# Quantum transfer learning¶

Author: PennyLane dev team. Last updated: 28 Jan 2021.

In this tutorial we apply a machine learning method, known as transfer learning, to an image classifier based on a hybrid classical-quantum network.

This example follows the general structure of the PyTorch tutorial on transfer learning by Sasank Chilamkurthy, with the crucial difference of using a quantum circuit to perform the final classification task.

More details on this topic can be found in the research paper  (Mari et al. (2019)).

## Introduction¶

Transfer learning is a well-established technique for training artificial neural networks (see e.g., Ref. ), which is based on the general intuition that if a pre-trained network is good at solving a given problem, then, with just a bit of additional training, it can be used to also solve a different but related problem.

As discussed in Ref. , this idea can be formalized in terms of two abstract networks $$A$$ and $$B$$, independently from their quantum or classical physical nature.

As sketched in the above figure, one can give the following general definition of the transfer learning method:

1. Take a network $$A$$ that has been pre-trained on a dataset $$D_A$$ and for a given task $$T_A$$.
2. Remove some of the final layers. In this way, the resulting truncated network $$A'$$ can be used as a feature extractor.
3. Connect a new trainable network $$B$$ at the end of the pre-trained network $$A'$$.
4. Keep the weights of $$A'$$ constant, and train the final block $$B$$ with a new dataset $$D_B$$ and/or for a new task of interest $$T_B$$.

When dealing with hybrid systems, depending on the physical nature (classical or quantum) of the networks $$A$$ and $$B$$, one can have different implementations of transfer learning as

summarized in following table:

Network A Network B Transfer learning scheme
Classical Classical CC - Standard classical method. See e.g., Ref. .
Classical Quantum CQ - Hybrid model presented in this tutorial.
Quantum Classical QC - Model studied in Ref. .
Quantum Quantum QQ - Model studied in Ref. .

## Classical-to-quantum transfer learning¶

We focus on the CQ transfer learning scheme discussed in the previous section and we give a specific example.

1. As pre-trained network $$A$$ we use ResNet18, a deep residual neural network introduced by Microsoft in Ref. , which is pre-trained on the ImageNet dataset.
2. After removing its final layer we obtain $$A'$$, a pre-processing block which maps any input high-resolution image into 512 abstract features.
3. Such features are classified by a 4-qubit “dressed quantum circuit” $$B$$, i.e., a variational quantum circuit sandwiched between two classical layers.
4. The hybrid model is trained, keeping $$A'$$ constant, on the Hymenoptera dataset (a small subclass of ImageNet) containing images of ants and bees.

A graphical representation of the full data processing pipeline is given in the figure below.

## General setup¶

Note

To use the PyTorch interface in PennyLane, you must first install PyTorch.

In addition to PennyLane, we will also need some standard PyTorch libraries and the plotting library matplotlib.

# Some parts of this code are based on the Python script:
# https://github.com/pytorch/tutorials/blob/master/beginner_source/transfer_learning_tutorial.py

import time
import os
import copy

# PyTorch
import torch
import torch.nn as nn
import torch.optim as optim
from torch.optim import lr_scheduler
import torchvision
from torchvision import datasets, transforms

# Pennylane
import pennylane as qml
from pennylane import numpy as np

torch.manual_seed(42)
np.random.seed(42)

# Plotting
import matplotlib.pyplot as plt

# OpenMP: number of parallel threads.


## Setting of the main hyper-parameters of the model¶

Note

To reproduce the results of Ref. , num_epochs should be set to 30 which may take a long time. We suggest to first try with num_epochs=1 and, if everything runs smoothly, increase it to a larger value.

n_qubits = 4                # Number of qubits
step = 0.0004               # Learning rate
batch_size = 4              # Number of samples for each training step
num_epochs = 1              # Number of training epochs
q_depth = 6                 # Depth of the quantum circuit (number of variational layers)
gamma_lr_scheduler = 0.1    # Learning rate reduction applied every 10 epochs.
q_delta = 0.01              # Initial spread of random quantum weights
start_time = time.time()    # Start of the computation timer


We initialize a PennyLane device with a default.qubit backend.

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


We configure PyTorch to use CUDA only if available. Otherwise the CPU is used.

device = torch.device("cuda:0" if torch.cuda.is_available() else "cpu")


Note

The dataset containing images of ants and bees can be downloaded here and should be extracted in the subfolder ../_data/hymenoptera_data.

This is a very small dataset (roughly 250 images), too small for training from scratch a classical or quantum model, however it is enough when using transfer learning approach.

The PyTorch packages torchvision and torch.utils.data are used for loading the dataset and performing standard preliminary image operations: resize, center, crop, normalize, etc.

data_transforms = {
"train": transforms.Compose(
[
# transforms.RandomResizedCrop(224),     # uncomment for data augmentation
# transforms.RandomHorizontalFlip(),     # uncomment for data augmentation
transforms.Resize(256),
transforms.CenterCrop(224),
transforms.ToTensor(),
# Normalize input channels using mean values and standard deviations of ImageNet.
transforms.Normalize([0.485, 0.456, 0.406], [0.229, 0.224, 0.225]),
]
),
"val": transforms.Compose(
[
transforms.Resize(256),
transforms.CenterCrop(224),
transforms.ToTensor(),
transforms.Normalize([0.485, 0.456, 0.406], [0.229, 0.224, 0.225]),
]
),
}

data_dir = "../_data/hymenoptera_data"
image_datasets = {
x if x == "train" else "validation": datasets.ImageFolder(
os.path.join(data_dir, x), data_transforms[x]
)
for x in ["train", "val"]
}
dataset_sizes = {x: len(image_datasets[x]) for x in ["train", "validation"]}
class_names = image_datasets["train"].classes

for x in ["train", "validation"]
}

# function to plot images
def imshow(inp, title=None):
"""Display image from tensor."""
inp = inp.numpy().transpose((1, 2, 0))
# Inverse of the initial normalization operation.
mean = np.array([0.485, 0.456, 0.406])
std = np.array([0.229, 0.224, 0.225])
inp = std * inp + mean
inp = np.clip(inp, 0, 1)
plt.imshow(inp)
if title is not None:
plt.title(title)


Let us show a batch of the test data, just to have an idea of the classification problem.

# Get a batch of training data

# Make a grid from batch
out = torchvision.utils.make_grid(inputs)

imshow(out, title=[class_names[x] for x in classes])

for x in ["train", "validation"]
} ## Variational quantum circuit¶

We first define some quantum layers that will compose the quantum circuit.

def H_layer(nqubits):
"""
for idx in range(nqubits):

def RY_layer(w):
"""Layer of parametrized qubit rotations around the y axis.
"""
for idx, element in enumerate(w):
qml.RY(element, wires=idx)

def entangling_layer(nqubits):
"""Layer of CNOTs followed by another shifted layer of CNOT.
"""
# In other words it should apply something like :
# CNOT  CNOT  CNOT  CNOT...  CNOT
#   CNOT  CNOT  CNOT...  CNOT
for i in range(0, nqubits - 1, 2):  # Loop over even indices: i=0,2,...N-2
qml.CNOT(wires=[i, i + 1])
for i in range(1, nqubits - 1, 2):  # Loop over odd indices:  i=1,3,...N-3
qml.CNOT(wires=[i, i + 1])


Now we define the quantum circuit through the PennyLane qnode decorator .

The structure is that of a typical variational quantum circuit:

• Embedding layer: All qubits are first initialized in a balanced superposition of up and down states, then they are rotated according to the input parameters (local embedding).
• Variational layers: A sequence of trainable rotation layers and constant entangling layers is applied.
• Measurement layer: For each qubit, the local expectation value of the $$Z$$ operator is measured. This produces a classical output vector, suitable for additional post-processing.
@qml.qnode(dev, interface="torch")
def quantum_net(q_input_features, q_weights_flat):
"""
The variational quantum circuit.
"""

# Reshape weights
q_weights = q_weights_flat.reshape(q_depth, n_qubits)

# Start from state |+> , unbiased w.r.t. |0> and |1>
H_layer(n_qubits)

# Embed features in the quantum node
RY_layer(q_input_features)

# Sequence of trainable variational layers
for k in range(q_depth):
entangling_layer(n_qubits)
RY_layer(q_weights[k])

# Expectation values in the Z basis
exp_vals = [qml.expval(qml.PauliZ(position)) for position in range(n_qubits)]
return tuple(exp_vals)


## Dressed quantum circuit¶

We can now define a custom torch.nn.Module representing a dressed quantum circuit.

This is a concatenation of:

• A classical pre-processing layer (nn.Linear).
• A classical activation function (torch.tanh).
• A constant np.pi/2.0 scaling.
• The previously defined quantum circuit (quantum_net).
• A classical post-processing layer (nn.Linear).

The input of the module is a batch of vectors with 512 real parameters (features) and the output is a batch of vectors with two real outputs (associated with the two classes of images: ants and bees).

class DressedQuantumNet(nn.Module):
"""
Torch module implementing the *dressed* quantum net.
"""

def __init__(self):
"""
Definition of the *dressed* layout.
"""

super().__init__()
self.pre_net = nn.Linear(512, n_qubits)
self.q_params = nn.Parameter(q_delta * torch.randn(q_depth * n_qubits))
self.post_net = nn.Linear(n_qubits, 2)

def forward(self, input_features):
"""
Defining how tensors are supposed to move through the *dressed* quantum
net.
"""

# obtain the input features for the quantum circuit
# by reducing the feature dimension from 512 to 4
pre_out = self.pre_net(input_features)
q_in = torch.tanh(pre_out) * np.pi / 2.0

# Apply the quantum circuit to each element of the batch and append to q_out
q_out = torch.Tensor(0, n_qubits)
q_out = q_out.to(device)
for elem in q_in:
q_out_elem = quantum_net(elem, self.q_params).float().unsqueeze(0)
q_out = torch.cat((q_out, q_out_elem))

# return the two-dimensional prediction from the postprocessing layer
return self.post_net(q_out)


## Hybrid classical-quantum model¶

We are finally ready to build our full hybrid classical-quantum network. We follow the transfer learning approach:

1. First load the classical pre-trained network ResNet18 from the torchvision.models zoo.
2. Freeze all the weights since they should not be trained.
3. Replace the last fully connected layer with our trainable dressed quantum circuit (DressedQuantumNet).

Note

The ResNet18 model is automatically downloaded by PyTorch and it may take several minutes (only the first time).

model_hybrid = torchvision.models.resnet18(pretrained=True)

for param in model_hybrid.parameters():

# Notice that model_hybrid.fc is the last layer of ResNet18
model_hybrid.fc = DressedQuantumNet()

# Use CUDA or CPU according to the "device" object.
model_hybrid = model_hybrid.to(device)


Out:

Downloading: "https://download.pytorch.org/models/resnet18-5c106cde.pth" to /home/circleci/.cache/torch/hub/checkpoints/resnet18-5c106cde.pth

0%|          | 0.00/44.7M [00:00<?, ?B/s]
30%|##9       | 13.4M/44.7M [00:00<00:00, 140MB/s]
61%|######    | 27.1M/44.7M [00:00<00:00, 142MB/s]
92%|#########2| 41.1M/44.7M [00:00<00:00, 144MB/s]
100%|##########| 44.7M/44.7M [00:00<00:00, 144MB/s]


## Training and results¶

Before training the network we need to specify the loss function.

We use, as usual in classification problem, the cross-entropy which is directly available within torch.nn.

criterion = nn.CrossEntropyLoss()


We also initialize the Adam optimizer which is called at each training step in order to update the weights of the model.

optimizer_hybrid = optim.Adam(model_hybrid.fc.parameters(), lr=step)


We schedule to reduce the learning rate by a factor of gamma_lr_scheduler every 10 epochs.

exp_lr_scheduler = lr_scheduler.StepLR(
optimizer_hybrid, step_size=10, gamma=gamma_lr_scheduler
)


What follows is a training function that will be called later. This function should return a trained model that can be used to make predictions (classifications).

def train_model(model, criterion, optimizer, scheduler, num_epochs):
since = time.time()
best_model_wts = copy.deepcopy(model.state_dict())
best_acc = 0.0
best_loss = 10000.0  # Large arbitrary number
best_acc_train = 0.0
best_loss_train = 10000.0  # Large arbitrary number
print("Training started:")

for epoch in range(num_epochs):

# Each epoch has a training and validation phase
for phase in ["train", "validation"]:
if phase == "train":
# Set model to training mode
model.train()
else:
# Set model to evaluate mode
model.eval()
running_loss = 0.0
running_corrects = 0

# Iterate over data.
n_batches = dataset_sizes[phase] // batch_size
it = 0
since_batch = time.time()
batch_size_ = len(inputs)
inputs = inputs.to(device)
labels = labels.to(device)

# Track/compute gradient and make an optimization step only when training
outputs = model(inputs)
_, preds = torch.max(outputs, 1)
loss = criterion(outputs, labels)
if phase == "train":
loss.backward()
optimizer.step()

# Print iteration results
running_loss += loss.item() * batch_size_
batch_corrects = torch.sum(preds == labels.data).item()
running_corrects += batch_corrects
print(
"Phase: {} Epoch: {}/{} Iter: {}/{} Batch time: {:.4f}".format(
phase,
epoch + 1,
num_epochs,
it + 1,
n_batches + 1,
time.time() - since_batch,
),
end="\r",
flush=True,
)
it += 1

# Print epoch results
epoch_loss = running_loss / dataset_sizes[phase]
epoch_acc = running_corrects / dataset_sizes[phase]
print(
"Phase: {} Epoch: {}/{} Loss: {:.4f} Acc: {:.4f}        ".format(
"train" if phase == "train" else "validation  ",
epoch + 1,
num_epochs,
epoch_loss,
epoch_acc,
)
)

# Check if this is the best model wrt previous epochs
if phase == "validation" and epoch_acc > best_acc:
best_acc = epoch_acc
best_model_wts = copy.deepcopy(model.state_dict())
if phase == "validation" and epoch_loss < best_loss:
best_loss = epoch_loss
if phase == "train" and epoch_acc > best_acc_train:
best_acc_train = epoch_acc
if phase == "train" and epoch_loss < best_loss_train:
best_loss_train = epoch_loss

# Update learning rate
if phase == "train":
scheduler.step()

# Print final results
time_elapsed = time.time() - since
print(
"Training completed in {:.0f}m {:.0f}s".format(time_elapsed // 60, time_elapsed % 60)
)
print("Best test loss: {:.4f} | Best test accuracy: {:.4f}".format(best_loss, best_acc))
return model


We are ready to perform the actual training process.

model_hybrid = train_model(
model_hybrid, criterion, optimizer_hybrid, exp_lr_scheduler, num_epochs=num_epochs
)


Out:

Training started:
Phase: train Epoch: 1/1 Iter: 1/62 Batch time: 0.9018
Phase: train Epoch: 1/1 Iter: 2/62 Batch time: 0.8789
Phase: train Epoch: 1/1 Iter: 3/62 Batch time: 0.8890
Phase: train Epoch: 1/1 Iter: 4/62 Batch time: 0.8741
Phase: train Epoch: 1/1 Iter: 5/62 Batch time: 0.8433
Phase: train Epoch: 1/1 Iter: 6/62 Batch time: 0.8261
Phase: train Epoch: 1/1 Iter: 7/62 Batch time: 0.8364
Phase: train Epoch: 1/1 Iter: 8/62 Batch time: 0.9237
Phase: train Epoch: 1/1 Iter: 9/62 Batch time: 0.8500
Phase: train Epoch: 1/1 Iter: 10/62 Batch time: 0.8984
Phase: train Epoch: 1/1 Iter: 11/62 Batch time: 0.8836
Phase: train Epoch: 1/1 Iter: 12/62 Batch time: 0.8144
Phase: train Epoch: 1/1 Iter: 13/62 Batch time: 0.8155
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Phase: train Epoch: 1/1 Iter: 16/62 Batch time: 0.8783
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Phase: train Epoch: 1/1 Iter: 18/62 Batch time: 1.2298
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Phase: train Epoch: 1/1 Loss: 0.6528 Acc: 0.6066
Phase: validation Epoch: 1/1 Iter: 1/39 Batch time: 0.2039
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Phase: validation Epoch: 1/1 Iter: 39/39 Batch time: 0.0659
Phase: validation   Epoch: 1/1 Loss: 0.5325 Acc: 0.7582
Training completed in 1m 12s
Best test loss: 0.5325 | Best test accuracy: 0.7582


## Visualizing the model predictions¶

We first define a visualization function for a batch of test data.

def visualize_model(model, num_images=6, fig_name="Predictions"):
images_so_far = 0
_fig = plt.figure(fig_name)
model.eval()
for _i, (inputs, labels) in enumerate(dataloaders["validation"]):
inputs = inputs.to(device)
labels = labels.to(device)
outputs = model(inputs)
_, preds = torch.max(outputs, 1)
for j in range(inputs.size()):
images_so_far += 1
ax = plt.subplot(num_images // 2, 2, images_so_far)
ax.axis("off")
ax.set_title("[{}]".format(class_names[preds[j]]))
imshow(inputs.cpu().data[j])
if images_so_far == num_images:
return


Finally, we can run the previous function to see a batch of images with the corresponding predictions.

visualize_model(model_hybrid, num_images=batch_size)
plt.show() 