Quantum embeddings and metric learning

Authors: Maria Schuld and Aroosa Ijaz

This tutorial illustrates the idea of quantum embeddings for metric learning presented in Lloyd, Schuld, Ijaz, Izaac, Killoran (2019), by training a hybrid classical-quantum data embedding to classify images of ants and bees. The example was inspired by Mari et al. (2019), (see also this tutorial), and reproduces some of the subplots in Figure 5 of Lloyd et al.

The tutorial requires the following imports:

# %matplotlib inline
import matplotlib.pyplot as plt
from mpl_toolkits.axes_grid1 import make_axes_locatable

import pennylane as qml
from pennylane import numpy as np
from pennylane import RX, RY, RZ, CNOT


Quantum metric learning trains a quantum embedding—for example, a quantum circuit that encodes classical data into quantum states—to separate different classes of data in the Hilbert space of the quantum system.


The trained embedding can be used for classification. A new data sample (red dot) gets mapped into Hilbert space via the same embedding, and a special measurement compares it to the two embedded classes. The decision boundary of the measurement in quantum state space is nearly linear (red dashed line).


Since a simple metric in Hilbert space corresponds to a potentially much more complex metric in the original data space, the simple decision boundary can translate to a non-trivial decision boundary in the original space of the data.


The best quantum measurement one could construct to classify new inputs depends on the loss defined for the classification task, as well as the metric used to optimize the separation of data.

For a linear cost function, data separated by the trace distance or \(\ell_1\) metric is best distinguished by a Helstrom measurement, while data separated by the Hilbert-Schmidt distance or \(\ell_2\) metric is best classified by a fidelity measurement. Here we show how to implement training and classification based on the \(\ell_2\) metric.


A quantum embedding is a representation of data points \(x\) from a data domain \(X\) as a (quantum) feature state \(| x \rangle\). Either the full embedding, or part of it, can be facilitated by a “quantum feature map”, a quantum circuit \(\Phi(x)\) that depends on the input. If the circuit has additional parameters \(\theta\) that are adaptable, \(\Phi = \Phi(x, \theta)\), the quantum feature map can be trained via optimization.

In this tutorial we investigate a trainable, hybrid classical-quantum embedding implemented by a partially pre-trained classical neural network, followed by a parametrized quantum circuit that implements the quantum feature map:


Following Mari et al. (2019), for the classical neural network we use PyTorch’s torch.models.resnet18(), setting pretrained=True. The final layer of the ResNet, which usually maps a 512-dimensional vector to 1000 nodes representing different image classes, is replaced by a linear layer of 2 output neurons. The classical part of the embedding therefore maps the images to a 2-dimensional intermediate feature space.

For the quantum part we use the QAOA embedding proposed in Lloyd et al. (2019). The feature map is represented by a layered variational circuit, which alternates a “feature-encoding Hamiltonian” and an “Ising-like” Hamiltonian with ZZ-entanglers (the two-qubit gates in the circuit diagram above) and RY gates as local fields.

def feature_encoding_hamiltonian(features, wires):

    for idx, w in enumerate(wires):
        RX(features[idx], wires=w)

def ising_hamiltonian(weights, wires, l):

        # ZZ coupling
        CNOT(wires=[wires[1], wires[0]])
        RZ(weights[l, 0], wires=wires[0])
        CNOT(wires=[wires[1], wires[0]])
        # local fields
        for idx, w in enumerate(wires):
            RY(weights[l, idx + 1], wires=w)

def QAOAEmbedding(features, weights, wires):

    repeat = len(weights)
    for l in range(repeat):
        # apply alternating Hamiltonians
        feature_encoding_hamiltonian(features, wires)
        ising_hamiltonian(weights, wires, l)
    # repeat the feature encoding once more at the end
    feature_encoding_hamiltonian(features, wires)


Instead of using the hand-coded QAOAEmbedding() function, PennyLane provides a built-in QAOAEmebedding template. To use it, simply replace the cell above by from pennylane.templates import QAOAEmbedding. This will also allow you to use a different number of qubits in your experiment.

Overall, the embedding has 1024 + 12 trainable parameters - 1024 for the classical part of the model and 12 for the four layers of the QAOA embedding.


The pretrained neural network has already learned to separate the data. The example does therefore not make any claims on the performance of the embedding, but aims to illustrate how a hybrid embedding can be trained.


We consider a binary supervised learning problem with examples \(\{a_1,...a_{M_a}\} \subseteq X\) from class \(A\) and examples \(\{b_1,...b_{M_b}\} \subseteq X\) from class \(B\). The data are images of ants (\(A\)) and bees (\(B\)), taken from Kaggle’s hymenoptera dataset. This is a sample of four images:


For convenience, instead of coding up the classical neural network, we load pre-extracted feature vectors of the images. These were created by resizing, cropping and normalizing the images, and passing them through PyTorch’s pretrained ResNet 512 (that is, without the final linear layer).

X = np.loadtxt("embedding_metric_learning/X_antbees.txt", ndmin=2)  #1  pre-extracted inputs
Y = np.loadtxt("embedding_metric_learning/Y_antbees.txt")  # labels
X_val = np.loadtxt(
    "embedding_metric_learning/X_antbees_test.txt", ndmin=2
)  # pre-extracted validation inputs
Y_val = np.loadtxt("embedding_metric_learning/Y_antbees_test.txt")  # validation labels

# split data into two classes
A = X[Y == -1]
B = X[Y == 1]
A_val = X_val[Y_val == -1]
B_val = X_val[Y_val == 1]



(83, 512)
(70, 512)


The distance metric underlying the notion of ‘separation’ is the \(\ell_2\) or Hilbert-Schmidt norm, which depends on overlaps of the embedded data points \(|a\rangle\) from class \(A\) and \(|b\rangle\) from class \(B\),

\[D_{\mathrm{hs}}(A, B) = \frac{1}{2} \big( \sum_{i, i'} |\langle a_i|a_{i'}\rangle|^2 + \sum_{j,j'} |\langle b_j|b_{j'}\rangle|^2 \big) - \sum_{i,j} |\langle a_i|b_j\rangle|^2.\]

To maximize the \(\ell_2\) distance between the two classes in Hilbert space, we minimize the cost \(C = 1 - \frac{1}{2}D_{\mathrm{hs}}(A, B)\).

To set up the “quantum part” of the cost function in PennyLane, we have to create a quantum node. Here, the quantum node is simulated on PennyLane’s 'default.qubit' backend.


One could also connect the quantum node to a hardware backend to find out if the noise of a physical implementation still allows us to train the embedding.

n_features = 2
n_qubits = 2 * n_features + 1

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

We use a SWAP test to measure the overlap \(|\langle \psi | \phi \rangle|^2\) between two quantum feature states \(|\psi\rangle\) and \(|\phi\rangle\), prepared by a QAOAEmbedding with weights q_weights.

def swap_test(q_weights, x1, x2):

    # load the two inputs into two different registers
    QAOAEmbedding(features=x1, weights=q_weights, wires=[1, 2])
    QAOAEmbedding(features=x2, weights=q_weights, wires=[3, 4])

    # perform the SWAP test
    for k in range(n_features):
        qml.CSWAP(wires=[0, k + 1, 2 + k + 1])

    return qml.expval(qml.PauliZ(0))

Before executing the swap test, the feature vectors have to be multiplied by a (2, 512)-dimensional matrix that represents the weights of the linear layer. This trainable classical pre-processing is executed before calling the swap test:

def overlaps(weights, X1=None, X2=None):

    linear_layer = weights[0]
    q_weights = weights[1]

    overlap = 0
    for x1 in X1:
        for x2 in X2:
            # multiply the inputs with the linear layer weight matrix
            w_x1 = linear_layer @ x1
            w_x2 = linear_layer @ x2
            # overlap of embedded intermediate features
            overlap += swap_test(q_weights, w_x1, w_x2)

    mean_overlap = overlap / (len(X1) * len(X2))
    return mean_overlap

In the overlaps() function, weights is a list of two arrays, the first representing the matrix of the linear layer, and the second containing the quantum circuit parameters.

With this we can define the cost function \(C\), which depends on inter- and intra-cluster overlaps.

def cost(weights, A=None, B=None):

    aa = overlaps(weights, X1=A, X2=A)
    bb = overlaps(weights, X1=B, X2=B)
    ab = overlaps(weights, X1=A, X2=B)

    d_hs = -2 * ab + (aa + bb)

    return 1 - 0.5 * d_hs


The initial parameters for the trainable classical and quantum part of the embedding are chosen at random. The number of layers in the quantum circuit is derived from the first dimension of init_pars_quantum.

# generate initial parameters for circuit
init_pars_quantum = np.random.normal(loc=0, scale=0.1, size=(4, 3))

# generate initial parameters for linear layer
init_pars_classical = np.random.normal(loc=0, scale=0.1, size=(2, 512))

init_pars = [init_pars_classical, init_pars_quantum]


You can alternatively use the utility function pennylane.init.qaoa_embedding_normal() to conveniently generate the correct shape of init_pars_quantum for pennylane.templates.QAOAEmbedding(). Import it with the statement from pennylane.init import qaoa_embedding_normal.

We can now train the embedding with an RMSPropOptimizer, sampling five training points from each class in every step, here shown for 2 steps.

optimizer = qml.RMSPropOptimizer(stepsize=0.01)
batch_size = 5
pars = init_pars

for i in range(2):

    # Sample a batch of training inputs from each class
    selectA = np.random.choice(range(len(A)), size=(batch_size,), replace=True)
    selectB = np.random.choice(range(len(B)), size=(batch_size,), replace=True)
    A_batch = [A[s] for s in selectA]
    B_batch = [B[s] for s in selectB]

    # Walk one optimization step
    pars = optimizer.step(lambda w: cost(w, A=A_batch, B=B_batch), pars)
    print("Step", i, "done.")

    # Print the validation cost every 10 steps
    if i % 5 == 0 and i != 0:
        cst = cost(pars, A=A_val, B=B_val)
        print("Cost on validation set {:2f}".format(cst))


Step 0 done.
Step 1 done.

Optimizing a hybrid quantum-classical model with 1024 + 12 parameters takes an awfully long time. We will therefore load a set of already trained parameters (from running the cell above for 1500 steps).


Training is sensitive to the hyperparameters such as the batch size, initial parameters and optimizer used.

pretrained_pars = np.load("embedding_metric_learning/pretrained_parameters.npy",



[array([[ 0.07892546,  0.18963997,  0.01267011, ..., -0.41917757,
         0.13794201, -0.31620719],
       [-0.49709026, -0.49160803, -0.16817771, ...,  0.35410417,
        -0.28687695,  0.25804757]])
 array([[ 1.28247918e-03, -2.53456415e-05, -2.53456415e-05],
       [ 1.55648560e-06,  1.65446338e-02,  1.55648553e-06],
       [ 8.12333186e-06, -1.99865981e-04,  6.71177010e-03],
       [-2.92851167e-03,  8.12333185e-06,  1.09834400e-03]])]


Let us analyze the effect of training. To speed up the script, we will only look at a reduced version of the training and validation set, selecting the first 10 points from either class.

select = 10

First of all, the final cost with the pre-trained parameters is as follows:

cost_train = cost(pretrained_pars, A=A[:select], B=B[:select])
cost_val = cost(pretrained_pars, A=A_val[:select], B=B_val[:select])
print("Cost for pretrained parameters on training set:", cost_train)
print("Cost for pretrained parameters on validation set:", cost_val)


Cost for pretrained parameters on training set: 5.8432370359184915e-05
Cost for pretrained parameters on validation set: 5.8432370359184915e-05

A useful way to visualize the distance of data points is to plot a Gram matrix of the overlaps of different feature states. For this we join the first 10 examples of each of the two classes.

A_B = np.r_[A[:select], B[:select]]

Before training, the separation between the classes is not recognizable in the Gram matrix:

gram_before = [[overlaps(init_pars, X1=[x1], X2=[x2]) for x1 in A_B] for x2 in A_B]

ax = plt.subplot(111)
im = ax.matshow(gram_before, vmin=0, vmax=1)
divider = make_axes_locatable(ax)
cax = divider.append_axes("right", size="5%", pad=0.05)
plt.colorbar(im, cax=cax)
tutorial embeddings metric learning

After training, the gram matrix clearly separates the two classes.

gram_after = [[overlaps(pretrained_pars, X1=[x1], X2=[x2]) for x1 in A_B] for x2 in A_B]

ax = plt.subplot(111)
im = ax.matshow(gram_after, vmin=0, vmax=1)
divider = make_axes_locatable(ax)
cax = divider.append_axes("right", size="5%", pad=0.05)
plt.colorbar(im, cax=cax)
tutorial embeddings metric learning

We can also visualize the “intermediate layer” of 2-dimensional vectors \((x_1, x_2)\), just before feeding them into the quantum circuit. Before training the (2, 512)-dimensional weight matrix of the linear layer, the classes are arbitrarily intermixed.

for a in A:
    intermediate_a = init_pars[0] @ a
    plt.scatter(intermediate_a[:][0], intermediate_a[:][1], c="red")

for b in B:
    intermediate_b = init_pars[0] @ b
    plt.scatter(intermediate_b[:][0], intermediate_b[:][1], c="blue")
tutorial embeddings metric learning

However, after training, the linear layer learned to arrange the intermediate feature vectors on a periodic grid.

for a in A:
    intermediate_a = pretrained_pars[0] @ a
    plt.scatter(intermediate_a[:][0], intermediate_a[:][1], c="red")

for b in B:
    intermediate_b = pretrained_pars[0] @ b
    plt.scatter(intermediate_b[:][0], intermediate_b[:][1], c="blue")
tutorial embeddings metric learning


Given a new input \(x \in X\), and its quantum feature state \(|x \rangle\), the trained embedding can be used to solve the binary classification problem of assigning \(x\) to either \(A\) or \(B\). For an embedding separating data via the \(\ell_2\) metric, a very simple measurement can be used for classification: one computes the overlap of \(|x \rangle\) with examples of \(|a \rangle\) and \(|b \rangle\). \(x\) is assigned to the class with which it has a larger average overlap in the space of the embedding.

Let us consider a picture of an ant from the validation set (assuming our model never saw it during training):


After passing it through the classical neural network (excluding the final linear layer), the 512-dimensional feature vector is given by A_val[0].

x_new = A_val[0]




We compare the new input with randomly selected samples. The more samples used, the smaller the variance in the prediction.

n_samples = 200

prediction = 0
for s in range(n_samples):

    # select a random sample from the training set
    sample_index = np.random.choice(len(X))
    x = X[sample_index]
    y = Y[sample_index]

    # compute the overlap between training sample and new input
    overlap = overlaps(pretrained_pars, X1=[x], X2=[x_new])

    # add the label weighed by the overlap to the prediction
    prediction += y * overlap

# normalize prediction
prediction = prediction / n_samples



Since the result is negative, the new data point is (correctly) predicted to be a picture of an ant, which was the class with -1 labels.


Seth Lloyd, Maria Schuld, Aroosa Ijaz, Josh Izaac, Nathan Killoran: “Quantum embeddings for machine learning” arXiv preprint arXiv:2001.03622.

Andrea Mari, Thomas R. Bromley, Josh Izaac, Maria Schuld, Nathan Killoran: “Transfer learning in hybrid classical-quantum neural networks” arXiv preprint arXiv:1912.08278


Previous versions of this tutorial may instead use the following gate sequence in the ising_hamiltonian function:

# ZZ coupling
RZ(2*weights[l, 0], wires=wires[0])

The current version fixes a bug in the CNOT wires and does not multiply the weight parameter by a factor 2. It is consistent with the in-built QAOAEmbedding of PennyLane v0.9 and higher.

Total running time of the script: ( 1 minutes 13.118 seconds)

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