirm), but it does not work if the entropy is called directly on a state. For example I tried the following:

import numpy as np
from qibo import K
from qibo.callbacks import EntanglementEntropy

# 3 qubit state
state1 = np.random.random(8) + 1j * np.random.random(8)
state1 = K.cast(state1 / np.sqrt(np.sum(np.abs(state1) ** 2)))
# 4 qubit state
state2 = np.random.random(16) + 1j * np.random.random(16)
state2 = K.cast(state2 / np.sqrt(np.sum(np.abs(state2) ** 2)))

entropy = EntanglementEntropy()
ent1 = entropy(state1)
ent2 = entropy(state2)

and I don't get the RuntimeError. I think the reason is that when entropy is called on a state, it calls set_nqubits which only calls the nqubits setter if self._nqubits is None. So we should either remove the if self._nqubits is None condition from there and always call the setter or move the new check in the set_nqubits function. I think the first is a better solution.

Note that in the above example I get a different error:

ValueError: cannot reshape array of size 16 into shape 8

[scarrazza]

Thanks, should work now.

[codecov]

Codecov Report

Merging #448 (1435bbe) into master (195aa16) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
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  Coverage   100.00%   100.00%           
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  Files           84        84           
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Impacted Files	Coverage Δ
src/qibo/core/callbacks.py	100.00% <100.00%> (ø)
src/qibo/tests/test_core_callbacks.py	100.00% <100.00%> (ø)

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[stavros11]

Thanks for the update, looks good to me now. Both the script from the above post and the following:

from qibo import callbacks, gates, models

entropy = callbacks.EntanglementEntropy()

c = models.Circuit(3)
c.add(gates.CallbackGate(entropy))

c = models.Circuit(4)
c.add(gates.CallbackGate(entropy))

raise the RuntimeError.