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Remove deprecated qubit auto-alignment #1043

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199 changes: 28 additions & 171 deletions qiskit_machine_learning/utils/adjust_num_qubits.py
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Original file line number Diff line number Diff line change
Expand Up @@ -10,16 +10,14 @@
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.
"""Helper functions to adjust number of qubits."""
"""Helper functions to derive the number of qubits, feature map, and ansatz."""

from __future__ import annotations

import warnings
from qiskit import QuantumCircuit
from qiskit.circuit.library import real_amplitudes, z_feature_map, zz_feature_map

from ..exceptions import QiskitMachineLearningError
from ..utils.deprecation import issue_deprecation_msg


# pylint: disable=invalid-name
Expand All @@ -29,194 +27,53 @@ def derive_num_qubits_feature_map_ansatz(
ansatz: QuantumCircuit | None = None,
) -> tuple[int, QuantumCircuit, QuantumCircuit]:
"""
Derives a correct number of qubits, feature map, and ansatz from the parameters.

If the number of qubits is not ``None``, then the feature map and ansatz are adjusted to this
number of qubits if required. If such an adjustment fails, an error is raised. Also, if the
feature map or ansatz or both are ``None``, then :func:`~qiskit.circuit.library.zz_feature_map`
and :func:`~qiskit.circuit.library.real_amplitudes` are created respectively. If there's just
one qubit, :func:`~qiskit.circuit.library.z_feature_map` is created instead.

If the number of qubits is ``None``, then the number of qubits is derived from the feature map
or ansatz. Both the feature map and ansatz in this case must have the same number of qubits.
If the number of qubits of the feature map is not the same as the number of qubits of
the ansatz, an error is raised. If only one of the feature map and ansatz are ``None``, then
:func:`~qiskit.circuit.library.zz_feature_map` or :func:`~qiskit.circuit.library.real_amplitudes`
are created respectively.

If the number of qubits is not ``None``, then the feature map and ansatz are adjusted to this
number of qubits if required. If such an adjustment fails, an error is raised. Also, if the
feature map or ansatz or both are ``None``, then :meth:`~qiskit.circuit.library.zz_feature_map`
and :meth:`~qiskit.circuit.library.real_amplitudes` are created respectively. If there's just
one qubit, :meth:`~qiskit.circuit.library.z_feature_map` is created instead.

If all the parameters are ``None`` an error is raised.

.. warning::

The ``num_qubits`` argument and automatic qubit alignment (padding or resizing of
``feature_map`` and ``ansatz`` circuits) are **deprecated** as of version ``0.9.1`` and
will be removed after a 6-month deprecation period.

In a future release, this function will require that ``feature_map`` and ``ansatz`` are
explicitly provided with the **same number of qubits**, and passing ``num_qubits`` or
relying on automatic circuit alignment will raise an error.

To ensure forward compatibility, remove the ``num_qubits`` argument (or set it to
``None``) and construct ``feature_map`` and ``ansatz`` with matching numbers of qubits
before calling this function.

See https://github.com/qiskit-community/qiskit-machine-learning/issues/1010 for details.
Derive the number of qubits, feature map, and ansatz from the parameters.

All provided arguments must agree on the number of qubits. If only some are
provided, the missing ones are constructed at the agreed qubit count using
:func:`~qiskit.circuit.library.zz_feature_map` (or
:func:`~qiskit.circuit.library.z_feature_map` for a single qubit) and
:func:`~qiskit.circuit.library.real_amplitudes`.

Args:
num_qubits: Number of qubits (deprecated).
num_qubits: Number of qubits.
feature_map: A feature map.
ansatz: An ansatz.

Returns:
A tuple of number of qubits, feature map, and ansatz.

Raises:
QiskitMachineLearningError: If correct values can not be derived from the parameters.
QiskitMachineLearningError: If no arguments are provided, or if the
provided arguments disagree on the number of qubits.
"""
candidates = {}

counts: dict[str, int] = {}
if num_qubits is not None:
counts["num_qubits"] = num_qubits
if feature_map is not None:
candidates["feature_map"] = feature_map.num_qubits
counts["feature_map"] = feature_map.num_qubits
if ansatz is not None:
candidates["ansatz"] = ansatz.num_qubits
if num_qubits is not None:
candidates["num_qubits"] = num_qubits

issue_deprecation_msg(
msg=(
"The num_qubits argument and the qubit auto-alignment of circuits are "
"deprecated and will be removed. "
"See https://github.com/qiskit-community/qiskit-machine-learning/issues/1010 "
"for more details."
),
version="0.10.0",
remedy=(
"Remove the num_qubits argument (or set to None), and make sure that the "
"feature_map and ansatz have the same number of qubits before passing "
"them as arguments."
),
period="6 months",
)
counts["ansatz"] = ansatz.num_qubits

if not candidates:
if not counts:
raise QiskitMachineLearningError(
"Unable to determine number of qubits: "
"provide `num_qubits` (int), `feature_map` (QuantumCircuit), "
"or `ansatz` (QuantumCircuit)."
"Unable to determine number of qubits: provide `num_qubits` (int), "
"`feature_map` (QuantumCircuit), or `ansatz` (QuantumCircuit)."
)

# Check consensus on num_qubits
unique_vals = set(candidates.values())
if len(unique_vals) > 1:
conflicts = ", ".join(f"{k}={v}" for k, v in candidates.items())
warnings.warn(
(
f"Inconsistent qubit numbers detected: {conflicts}. "
"Ensure all inputs agree on the number of qubits."
),
UserWarning,
)
issue_deprecation_msg(
msg=(
"Qubit auto-alignment of circuits is deprecated and will be removed. "
"In the future, an error will be raised if the number of qubits in "
"feature_map and ansatz are note the same. "
"See https://github.com/qiskit-community/qiskit-machine-learning/issues/1010 "
"for more details."
),
version="0.10.0",
remedy=(
"Ensure that the feature_map and ansatz have the same number of qubits "
"before passing them as arguments."
),
period="6 months",
unique_counts = set(counts.values())
if len(unique_counts) > 1:
details = ", ".join(f"{k}={v}" for k, v in counts.items())
raise QiskitMachineLearningError(
f"Inconsistent qubit counts: {details}. "
"Adjust the inputs to match before passing them as arguments."
)

# Final resolved number of qubits
resolved_num_qubits = max(unique_vals)

def default_feature_map(n: int) -> QuantumCircuit:
return z_feature_map(n) if n == 1 else zz_feature_map(n)

def default_ansatz(n: int) -> QuantumCircuit:
return real_amplitudes(n)
resolved = next(iter(unique_counts))

if feature_map is None:
feature_map = default_feature_map(resolved_num_qubits)
candidates["feature_map"] = feature_map.num_qubits
else:
feature_map = _pad_if_needed(feature_map, resolved_num_qubits)

feature_map = z_feature_map(resolved) if resolved == 1 else zz_feature_map(resolved)
if ansatz is None:
ansatz = default_ansatz(resolved_num_qubits)
candidates["ansatz"] = ansatz.num_qubits
else:
ansatz = _pad_if_needed(ansatz, resolved_num_qubits)
ansatz = real_amplitudes(resolved)

# Mismatch in the circuits' num_qubits is unacceptable
if candidates["feature_map"] != candidates["ansatz"]:
raise QiskitMachineLearningError(
f"Inconsistent qubit numbers detected between the feature map ({candidates['feature_map']}) "
f"and the ansatz ({candidates['ansatz']}). These must match at all times."
)

return resolved_num_qubits, feature_map, ansatz


def _pad_if_needed(circ: QuantumCircuit, requested_num_qubits: int) -> QuantumCircuit | None:
"""
.. warning::

This function is **deprecated** as of version ``0.9.1``.
See https://github.com/qiskit-community/qiskit-machine-learning/issues/1010 for details.
"""
circ_nq = circ.num_qubits

if requested_num_qubits == circ_nq:
return circ

if requested_num_qubits < circ_nq:
raise QiskitMachineLearningError(
f"Requesting num_qubits={requested_num_qubits} to a circuit with {circ_nq} qubits. "
f"Circuit cutting is not supported by default. Please, remove qubit registers manually."
)

warnings.warn(
(
f"Requesting num_qubits={requested_num_qubits} to a circuit with {circ_nq} qubits. "
f"Padding with {requested_num_qubits - circ_nq} idle qubits."
),
UserWarning,
)
padded = QuantumCircuit(requested_num_qubits, circ.num_clbits, name=circ.name)
padded.compose(circ, inplace=True)
return padded


# pylint: disable=unused-argument
def _adjust_num_qubits(circuit: QuantumCircuit, circuit_name: str, num_qubits: int) -> None:
"""
Tries to adjust the number of qubits of the circuit by trying to set ``num_qubits`` properties.

Args:
circuit: A circuit to adjust.
circuit_name: A circuit name, used in the error description.
num_qubits: A number of qubits to set.

Raises:
QiskitMachineLearningError: if number of qubits can't be adjusted.

"""
issue_deprecation_msg(
msg="No longer in use",
version="0.9.0",
remedy="Check ",
period="0 months",
)
return resolved, feature_map, ansatz
22 changes: 22 additions & 0 deletions releasenotes/notes/remove-qubit-auto-alignment-1024.yaml
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@@ -0,0 +1,22 @@
---
upgrade:
- |
The qubit auto-alignment behavior of
:func:`~qiskit_machine_learning.utils.derive_num_qubits_feature_map_ansatz`
has been removed, as previously announced in version ``0.9.1``. The
function no longer accepts a ``num_qubits`` argument; the number of qubits
is now derived directly from ``feature_map`` and/or ``ansatz``. If both
are provided, they must have the same number of qubits, otherwise a
``QiskitMachineLearningError`` is raised.
- |
The ``num_qubits`` argument is no longer supported by the constructors of
:class:`~qiskit_machine_learning.algorithms.classifiers.VQC` and
:class:`~qiskit_machine_learning.algorithms.regressors.VQR`, nor by
:func:`~qiskit_machine_learning.circuit.library.qnn_circuit`. Passing it
raises a ``QiskitMachineLearningError``. Construct ``feature_map`` and
``ansatz`` with the desired number of qubits before passing them to these
APIs.
fixes:
- |
Removes the deprecated qubit auto-alignment feature. See
`#1024 <https://github.com/qiskit-community/qiskit-machine-learning/issues/1024>`__.
79 changes: 41 additions & 38 deletions test/utils/test_adjust_num_qubits.py
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Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,7 @@
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.
"""Tests for adjusting number of qubits in a feature map / ansatz."""
"""Tests for derive_num_qubits_feature_map_ansatz."""

from test import QiskitMachineLearningTestCase
import itertools
Expand All @@ -22,7 +22,7 @@


@ddt
class TestAdjustNumQubits(QiskitMachineLearningTestCase):
class TestDeriveNumQubits(QiskitMachineLearningTestCase):
"""Tests for the derive_num_qubits_feature_map_ansatz function."""

def setUp(self) -> None:
Expand All @@ -35,13 +35,13 @@ def setUp(self) -> None:
}

def test_all_none(self):
"""Test when all parameters are ``None``."""
"""All parameters None must raise."""
self.assertRaises(
QiskitMachineLearningError, derive_num_qubits_feature_map_ansatz, None, None, None
)

def test_incompatible_feature_map_ansatz(self):
"""Test when feature map and ansatz are incompatible."""
def test_mismatch_feature_map_ansatz(self):
"""feature_map and ansatz with different qubit counts must raise."""
self.assertRaises(
QiskitMachineLearningError,
derive_num_qubits_feature_map_ansatz,
Expand All @@ -50,6 +50,26 @@ def test_incompatible_feature_map_ansatz(self):
self.properties["ra2"],
)

def test_mismatch_num_qubits_feature_map(self):
"""num_qubits != feature_map.num_qubits must raise."""
self.assertRaises(
QiskitMachineLearningError,
derive_num_qubits_feature_map_ansatz,
2,
self.properties["z1"],
None,
)

def test_mismatch_num_qubits_ansatz(self):
"""num_qubits != ansatz.num_qubits must raise."""
self.assertRaises(
QiskitMachineLearningError,
derive_num_qubits_feature_map_ansatz,
2,
None,
self.properties["ra1"],
)

@idata(
itertools.chain(
itertools.product([1], [None, "z1"], [None, "ra1"]),
Expand All @@ -58,45 +78,28 @@ def test_incompatible_feature_map_ansatz(self):
)
@unpack
def test_num_qubits_is_set(self, num_qubits, feature_map, ansatz):
"""Test when the number of qubits is set."""
"""When num_qubits is set and inputs agree."""
feature_map = self.properties.get(feature_map)
ansatz = self.properties.get(ansatz)

# derived objects
num_qubits_der, feature_map_der, ansatz_der = derive_num_qubits_feature_map_ansatz(
num_qubits, feature_map, ansatz
)
self.assertEqual(num_qubits_der, num_qubits)

self._test_feature_map(feature_map_der, feature_map, num_qubits)
self._test_ansatz(ansatz_der, num_qubits)
nq, fm, anz = derive_num_qubits_feature_map_ansatz(num_qubits, feature_map, ansatz)
self.assertEqual(nq, num_qubits)
self.assertEqual(fm.num_qubits, num_qubits)
self.assertEqual(anz.num_qubits, num_qubits)
self.assertIsInstance(fm, QuantumCircuit)
self.assertIsInstance(anz, QuantumCircuit)

@idata([(None, "ra1"), (None, "ra2"), ("z1", None), ("z1", "ra1"), ("z2", None), ("z2", "ra2")])
@unpack
def test_num_qubits_isnot_set(self, feature_map, ansatz):
"""Test when the number of qubits is not set."""
ansatz = self.properties.get(ansatz)
"""When num_qubits is None and inputs determine the count."""
feature_map = self.properties.get(feature_map)
ansatz = self.properties.get(ansatz)
expected = feature_map.num_qubits if feature_map is not None else ansatz.num_qubits

num_qubits_der, feature_map_der, ansatz_der = derive_num_qubits_feature_map_ansatz(
None, feature_map, ansatz
)
num_qubits = feature_map.num_qubits if feature_map is not None else ansatz.num_qubits

self.assertEqual(num_qubits_der, num_qubits)
self._test_feature_map(feature_map_der, feature_map, num_qubits)
self._test_ansatz(ansatz_der, num_qubits)

def _test_feature_map(self, feature_map_der, feature_map_org, num_qubits_expected):
self.assertIsNotNone(feature_map_der)
self.assertEqual(feature_map_der.num_qubits, num_qubits_expected)

if feature_map_org is None and num_qubits_expected == 1:
self.assertIsInstance(feature_map_der, QuantumCircuit)
if feature_map_org is None and num_qubits_expected == 2:
self.assertIsInstance(feature_map_der, QuantumCircuit)

def _test_ansatz(self, ansatz_der, num_qubits_expected):
self.assertIsNotNone(ansatz_der)
self.assertEqual(ansatz_der.num_qubits, num_qubits_expected)
self.assertIsInstance(ansatz_der, QuantumCircuit)
nq, fm, anz = derive_num_qubits_feature_map_ansatz(None, feature_map, ansatz)
self.assertEqual(nq, expected)
self.assertEqual(fm.num_qubits, expected)
self.assertEqual(anz.num_qubits, expected)
self.assertIsInstance(fm, QuantumCircuit)
self.assertIsInstance(anz, QuantumCircuit)