Fix qudit surface/toric codes (#197)

* fix parity checks for rotated surface/toric codes

* fix tests and coverage

qldpc/circuits_test.py

@@ -115,7 +115,7 @@ def test_finding_circuit(pytestconfig: pytest.Config) -> None:
 
 def test_deformed_decoder() -> None:
     """Deform a code in such a way as to preserve its logicals, but change its stabilizers."""
-    code = codes.CSSCode([[1] * 6], [[1] * 6])
+    code = codes.CSSCode([[1] * 6], [[1] * 6], field=2)
     code.set_logical_ops_xz(
         [
             [1, 1, 0, 0, 0, 0],

qldpc/codes/common.py

@@ -1701,13 +1701,14 @@ def concatenate(
         operators of the concatenated code.  Otherwise, logical operators of the concatenated code
         get recomputed from scratch.
         """
+        if not isinstance(inner, CSSCode) or not isinstance(outer, CSSCode):
+            raise TypeError("CSSCode.concatenate requires CSSCode inputs")
+
         # stack copies of the inner and outer codes (if necessary) and permute inner logicals
         inner, outer = QuditCode._standardize_concatenation_inputs(
             inner, outer, outer_physical_to_inner_logical
         )
-
-        if not isinstance(inner, CSSCode) or not isinstance(outer, CSSCode):
-            raise TypeError("CSSCode.concatenate requires CSSCode inputs")
+        assert isinstance(inner, CSSCode) and isinstance(outer, CSSCode)
 
         """
         Parity checks inherited from the outer code are nominally defined in terms of their support

qldpc/codes/common_test.py

@@ -243,7 +243,8 @@ def test_qudit_code() -> None:
             [0, 1, 0, 0, 1, 0, 0, 1, 1, 0],
             [1, 0, 1, 0, 0, 0, 0, 0, 1, 1],
             [0, 1, 0, 1, 0, 1, 0, 0, 0, 1],
-        ]
+        ],
+        field=2,
     )
     assert np.array_equal(code.matrix, equiv_code.matrix)
 
@@ -439,7 +440,7 @@ def test_css_ops() -> None:
     # the 2x2 toric code has redundant stabilizers
     code = codes.ToricCode(2)
     assert code.num_checks == 4
-    assert codes.CSSCode.equiv(code, codes.CSSCode([[1, 1, 1, 1]], [[1, 1, 1, 1]]))
+    assert code.canonicalized().num_checks == 2
 
 
 def test_distance_css() -> None:

qldpc/codes/quantum.py

@@ -22,12 +22,14 @@
 import os
 from collections.abc import Collection, Sequence
 
+import galois
 import networkx as nx
 import numpy as np
 import numpy.typing as npt
 import sympy
 
 from qldpc import abstract
+from qldpc.abstract import DEFAULT_FIELD_ORDER
 from qldpc.objects import CayleyComplex, ChainComplex, Node, Pauli, QuditOperator
 
 from .classical import HammingCode, RepetitionCode, RingCode, TannerCode
@@ -998,6 +1000,12 @@ def __init__(
                 if (row + col) % 2
             ]
 
+            # invert Z-type Pauli on every other qubit
+            code_field = galois.GF(field or DEFAULT_FIELD_ORDER)
+            if code_field.order > 2:
+                matrix_z = code_field(matrix_z)
+                matrix_z[:, self._default_conjugate] *= -1
+
         else:
             # "original" surface code
             code_a = RepetitionCode(rows, field)
@@ -1007,7 +1015,7 @@ def __init__(
             matrix_z = code_ab.matrix_z
             self._default_conjugate = slice(code_ab.sector_size[0, 0], None)
 
-        CSSCode.__init__(self, matrix_x, matrix_z, field=field, validate=False)
+        CSSCode.__init__(self, matrix_x, matrix_z, field=field)
 
     @classmethod
     def get_rotated_checks(
@@ -1107,6 +1115,12 @@ def __init__(
                 if (row + col) % 2
             ]
 
+            # invert Z-type Pauli on every other qubit
+            code_field = galois.GF(field or DEFAULT_FIELD_ORDER)
+            if code_field.order > 2:
+                matrix_z = code_field(matrix_z)
+                matrix_z[:, self._default_conjugate] *= -1
+
         else:
             # "original" toric code
             code_a = RingCode(rows, field)
@@ -1116,7 +1130,7 @@ def __init__(
             matrix_z = code_ab.matrix_z
             self._default_conjugate = slice(code_ab.sector_size[0, 0], None)
 
-        CSSCode.__init__(self, matrix_x, matrix_z, field=field, validate=False)
+        CSSCode.__init__(self, matrix_x, matrix_z, field=field)
 
     @classmethod
     def get_rotated_checks(
@@ -1188,4 +1202,4 @@ def __init__(
         matrix_x, matrix_z = chain.op(1), chain.op(2).T
         assert not isinstance(matrix_x, abstract.Protograph)
         assert not isinstance(matrix_z, abstract.Protograph)
-        CSSCode.__init__(self, matrix_x, matrix_z, field, validate=False)
+        CSSCode.__init__(self, matrix_x, matrix_z, field)

qldpc/codes/quantum_test.py

@@ -388,14 +388,14 @@ def test_surface_codes(rows: int = 3, cols: int = 2) -> None:
     assert code.conjugated() == codes.HGPCode(*rep_codes).conjugated()
 
     # rotated surface code
-    code = codes.SurfaceCode(rows, cols, rotated=True)
+    code = codes.SurfaceCode(rows, cols, rotated=True, field=3)
     assert code.dimension == 1
     assert code.num_qudits == rows * cols
     assert codes.CSSCode.get_distance(code, Pauli.X) == cols
     assert codes.CSSCode.get_distance(code, Pauli.Z) == rows
 
     # test that the conjugated rotated surface code is an XZZX code
-    code = codes.SurfaceCode(max(rows, cols), rotated=True, field=2)
+    code = codes.SurfaceCode(max(rows, cols), rotated=True)
     for row in code.conjugated().matrix:
         row_x, row_z = row[: code.num_qudits], row[-code.num_qudits :]
         assert np.count_nonzero(row_x) == np.count_nonzero(row_z)
@@ -420,7 +420,7 @@ def test_toric_codes() -> None:
 
     # rotated toric code
     distance = 4
-    code = codes.ToricCode(distance, rotated=True)
+    code = codes.ToricCode(distance, rotated=True, field=3)
     assert code.dimension == 2
     assert code.num_qudits == distance**2
     assert codes.CSSCode.get_distance(code) == distance