Qiskit · mrossinek · Jan 6, 2025 · Jan 6, 2025
diff --git a/qiskit_addon_mpf/backends/tenpy_layers/model.py b/qiskit_addon_mpf/backends/tenpy_layers/model.py
@@ -1,6 +1,6 @@
 # This code is a Qiskit project.
 #
-# (C) Copyright IBM 2024.
+# (C) Copyright IBM 2024, 2025.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -37,9 +37,6 @@ def init_sites(self, model_params: Config) -> Site:
 
         See :external:meth:`~tenpy.models.model.CouplingMPOModel.init_sites` for more details.
 
-        .. caution::
-           Currently, this enforces ``Sz`` conservation on all sites.
-
         Args:
             model_params: the model parameters.
 
@@ -48,8 +45,6 @@ def init_sites(self, model_params: Config) -> Site:
         """
         # WARN: we use our own default to NOT sort charges (contrary to TeNPy default: `True`)
         sort_charge = model_params.get("sort_charge", False, bool)
-        # TODO: currently, this default Sz conservation is coupled to the LegCharge definition in
-        # MPOState.initialize_from_lattice. Not conserving Sz errors with 'Different ChargeInfo'.
         conserve = model_params.get("conserve", "Sz", str)
         return SpinHalfSite(conserve=conserve, sort_charge=sort_charge)
 

diff --git a/qiskit_addon_mpf/backends/tenpy_tebd/state.py b/qiskit_addon_mpf/backends/tenpy_tebd/state.py
@@ -1,6 +1,6 @@
 # This code is a Qiskit project.
 #
-# (C) Copyright IBM 2024.
+# (C) Copyright IBM 2024, 2025.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -32,14 +32,17 @@ class MPOState(MPO, State):
     """
 
     @classmethod
-    def initialize_from_lattice(cls, lat: Lattice) -> MPOState:
+    def initialize_from_lattice(cls, lat: Lattice, *, conserve: bool = True) -> MPOState:
         """Construct an identity :class:`MPOState` instance matching the provided lattice shape.
 
         Given a lattice, this method constructs a new MPO identity matching the shape of the
         lattice.
 
         Args:
             lat: the lattice describing the MPO sites.
+            conserve: whether to conserve ``Sz``. This is a simplified version of the more elaborate
+                ``conserve`` property of :class:`~tenpy.networks.site.SpinHalfSite`. The boolean
+                value simply indicates ``Sz`` (``True``) or ``None`` conservation (``False``)
 
         Returns:
             An identity MPO.
@@ -53,24 +56,22 @@ def initialize_from_lattice(cls, lat: Lattice) -> MPOState:
 
         labels = ["wL", "p", "p*", "wR"]
 
-        # creates a list of tensor leg charge objects encoding charges + conjugations for tensor
-        # legs (i.e. dimensions)
-        leg_charge = [
-            # e.g. charge information for tensor leg / dimension [1] and label ["2*Sz"]
-            # creates a LegCharge object from the flattened list of charges
-            # one for each of four legs or dimensions on B
-            npc.LegCharge.from_qflat(npc.ChargeInfo([1], ["2*Sz"]), [1], qconj=1),
-            npc.LegCharge.from_qflat(npc.ChargeInfo([1], ["2*Sz"]), [1, -1], qconj=1),
-            npc.LegCharge.from_qflat(npc.ChargeInfo([1], ["2*Sz"]), [1, -1], qconj=-1),
-            npc.LegCharge.from_qflat(npc.ChargeInfo([1], ["2*Sz"]), [1], qconj=-1),
-        ]
-
-        B_array = npc.Array.from_ndarray(B, legcharges=leg_charge, labels=labels)
-
-        # FIXME: the following is supposed to allow `conserve=None` for our `SpinHalfSite` but it
-        # does not seem to work, even in the `conserve="Sz"` case.
-        #
-        # B_array = npc.Array.from_ndarray_trivial(B, labels=labels)
+        if conserve:
+            # creates a list of tensor leg charge objects encoding charges + conjugations for tensor
+            # legs (i.e. dimensions)
+            leg_charge = [
+                # e.g. charge information for tensor leg / dimension [1] and label ["2*Sz"]
+                # creates a LegCharge object from the flattened list of charges
+                # one for each of four legs or dimensions on B
+                npc.LegCharge.from_qflat(npc.ChargeInfo([1], ["2*Sz"]), [1], qconj=1),
+                npc.LegCharge.from_qflat(npc.ChargeInfo([1], ["2*Sz"]), [1, -1], qconj=1),
+                npc.LegCharge.from_qflat(npc.ChargeInfo([1], ["2*Sz"]), [1, -1], qconj=-1),
+                npc.LegCharge.from_qflat(npc.ChargeInfo([1], ["2*Sz"]), [1], qconj=-1),
+            ]
+
+            B_array = npc.Array.from_ndarray(B, legcharges=leg_charge, labels=labels)
+        else:
+            B_array = npc.Array.from_ndarray_trivial(B, labels=labels)
 
         num_sites = lat.N_sites
         # initialize the MPO psi with the wavepacket and an identity operator

diff --git a/releasenotes/notes/tenpy-conserve-none-f34f73b38b89111c.yaml b/releasenotes/notes/tenpy-conserve-none-f34f73b38b89111c.yaml
@@ -0,0 +1,8 @@
+---
+features:
+  - |
+    TeNPy can disable ``Sz`` conservation. Previously, the
+    :meth:`~qiskit_addon_mpf.backends.tenpy_tebd.MPOState.initialize_from_lattice`
+    method did not support this, but a new keyword argument ``conserve`` has
+    been added which allows the disabling of ``Sz`` conservation. The default
+    remains to be ``True``.
diff --git a/test/backends/tenpy_layers/test_e2e.py b/test/backends/tenpy_layers/test_e2e.py
@@ -1,6 +1,6 @@
 # This code is a Qiskit project.
 #
-# (C) Copyright IBM 2024.
+# (C) Copyright IBM 2024, 2025.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -24,6 +24,7 @@
     from qiskit_addon_mpf.backends.tenpy_layers import LayerModel, LayerwiseEvolver
     from qiskit_addon_mpf.backends.tenpy_tebd import MPOState, MPS_neel_state, TEBDEvolver
     from tenpy.models import XXZChain2
+    from tenpy.networks.site import SpinHalfSite
 
 
 def gen_ext_field_layer(n, hz):
@@ -52,32 +53,51 @@ def gen_even_coupling_layer(n, Jxx, Jz, J):
     return qc
 
 
+class ConserveXXZChain2(XXZChain2):
+    """TeNPy's XXZChain2 hard-codes Sz conservation. This subclass makes it configurable."""
+
+    def init_sites(self, model_params):
+        conserve = model_params.get("conserve", "Sz", bool)
+        sort_charge = model_params.get("sort_charge", True, bool)
+        return SpinHalfSite(conserve=conserve, sort_charge=sort_charge)  # use predefined Site
+
+
 @pytest.mark.skipif(not HAS_TENPY, reason="TeNPy is required for these unittests")
 class TestEndToEnd:
     @pytest.mark.parametrize(
-        ["time", "expected_A", "expected_b", "expected_coeffs"],
+        ["time", "expected_A", "expected_b", "expected_coeffs", "conserve"],
         [
             (
                 0.5,
                 [[1.0, 0.9997562], [0.9997562, 1.0]],
                 [0.99944125, 0.99857914],
                 [2.2680558, -1.26805551],
+                "None",
+            ),
+            (
+                0.5,
+                [[1.0, 0.9997562], [0.9997562, 1.0]],
+                [0.99944125, 0.99857914],
+                [2.2680558, -1.26805551],
+                "Sz",
             ),
             (
                 1.0,
                 [[1.0, 0.99189288], [0.99189288, 1.0]],
                 [0.98478594, 0.9676077],
                 [1.55870387, -0.55870387],
+                "Sz",
             ),
             (
                 1.5,
                 [[1.0, 0.95352741], [0.95352741, 1.0]],
                 [0.9032996, 0.71967399],
                 [2.47563369, -1.47563369],
+                "Sz",
             ),
         ],
     )
-    def test_end_to_end(self, time, expected_A, expected_b, expected_coeffs):
+    def test_end_to_end(self, time, expected_A, expected_b, expected_coeffs, conserve):
         np.random.seed(0)
 
         # constants
@@ -97,13 +117,14 @@ def test_end_to_end(self, time, expected_A, expected_b, expected_coeffs):
 
         # This is the full model that we want to simulate. It is used for the "exact" time evolution
         # (which is approximated via a fourth-order Suzuki-Trotter formula).
-        exact_model = XXZChain2(
+        exact_model = ConserveXXZChain2(
             {
                 "L": L,
                 "Jz": 4.0 * Jz * J,
                 "Jxx": 4.0 * Jxx * J,
                 "hz": 2.0 * hz,
                 "bc_MPS": "finite",
+                "conserve": conserve,
                 "sort_charge": False,
             }
         )
@@ -112,22 +133,26 @@ def test_end_to_end(self, time, expected_A, expected_b, expected_coeffs):
         # Trotter circuit and sliced it using `qiskit_addon_utils.slicing`.
         odd_coupling_layer = LayerModel.from_quantum_circuit(
             gen_odd_coupling_layer(L, Jxx, Jz, J),
+            conserve=conserve,
             bc_MPS="finite",
         )
         even_coupling_layer = LayerModel.from_quantum_circuit(
             gen_even_coupling_layer(L, Jxx, Jz, 2.0 * J),  # factor 2 because its the central layer
+            conserve=conserve,
             bc_MPS="finite",
         )
         odd_onsite_layer = LayerModel.from_quantum_circuit(
             gen_ext_field_layer(L, hz),
             keep_only_odd=True,
             scaling_factor=0.5,
+            conserve=conserve,
             bc_MPS="finite",
         )
         even_onsite_layer = LayerModel.from_quantum_circuit(
             gen_ext_field_layer(L, hz),
             keep_only_odd=False,
             scaling_factor=0.5,
+            conserve=conserve,
             bc_MPS="finite",
         )
         # Our layers combine to form a second-order Suzuki-Trotter formula as follows:
@@ -159,7 +184,7 @@ def test_end_to_end(self, time, expected_A, expected_b, expected_coeffs):
         model = setup_dynamic_lse(
             [4, 3],
             time,
-            partial(MPOState.initialize_from_lattice, exact_model.lat),
+            partial(MPOState.initialize_from_lattice, exact_model.lat, conserve=conserve == "Sz"),
             partial(
                 TEBDEvolver,
                 model=exact_model,

diff --git a/test/backends/tenpy_tebd/test_e2e.py b/test/backends/tenpy_tebd/test_e2e.py
@@ -1,6 +1,6 @@
 # This code is a Qiskit project.
 #
-# (C) Copyright IBM 2024.
+# (C) Copyright IBM 2024, 2025.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -21,34 +21,54 @@
 if HAS_TENPY:
     from qiskit_addon_mpf.backends.tenpy_tebd import MPOState, MPS_neel_state, TEBDEvolver
     from tenpy.models import XXZChain2
+    from tenpy.networks.site import SpinHalfSite
+
+
+class ConserveXXZChain2(XXZChain2):
+    """TeNPy's XXZChain2 hard-codes Sz conservation. This subclass makes it configurable."""
+
+    def init_sites(self, model_params):
+        conserve = model_params.get("conserve", "Sz", bool)
+        sort_charge = model_params.get("sort_charge", True, bool)
+        return SpinHalfSite(conserve=conserve, sort_charge=sort_charge)  # use predefined Site
 
 
 @pytest.mark.skipif(not HAS_TENPY, reason="TeNPy is required for these unittests")
 class TestEndToEnd:
     @pytest.mark.parametrize(
-        ["time", "expected_A", "expected_b", "expected_coeffs"],
+        ["time", "expected_A", "expected_b", "expected_coeffs", "conserve"],
         [
             (
                 0.5,
                 [[1.0, 0.9997562], [0.9997562, 1.0]],
                 [0.99944125, 0.99857914],
                 [2.26805572, -1.26805543],
+                "None",
+            ),
+            (
+                0.5,
+                [[1.0, 0.9997562], [0.9997562, 1.0]],
+                [0.99944125, 0.99857914],
+                [2.26805572, -1.26805543],
+                "Sz",
             ),
             (
                 1.0,
                 [[1.0, 0.99189288], [0.99189288, 1.0]],
                 [0.98478594, 0.9676077],
                 [1.55870386, -0.55870386],
+                "Sz",
             ),
             (
                 1.5,
                 [[1.0, 0.95352741], [0.95352741, 1.0]],
                 [0.93918471, 0.71967399],
                 [2.8617227, -1.8617227],
+                "Sz",
             ),
         ],
     )
-    def test_end_to_end(self, time, expected_A, expected_b, expected_coeffs):
+    def test_end_to_end(self, time, expected_A, expected_b, expected_coeffs, conserve):
         np.random.seed(0)
 
         # constants
@@ -68,13 +88,14 @@ def test_end_to_end(self, time, expected_A, expected_b, expected_coeffs):
 
         # This is the full model that we want to simulate. It is used for the "exact" time evolution
         # (which is approximated via a fourth-order Suzuki-Trotter formula).
-        exact_model = XXZChain2(
+        exact_model = ConserveXXZChain2(
             {
                 "L": L,
                 "Jz": 4.0 * Jz * J,
                 "Jxx": 4.0 * Jxx * J,
                 "hz": 2.0 * hz,
                 "bc_MPS": "finite",
+                "conserve": conserve,
                 "sort_charge": False,
             }
         )
@@ -98,7 +119,7 @@ def test_end_to_end(self, time, expected_A, expected_b, expected_coeffs):
         model = setup_dynamic_lse(
             [4, 3],
             time,
-            partial(MPOState.initialize_from_lattice, exact_model.lat),
+            partial(MPOState.initialize_from_lattice, exact_model.lat, conserve=conserve == "Sz"),
             partial(
                 TEBDEvolver,
                 model=exact_model,