reduce decimal for probabilities_dict in Sampler

[1ucian0]

The example in #10584 is:

import numpy as np
from qiskit import QuantumCircuit

# 1. A quantum circuit for preparing the quantum state |000> + i |111>
qc_example = QuantumCircuit(3)
qc_example.h(0) # generate superpostion
qc_example.cx(0,1) # condition 1st qubit on 0th qubit
qc_example.cx(0,2) # condition 2nd qubit on 0th qubit

qc_measured = qc_example.measure_all(inplace=False)

# 3. Execute using the Sampler primitive
from qiskit.primitives.sampler import Sampler
sampler = Sampler()
job = sampler.run(qc_measured)
result = job.result()
print("> Quasi probability distribution:", result.quasi_dists)

> Quasi probability distribution: [{0: 0.4999999999999999, 7: 0.4999999999999999}]

The rounding error is coming from slightly high np.round(..., decimals=16), original introduced in #9248. Moving that to 15 solved the issue.

> Quasi probability distribution: [{0: 0.5, 7: 0.5}]

Not sure if reno-worthy Actually, it might change the output of QAOA, so adding a update note.

[qiskit-bot]

One or more of the the following people are requested to review this:

• @Qiskit/terra-core
• @ajavadia
• @ikkoham
• @levbishop
• @t-imamichi

[coveralls]

Pull Request Test Coverage Report for Build 5822445480

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 16 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.008%) to 87.242%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	93.76%
crates/qasm2/src/lex.rs	3	91.14%
crates/qasm2/src/parse.rs	12	96.67%

Totals
Change from base Build 5820521776:	-0.008%
Covered Lines:	74274
Relevant Lines:	85136

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💛 - Coveralls

[Cryoris]

Since this seems to be mainly a visualization issue, would it be better to change the printed values instead of the actual values? Otherwise, as you noted, this small changes can add up 🤔

[1ucian0]

Since this seems to be mainly a visualization issue, would it be better to change the printed values instead of the actual values? Otherwise, as you noted, this small changes can add up 🤔

Is there a way to "parametrize" the visualisation of a result?

[Cryoris]

Yeah e.g. via QuasiDist.__str__ 🙂

[1ucian0]

Yeah e.g. via QuasiDist.__str__ 🙂

I mean, to tell how many decimals I want.

[Cryoris]

NumPy allows to set the precision of printed numbers, but I think this is only a global setting. With __str__ I meant you could do something like

# in QuasiDist.__str__
truncated = {key: np.round(value, decimals=10) for key, value in self.items}
return str(truncated)

[1ucian0]

Sure... I'm still curious why changing the rounding in #9248 changes the output. But a fight for another day. The __repr__ approach implemented in 7787b8b

[t-imamichi]

It might be good to set the decimal with a class variable __decimal__ or __ndigits__ (instead of instance variable self.decimal) similar to Pauli.__repr__.

qiskit/qiskit/quantum_info/operators/symplectic/pauli.py

Lines 96 to 101 in 55d4e98

	Using ``str`` to convert a ``Pauli`` to a string will truncate the
	returned string for large numbers of qubits while :meth:`to_label`
	will return the full string with no truncation. The default
	truncation length is 50 characters. The default value can be
	changed by setting the class `__truncate__` attribute to an integer
	value. If set to ``0`` no truncation will be performed.

Do you have any suggestion, @ikkoham?

[ikkoham]

I agree with @t-imamichi. Class variable is better than instance variable. Variable names can be something like dunder/magic (__decimal__), or they can be normal (decimal).

[1ucian0]

Great idea! done in d2866a3

Thanks!

[coveralls]

Pull Request Test Coverage Report for Build 6334811226

• 2 of 3 (66.67%) changed or added relevant lines in 1 file are covered.
• 20 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.009%) to 86.998%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/result/distributions/quasi.py	2	3	66.67%

Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	2	90.15%
crates/qasm2/src/parse.rs	18	96.67%

Totals
Change from base Build 6332583014:	-0.009%
Covered Lines:	74079
Relevant Lines:	85150

---

💛 - Coveralls

[t-imamichi]

Thanks. LGTM.

[ElePT]

LGTM too