# qml.tape¶

Warning

The new PennyLane tape mode is experimental, and does not currently have feature-parity with the existing QNode. Feedback and bug reports are encouraged and will help improve the new tape mode.

Tape mode can be enabled globally via the enable_tape() function, without changing your PennyLane code:

>>> qml.enable_tape()


Once enabled, tape mode can be disabled via disable_tape().

## Tape-mode QNodes¶

The PennyLane tape module provides a new QNode class, rewritten from the ground-up, that uses a QuantumTape to represent the internal variational quantum circuit. Tape mode provides several advantages over the standard PennyLane QNode.

• Support for in-QNode classical processing: Tape mode allows for differentiable classical processing within the QNode.

qml.enable_tape()
dev = qml.device("default.qubit", wires=1)

@qml.qnode(dev, interface="tf")
def circuit(p):
qml.RX(tf.sin(p[0])**2 + p[1], wires=0)
return qml.expval(qml.PauliZ(0))


The classical processing functions used within the QNode must match the QNode interface. Here, we use TensorFlow:

>>> params = tf.Variable([0.5, 0.1], dtype=tf.float64)
...     res = circuit(params)
>>> print(res)
tf.Tensor(0.9460913127754935, shape=(), dtype=float64)
tf.Tensor([-0.27255248 -0.32390003], shape=(2,), dtype=float64)


As a result of this change, quantum decompositions that require classical processing are fully supported and end-to-end differentiable in tape mode.

• No more Variable wrapping: In tape mode, QNode arguments no longer become Variable objects within the QNode.

qml.enable_tape()
dev = qml.device("default.qubit", wires=1)

@qml.qnode(dev)
def circuit(x):
print("Parameter value:", x)
qml.RX(x, wires=0)
return qml.expval(qml.PauliZ(0))


Internal QNode parameters can be easily inspected, printed, and manipulated:

>>> circuit(0.5)
Parameter value: 0.5

• Return the quantum state: In tape mode, QNodes bound to statevector simulators can return the quantum state using the state() function:

qml.enable_tape()
dev = qml.device("default.qubit", wires=2)

@qml.qnode(dev)
def circuit():
return qml.state()

>>> circuit()
array([0.70710678+0.j, 0.70710678+0.j, 0.        +0.j, 0.        +0.j])


Calculating the derivative of state() is currently only supported when using the classical backpropagation differentiation method (diff_method="backprop") with a compatible device.

• Less restrictive QNode signatures: There is no longer any restriction on the QNode signature; the QNode can be defined and called following the same rules as standard Python functions.

For example, the following QNode uses positional, named, and variable keyword arguments:

qml.enable_tape()

@qml.qnode(dev, interface="torch")
def circuit(p1, p2=y, **kwargs):
qml.RX(p1, wires=0)
qml.RY(p2[0] * p2[1], wires=0)
qml.RX(kwargs["p3"], wires=0)
return qml.var(qml.PauliZ(0))


When we call the QNode, we may pass the arguments by name even if defined positionally; any argument not provided will use the default value.

>>> res = circuit(p1=x, p3=z)
>>> print(res)
>>> res.backward()
tensor(0.8396) tensor([0.0289, 0.0193]) tensor(0.8387)

• Unifying all QNodes: The tape-mode QNode merges all QNodes (including the JacobianQNode and the PassthruQNode) into a single unified QNode, with identical behaviour regardless of the differentiation type.

In addition, it is now possible to inspect the internal variational quantum circuit structure of QNodes when using classical backpropagation (which is not support in the standard PassthruQNode).

• Optimizations: Tape mode provides various performance optimizations, reducing pre- and post-processing overhead, and reduces the number of quantum evaluations in certain cases.

Warning

In tape-mode, the QNode does not yet have feature-parity with the standard PennyLane QNode. Features currently not available in tape mode include:

• Metric tensor computation

• The ability to automatically extract the layer structure of variational circuits

## Quantum tapes¶

Under the hood, tape mode is able to provide these new features by significantly overhauling the internal structure of the QNode. When tape mode is enabled, the QNode is no longer responsible for recording quantum operations, executing devices, or computing gradients—these tasks have been delegated to an internal object that is created by the QNode, the quantum tape.

In addition to being created internally by QNodes in tape mode, quantum tapes can also be created, nested, expanded (via expand()), and executed manually. Tape subclasses also provide additional gradient methods:

 QuantumTape([name]) A quantum tape recorder, that records, validates and executes variational quantum programs. QubitParamShiftTape([name]) Quantum tape for qubit parameter-shift analytic differentiation method. CVParamShiftTape([name]) Quantum tape for CV parameter-shift analytic differentiation method. ReversibleTape([name]) Quantum tape for computing gradients via reversible analytic differentiation.

Finally, quantum tapes are fully compatible with autodifferentiating via NumPy/Autograd, TensorFlow, and PyTorch:

 Mixin class for applying an TensorFlow interface to a JacobianTape. Mixin class for applying an Torch interface to a JacobianTape. Mixin class for applying an autograd interface to a JacobianTape.

For more details and examples, please see the tape documentation.

## pennylane.tape Package¶

### Functions¶

 density_matrix(wires) Quantum density matrix in the computational basis. draw(_qnode[, charset, wire_order]) draw(qnode, charset=”unicode”): Create a function that draws the given _qnode. qnode(device[, interface, diff_method]) Decorator for creating QNodes. Quantum state in the computational basis. Returns whether tape mode is enabled.

### Classes¶

 Lightweight class that maintains a basic queue of operations, in addition to metadata annotations. CVParamShiftTape([name]) Quantum tape for CV parameter-shift analytic differentiation method. JacobianTape([name]) A quantum tape recorder, that records, validates, executes, and differentiates variational quantum programs. MeasurementProcess(return_type[, obs, …]) Represents a measurement process occurring at the end of a quantum variational circuit. QNode(func, device[, interface, diff_method]) Represents a quantum node in the hybrid computational graph. QuantumTape([name]) A quantum tape recorder, that records, validates and executes variational quantum programs. QubitParamShiftTape([name]) Quantum tape for qubit parameter-shift analytic differentiation method. Lightweight class that maintains a basic queue of objects. QueuingContext Abstract base class for classes that exposes a queue for objects. ReversibleTape([name]) Quantum tape for computing gradients via reversible analytic differentiation. TapeCircuitGraph(ops, obs, wires[, …]) Represents a quantum circuit as a directed acyclic graph.