Add null qubit (#1179)

**Context:** Implement a Catalyst runtime plugin that mocks out all
functions in the QuantumDevice interface and implement a PennyLane
device plugin (implemented in
[qjit_device.py](https://github.com/PennyLaneAI/catalyst/blob/main/frontend/catalyst/device/qjit_device.py))
providing access to the Null runtime device.

**Description of the Change:**
- [x] Rename the device from "dummy" to "null".
- [x] Make sure the device is up to date with the latest version of the
QuantumDevice interface.
- [x] Ensure proper unit testing of the device in C++.
- [x] Package and ship this plugin for both development builds and
wheels.

**Benefits:**
Such a device will be useful for a few reasons:

* "execute" programs without needing to install additional plugins
* Simplify and speed up integration tests when numerical results aren't
needed
* Benchmark programs while excluding the cost of simulation -> exposes
the compilation and infrastructure overhead
* Can also act as an instruction tracer / program printer

**Possible Drawbacks:** It is a surprise.

**Related GitHub Issues:**

[sc-72829] [sc-72830]

---------

Co-authored-by: erick-xanadu <[email protected]>
Co-authored-by: David Ittah <[email protected]>

.github/workflows/build-wheel-linux-x86_64.yaml

@@ -348,7 +348,7 @@ jobs:
               -DENABLE_OPENMP=OFF \
               -DLQ_ENABLE_KERNEL_OMP=OFF
 
-        cmake --build runtime-build --target rt_capi rtd_lightning rtd_openqasm rtd_dummy
+        cmake --build runtime-build --target rt_capi rtd_lightning rtd_openqasm rtd_null_qubit
 
     # Build OQC-Runtime
     - name: Build OQC-Runtime

.github/workflows/build-wheel-macos-arm64.yaml

@@ -308,7 +308,7 @@ jobs:
               -DENABLE_OPENMP=OFF \
               -DLQ_ENABLE_KERNEL_OMP=OFF
 
-        cmake --build runtime-build --target rt_capi rtd_lightning rtd_openqasm rtd_dummy
+        cmake --build runtime-build --target rt_capi rtd_lightning rtd_openqasm rtd_null_qubit
 
     - name: Test Catalyst-Runtime
       run: |

.github/workflows/build-wheel-macos-x86_64.yaml

@@ -299,7 +299,7 @@ jobs:
               -DENABLE_OPENMP=OFF \
               -DLQ_ENABLE_KERNEL_OMP=OFF
 
-        cmake --build runtime-build --target rt_capi rtd_lightning rtd_openqasm rtd_dummy
+        cmake --build runtime-build --target rt_capi rtd_lightning rtd_openqasm rtd_null_qubit
 
     # Build OQC-Runtime
     - name: Build OQC-Runtime

.github/workflows/check-catalyst.yaml

@@ -57,15 +57,6 @@ jobs:
           ENABLE_ASAN=OFF \
           make runtime
 
-          # This is needed in the artifact for the pytests
-          # Note the lack of sanitizers.
-          # Left other flags the same.
-          COMPILER_LAUNCHER="" \
-          C_COMPILER=$(which ${{ needs.constants.outputs[format('c_compiler.{0}', matrix.compiler)] }}) \
-          CXX_COMPILER=$(which ${{ needs.constants.outputs[format('cxx_compiler.{0}', matrix.compiler)] }}) \
-          RT_BUILD_DIR="$(pwd)/runtime-build" \
-          make dummy_device
-
           COMPILER_LAUNCHER="" \
           C_COMPILER=$(which ${{ needs.constants.outputs[format('c_compiler.{0}', matrix.compiler)] }}) \
           CXX_COMPILER=$(which ${{ needs.constants.outputs[format('cxx_compiler.{0}', matrix.compiler)] }}) \

.github/workflows/scripts/linux_arm64/rh8/build_catalyst.sh

@@ -14,21 +14,21 @@ export PYTHON_PACKAGE=$4
 export PYTHON_ALTERNATIVE_VERSION=$5
 
 # Install system dependencies
-dnf update -y 
+dnf update -y
 dnf install -y libzstd-devel gcc-toolset-${GCC_VERSION}
 if [ "$PYTHON_VERSION" != "3.10" ]; then
     dnf install -y ${PYTHON_PACKAGE} ${PYTHON_PACKAGE}-devel
 fi
 dnf clean all -y
 
 # Make GCC the default compiler
-source /opt/rh/gcc-toolset-${GCC_VERSION}/enable -y 
-export C_COMPILER=/opt/rh/gcc-toolset-${GCC_VERSION}/root/usr/bin/gcc 
+source /opt/rh/gcc-toolset-${GCC_VERSION}/enable -y
+export C_COMPILER=/opt/rh/gcc-toolset-${GCC_VERSION}/root/usr/bin/gcc
 export CXX_COMPILER=/opt/rh/gcc-toolset-${GCC_VERSION}/root/usr/bin/g++
 
 # Set the right Python interpreter
 rm -rf /usr/bin/python3
-ln -s /opt/_internal/cpython-${PYTHON_VERSION}.${PYTHON_SUBVERSION}/bin/python3 /usr/bin/python3 
+ln -s /opt/_internal/cpython-${PYTHON_VERSION}.${PYTHON_SUBVERSION}/bin/python3 /usr/bin/python3
 export PYTHON=/usr/bin/python3
 
 # Add LLVM, Python and GCC to the PATH env var
@@ -51,7 +51,7 @@ cmake -S runtime -B runtime-build -G Ninja \
     -DENABLE_OPENQASM=ON \
     -DENABLE_OPENMP=OFF \
     -DLQ_ENABLE_KERNEL_OMP=OFF
-cmake --build runtime-build --target rt_capi rtd_lightning rtd_openqasm rtd_dummy
+cmake --build runtime-build --target rt_capi rtd_lightning rtd_openqasm rtd_null_qubit
 
 # Build OQC
 export OQC_BUILD_DIR="/catalyst/oqc-build"

.gitignore

@@ -24,3 +24,6 @@ frontend/mlir_quantum
 doc/_build
 doc/code/api
 .ipynb*
+
+# Development
+venv

Makefile

@@ -66,7 +66,6 @@ help:
 	@echo "  mlir               to build MLIR and custom Catalyst dialects"
 	@echo "  runtime            to build Catalyst Runtime"
 	@echo "  oqc                to build Catalyst-OQC Runtime"
-	@echo "  dummy_device       needed for frontend tests"
 	@echo "  test               to run the Catalyst test suites"
 	@echo "  docs               to build the documentation for Catalyst"
 	@echo "  clean              to uninstall Catalyst and delete all temporary and cache files"
@@ -112,9 +111,6 @@ dialects:
 runtime:
 	$(MAKE) -C runtime runtime
 
-dummy_device:
-	$(MAKE) -C runtime dummy_device
-
 oqc:
 	$(MAKE) -C frontend/catalyst/third_party/oqc/src oqc
 
@@ -268,11 +264,11 @@ format-frontend:
 ifdef check
 	$(PYTHON) ./bin/format.py --check $(if $(version:-=),--cfversion $(version)) ./frontend/catalyst/utils
 	black --check --verbose .
-	isort --check --diff . 
+	isort --check --diff .
 else
 	$(PYTHON) ./bin/format.py $(if $(version:-=),--cfversion $(version)) ./frontend/catalyst/utils
 	black .
-	isort . 
+	isort .
 endif
 
 .PHONY: docs clean-docs

doc/dev/custom_devices.rst

@@ -168,7 +168,7 @@ The Pennylane device API allows you to build a QJIT compatible device in a simpl
 .. code-block:: python
 
     class CustomDevice(qml.devices.Device):
-        """Dummy Device"""
+        """Custom Device"""
 
         config = pathlib.Path("absolute/path/to/configuration/file.toml")
 
@@ -178,7 +178,7 @@ The Pennylane device API allows you to build a QJIT compatible device in a simpl
             the location to the shared object with the C/C++ device implementation.
             """
 
-            return "CustomDevice", "absolute/path/to/librtd_dummy.so"
+            return "CustomDevice", "absolute/path/to/librtd_custom.so"
 
         def __init__(self, shots=None, wires=None):
             super().__init__(wires=wires, shots=shots)

frontend/catalyst/device/qjit_device.py

@@ -126,6 +126,7 @@
 # TODO: This should be removed after implementing `get_c_interface`
 # for the following backend devices:
 SUPPORTED_RT_DEVICES = {
+    "null.qubit": ("NullQubit", "librtd_null_qubit"),
     "lightning.qubit": ("LightningSimulator", "librtd_lightning"),
     "braket.aws.qubit": ("OpenQasmDevice", "librtd_openqasm"),
     "braket.local.qubit": ("OpenQasmDevice", "librtd_openqasm"),

frontend/test/conftest.py

@@ -23,9 +23,6 @@
 
 # pylint: disable=unused-import,wrong-import-position
 import platform
-
-import numpy as np
-import pennylane as qml
 import pytest
 
 

frontend/test/lit/test_decomposition.py

@@ -70,9 +70,9 @@ def get_c_interface():
             """
             system_extension = ".dylib" if platform.system() == "Darwin" else ".so"
             lib_path = (
-                get_lib_path("runtime", "RUNTIME_LIB_DIR") + "/librtd_dummy" + system_extension
+                get_lib_path("runtime", "RUNTIME_LIB_DIR") + "/librtd_null_qubit" + system_extension
             )
-            return "dummy.remote", lib_path
+            return "NullQubit", lib_path
 
         def execute(self, circuits, execution_config):
             """Execution."""

frontend/test/lit/test_device_api.py

@@ -16,6 +16,8 @@
 
 """Test for the device API.
 """
+import os
+import pathlib
 import platform
 from typing import Optional
 
@@ -27,11 +29,14 @@
 from catalyst import qjit
 from catalyst.compiler import get_lib_path
 
+TEST_PATH = os.path.dirname(__file__)
+CONFIG_CUSTOM_DEVICE = pathlib.Path(f"{TEST_PATH}/../custom_device/custom_device.toml")
 
-class DummyDevice(Device):
-    """A dummy device from the device API."""
 
-    config = get_lib_path("runtime", "RUNTIME_LIB_DIR") + "/backend/dummy_device.toml"
+class CustomDevice(Device):
+    """A custom device that does nothing."""
+
+    config = CONFIG_CUSTOM_DEVICE
 
     def __init__(self, wires, shots=1024):
         super().__init__(wires=wires, shots=shots)
@@ -42,11 +47,11 @@ def get_c_interface():
         the location to the shared object with the C/C++ device implementation.
         """
         system_extension = ".dylib" if platform.system() == "Darwin" else ".so"
-        lightning_lib_path = (
-            get_lib_path("runtime", "RUNTIME_LIB_DIR") + "/librtd_lightning" + system_extension
+        null_qubit_lib_path = (
+            get_lib_path("runtime", "RUNTIME_LIB_DIR") + "/librtd_null_qubit" + system_extension
         )
 
-        return "dummy.remote", lightning_lib_path
+        return "Custom", null_qubit_lib_path
 
     def execute(self, circuits, execution_config):
         """Execute"""
@@ -65,8 +70,8 @@ def preprocess(self, execution_config: Optional[ExecutionConfig] = None):
 def test_circuit():
     """Test a circuit compilation to MLIR when using the new device API."""
 
-    # CHECK:    quantum.device["[[PATH:.*]]librtd_lightning.{{so|dylib}}", "dummy.remote", "{'shots': 2048}"]
-    dev = DummyDevice(wires=2, shots=2048)
+    # CHECK:    quantum.device["[[PATH:.*]]librtd_null_qubit.{{so|dylib}}", "Custom", "{'shots': 2048}"]
+    dev = CustomDevice(wires=2, shots=2048)
 
     @qjit(target="mlir")
     @qml.qnode(device=dev)
@@ -90,8 +95,8 @@ def test_preprocess():
     using the new device API.
     TODO: we need to readd the two check-not once we accept the device preprocessing."""
 
-    # CHECK:    quantum.device["[[PATH:.*]]librtd_lightning.{{so|dylib}}", "dummy.remote", "{'shots': 2048}"]
-    dev = DummyDevice(wires=2, shots=2048)
+    # CHECK:    quantum.device["[[PATH:.*]]librtd_null_qubit.{{so|dylib}}", "Custom", "{'shots': 2048}"]
+    dev = CustomDevice(wires=2, shots=2048)
 
     @qjit(target="mlir")
     @qml.qnode(device=dev)

frontend/test/pytest/conftest.py

@@ -0,0 +1,22 @@
+# Copyright 2023 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.
+"""
+Pytest configuration file for Catalyst test suite.
+"""
+
+import os
+import pathlib
+
+TEST_PATH = os.path.dirname(__file__)
+CONFIG_CUSTOM_DEVICE = pathlib.Path(f"{TEST_PATH}/../custom_device/custom_device.toml")

frontend/test/pytest/device/test_decomposition.py

@@ -172,9 +172,9 @@ def get_c_interface():
             """
             system_extension = ".dylib" if platform.system() == "Darwin" else ".so"
             lib_path = (
-                get_lib_path("runtime", "RUNTIME_LIB_DIR") + "/librtd_dummy" + system_extension
+                get_lib_path("runtime", "RUNTIME_LIB_DIR") + "/librtd_null_qubit" + system_extension
             )
-            return "dummy.remote", lib_path
+            return "NullQubit", lib_path
 
         def execute(self, circuits, execution_config):
             """Execution."""

frontend/test/pytest/test_config_functions.py

@@ -18,7 +18,6 @@
 from tempfile import TemporaryDirectory
 from textwrap import dedent
 
-import pennylane as qml
 import pytest
 
 from catalyst.utils.exceptions import CompileError
@@ -32,23 +31,6 @@
 )
 
 
-class DeviceToBeTested(qml.devices.QubitDevice):
-    """Test device"""
-
-    name = "Dummy Device"
-    short_name = "dummy.device"
-    pennylane_requires = "0.33.0"
-    version = "0.0.1"
-    author = "Dummy"
-
-    operations = []
-    observables = []
-
-    def apply(self, operations, **kwargs):
-        """Unused"""
-        raise RuntimeError("Only C/C++ interface is defined")
-
-
 def get_test_config(config_text: str) -> TOMLDocument:
     """Parse test config into the TOMLDocument structure"""
     with TemporaryDirectory() as d: