Add importer for OpenQASM 3

qiskit/qasm3/__init__.py

@@ -11,34 +11,126 @@
 # that they have been altered from the originals.
 
 """
-==========================
-Qasm (:mod:`qiskit.qasm3`)
-==========================
+================================
+OpenQASM 3 (:mod:`qiskit.qasm3`)
+================================
 
 .. currentmodule:: qiskit.qasm3
 
-.. autosummary::
-    :toctree: ../stubs/
+Qiskit provides some tools for converting between `OpenQASM 3 <https://openqasm.com>`__
+representations of quantum programs, and the :class:`.QuantumCircuit` class.  These will continue to
+evolve as Qiskit's support for the dynamic-circuit capabilities expressed by OpenQASM 3 increases.
 
-    Exporter
-    dumps
-    dump
+
+Exporting to OpenQASM 3
+=======================
+
+The high-level functions are simply :func:`dump` and :func:`dumps`, which respectively export to a
+file (given as a filename) and to a Python string.
+
+.. autofunction:: dump
+.. autofunction:: dumps
+
+Both of these exporter functions are single-use wrappers around the main :class:`Exporter` class.
+For more complex exporting needs, including dumping multiple circuits in a single session, it may be
+more convenient or faster to use the complete interface.
+
+.. autoclass:: Exporter
+    :members:
+
+All of these interfaces will raise :exc:`QASM3ExporterError` on failure.
+
+.. autoexception:: QASM3ExporterError
+
+
+Importing from OpenQASM 3
+=========================
+
+Currently only two high-level functions are offered, as Qiskit support for importing from OpenQASM 3
+is in its infancy, and the implementation is expected to change significantly.  The two functions
+are :func:`load` and :func:`loads`, which are direct counterparts of :func:`dump` and :func:`dumps`,
+respectively loading a program indirectly from a named file and directly from a given string.
+
+.. note::
+
+    While we are still in the exploratory release period, to use either function, the package
+    ``qiskit_qasm3_import`` must be installed.  This can be done by installing Qiskit Terra with the
+    ``qasm3-import`` extra, such as by:
+
+    .. code-block:: text
+
+        pip install qiskit-terra[qasm3-import]
+
+    We expect that this functionality will eventually be merged into core Terra, and no longer
+    require an optional import, but we do not yet have a timeline for this.
+
+.. autofunction:: load
+.. autofunction:: loads
+
+Both of these two functions raise :exc:`QASM3ImporterError` on failure.
+
+.. autoexception:: QASM3ImporterError
+
+For example, we can define a quantum program using OpenQASM 3, and use :func:`loads` to directly
+convert it into a :class:`.QuantumCircuit`:
+
+.. plot::
+    :include-source:
+
+    import qiskit.qasm3
+
+    program = \"\"\"
+        OPENQASM 3.0;
+        include "stdgates.inc";
+
+        input float[64] a;
+        qubit[3] q;
+        bit[2] mid;
+        bit[3] out;
+
+        let aliased = q[0:1];
+
+        gate my_gate(a) c, t {
+          gphase(a / 2);
+          ry(a) c;
+          cx c, t;
+        }
+        gate my_phase(a) c {
+          ctrl @ inv @ gphase(a) c;
+        }
+
+        my_gate(a * 2) aliased[0], q[{1, 2}][0];
+        measure q[0] -> mid[0];
+        measure q[1] -> mid[1];
+
+        while (mid == "00") {
+          reset q[0];
+          reset q[1];
+          my_gate(a) q[0], q[1];
+          my_phase(a - pi/2) q[1];
+          mid[0] = measure q[0];
+          mid[1] = measure q[1];
+        }
+
+        if (mid[0]) {
+          let inner_alias = q[{0, 1}];
+          reset inner_alias;
+        }
+
+        out = measure q;
+    \"\"\"
+    circuit = qiskit.qasm3.loads(program)
+    circuit.draw("mpl")
 """
 
+from qiskit.utils import optionals as _optionals
 from .exporter import Exporter
-from .exceptions import QASM3Error, QASM3ExporterError
+from .exceptions import QASM3Error, QASM3ImporterError, QASM3ExporterError
 
 
 def dumps(circuit, **kwargs) -> str:
     """Serialize a :class:`~qiskit.circuit.QuantumCircuit` object in an OpenQASM3 string.
 
-    .. note::
-
-        This is a quick interface to the main :obj:`.Exporter` interface.  All keyword arguments to
-        this function are inherited from the constructor of that class, and if you have multiple
-        circuits to export, it will be faster to create an :obj:`.Exporter` instance, and use its
-        :obj:`.Exporter.dumps` method.
-
     Args:
         circuit (QuantumCircuit): Circuit to serialize.
         **kwargs: Arguments for the :obj:`.Exporter` constructor.
@@ -53,17 +145,58 @@ def dump(circuit, stream, **kwargs) -> None:
     """Serialize a :class:`~qiskit.circuit.QuantumCircuit` object as a OpenQASM3 stream to file-like
     object.
 
-    .. note::
-
-        This is a quick interface to the main :obj:`.Exporter` interface.  All keyword arguments to
-        this function are inherited from the constructor of that class, and if you have multiple
-        circuits to export, it will be faster to create an :obj:`.Exporter` instance, and use its
-        :obj:`.Exporter.dump` method.
-
     Args:
         circuit (QuantumCircuit): Circuit to serialize.
         stream (TextIOBase): stream-like object to dump the OpenQASM3 serialization
         **kwargs: Arguments for the :obj:`.Exporter` constructor.
 
     """
     Exporter(**kwargs).dump(circuit, stream)
+
+
+@_optionals.HAS_QASM3_IMPORT.require_in_call("loading from OpenQASM 3")
+def load(filename: str):
+    """Load an OpenQASM 3 program from the file ``filename``.
+
+    Args:
+        filename: the filename to load the program from.
+
+    Returns:
+        QuantumCircuit: a circuit representation of the OpenQASM 3 program.
+
+    Raises:
+        QASM3ImporterError: if the OpenQASM 3 file is invalid, or cannot be represented by a
+            :class:`.QuantumCircuit`.
+    """
+
+    import qiskit_qasm3_import
+
+    with open(filename, "r") as fptr:
+        program = fptr.read()
+    try:
+        return qiskit_qasm3_import.parse(program)
+    except qiskit_qasm3_import.ConversionError as exc:
+        raise QASM3ImporterError(str(exc)) from exc
+
+
+@_optionals.HAS_QASM3_IMPORT.require_in_call("loading from OpenQASM 3")
+def loads(program: str):
+    """Load an OpenQASM 3 program from the given string.
+
+    Args:
+        program: the OpenQASM 3 program.
+
+    Returns:
+        QuantumCircuit: a circuit representation of the OpenQASM 3 program.
+
+    Raises:
+        QASM3ImporterError: if the OpenQASM 3 file is invalid, or cannot be represented by a
+            :class:`.QuantumCircuit`.
+    """
+
+    import qiskit_qasm3_import
+
+    try:
+        return qiskit_qasm3_import.parse(program)
+    except qiskit_qasm3_import.ConversionError as exc:
+        raise QASM3ImporterError(str(exc)) from exc

qiskit/qasm3/exceptions.py

@@ -21,3 +21,7 @@ class QASM3Error(QiskitError):
 
 class QASM3ExporterError(QASM3Error):
     """An error raised during running the OpenQASM 3 exporter."""
+
+
+class QASM3ImporterError(QASM3Error):
+    """An error raised during the OpenQASM 3 importer."""

qiskit/utils/optionals.py

@@ -117,6 +117,11 @@
       - Various LaTeX-based visualizations, especially the circuit drawers, need access to the
         `pylatexenc <https://github.com/phfaist/pylatexenc>`__ project to work correctly.
 
+    * - .. py:data:: HAS_QASM3_IMPORT
+      - The functions :func:`.qasm3.load` and :func:`.qasm3.loads` for importing OpenQASM 3 programs
+        into :class:`.QuantumCircuit` instances use `an external importer package
+        <https://qiskit.github.io/qiskit-qasm3-import>`__.
+
     * - .. py:data:: HAS_SEABORN
       - Qiskit Terra provides several visualisation tools in the :mod:`.visualization` module.  Some
         of these are built using `Seaborn <https://seaborn.pydata.org/>`__, which must be installed
@@ -264,6 +269,9 @@
     name="pylatexenc",
     install="pip install pylatexenc",
 )
+HAS_QASM3_IMPORT = _LazyImportTester(
+    "qiskit_qasm3_import", install="pip install qiskit_qasm3_import"
+)
 HAS_SEABORN = _LazyImportTester("seaborn", install="pip install seaborn")
 HAS_SKLEARN = _LazyImportTester(
     {"sklearn.linear_model": ("Ridge", "Lasso")},

releasenotes/notes/qasm3-import-0e7e01cb75aa6251.yaml

@@ -0,0 +1,48 @@
+---
+features:
+  - |
+    Support for importing OpenQASM 3 programs into Qiskit has been added.  This can most easily be
+    accessed using the functions :func:`.qasm3.loads` and :func:`.qasm3.load`, to load a program
+    directly from a string and indirectly from a filename, respectively.  For example, one can now
+    do::
+
+      from qiskit import qasm3
+
+      circuit = qasm3.loads("""
+        OPENQASM 3.0;
+        include "stdgates.inc";
+
+        qubit q;
+        qubit[5] qr;
+        bit c;
+        bit[5] cr;
+
+        h q;
+        c = measure q;
+
+        if (c) {
+          h qr[0];
+          cx qr[0], qr[1];
+          cx qr[0], qr[2];
+          cx qr[0], qr[3];
+          cx qr[0], qr[4];
+        } else {
+          h qr[4];
+          cx qr[4], qr[3];
+          cx qr[4], qr[2];
+          cx qr[4], qr[1];
+          cx qr[4], qr[0];
+        }
+        cr = measure qr;
+      """)
+
+    This will load the program into a :class:`.QuantumCircuit` instance in the variable ``circuit``.
+
+    Not all OpenQASM 3 features are supported at first, because Qiskit does not yet have a way to
+    represent advanced classical data processing.  The capabilities of the importer will increase
+    along with the capabilities of the rest of Qiskit.  The initial feature set of the importer is
+    approximately the same set of features that would be output by the exporter (:func:`.qasm3.dump`
+    and :func:`.qasm3.dumps`).
+
+    Note that Qiskit's support of OpenQASM 3 is not meant to provide a totally lossless
+    representation of :class:`.QuantumCircuit`\ s.  For that, consider using :mod:`qiskit.qpy`.

requirements-dev.txt

@@ -26,3 +26,4 @@ scikit-quant<=0.7;platform_system != 'Windows'
 jax;platform_system != 'Windows'
 jaxlib;platform_system != 'Windows'
 docplex
+qiskit-qasm3-import; python_version>='3.8'

setup.py

@@ -33,6 +33,9 @@
     )
 
 
+qasm3_import_extras = [
+    "qiskit-qasm3-import>=0.1.0",
+]
 visualization_extras = [
     "matplotlib>=3.3",
     "ipywidgets>=7.3.0",
@@ -81,14 +84,15 @@
     include_package_data=True,
     python_requires=">=3.7",
     extras_require={
+        "qasm3-import": qasm3_import_extras,
         "visualization": visualization_extras,
         "bip-mapper": bip_requirements,
         "crosstalk-pass": z3_requirements,
         "csp-layout-pass": csp_requirements,
         "toqm": toqm_requirements,
         # Note: 'all' only includes extras that are stable and work on the majority of Python
         # versions and OSes supported by Terra. You have to ask for anything else explicitly.
-        "all": visualization_extras + z3_requirements + csp_requirements,
+        "all": visualization_extras + z3_requirements + csp_requirements + qasm3_import_extras,
     },
     project_urls={
         "Bug Tracker": "https://github.com/Qiskit/qiskit-terra/issues",