Gradients and training¶
PennyLane offers seamless integration between classical and quantum computations. Code up quantum circuits in PennyLane, compute gradients of quantum circuits, and connect them easily to the top scientific computing and machine learning libraries.
Training and interfaces¶
The bridge between the quantum and classical worlds is provided in PennyLane via interfaces to automatic differentiation libraries. Currently, four libraries are supported: NumPy, PyTorch, JAX, and TensorFlow. PennyLane makes each of these libraries quantumaware, allowing quantum circuits to be treated just like any other operation. Any automatic differentiation framework can be chosen with any device.
In PennyLane, an automatic differentiation framework is declared using the interface
argument when creating
a QNode
, e.g.,
@qml.qnode(dev, interface="tf")
def my_quantum_circuit(...):
...
Note
If no interface is specified, PennyLane will default to the NumPy interface (powered by the autograd library).
This will allow native numerical objects of the specified library (NumPy arrays, JAX arrays, Torch Tensors, or TensorFlow Tensors) to be passed as parameters to the quantum circuit. It also makes the gradients of the quantum circuit accessible to the classical library, enabling the optimization of arbitrary hybrid circuits by making use of the library’s native optimizers.
When specifying an interface, the objects of the chosen framework are converted into NumPy objects and are passed to a device in most cases. Exceptions include cases when the devices support endtoend computations in a framework. Such devices may be referred to as backpropagation or passthru devices.
See the links below for walkthroughs of each specific interface:
In addition to the core automatic differentiation frameworks discussed above,
PennyLane also provides higherlevel classes for converting QNodes into both Keras and torch.nn
layers:

Converts a 

Converts a 
Note
QNodes that allow for automatic differentiation will always incur a small overhead on evaluation.
If you do not need to compute quantum gradients of a QNode, specifying interface=None
will remove
this overhead and result in a slightly faster evaluation. However, gradients will no
longer be available.
Optimizers¶
Optimizers are objects which can be used to automatically update the parameters of a quantum or hybrid machine learning model. The optimizers you should use are dependent on your choice of the classical autodifferentiation library, and are available from different access points.
NumPy¶
When using the standard NumPy framework, PennyLane offers some builtin optimizers.
Some of these are specific to quantum optimization, such as the QNGOptimizer
, LieAlgebraOptimizer
RotosolveOptimizer
, RotoselectOptimizer
, ShotAdaptiveOptimizer
, and QNSPSAOptimizer
.
Gradientdescent optimizer with pastgradientdependent learning rate in each dimension. 

Gradientdescent optimizer with adaptive learning rate, first and second moment. 

Basic gradientdescent optimizer. 

Lie algebra optimizer. 

Gradientdescent optimizer with momentum. 

Gradientdescent optimizer with Nesterov momentum. 

Optimizer with adaptive learning rate, via calculation of the diagonal or blockdiagonal approximation to the FubiniStudy metric tensor. 

Root mean squared propagation optimizer. 

Rotosolve gradientfree optimizer. 

Rotoselect gradientfree optimizer. 

Optimizer where the shot rate is adaptively calculated using the variances of the parametershift gradient. 

The Simultaneous Perturbation Stochastic Approximation method (SPSA) is a stochastic approximation algorithm for optimizing cost functions whose evaluation may involve noise. 

Quantum natural SPSA (QNSPSA) optimizer. 
PyTorch¶
If you are using the PennyLane PyTorch framework, you should import one of the native
PyTorch optimizers (found in torch.optim
).
TensorFlow¶
When using the PennyLane TensorFlow framework, you will need to leverage one of
the TensorFlow optimizers
(found in tf.keras.optimizers
).
JAX¶
Check out the JAXopt and the Optax packages to find optimizers for the PennyLane JAX framework.
Gradients¶
The interface between PennyLane and automatic differentiation libraries relies on PennyLane’s ability to compute or estimate gradients of quantum circuits. There are different strategies to do so, and they may depend on the device used.
When creating a QNode, you can specify the differentiation method like this:
@qml.qnode(dev, diff_method="parametershift")
def circuit(x):
qml.RX(x, wires=0)
return qml.probs(wires=0)
PennyLane currently provides the following differentiation methods for QNodes:
Simulationbased differentiation¶
The following methods use reverse accumulation to compute
gradients; a wellknown example of this approach is backpropagation. These methods are not hardware compatible; they are only supported on
statevector simulator devices such as default.qubit
.
However, for rapid prototyping on simulators, these methods typically outperform forwardmode accumulators such as the parametershift rule and finitedifferences. For more details, see the quantum backpropagation demonstration.
"backprop"
: Use standard backpropagation.This differentiation method is only allowed on simulator devices that are classically endtoend differentiable, for example
default.qubit
. This method does not work on devices that estimate measurement statistics using a finite number of shots; please use theparametershift
rule instead."adjoint"
: Use a form of backpropagation that takes advantage of the unitary or reversible nature of quantum computation.The adjoint method reverses through the circuit after a forward pass by iteratively applying the inverse (adjoint) gate. This method is similar to
"backprop"
, but has significantly lower memory usage and a similar runtime.
Hardwarecompatible differentiation¶
The following methods support both quantum hardware and simulators, and are examples of forward accumulation. However, when using a simulator, you may notice that the time required to compute the gradients with these methods scales linearly with the number of trainable circuit parameters.
"parametershift"
: Use the analytic parametershift rule for all supported quantum operation arguments, with finitedifference as a fallback."finitediff"
: Use numerical finitedifferences for all quantum operation arguments.
Device gradients¶
"device"
: Queries the device directly for the gradient. Only allowed on devices that provide their own gradient computation.
Note
If not specified, the default differentiation method is diff_method="best"
. PennyLane
will attempt to determine the best differentiation method given the device and interface.
Typically, PennyLane will prioritize deviceprovided gradients, backpropagation, parametershift
rule, and finally finite differences, in that order.
Gradient transforms¶
In addition to registering the differentiation method of QNodes to be used with autodifferentiation
frameworks, PennyLane also provides a library of gradient transforms via the
qml.gradients
module.
Quantum gradient transforms are strategies for computing the gradient of a quantum circuit that work by transforming the quantum circuit into one or more gradient circuits. They accompany these circuits with a function that postprocesses their output. These gradient circuits, once executed and postprocessed, return the gradient of the original circuit.
Examples of quantum gradient transforms include finitedifference rules and parametershift rules; these can be applied directly to QNodes:
dev = qml.device("default.qubit", wires=2)
@qml.qnode(dev)
def circuit(weights):
qml.RX(weights[0], wires=0)
qml.RY(weights[1], wires=1)
qml.CNOT(wires=[0, 1])
qml.RX(weights[2], wires=1)
return qml.probs(wires=1)
>>> weights = np.array([0.1, 0.2, 0.3], requires_grad=True)
>>> circuit(weights)
tensor([0.9658079, 0.0341921], requires_grad=True)
>>> qml.gradients.param_shift(circuit)(weights)
tensor([[0.04673668, 0.09442394, 0.14409127],
[ 0.04673668, 0.09442394, 0.14409127]], requires_grad=True)
Note that, while gradient transforms allow quantum gradient rules to be applied directly to QNodes,
this is not a replacement — and should not be used instead of — standard training workflows (for example,
qml.grad()
if using Autograd, loss.backward()
for PyTorch, or tape.gradient()
for TensorFlow).
This is because gradient transforms do not take into account classical computation nodes, and only
support gradients of QNodes.
For more details on available gradient transforms, as well as learning how to define your own
gradient transform, please see the qml.gradients
documentation.
Supported configurations¶
The table below show all the currently supported functionality for the "default.qubit"
device.
At the moment, it takes into account the following parameters:
The interface, e.g.
"jax"
The differentiation method, e.g.
"parametershift"
The return value of the QNode, e.g.
qml.expval()
orqml.probs()
The number of shots, either None or an integer > 0
Return type 


Interface 
Differentiation method 
state 
density matrix 
probs 
sample 
expval (obs) 
expval (herm) 
expval (proj) 
var 
vn entropy 
mutual info 


1 
1 
1 
1 
1 
1 
1 
1 
1 
1 

1 
1 
1 
1 
1 
1 
1 
1 
1 
1 


2 
2 
2 
2 
2 
2 
2 
2 
2 
2 


2 
2 
2 
2 
2 
2 
2 
2 
2 
2 


2 
2 
2 
2 
2 
2 
2 
2 
2 
2 



3 
3 
3 
3 
3 
3 
3 
3 
3 
3 

4 
4 
5 
9 
5 
5 
5 
5 
5 
5 


6 
6 
6 
6 
7 
7 
7 
6 
6 
6 


10 
10 
8 
9 
8 
8 
8 
8 
10 
10 


10 
10 
8 
9 
8 
8 
8 
8 
8 
8 



3 
3 
3 
3 
3 
3 
3 
3 
3 
3 

5 
5 
5 
9 
5 
5 
5 
5 
5 
5 


6 
6 
6 
6 
7 
7 
7 
6 
6 
6 


10 
10 
8 
9 
8 
8 
8 
8 
10 
10 


10 
10 
8 
9 
8 
8 
8 
8 
8 
8 



3 
3 
3 
3 
3 
3 
3 
3 
3 
3 

5 
5 
5 
9 
5 
5 
5 
5 
5 
5 


6 
6 
6 
6 
7 
7 
7 
6 
6 
6 


10 
10 
8 
9 
8 
8 
8 
8 
10 
10 


10 
10 
8 
9 
8 
8 
8 
8 
8 
8 



3 
3 
3 
3 
3 
3 
3 
3 
3 
3 

5 
5 
5 
9 
5 
5 
5 
5 
5 
5 


6 
6 
6 
6 
7 
7 
7 
6 
6 
6 


10 
10 
8 
9 
8 
8 
8 
8 
10 
10 


10 
10 
8 
9 
8 
8 
8 
8 
8 
8 
Not supported. Gradients are not computed even though
diff_method
is provided. Fails with error.Not supported. Gradients are not computed even though
diff_method
is provided. Warns that no autodifferentiation framework is being used, but does not fail. Forward pass is still supported.Not supported. The
default.qubit
device does not provide a native way to compute gradients. See Device jacobian for details.Supported, but only when
shots=None
. See Backpropagation for details.If the circuit returns a state, then the circuit itself is not differentiable directly. However, any real scalarvalued postprocessing done to the output of the circuit will be differentiable. See State gradients for details.
Supported, but only when
shots=None
. See Backpropagation for details.Not supported. The adjoint differentiation algorithm is only implemented for computing the expectation values of observables. See Adjoint differentation for details.
Supported. Raises warning when
shots>0
since the gradient is always computed analytically. See Adjoint differentation for details.Supported.
Not supported. The discretization of the output caused by wave function collapse is not differentiable. The forward pass is still supported. See Sample gradients for details.
Not supported. “We just don’t have the theory yet.”