QCQMC Part 4 (#346)

* Add shadow tomography module.

* Don't depend on trial_wf.

* Add test.

* Fix import.

* Incorporate docstring suggestions.

* Remove quaff test from CI.

.github/workflows/pythonpackage.yml

@@ -51,11 +51,6 @@ jobs:
           # exit-zero treats all errors as warnings. The GitHub editor is 127 chars wide
           flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
 
-      - name: Test third_party 
-        run: |
-          pip install pytest
-          pytest third_party/quaff
-
       - name: Test with pytest
         run: |
           pip install pytest

recirq/qcqmc/bitstrings.py

@@ -0,0 +1,40 @@
+# Copyright 2024 Google
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import itertools
+from typing import Iterable, Tuple
+
+
+def get_bitstrings_a_b(*, n_orb: int, n_elec: int) -> Iterable[Tuple[bool, ...]]:
+    """Iterates over bitstrings with the right symmetry assuming a_b ordering.
+
+    This function assumes that the first n_orb qubits correspond to the alpha
+    orbitals and the second n_orb qubits correspond to the beta orbitals. The
+    ordering within the alpha and beta sectors doesn't matter (because we
+    iterate over all bitstrings with Hamming weight n_elec//2 in each sector.
+    """
+
+    if n_orb != n_elec:
+        raise NotImplementedError("n_orb must equal n_elec.")
+
+    initial_bitstring = tuple(False for _ in range(n_orb - n_elec // 2)) + tuple(
+        True for _ in range(n_elec // 2)
+    )
+
+    spin_sector_bitstrings = set()
+    for perm in itertools.permutations(initial_bitstring):
+        spin_sector_bitstrings.add(perm)
+
+    for bitstring_a, bitstring_b in itertools.product(spin_sector_bitstrings, repeat=2):
+        yield bitstring_a + bitstring_b

recirq/qcqmc/bitstrings_test.py

@@ -0,0 +1,20 @@
+import numpy as np
+import pytest
+import scipy
+
+from recirq.qcqmc.bitstrings import get_bitstrings_a_b
+
+
+def test_get_bitstrings_a_b():
+    with pytest.raises(NotImplementedError):
+        list(get_bitstrings_a_b(n_orb=4, n_elec=3))
+
+    bitstrings = np.array(list(get_bitstrings_a_b(n_orb=4, n_elec=4)))
+
+    assert bitstrings.shape[0] == scipy.special.binom(4, 2) ** 2
+    assert bitstrings.shape[1] == 2 * 4  # n_qubits columns = 2 * n_orb.
+    hamming_weight_left = np.sum(bitstrings[:, 0:4], axis=1)
+    hamming_weight_right = np.sum(bitstrings[:, 4:8], axis=1)
+
+    assert np.all(hamming_weight_left == 2)
+    assert np.all(hamming_weight_right == 2)