Add qml.to_openqasm transform

doc/releases/changelog-dev.md

@@ -4,6 +4,36 @@
 
 <h3>New features since last release</h3>
 
+* A new function called `qml.to_openqasm` has been added, which allows for converting PennyLane circuits to OpenQASM 2.0 programs.
+  [(#7393)](https://github.com/PennyLaneAI/pennylane/pull/7393)
+
+  Consider this simple circuit in PennyLane:
+  ```python
+  dev = qml.device("default.qubit", wires=2, shots=100)
+
+  @qml.qnode(dev)
+  def circuit(theta, phi):
+      qml.RX(theta, wires=0)
+      qml.CNOT(wires=[0,1])
+      qml.RZ(phi, wires=1)
+      return qml.sample()
+  ```
+
+  This can be easily converted to OpenQASM 2.0 with `qml.to_openqasm`:
+  ```pycon
+  >>> openqasm_circ = qml.to_openqasm(circuit)(1.2, 0.9)
+  >>> print(openqasm_circ)
+  OPENQASM 2.0;
+  include "qelib1.inc";
+  qreg q[2];
+  creg c[2];
+  rx(1.2) q[0];
+  cx q[0],q[1];
+  rz(0.9) q[1];
+  measure q[0] -> c[0];
+  measure q[1] -> c[1];
+  ```
+
 * A new template called :class:`~.SelectPauliRot` that applies a sequence of uniformly controlled rotations to a target qubit 
   is now available. This operator appears frequently in unitary decomposition and block encoding techniques. 
   [(#7206)](https://github.com/PennyLaneAI/pennylane/pull/7206)
@@ -365,6 +395,7 @@ Astral Cai,
 Yushao Chen,
 Lillian Frederiksen,
 Pietropaolo Frisoni,
+Simone Gasperini,
 Korbinian Kottmann,
 Christina Lee,
 Lee J. O'Riordan,

pennylane/__init__.py

@@ -78,6 +78,7 @@
 from pennylane.io import (
     from_pyquil,
     from_qasm,
+    to_openqasm,
     from_qiskit,
     from_qiskit_noise,
     from_qiskit_op,

pennylane/io/__init__.py

@@ -18,6 +18,7 @@
 from .io import (
     from_pyquil,
     from_qasm,
+    to_openqasm,
     from_qiskit,
     from_qiskit_noise,
     from_qiskit_op,

pennylane/io/io.py

@@ -16,8 +16,13 @@
 PennyLane templates.
 """
 from collections import defaultdict
+from collections.abc import Callable
+from functools import wraps
 from importlib import metadata
 from sys import version_info
+from typing import Any, Optional
+
+from pennylane.wires import WiresLike
 
 # Error message to show when the PennyLane-Qiskit plugin is required but missing.
 _MISSING_QISKIT_PLUGIN_MESSAGE = (
@@ -613,6 +618,126 @@ def circuit(x):
     return plugin_converter(quantum_circuit, measurements=measurements)
 
 
+def to_openqasm(
+    qnode,
+    wires: Optional[WiresLike] = None,
+    rotations: bool = True,
+    measure_all: bool = True,
+    precision: Optional[int] = None,
+) -> Callable[[Any], str]:
+    """Convert a circuit to an OpenQASM 2.0 program.
+
+    .. note::
+      Terminal measurements are assumed to be performed on all qubits in the computational basis.
+      An optional ``rotations`` argument can be provided so that the output of the OpenQASM circuit
+      is diagonal in the eigenbasis of the quantum circuit's observables.
+      The measurement outputs can be restricted to only those specified in the circuit by setting ``measure_all=False``.
+
+    Args:
+        wires (Wires or None): the wires to use when serializing the circuit.
+            Default is ``None``, such that all the wires are used for serialization.
+        rotations (bool): if ``True``, add gates that diagonalize the measured wires to the eigenbasis
+            of the circuit's observables. Default is ``True``.
+        measure_all (bool): if ``True``, add a computational basis measurement on all the qubits.
+            Default is ``True``.
+        precision (int or None): number of decimal digits to display for the parameters.
+
+    Returns:
+        str: OpenQASM 2.0 program corresponding to the circuit.
+
+    **Example**
+
+    The following QNode can be serialized to an OpenQASM 2.0 program:
+
+    .. code-block:: python
+
+        dev = qml.device("default.qubit", wires=2, shots=100)
+
+        @qml.qnode(dev)
+        def circuit(theta, phi):
+            qml.RX(theta, wires=0)
+            qml.CNOT(wires=[0,1])
+            qml.RZ(phi, wires=1)
+            return qml.sample()
+
+    >>> print(qml.to_openqasm(circuit)(1.2, 0.9))
+    OPENQASM 2.0;
+    include "qelib1.inc";
+    qreg q[2];
+    creg c[2];
+    rx(1.2) q[0];
+    cx q[0],q[1];
+    rz(0.9) q[1];
+    measure q[0] -> c[0];
+    measure q[1] -> c[1];
+
+    .. details::
+        :title: Usage Details
+
+        By default, the resulting OpenQASM code will have terminal measurements on all qubits, where all the measurements are performed in the computational basis.
+        However, if terminal measurements in the QNode act only on a subset of the qubits and ``measure_all=False``,
+        the OpenQASM code will include measurements on those specific qubits only.
+
+        .. code-block:: python
+
+            dev = qml.device("default.qubit", wires=2, shots=100)
+
+            @qml.qnode(dev)
+            def circuit():
+                qml.Hadamard(0)
+                qml.CNOT(wires=[0,1])
+                return qml.sample(wires=1)
+
+        >>> print(qml.to_openqasm(circuit, measure_all=False)())
+        OPENQASM 2.0;
+        include "qelib1.inc";
+        qreg q[2];
+        creg c[2];
+        h q[0];
+        cx q[0],q[1];
+        measure q[1] -> c[1];
+
+        If the ``QNode`` returns an expectation value of a given observable and ``rotations=True``, the OpenQASM program will also
+        include the gates that diagonalize the measured wires such that they are in the eigenbasis of the measured observable.
+
+        .. code-block:: python
+
+            dev = qml.device("default.qubit", wires=2, shots=100)
+
+            @qml.qnode(dev)
+            def circuit():
+                qml.Hadamard(0)
+                qml.CNOT(wires=[0,1])
+                return qml.expval(qml.PauliX(0) @ qml.PauliY(1))
+
+        >>> print(qml.to_openqasm(circuit, rotations=True)())
+        OPENQASM 2.0;
+        include "qelib1.inc";
+        qreg q[2];
+        creg c[2];
+        h q[0];
+        cx q[0],q[1];
+        h q[0];
+        z q[1];
+        s q[1];
+        h q[1];
+        measure q[0] -> c[0];
+        measure q[1] -> c[1];
+    """
+
+    # pylint: disable=import-outside-toplevel
+    from pennylane.workflow import construct_tape
+
+    @wraps(qnode)
+    def wrapper(*args, **kwargs) -> str:
+        tape = construct_tape(qnode)(*args, **kwargs)
+        return tape.to_openqasm(
+            wires=wires, rotations=rotations, measure_all=measure_all, precision=precision
+        )
+
+    return wrapper
+
+
 def from_pyquil(pyquil_program):
     """Loads pyQuil Program objects by using the converter in the
     PennyLane-Rigetti plugin.

tests/io/test_io.py

@@ -14,6 +14,7 @@
 """
 Unit tests for the :mod:`pennylane.io` module.
 """
+from textwrap import dedent
 from unittest.mock import Mock
 
 import pytest
@@ -193,3 +194,89 @@ def test_convenience_function_arguments(
 
             if mock_plugin_converters[plugin_converter].called:
                 raise RuntimeError(f"The other plugin converter {plugin_converter} was called.")
+
+
+class TestToOpenQasm:
+    """Test the qml.to_openqasm function."""
+
+    dev = qml.device("default.qubit", wires=2, shots=100)
+
+    def test_basic_example(self):
+        """Test basic usage on simple circuit with parameters."""
+
+        @qml.qnode(self.dev)
+        def circuit(theta, phi):
+            qml.RX(theta, wires=0)
+            qml.CNOT(wires=[0, 1])
+            qml.RZ(phi, wires=1)
+            return qml.sample()
+
+        qasm = qml.to_openqasm(circuit)(1.2, 0.9)
+
+        expected = dedent(
+            """\
+            OPENQASM 2.0;
+            include "qelib1.inc";
+            qreg q[2];
+            creg c[2];
+            rx(1.2) q[0];
+            cx q[0],q[1];
+            rz(0.9) q[1];
+            measure q[0] -> c[0];
+            measure q[1] -> c[1];
+            """
+        )
+        assert qasm == expected
+
+    def test_measure_qubits_subset_only(self):
+        """Test OpenQASM program includes measurements only over the qubits subset specified in the QNode."""
+
+        @qml.qnode(self.dev)
+        def circuit():
+            qml.Hadamard(0)
+            qml.CNOT(wires=[0, 1])
+            return qml.sample(wires=1)
+
+        qasm = qml.to_openqasm(circuit, measure_all=False)()
+
+        expected = dedent(
+            """\
+            OPENQASM 2.0;
+            include "qelib1.inc";
+            qreg q[2];
+            creg c[2];
+            h q[0];
+            cx q[0],q[1];
+            measure q[1] -> c[1];
+            """
+        )
+        assert qasm == expected
+
+    def test_rotations_with_expval(self):
+        """Test OpenQASM program includes gates that make the measured observables diagonal in the computational basis."""
+
+        @qml.qnode(self.dev)
+        def circuit():
+            qml.Hadamard(0)
+            qml.CNOT(wires=[0, 1])
+            return qml.expval(qml.PauliX(0) @ qml.PauliY(1))
+
+        qasm = qml.to_openqasm(circuit, rotations=True)()
+
+        expected = dedent(
+            """\
+            OPENQASM 2.0;
+            include "qelib1.inc";
+            qreg q[2];
+            creg c[2];
+            h q[0];
+            cx q[0],q[1];
+            h q[0];
+            z q[1];
+            s q[1];
+            h q[1];
+            measure q[0] -> c[0];
+            measure q[1] -> c[1];
+            """
+        )
+        assert qasm == expected