Source code for pennylane.queuing

# Copyright 2018-2021 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.
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This module contains the classes for placing objects into queues.


Users provide *quantum functions* which PennyLane needs to convert into a circuit representation capable
of being executed by a device. A quantum function is any callable that:

* accepts classical inputs
* constructs any number of quantum :class:`~.Operator` objects
* returns one or more :class:`~.MeasurementProcess` objects.

For example:

.. code-block:: python

    def qfunc(x, scale_value=1):
        qml.RX(x * scale_value, wires=0)
        if (1 != 2):
        return qml.expval(qml.PauliZ(0)), qml.expval(qml.PauliX(1))

To convert from a quantum function to a representation of a circuit, we use queuing.

A *queuable object* is anything that can be placed into a queue. These will be :class:`~.Operator`,
:class:`~.MeasurementProcess`, and :class:`~.QuantumTape` objects. :class:`~.Operator` and
:class:`~.MeasurementProcess` objects achieve queuing via a :meth:`~.Operator.queue` method called upon construction.
Note that even though :class:`~.QuantumTape` is a queuable object, it does not have a ``queue`` method.

When an object is queued, it sends itself to the :class:`~.QueuingManager`. The :class:`~.QueuingManager`
is a global singleton class that facilitates placing objects in the queue. All of :class:`~.QueuingManager`'s methods
and properties are class methods and properties, so all instances will access the same information.

The :meth:`~.QueuingManager.active_context` is the queue where any new objects are placed.
The :class:`~.QueuingManager` is said to be *recording* if an active context exists.

Active contexts are :class:`~.AnnotatedQueue` instances. They are *context managers* where recording occurs
within a ``with`` block.

Let's take a look at an example. If we query the :class:`~.QueuingManager` outside of an
:class:`~.AnnotatedQueue`'s context, we can see that nothing is recording and no active context exists.

>>> print("Are we recording? ", qml.QueuingManager.recording())
Are we recording?  False
>>> print("What's the active context? ", qml.QueuingManager.active_context())
What's the active context?  None

Inside of a context, we can see the active recording context:

>>> with qml.queuing.AnnotatedQueue() as q:
...     print("Are we recording? ", qml.QueuingManager.recording())
...     print("Is q the active queue? ", q is qml.QueuingManager.active_context())
Are we recording?  True
Is q the active queue?  True

If we have nested :class:`~.AnnotatedQueue` contexts, only the innermost one will be recording.
Once the currently active queue exits, any outer queue will resume recording.

>>> with qml.queuing.AnnotatedQueue() as q1:
...     print("Is q1 recording? ", q1 is qml.QueuingManager.active_context())
...     with qml.queuing.AnnotatedQueue() as q2:
...         print("Is q1 recording? ", q1 is qml.QueuingManager.active_context())
...     print("Is q1 recording? ", q1 is qml.QueuingManager.active_context())
Is q1 recording?  True
Is q1 recording?  False
Is q1 recording?  True

If we construct an operator inside the recording context, we can see it is added to the queue:

>>> with qml.queuing.AnnotatedQueue() as q:
...     op = qml.PauliX(0)
>>> q.queue

If an operator is constructed outside of the context, we can manually add it to the queue by
calling the :meth:`~.Operator.queue` method. The :meth:`~.Operator.queue` method is automatically
called upon initialization, but it can also be manually called at a later time.

>>> op = qml.PauliX(0)
>>> with qml.queuing.AnnotatedQueue() as q:
...     op.queue()
>>> q.queue

An object can only exist up to *once* in the queue, so calling queue multiple times will
not do anything.

>>> op = qml.PauliX(0)
>>> with qml.queuing.AnnotatedQueue() as q:
...     op.queue()
...     op.queue()
>>> q.queue

The :func:`~.apply` method allows a single object to be queued multiple times in a circuit.
The function queues a copy of the original object if it already in the queue.

>>> op = qml.PauliX(0)
>>> with qml.queuing.AnnotatedQueue() as q:
...     qml.apply(op)
...     qml.apply(op)
>>> q.queue
[PauliX(wires=[0]), PauliX(wires=[0])]
>>> q.queue[0] is q.queue[1]

In the case of operators composed of other operators, like with :class:`~.SymbolicOp` and
:class:`~.CompositeOp`, the new nested operation removes its constituents from the queue.
Only the operators that will end up in the circuit will remain.

>>> with qml.queuing.AnnotatedQueue() as q:
...     base = qml.PauliX(0)
...     print(q.queue)
...     pow_op = base ** 1.5
...     print(q.queue)

Once the queue is constructed, the :func:`~.process_queue` function converts it into the operations,
measurements, and state prep present in the final circuit. This step eliminates any object that has an owner.

>>> with qml.queuing.AnnotatedQueue() as q:
...     qml.QubitStateVector(np.array([1.0, 0]), wires=0)
...     base = qml.PauliX(0)
...     pow_op = base ** 1.5
...     qml.expval(qml.PauliZ(0) @ qml.PauliX(1))
>>> ops, measurements, prep = qml.queuing.process_queue(q)
>>> ops
>>> measurements
[expval(PauliZ(wires=[0]) @ PauliX(wires=[1]))]
>>> prep
[QubitStateVector(tensor([1., 0.], requires_grad=True), wires=[0])]

These three lists can be used to construct a :class:`~.QuantumScript`:

>>> qml.tape.QuantumScript(ops, measurements, prep)
<QuantumScript: wires=[0, 1], params=1>

In order to construct new operators within a recording, but without queuing them, either
use the :meth:`~.QueuingManager.stop_recording` context or specify `do_queue=False` upon construction:

>>> with qml.queuing.AnnotatedQueue() as q:
...     qml.PauliX(0, do_queue=False)
...     with qml.QueuingManager.stop_recording():
...         qml.PauliY(1)
>>> q.queue


import copy
from collections import OrderedDict
from contextlib import contextmanager
from threading import RLock

[docs]class QueuingError(Exception): """Exception that is raised when there is a queuing error"""
[docs]class QueuingManager: """Singleton global entry point for managing active recording contexts. This class consists purely of class methods. It both maintains a list of recording queues and allows communication with the currently active object. Queueable objects, like :class:`~.operation.Operator` and :class:`~.measurements.MeasurementProcess`, should use ``QueuingManager`` as an entry point for accessing the active queue. See also: :class:`~.AnnotatedQueue`, :class:`~.tape.QuantumTape`, :meth:`~.operation.Operator.queue`. Recording queues, such as :class:`~.AnnotatedQueue`, must define the following methods: * ``append``: define an action to perform when an object append request is made. * ``remove``: define an action to perform when an object removal request is made. * ``get_info``: retrieve the object's metadata * ``update_info``: Update an object's metadata if it is already queued. To start and end recording, the recording queue can use the :meth:`add_active_queue` and :meth:`remove_active_queue` methods. """ _active_contexts = [] """The stack of contexts that are currently active."""
[docs] @classmethod def add_active_queue(cls, queue): """Makes a queue the currently active recording context.""" cls._active_contexts.append(queue)
[docs] @classmethod def remove_active_queue(cls): """Ends recording on the currently active recording queue.""" return cls._active_contexts.pop()
[docs] @classmethod def recording(cls): """Whether a queuing context is active and recording operations""" return bool(cls._active_contexts)
[docs] @classmethod def active_context(cls): """Returns the currently active queuing context.""" return cls._active_contexts[-1] if cls.recording() else None
[docs] @classmethod @contextmanager def stop_recording(cls): """A context manager and decorator to ensure that contained logic is non-recordable or non-queueable within a QNode or quantum tape context. **Example:** Consider the function: >>> def list_of_ops(params, wires): ... return [ ... qml.RX(params[0], wires=wires), ... qml.RY(params[1], wires=wires), ... qml.RZ(params[2], wires=wires) ... ] If executed in a recording context, the operations constructed in the function will be queued: >>> dev = qml.device("default.qubit", wires=2) >>> @qml.qnode(dev) ... def circuit(params): ... ops = list_of_ops(params, wires=0) ... qml.apply(ops[-1]) # apply the last operation from the list again ... return qml.expval(qml.PauliZ(0)) >>> print(qml.draw(circuit)([1, 2, 3])) 0: ──RX(1.00)──RY(2.00)──RZ(3.00)──RZ(3.00)─┤ <Z> Using the ``stop_recording`` context manager, all logic contained inside is not queued or recorded. >>> @qml.qnode(dev) ... def circuit(params): ... with qml.QueuingManager.stop_recording(): ... ops = list_of_ops(params, wires=0) ... qml.apply(ops[-1]) ... return qml.expval(qml.PauliZ(0)) >>> print(qml.draw(circuit)([1, 2, 3])) 0: ──RZ(3.00)─┤ <Z> The context manager can also be used as a decorator on a function: >>> @qml.QueuingManager.stop_recording() ... def list_of_ops(params, wires): ... return [ ... qml.RX(params[0], wires=wires), ... qml.RY(params[1], wires=wires), ... qml.RZ(params[2], wires=wires) ... ] >>> @qml.qnode(dev) ... def circuit(params): ... ops = list_of_ops(params, wires=0) ... qml.apply(ops[-1]) ... return qml.expval(qml.PauliZ(0)) >>> print(qml.draw(circuit)([1, 2, 3])) 0: ──RZ(3.00)─┤ <Z> """ previously_active_contexts = cls._active_contexts cls._active_contexts = [] try: yield except Exception as e: cls._active_contexts = previously_active_contexts raise e cls._active_contexts = previously_active_contexts
[docs] @classmethod def append(cls, obj, **kwargs): """Append an object to the queue(s). Args: obj: the object to be appended """ if cls.recording(): cls.active_context().append(obj, **kwargs)
[docs] @classmethod def remove(cls, obj): """Remove an object from the queue(s) if it is in the queue(s). Args: obj: the object to be removed """ if cls.recording(): cls.active_context().remove(obj)
[docs] @classmethod def update_info(cls, obj, **kwargs): """Updates information of an object in the active queue if it is already in the queue. Args: obj: the object with metadata to be updated """ if cls.recording(): cls.active_context().update_info(obj, **kwargs)
[docs] @classmethod def get_info(cls, obj): """Retrieves information of an object in the active queue. Args: obj: the object with metadata to be retrieved Returns: object metadata """ return cls.active_context().get_info(obj) if cls.recording() else None
[docs]class AnnotatedQueue(OrderedDict): """Lightweight class that maintains a basic queue of operations, in addition to metadata annotations.""" _lock = RLock() """threading.RLock: Used to synchronize appending to/popping from global QueueingContext.""" def __enter__(self): """Adds this instance to the global list of active contexts. Returns: AnnotatedQueue: this instance """ AnnotatedQueue._lock.acquire() QueuingManager.add_active_queue(self) return self def __exit__(self, exception_type, exception_value, traceback): """Remove this instance from the global list of active contexts.""" QueuingManager.remove_active_queue() AnnotatedQueue._lock.release()
[docs] def append(self, obj, **kwargs): """Append ``obj`` into the queue with ``kwargs`` metadata.""" self[obj] = kwargs
[docs] def remove(self, obj): """Remove ``obj`` from the queue. Passes silently if the object is not in the queue.""" if obj in self: del self[obj]
[docs] def update_info(self, obj, **kwargs): """Update ``obj``'s metadata with ``kwargs`` if it exists in the queue.""" if obj in self: self[obj].update(kwargs)
[docs] def get_info(self, obj): """Retrieve the metadata for ``obj``. Raises a ``QueuingError`` if obj is not in the queue.""" if obj not in self: raise QueuingError(f"Object {obj} not in the queue.") return self[obj]
@property def queue(self): """Returns a list of objects in the annotated queue""" return list(self.keys())
[docs]def apply(op, context=QueuingManager): """Apply an instantiated operator or measurement to a queuing context. Args: op (.Operator or .MeasurementProcess): the operator or measurement to apply/queue context (.QueuingManager): The queuing context to queue the operator to. Note that if no context is specified, the operator is applied to the currently active queuing context. Returns: .Operator or .MeasurementProcess: the input operator is returned for convenience **Example** In PennyLane, **operations and measurements are 'queued' or added to a circuit when they are instantiated**. The ``apply`` function can be used to add operations that might have already been instantiated elsewhere to the QNode: .. code-block:: python op = qml.RX(0.4, wires=0) dev = qml.device("default.qubit", wires=2) @qml.qnode(dev) def circuit(x): qml.RY(x, wires=0) # applied during instantiation qml.apply(op) # manually applied return qml.expval(qml.PauliZ(0)) >>> print(qml.draw(circuit)(0.6)) 0: ──RY(0.6)──RX(0.4)──┤ ⟨Z⟩ It can also be used to apply functions repeatedly: .. code-block:: python @qml.qnode(dev) def circuit(x): qml.apply(op) qml.RY(x, wires=0) qml.apply(op) return qml.expval(qml.PauliZ(0)) >>> print(qml.draw(circuit)(0.6)) 0: ──RX(0.4)──RY(0.6)──RX(0.4)──┤ ⟨Z⟩ .. details:: :title: Usage Details Instantiated measurements can also be applied to queuing contexts using ``apply``: .. code-block:: python meas = qml.expval(qml.PauliZ(0) @ qml.PauliY(1)) dev = qml.device("default.qubit", wires=2) @qml.qnode(dev) def circuit(x): qml.RY(x, wires=0) qml.CNOT(wires=[0, 1]) return qml.apply(meas) >>> print(qml.draw(circuit)(0.6)) 0: ──RY(0.6)──╭●──╭┤ ⟨Z ⊗ Y⟩ 1: ───────────╰X──╰┤ ⟨Z ⊗ Y⟩ By default, ``apply`` will queue operators to the currently active queuing context. When working with low-level queuing contexts such as quantum tapes, the desired context to queue the operation to can be explicitly passed: .. code-block:: python with qml.tape.QuantumTape() as tape1: qml.Hadamard(wires=1) with qml.tape.QuantumTape() as tape2: # Due to the nesting behaviour of queuing contexts, # tape2 will be queued to tape1. # The following PauliX operation will be queued # to the active queuing context, tape2, during instantiation. op1 = qml.PauliX(wires=0) # We can use qml.apply to apply the same operation to tape1 # without leaving the tape2 context. qml.apply(op1, context=tape1) qml.RZ(0.2, wires=0) qml.CNOT(wires=[0, 1]) >>> tape1.operations [Hadamard(wires=[1]), <QuantumTape: wires=[0], params=1>, PauliX(wires=[0]), CNOT(wires=[0, 1])] >>> tape2.operations [PauliX(wires=[0]), RZ(0.2, wires=[0])] """ if not QueuingManager.recording(): raise RuntimeError("No queuing context available to append operation to.") if op in getattr(context, "queue", QueuingManager.active_context().queue): # Queuing contexts can only contain unique objects. # If the object to be queued already exists, copy it. op = copy.copy(op) if hasattr(op, "queue"): # operator provides its own logic for queuing op.queue(context=context) else: # append the operator directly to the relevant queuing context context.append(op) return op
# pylint: disable=protected-access
[docs]def process_queue(queue: AnnotatedQueue): """Process the annotated queue, creating a list of quantum operations and measurement processes. Args: queue (.AnnotatedQueue): The queue to be processed into individual lists Returns: tuple[list(.Operation), list(.MeasurementProcess)], list(.Operation): The list of main tape operations, the list of tape measurements, and the list of preparation operations """ lists = {"_prep": [], "_ops": [], "_measurements": []} list_order = {"_prep": 0, "_ops": 1, "_measurements": 2} current_list = "_prep" for obj, info in queue.items(): if "owner" not in info and getattr(obj, "_queue_category", None) is not None: if list_order[obj._queue_category] > list_order[current_list]: current_list = obj._queue_category elif list_order[obj._queue_category] < list_order[current_list]: raise ValueError( f"{obj._queue_category[1:]} operation {obj} must occur prior " f"to {current_list[1:]}. Please place earlier in the queue." ) lists[obj._queue_category].append(obj) return lists["_ops"], lists["_measurements"], lists["_prep"]