✨ Portable Layout Information (#76)

* ✨ return `QuantumCircuit` if Qiskit is available (including Layout information)
* 🙈 add virtual environments to .gitignore file
* 🔧🐍 make `qiskit-terra` a project dependency
* 🚨 fix LGTM warning
* ♻️ replace soon to be deprecated `qiskit.test.mock` imports
* 📝 update documentation
* ⬆️ QFR 📦 with a 🐛 fix for Qiskit `Layout` import
* 🐛 fix output permutation bug in heuristic mapper
* ⬆️ QFR 📦 with a 🐛 fix for the `cancelCNOTs` optimization
* ✅ add tests verifying the correctness of the circuits produced by both mappers

Author: burgholzer

.gitignore

@@ -21,3 +21,11 @@ var/
 .installed.cfg
 *.egg
 .pytest_cache/
+
+.env
+.venv
+env/
+venv/
+ENV/
+env.bak/
+venv.bak/

README.md

@@ -66,13 +66,13 @@ MQT QMAP is developed as a C++ library with an easy to use Python interface.
 - Once installed, start using it in Python:
     ```python
     from mqt import qmap
-    results = qmap.compile(circ, arch)
+    circ_mapped, results = qmap.compile(circ, arch)
     ```
 
 where `circ` is either a Qiskit `QuantumCircuit` object or the path to an input file (in any of the formats listed above)
 and `arch` is either
 
-- a Qiskit `Backend` instance such as those defined under `qiskit.test.mock` **(recommended)**,
+- a Qiskit `Backend` instance such as those defined under `qiskit.providers.fake_provider` **(recommended)**,
 - one of the pre-defined architectures (see below), or
 - the path to a file containing the number of qubits and a line-by-line enumeration of the qubit connections.
 

extern/qfr

@@ -1,5 +1,5 @@
 import warnings
-from mqt import *
+from mqt import qmap
 
 warnings.simplefilter('always', DeprecationWarning)
 warnings.warn('Usage via `import jkq` is deprecated in favor of the new prefix. Please use `import mqt` instead.', DeprecationWarning)

jkq/__init__.py

@@ -4,23 +4,42 @@
 #
 import pickle
 from pathlib import Path
-from typing import Union, Optional, Set
+from typing import Union, Optional, Set, List, Tuple
+
+from qiskit import QuantumCircuit, QuantumRegister
+from qiskit.providers import Backend
+from qiskit.providers.models import BackendProperties
+from qiskit.transpiler.target import Target
+from qiskit.transpiler import Layout
+
 from mqt.qmap.pyqmap import map, Method, InitialLayout, Layering, Arch, Encoding, CommanderGrouping, SwapReduction, Configuration, MappingResults, Architecture
 
-try:
-    from qiskit.providers import Backend
-    from qiskit.providers.models import BackendProperties
-    from qiskit.transpiler.target import Target
 
-    PossibleArchitectureTypes = Union[str, Arch, Architecture, Backend]
-    PossibleCalibrationTypes = Union[str, BackendProperties, Target]
-except ModuleNotFoundError:
-    PossibleArchitectureTypes = Union[str, Arch, Architecture]
-    PossibleCalibrationTypes = Union[str]
+def extract_initial_layout_from_qasm(qasm: str, qregs: List[QuantumRegister]) -> Layout:
+    """
+    Extracts the initial layout resulting from compiling a circuit from a QASM file.
+    :param qasm: QASM file
+    :type qasm: str
+    :param qregs: The quantum registers to apply the layout to.
+    :type qregs: List[QuantumRegister]
+    :return: layout to be used in Qiskit
+    """
+    for line in qasm.split("\n"):
+        if line.startswith("// i "):
+            # strip away initial part of line
+            line = line[5:]
+            # split line into tokens
+            tokens = line.split(" ")
+            # convert tokens to integers
+            tokens = [int(token) for token in tokens]
+            # create an empty layout
+            layout = Layout().from_intlist(tokens, *qregs)
+            return layout
 
 
-def compile(circ, arch: Optional[PossibleArchitectureTypes],
-            calibration: Optional[PossibleCalibrationTypes] = None,
+def compile(circ: Union[QuantumCircuit, str],
+            arch: Optional[Union[str, Arch, Architecture, Backend]],
+            calibration: Optional[Union[str, BackendProperties, Target]] = None,
             method: Union[str, Method] = "heuristic",
             initial_layout: Union[str, InitialLayout] = "dynamic",
             layering: Union[str, Layering] = "individual_gates",
@@ -38,14 +57,15 @@ def compile(circ, arch: Optional[PossibleArchitectureTypes],
             pre_mapping_optimizations: bool = True,
             post_mapping_optimizations: bool = True,
             verbose: bool = False
-            ) -> MappingResults:
+            ) -> Tuple[QuantumCircuit, MappingResults]:
     """Interface to the MQT QMAP tool for mapping quantum circuits
 
-    :param circ: Path to circuit file, path to Qiskit QuantumCircuit pickle, or Qiskit QuantumCircuit object
+    :param circ: Qiskit QuantumCircuit object, path to circuit file, or path to Qiskit QuantumCircuit pickle
+    :type circ: Union[QuantumCircuit, str]
     :param arch: Architecture to map to. Either a path to a file with architecture information, one of the available architectures (Arch), qmap.Architecture, or `qiskit.providers.backend` (if Qiskit is installed)
-    :type arch: Optional[PossibleArchitectureTypes]
+    :type arch: Optional[Union[str, Arch, Architecture, Backend]]
     :param calibration: Path to file containing calibration information, `qiskit.providers.models.BackendProperties` object (if Qiskit is installed), or `qiskit.transpiler.target.Target` object (if Qiskit is installed)
-    :type calibration: Optional[PossibleCalibrationTypes]
+    :type calibration: Optional[Union[str, BackendProperties, Target]]
     :param method: Mapping technique to use (*heuristic* | exact)
     :type method: Union[str, Method]
     :param initial_layout: Strategy to use for determining initial layout in heuristic mapper (identity | static | *dynamic*)
@@ -77,12 +97,13 @@ def compile(circ, arch: Optional[PossibleArchitectureTypes],
     :type post_mapping_optimizations: bool
     :param verbose: Print more detailed information during the mapping process
     :type verbose: bool
-    :return: Object containing all the results
-    :rtype: MappingResults
+    :return: Mapped circuit (as Qiskit `QuantumCircuit`) and results
+    :rtype: Tuple[QuantumCircuit, MappingResults]
     """
 
     if subgraph is None:
         subgraph = set()
+
     if type(circ) == str and Path(circ).suffix == '.pickle':
         circ = pickle.load(open(circ, "rb"))
 
@@ -100,34 +121,26 @@ def compile(circ, arch: Optional[PossibleArchitectureTypes],
             architecture.load_coupling_map(arch)
         elif isinstance(arch, Architecture):
             architecture = arch
+        elif isinstance(arch, Backend):
+            from mqt.qmap.qiskit.backend import import_backend
+
+            architecture = import_backend(arch)
         else:
-            try:
-                from qiskit.providers.backend import Backend
-                from mqt.qmap.qiskit.backend import import_backend
-                if isinstance(arch, Backend):
-                    architecture = import_backend(arch)
-                else:
-                    raise ValueError("No compatible type for architecture:", type(arch))
-            except ModuleNotFoundError:
-                raise ValueError("No compatible type for architecture:", type(arch))
+            raise ValueError("No compatible type for architecture:", type(arch))
 
     if calibration is not None:
         if type(calibration) == str:
             architecture.load_properties(calibration)
+        elif isinstance(calibration, BackendProperties):
+            from mqt.qmap.qiskit.backend import import_backend_properties
+
+            architecture.load_properties(import_backend_properties(calibration))
+        elif isinstance(calibration, Target):
+            from mqt.qmap.qiskit.backend import import_target
+
+            architecture.load_properties(import_target(calibration))
         else:
-            try:
-                from qiskit.providers.models import BackendProperties
-                from qiskit.transpiler.target import Target
-                from mqt.qmap.qiskit.backend import import_backend_properties, import_target
-
-                if isinstance(calibration, BackendProperties):
-                    architecture.load_properties(import_backend_properties(calibration))
-                elif isinstance(calibration, Target):
-                    architecture.load_properties(import_target(calibration))
-                else:
-                    raise ValueError("No compatible type for calibration:", type(calibration))
-            except ModuleNotFoundError:
-                raise ValueError("No compatible type for calibration:", type(calibration))
+            raise ValueError("No compatible type for calibration:", type(calibration))
 
     config = Configuration()
     config.method = Method(method)
@@ -148,4 +161,10 @@ def compile(circ, arch: Optional[PossibleArchitectureTypes],
     config.post_mapping_optimizations = post_mapping_optimizations
     config.verbose = verbose
 
-    return map(circ, architecture, config)
+    results = map(circ, architecture, config)
+
+    circ = QuantumCircuit.from_qasm_str(results.mapped_circuit)
+    layout = extract_initial_layout_from_qasm(results.mapped_circuit, circ.qregs)
+    circ._layout = layout
+
+    return circ, results

mqt/qmap/compile.py

@@ -15,7 +15,7 @@ build = "cp3*"
 archs = "auto64"
 skip = "*-musllinux*"
 test-skip = "*-macosx_arm64 *-musllinux* *aarch64"
-test-extras = ["tests"]
+test-extras = ["test"]
 test-command = "python -m pytest {project}/test/python"
 environment = { DEPLOY = "ON" }
 build-frontend = "build"

pyproject.toml

@@ -103,8 +103,10 @@ def build_extension(self, ext):
     cmdclass={"build_ext": CMakeBuild},
     zip_safe=False,
     packages=find_namespace_packages(include=['mqt.*']),
+    install_requires=["qiskit-terra~=0.20.2"],
     extras_require={
-        "tests": ["pytest~=7.1.1", "qiskit-terra>=0.19.2,<0.21.0"],
+        "test": ["pytest~=7.1.1", "mqt.qcec~=2.0.0rc4"],
+        "dev": ["mqt.qmap[test]"]  # requires Pip 21.2 or newer
     },
     classifiers=[
         'Development Status :: 5 - Production/Stable',

setup.py

@@ -111,6 +111,18 @@ void HeuristicMapper::map(const Configuration& ms) {
         }
     }
 
+    // infer output permutation from qubit locations
+    qcMapped.outputPermutation.clear();
+    std::size_t count = 0U;
+    for (std::size_t i = 0U; i < architecture.getNqubits(); ++i) {
+        if (qubits[i] != -1) {
+            qcMapped.outputPermutation[static_cast<dd::Qubit>(i)] = static_cast<dd::Qubit>(qubits[i]);
+        } else {
+            qcMapped.setLogicalQubitGarbage(qc.getNqubits() + count);
+            ++count;
+        }
+    }
+
     // fix single qubit gates
     if (!gatesToAdjust.empty()) {
         gateidx--; // index of last operation
@@ -153,18 +165,6 @@ void HeuristicMapper::map(const Configuration& ms) {
         }
     }
 
-    // infer output permutation from qubit locations
-    qcMapped.outputPermutation.clear();
-    std::size_t count = 0U;
-    for (std::size_t i = 0U; i < architecture.getNqubits(); ++i) {
-        if (qubits[i] != -1) {
-            qcMapped.outputPermutation[static_cast<dd::Qubit>(i)] = static_cast<dd::Qubit>(qubits[i]);
-        } else {
-            qcMapped.setLogicalQubitGarbage(qc.getNqubits() + count);
-            ++count;
-        }
-    }
-
     postMappingOptimizations(config);
     countGates(qcMapped, results.output);
     finalizeMappedCircuit();

src/heuristic/HeuristicMapper.cpp

@@ -1,5 +1,5 @@
 from qiskit import QuantumCircuit
-from qiskit.test.mock.backends import FakeLondon
+from qiskit.providers.fake_provider import FakeLondon
 
 from mqt import qmap
 
@@ -12,8 +12,5 @@
     print(qc.draw(fold=-1))
 
     # compile the circuit
-    results = qmap.compile(qc, arch=FakeLondon())
-
-    # get the mapped circuit
-    qc_mapped = QuantumCircuit.from_qasm_str(results.mapped_circuit)
+    qc_mapped, results = qmap.compile(qc, arch=FakeLondon())
     print(qc_mapped.draw(fold=-1))

test/python/test.py

@@ -0,0 +1,60 @@
+import pytest
+from qiskit import QuantumCircuit
+from qiskit.providers.fake_provider import FakeLondon
+
+from mqt import qmap, qcec
+
+
+def test_exact_no_swaps_trivial_layout():
+    """Verify that the exact mapper works on a simple circuit that requires no swaps on a trivial initial layout."""
+    qc = QuantumCircuit(3)
+    qc.h(0)
+    qc.cx(0, 1)
+    qc.cx(1, 2)
+    qc.measure_all()
+
+    qc_mapped, results = qmap.compile(qc, arch=FakeLondon(), method="exact")
+    assert results.timeout is False
+    assert results.mapped_circuit != ""
+    assert results.output.swaps == 0
+
+    result = qcec.verify(qc, qc_mapped)
+    assert result.considered_equivalent() is True
+
+
+def test_exact_no_swaps_non_trivial_layout():
+    """Verify that the exact mapper works on a simple circuit that requires a non-trivial layout to achieve no swaps."""
+    qc = QuantumCircuit(4)
+    qc.h(0)
+    qc.cx(0, 1)
+    qc.cx(0, 2)
+    qc.cx(0, 3)
+    qc.measure_all()
+
+    qc_mapped, results = qmap.compile(qc, arch=FakeLondon(), method="exact")
+
+    assert results.timeout is False
+    assert results.mapped_circuit != ""
+    assert results.output.swaps == 0
+
+    result = qcec.verify(qc, qc_mapped)
+    assert result.considered_equivalent() is True
+
+
+def test_exact_non_trivial_swaps():
+    """Verify that the exact mapper works on a simple circuit that requires at least a single SWAP."""
+    qc = QuantumCircuit(3)
+    qc.h(0)
+    qc.cx(0, 1)
+    qc.cx(1, 2)
+    qc.cx(2, 0)
+    qc.measure_all()
+
+    qc_mapped, results = qmap.compile(qc, arch=FakeLondon(), method="exact")
+
+    assert results.timeout is False
+    assert results.mapped_circuit != ""
+    assert results.output.swaps == 1
+
+    result = qcec.verify(qc, qc_mapped)
+    assert result.considered_equivalent() is True