Creating the LightningTensor device class based on the new API (#671)

* empty commit (triggering CI)

* Auto update version

* Definition of the two front-end classes

* adding the `lightning_tensor` string to the supported backends in `setup.py`

* adding `__init__.py` file to directly import the `lightning_tensor` device class

* re-naming file

* Auto update version

* Creating the first prototype of initial MPS state tensor using `quimb`

* providing the `backend`, `method` parameters and making `wires` optional

* Changing names and structure

* Auto update version

* adding method required by the new device API design

* Auto update version

* using the `kwargs` parameter in `LightningTensor` and `CircuitMPS` in `quimb`

* taking some further inputs from the new device API

* Perhaps decided the overall structure of `LIghtningTensor`

* Auto update version

* adding docs to methods

* temporary changes so that `pylint` does not complain at this stage

* running `isort`

* re-running formatter after `isort`

* re-running formatter after `isort`

* Applying suggested formatting change from CI

* adding tmp unit tests

* Adding `quimb` in `requirements.txt`

* runing `isort` on mps test

* removing `quimb` from requirement and deleting unit tests for `lightning.tensor`

* Auto update version

* re-inserting unit tests with an additional `yml` file

* running isort on quimb test

* changing name of yml file

* preventing error in import

* updating yml file

* inserting `quimb` package in requirements-dev

* strange error with `quimb`

* strange error with `quimb`

* specifying scipy version

* removing installation of scipy from yml file

* removing the new `yml` file

* testing if tests are tested

* Covering all lines in tests

* forgot final line for formatter

* Python formatter on CI complaints

* covering missing lines

* formatter on CI complaints

* Trying not to skip test if Cpp is enabled

* skipping tests if Cpp is enabled

* removing the only line not covered by tests so far

* Auto update version

* Applying suggestions from code review and making the `state` attribute private (new API design)

* Python formatter

* removing params from `QuimbMPS`

* Auto update version

* removing `**kwargs` from `QuimbMPS`

* removing unnecessary param at this stage

* covering test line

* formatter...

* removing param description

* Making `pylint` happy

* forgot new arg in test

* Updating base class and `preprocess` function

* Updating `LightningTensor` class with new names from more advanced PR

* Auto update version

* Auto update version

* Triggering CI

* Auto update version

* Trying to remove pin from `quimb` in `requirements.dev`

* Auto update version

* Auto update version

* Removing infos on derivatives and using config options to pass parameters to interface

* Usual `pylint` failures

* Trying to solve formatting errors

* typo in docstring

* Sunday update: improved docstrings and structure

* Removing method that was supposed to be in next PR

* removing old TODO comment

* Removing changes from the `setup.py` file

* restoring previous format to `setup.py`

* Auto update version from '0.36.0-dev34' to '0.36.0-dev41'

* Auto update version from '0.36.0-dev40' to '0.36.0-dev41'

* Removing kwargs as suggested from code review

* Addressing comments from CR

* Skipping tests if CPP binary is available

* Auto update version from '0.36.0-dev42' to '0.36.0-dev43'

* Auto update version from '0.36.0-dev43' to '0.36.0-dev44'

* Restoring name in changelog (?)

* Increasing time limit for Python tests

* Applying suggestions from code review

---------

Co-authored-by: Dev version update bot <github-actions[bot]@users.noreply.github.com>
Co-authored-by: ringo-but-quantum <[email protected]>

.github/CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ### New features since last release
 
+* Add Python class for the `lightning.tensor` device which uses the new device API and the interface for `quimb` based on the MPS method.
+  [(#671)](https://github.com/PennyLaneAI/pennylane-lightning/pull/671)
+
 * Add compile-time support for AVX2/512 streaming operations in `lightning.qubit`.
   [(#664)](https://github.com/PennyLaneAI/pennylane-lightning/pull/664)
 
@@ -133,7 +136,7 @@
 
 This release contains contributions from (in alphabetical order):
 
-Ali Asadi, Amintor Dusko, Christina Lee, Vincent Michaud-Rioux, Lee James O'Riordan, Mudit Pandey, Shuli Shu
+Ali Asadi, Amintor Dusko, Pietropaolo Frisoni, Christina Lee, Vincent Michaud-Rioux, Lee James O'Riordan, Mudit Pandey, Shuli Shu
 
 ---
 

.github/workflows/tests_linux_python.yml

@@ -37,7 +37,7 @@ jobs:
       matrix:
         os: [ubuntu-22.04]
         pl_backend: ["lightning_qubit"]
-    timeout-minutes: 60
+    timeout-minutes: 75
     name: Python tests
     runs-on: ${{ matrix.os }}
 
@@ -144,7 +144,7 @@ jobs:
       matrix:
         os: [ubuntu-22.04]
         pl_backend: ["lightning_qubit"]
-    timeout-minutes: 60
+    timeout-minutes: 75
     name: Python tests with OpenBLAS
     runs-on: ${{ matrix.os }}
 
@@ -257,7 +257,7 @@ jobs:
         exclude:
           - pl_backend: ["all"]
             exec_model: OPENMP
-    timeout-minutes: 60
+    timeout-minutes: 75
     name: Python tests with Kokkos
     runs-on: ${{ matrix.os }}
 

pennylane_lightning/core/_version.py

@@ -16,4 +16,4 @@
    Version number (major.minor.patch[-label])
 """
 
-__version__ = "0.36.0-dev43"
+__version__ = "0.36.0-dev44"

pennylane_lightning/lightning_tensor/__init__.py

@@ -0,0 +1,18 @@
+# Copyright 2018-2024 Xanadu Quantum Technologies Inc.
+
+# 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
+
+#     http://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.
+"""PennyLane lightning_tensor package."""
+
+from pennylane_lightning.core import __version__
+
+from .lightning_tensor import LightningTensor

pennylane_lightning/lightning_tensor/backends/quimb/_mps.py

@@ -0,0 +1,100 @@
+# Copyright 2018-2024 Xanadu Quantum Technologies Inc.
+
+# 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
+
+#     http://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.
+"""
+Class implementation for the Quimb MPS interface for simulating quantum circuits while keeping the state always in MPS form.
+"""
+
+import numpy as np
+import pennylane as qml
+import quimb.tensor as qtn
+from pennylane.wires import Wires
+
+_operations = frozenset({})  # pragma: no cover
+# The set of supported operations.
+
+_observables = frozenset({})  # pragma: no cover
+# The set of supported observables.
+
+
+def stopping_condition(op: qml.operation.Operator) -> bool:
+    """A function that determines if an operation is supported by ``lightning.tensor`` for this interface."""
+    return op.name in _operations  # pragma: no cover
+
+
+def accepted_observables(obs: qml.operation.Operator) -> bool:
+    """A function that determines if an observable is supported by ``lightning.tensor`` for this interface."""
+    return obs.name in _observables  # pragma: no cover
+
+
+class QuimbMPS:
+    """Quimb MPS class.
+
+    Used internally by the `LightningTensor` device.
+    Interfaces with `quimb` for MPS manipulation, and provides methods to execute quantum circuits.
+
+    Args:
+        num_wires (int): the number of wires in the circuit.
+        interf_opts (dict): dictionary containing the interface options.
+        dtype (np.dtype): the complex type used for the MPS.
+    """
+
+    def __init__(self, num_wires, interf_opts, dtype=np.complex128):
+
+        if dtype not in [np.complex64, np.complex128]:  # pragma: no cover
+            raise TypeError(f"Unsupported complex type: {dtype}")
+
+        self._wires = Wires(range(num_wires))
+        self._dtype = dtype
+
+        self._init_state_ops = {
+            "binary": "0" * max(1, len(self._wires)),
+            "dtype": self._dtype.__name__,
+            "tags": [str(l) for l in self._wires.labels],
+        }
+
+        self._gate_opts = {
+            "contract": "swap+split",
+            "parametrize": None,
+            "cutoff": interf_opts["cutoff"],
+            "max_bond": interf_opts["max_bond_dim"],
+        }
+
+        self._expval_opts = {
+            "dtype": self._dtype.__name__,
+            "simplify_sequence": "ADCRS",
+            "simplify_atol": 0.0,
+        }
+
+        self._circuitMPS = qtn.CircuitMPS(psi0=self._initial_mps())
+
+    @property
+    def state(self):
+        """Current MPS handled by the interface."""
+        return self._circuitMPS.psi
+
+    def state_to_array(self) -> np.ndarray:
+        """Contract the MPS into a dense array."""
+        return self._circuitMPS.to_dense()
+
+    def _initial_mps(self) -> qtn.MatrixProductState:
+        r"""
+        Returns an initial state to :math:`\ket{0}`.
+
+        Internally, it uses `quimb`'s `MPS_computational_state` method.
+
+        Returns:
+            MatrixProductState: The initial MPS of a circuit.
+        """
+
+        return qtn.MPS_computational_state(**self._init_state_ops)