Probabilities >1 in the sampler (and statevector)

[Cryoris]

Environment

• Qiskit Terra version: main @ 5c49d14
• Python version: 3.10
• Operating system: macOS Ventura

What is happening?

The Statevector.probabilities method sometimes contains roundoff errors, leading to probabilities larger than 1. This breaks the Sampler if shots are emulated, as larger than 1 probabilities are not valid inputs to NumPy's multinomial library.

How can we reproduce the issue?

from qiskit.circuit import QuantumCircuit
from qiskit.primitives import Sampler
import numpy as np

np.random.seed(1)
sampler = Sampler(options={"shots": 1024})

for _ in range(1000):
    try:
        angle = np.random.random(1)[0]
        # this angle breaks on my laptop for example:
        # angle = 0.10233442882782584
        qc = QuantumCircuit(1)
        qc.ry(angle, 0)
        qc.ry(-angle, 0)
        qc.measure_all()
        _ = sampler.run(qc).result()
    except Exception as exc:
        print("Failed on", angle)

The error raised is

  File "/Users/jul/Qiskit/qiskit-terra/qiskit/primitives/sampler.py", line 130, in _call
    counts = rng.multinomial(shots, np.fromiter(prob_dict.values(), dtype=float))
  File "_generator.pyx", line 3916, in numpy.random._generator.Generator.multinomial
  File "_common.pyx", line 391, in numpy.random._common.check_array_constraint
  File "_common.pyx", line 377, in numpy.random._common._check_array_cons_bounded_0_1
ValueError: pvals < 0, pvals > 1 or pvals contains NaNs

What should happen?

The probabilities should not exceed 1.

Any suggestions?

Clip the probabilities.