MPS emulator for LUCJ circuits

docs/api/ffsim.tenpy.rst

@@ -0,0 +1,7 @@
+ffsim.tenpy
+===========
+
+.. automodule:: ffsim.tenpy
+   :members:
+   :special-members:
+   :show-inheritance:

docs/api/index.md

@@ -9,5 +9,6 @@ ffsim.linalg
 ffsim.optimize
 ffsim.qiskit
 ffsim.random
+ffsim.tenpy
 ffsim.testing
 ```

docs/how-to-guides/index.md

@@ -4,6 +4,7 @@
 :maxdepth: 1
 
 lucj
+lucj_mps
 entanglement-forging
 fermion-operator
 qiskit-circuits

pyproject.toml

@@ -26,6 +26,7 @@ dependencies = [
     "numpy",
     "opt_einsum",
     "orjson",
+    "physics-tenpy",
     "pyscf >= 2.7",
     "qiskit >= 1.1",
     "scipy",

python/ffsim/__init__.py

@@ -10,7 +10,7 @@
 
 """ffsim is a software library for fast simulation of fermionic quantum circuits."""
 
-from ffsim import contract, linalg, optimize, qiskit, random, testing
+from ffsim import contract, linalg, optimize, qiskit, random, tenpy, testing
 from ffsim.cistring import init_cache
 from ffsim.gates import (
     apply_diag_coulomb_evolution,
@@ -175,6 +175,7 @@
     "slater_determinant_rdms",
     "spin_square",
     "strings_to_addresses",
+    "tenpy",
     "testing",
     "trace",
 ]

python/ffsim/tenpy/__init__.py

@@ -0,0 +1,41 @@
+# (C) Copyright IBM 2024.
+#
+# 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
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# 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.
+
+"""Code that uses TeNPy, e.g. for emulating quantum circuits."""
+
+from ffsim.tenpy.gates.abstract_gates import (
+    apply_single_site,
+    apply_two_site,
+)
+from ffsim.tenpy.gates.basic_gates import (
+    givens_rotation,
+    num_interaction,
+    num_num_interaction,
+    on_site_interaction,
+)
+from ffsim.tenpy.gates.diag_coulomb import apply_diag_coulomb_evolution
+from ffsim.tenpy.gates.orbital_rotation import apply_orbital_rotation
+from ffsim.tenpy.gates.ucj import apply_ucj_op_spin_balanced
+from ffsim.tenpy.hamiltonians.molecular_hamiltonian import MolecularHamiltonianMPOModel
+from ffsim.tenpy.util import bitstring_to_mps
+
+__all__ = [
+    "apply_ucj_op_spin_balanced",
+    "apply_diag_coulomb_evolution",
+    "apply_orbital_rotation",
+    "apply_single_site",
+    "apply_two_site",
+    "bitstring_to_mps",
+    "givens_rotation",
+    "MolecularHamiltonianMPOModel",
+    "num_interaction",
+    "num_num_interaction",
+    "on_site_interaction",
+]

python/ffsim/tenpy/gates/__init__.py

@@ -0,0 +1,9 @@
+# (C) Copyright IBM 2024.
+#
+# 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
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# 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.

python/ffsim/tenpy/gates/abstract_gates.py

@@ -0,0 +1,84 @@
+# (C) Copyright IBM 2024.
+#
+# 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
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# 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.
+
+"""TeNPy abstract gates."""
+
+import numpy as np
+import tenpy.linalg.np_conserved as npc
+from tenpy.algorithms.tebd import TEBDEngine
+from tenpy.linalg.charges import LegPipe
+from tenpy.networks.site import SpinHalfFermionSite
+
+# ignore lowercase argument and variable checks to maintain TeNPy naming conventions
+# ruff: noqa: N803, N806
+
+# define sites
+shfs = SpinHalfFermionSite(cons_N="N", cons_Sz="Sz")
+shfsc = LegPipe([shfs.leg, shfs.leg])
+
+
+def apply_single_site(eng: TEBDEngine, U1: np.ndarray, site: int) -> None:
+    r"""Apply a single-site gate to an MPS.
+
+    Args:
+        eng: The TEBD engine.
+        U1: The single-site quantum gate.
+        site: The gate will be applied to `site` on the MPS.
+
+    Returns:
+        None
+    """
+    U1_npc = npc.Array.from_ndarray(U1, [shfs.leg, shfs.leg.conj()], labels=["p", "p*"])
+    psi = eng.get_resume_data()["psi"]
+    psi.apply_local_op(site, U1_npc)
+
+
+def apply_two_site(
+    eng: TEBDEngine,
+    U2: np.ndarray,
+    sites: tuple[int, int],
+    *,
+    norm_tol: float = 1e-8,
+) -> None:
+    r"""Apply a two-site gate to an MPS.
+
+    Args:
+        eng: The TEBD engine.
+        U2: The two-site quantum gate.
+        sites: The gate will be applied to adjacent sites `(site1, site2)` on the MPS.
+        norm_tol: The norm error above which we recanonicalize the MPS. In general, the
+         application of a two-site gate to an MPS with truncation may degrade its
+         canonical form. To mitigate this, we explicitly bring the MPS back into
+         canonical form, if the Frobenius norm of the `site-resolved norm errors array <https://tenpy.readthedocs.io/en/latest/reference/tenpy.networks.mps.MPS.html#tenpy.networks.mps.MPS.norm_test>`_
+         is greater than `norm_tol`.
+
+    Returns:
+        None
+    """
+
+    # check that sites are adjacent
+    if abs(sites[0] - sites[1]) != 1:
+        raise ValueError("sites must be adjacent")
+
+    # check whether to transpose gate
+    if sites[0] > sites[1]:
+        U2 = U2.T
+
+    # apply NN gate between (site1, site2)
+    U2_npc = npc.Array.from_ndarray(
+        U2, [shfsc, shfsc.conj()], labels=["(p0.p1)", "(p0*.p1*)"]
+    )
+    U2_npc_split = U2_npc.split_legs()
+    eng.update_bond(max(sites), U2_npc_split)
+
+    # recanonicalize psi if below error threshold
+    psi = eng.get_resume_data()["psi"]
+    if np.linalg.norm(psi.norm_test()) > norm_tol:
+        psi.canonical_form_finite()