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Fix ordering of dims in QuantumState.probabilities_dict (backport #9214) #9226

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Dec 1, 2022
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Fix ordering of dims in QuantumState.probabilities_dict (backport #9214) #9226

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2 changes: 1 addition & 1 deletion qiskit/quantum_info/states/quantum_state.py
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Original file line number Diff line number Diff line change
Expand Up @@ -454,7 +454,7 @@ def _subsystem_probabilities(probs, dims, qargs=None):
if qargs is None:
return probs
# Convert qargs to tensor axes
probs_tens = np.reshape(probs, dims)
probs_tens = np.reshape(probs, list(reversed(dims)))
ndim = probs_tens.ndim
qargs_axes = [ndim - 1 - i for i in reversed(qargs)]
# Get sum axis for marginalized subsystems
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7 changes: 7 additions & 0 deletions releasenotes/notes/probabilities_dict_bug_fix-aac3b3d3853828dc.yaml
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Original file line number Diff line number Diff line change
@@ -0,0 +1,7 @@
---
fixes:
- |
Fixed an issue with the :meth:`.Statevector.probabilities_dict` and :meth:`.DensityMatrix.probabilities_dict`
method where it would return incorrect results for non-qubit systems when the ``qargs`` argument was
specified.
Fixed `#9210 <https://github.com/Qiskit/qiskit-terra/issues/9210>`__
61 changes: 61 additions & 0 deletions test/python/quantum_info/states/test_densitymatrix.py
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Expand Up @@ -678,6 +678,67 @@ def test_sample_counts_w(self):
counts = state.sample_counts(shots, qargs=qargs)
self.assertDictAlmostEqual(counts, target, threshold)

def test_probabilities_dict_unequal_dims(self):
"""Test probabilities_dict for a state with unequal subsystem dimensions."""

vec = np.zeros(60, dtype=float)
vec[15:20] = np.ones(5)
vec[40:46] = np.ones(6)
state = DensityMatrix(vec / np.sqrt(11.0), dims=[3, 4, 5])

p = 1.0 / 11.0

self.assertDictEqual(
state.probabilities_dict(),
{
s: p
for s in [
"110",
"111",
"112",
"120",
"121",
"311",
"312",
"320",
"321",
"322",
"330",
]
},
)

# differences due to rounding
self.assertDictAlmostEqual(
state.probabilities_dict(qargs=[0]), {"0": 4 * p, "1": 4 * p, "2": 3 * p}, delta=1e-10
)

self.assertDictAlmostEqual(
state.probabilities_dict(qargs=[1]), {"1": 5 * p, "2": 5 * p, "3": p}, delta=1e-10
)

self.assertDictAlmostEqual(
state.probabilities_dict(qargs=[2]), {"1": 5 * p, "3": 6 * p}, delta=1e-10
)

self.assertDictAlmostEqual(
state.probabilities_dict(qargs=[0, 1]),
{"10": p, "11": 2 * p, "12": 2 * p, "20": 2 * p, "21": 2 * p, "22": p, "30": p},
delta=1e-10,
)

self.assertDictAlmostEqual(
state.probabilities_dict(qargs=[1, 0]),
{"01": p, "11": 2 * p, "21": 2 * p, "02": 2 * p, "12": 2 * p, "22": p, "03": p},
delta=1e-10,
)

self.assertDictAlmostEqual(
state.probabilities_dict(qargs=[0, 2]),
{"10": 2 * p, "11": 2 * p, "12": p, "31": 2 * p, "32": 2 * p, "30": 2 * p},
delta=1e-10,
)

def test_sample_counts_qutrit(self):
"""Test sample_counts method for qutrit state"""
p = 0.3
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61 changes: 61 additions & 0 deletions test/python/quantum_info/states/test_statevector.py
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Original file line number Diff line number Diff line change
Expand Up @@ -710,6 +710,67 @@ def test_sample_counts_w(self):
counts = state.sample_counts(shots, qargs=qargs)
self.assertDictAlmostEqual(counts, target, threshold)

def test_probabilities_dict_unequal_dims(self):
"""Test probabilities_dict for a state with unequal subsystem dimensions."""

vec = np.zeros(60, dtype=float)
vec[15:20] = np.ones(5)
vec[40:46] = np.ones(6)
state = Statevector(vec / np.sqrt(11.0), dims=[3, 4, 5])

p = 1.0 / 11.0

self.assertDictEqual(
state.probabilities_dict(),
{
s: p
for s in [
"110",
"111",
"112",
"120",
"121",
"311",
"312",
"320",
"321",
"322",
"330",
]
},
)

# differences due to rounding
self.assertDictAlmostEqual(
state.probabilities_dict(qargs=[0]), {"0": 4 * p, "1": 4 * p, "2": 3 * p}, delta=1e-10
)

self.assertDictAlmostEqual(
state.probabilities_dict(qargs=[1]), {"1": 5 * p, "2": 5 * p, "3": p}, delta=1e-10
)

self.assertDictAlmostEqual(
state.probabilities_dict(qargs=[2]), {"1": 5 * p, "3": 6 * p}, delta=1e-10
)

self.assertDictAlmostEqual(
state.probabilities_dict(qargs=[0, 1]),
{"10": p, "11": 2 * p, "12": 2 * p, "20": 2 * p, "21": 2 * p, "22": p, "30": p},
delta=1e-10,
)

self.assertDictAlmostEqual(
state.probabilities_dict(qargs=[1, 0]),
{"01": p, "11": 2 * p, "21": 2 * p, "02": 2 * p, "12": 2 * p, "22": p, "03": p},
delta=1e-10,
)

self.assertDictAlmostEqual(
state.probabilities_dict(qargs=[0, 2]),
{"10": 2 * p, "11": 2 * p, "12": p, "31": 2 * p, "32": 2 * p, "30": 2 * p},
delta=1e-10,
)

def test_sample_counts_qutrit(self):
"""Test sample_counts method for qutrit state"""
p = 0.3
Expand Down