Predicting Open-Hole Laminates Failure Using Support Vector Machines With Classical and Quantum Kernels (2024)

Giorgio Tosti Balducci
Aerospace Structure and Materials
Delft University of Technology
Kluyverweg 1, 2629HS,
Delft, The Netherlands
&Boyang Chen
Aerospace Structure and Materials
Delft University of Technology
Kluyverweg 1, 2629HS,
Delft, The Netherlands
&Matthias Möller
Applied Mathematics
Delft University of Technology
Mekelweg 4, 2628CD,
Delft, The Netherlands
&Marc Gerritsma
Flow Physics and Technology
Delft University of Technology
Kluyverweg 1, 2629HS,
Delft, The Netherlands
&Roeland De Breuker
Aerospace Structure and Materials
Delft University of Technology
Kluyverweg 1, 2629HS,
Delft, The Netherlands
Corresponding author. Email: b.chen-2@tudelft.nl

Abstract

Modeling open hole failure of composites is a complex task, consisting in a highly nonlinear response with interacting failure modes. Numerical modeling of this phenomenon has traditionally been based on the finite element method, but requires to tradeoff between high fidelity and computational cost. To mitigate this shortcoming, recent work has leveraged machine learning to predict the strength of open hole composite specimens. Here, we also propose using data-based models but to tackle open hole composite failure from a classification point of view. More specifically, we show how to train surrogate models to learn the ultimate failure envelope of an open hole composite plate under in-plane loading. To achieve this, we solve the classification problem via support vector machine (SVM) and test different classifiers by changing the SVM kernel function. The flexibility of kernel-based SVM also allows us to integrate the recently developed quantum kernels in our algorithm and compare them with the standard radial basis function (RBF) kernel. Finally, thanks to kernel-target alignment optimization, we tune the free parameters of all kernels to best separate safe and failure-inducing loading states. The results show classification accuracies higher than 90% for RBF, especially after alignment, followed closely by the quantum kernel classifiers.

Keywords Composites \cdotSupport Vector Machines \cdotQuantum Machine Learning

1 Introduction

Modern aviation industry makes wide use of composite materials, thanks to their lightweight and favorable mechanical properties. Frequently, aeronautical structural elements are often not textbook flat composite panels, but tailored components with complex mechanical responses. For instance, composite panels often show cutouts in order to allow fastening or lightening the structure or even for allowing the passage of wiring or cables. However, the presence of holes in a composite plate induces stress concentrations that can initiate damage which can propagate into intricate failure mechanisms involving different modes.

Models for open hole composite failure have developed in different directions.On the one hand, semi-empirical models were proposed to predict the allowables of these structures, such as ultimate stength, and their statistical distribution with respect to hole geometry, loading conditions, stacking sequence, ply thickness, etc. Early attempts required experimental properties from testing both the unnotched and notched laminate [1], while later models removed the need of directly testing the open hole laminate [2, 3] or just required the ply properties [4]. Despite being fast to evaluate and suitable for preliminary design, semi-empirical models can make large errors when extensive delaminations propagate from the notch, as it happens with ply-scaled laminates.

Finite Element (FE) simulations allow for improved modeling of open hole laminates failure. Open hole tension (OHT) has been extensively studied numerically both for capturing the in-plane [5] and thickness size effects [6, 7, 8] on the ultimate strength and for reproducing the different failure modes and their interactions [9, 10] with increasing detail. Furthermore, FE simulations managed to quite accurately predict open-hole compression (OHC), even though still struggling to predict the precise kink band formation[11, 12, 13]. However, the accuracy offered by FE models generally comes at the price of high computational costs, possibly making them unfeasible when many design iterations are required.

Therefore, there is a practical need for computationally efficient yet accurate models that can simulate open hole composite laminates. A possibility is offerred by machine learning surrogates, which have been employed in composite design and optimization [14, 15, 16], constitutive law modeling and multiscale analyses (see [17] for a comprehensive review) and damage characterisation [18, 19]. Concerning open-hole composite failure, Furtado et al. proposed a methodology to define allowables using four different machine learning models [20]. Their methodology was applied to open-hole tensile strength prediction for different dimensions, layups and material properties. While their methods are demonstrated on data generated analytically [4], the authors suggest using high fidelity finite element analyses for training, potentially providing accurate data-based models.

Similarly, in this work we propose a machine learning surrogate for open hole composites, which is accurate and efficient in inference. Differently from [20] however, the approach we suggest is not to have a fast allowables generator, but a classifier for ultimate failure of open hole composite laminates. More precisely, our trained model takes a loading state as input, such as the far field hom*ogenized plane strain components and returns a binary valuable (±1plus-or-minus1\pm 1± 1) as output, depending on whether the load applied is lower or higher than the notched laminate strength. In this sense, the surrogate acts as a data-based generalized failure criterion which predicts at the structural component level, rather than at the material level.

This paper also aims at comparing classical and quantum computation for a classification problem in composite mechanics. To do this, we train the machine learning surrogate using kernel-based support vector machines (SVMs) [21], where the kernel function can be computed both in classical and quantum logic. As it will be clear in the next sections, quantum computation offers a way to encode information into exponentially large Hilbert spaces and to define an inner product in this spaces, effectively generating a kernel. This allows to explore the generalization potential of quantum machine learning, while leaving the SVM optimization to well-established classical quadratic optimization algorithms.

The rest of the paper is structured as follows. Section2 describes the machine learning problem, by defining the input, the data sampling strategy and the labeling criterion. Section3 briefly introduces the SVM dual problem, the RBF kernels and the quantum kernels. More details about these methods are available in the appendices following the main body of the manuscript. Finally, Section4 presents the classification results for all kernels and Section5 outlines conclusions and future work.

All data and code used in this work are made publicly available (see [22], [23] respectively).

2 Machine learning problem

Predicting Open-Hole Laminates Failure Using Support Vector Machines With Classical and Quantum Kernels (1)

Our method was applied to predict failure of an open hole composite specimen similar in geometry and material properties to the one experimentally tested in [24]. The specimen was modeled and meshed with the Abaqus finite element code [25] and it was loaded with different combinations of axial and shear strains and constrained with periodic boundary conditions. All the details of the specimen properties and of the finite element analyses are left to AppendixA.

The input of our surrogate models are hom*ogenized far field strains 𝜺=[ε11,ε22,γ12]𝜺superscriptsubscript𝜀11subscript𝜀22subscript𝛾12top\bm{\varepsilon}=\left[\varepsilon_{11},\,\varepsilon_{22},\,\gamma_{12}\right%]^{\top}bold_italic_ε = [ italic_ε start_POSTSUBSCRIPT 11 end_POSTSUBSCRIPT , italic_ε start_POSTSUBSCRIPT 22 end_POSTSUBSCRIPT , italic_γ start_POSTSUBSCRIPT 12 end_POSTSUBSCRIPT ] start_POSTSUPERSCRIPT ⊤ end_POSTSUPERSCRIPT, which derive from enforcing periodic boundary conditions on opposite faces of the plate. The displacements of the left/right and top/bottom faces respectively can be linked through some reference degrees of freedom

U1subscript𝑈1\displaystyle U_{1}italic_U start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT=u1Ru1Labsentsuperscriptsubscript𝑢1𝑅superscriptsubscript𝑢1𝐿\displaystyle=u_{1}^{R}-u_{1}^{L}= italic_u start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_R end_POSTSUPERSCRIPT - italic_u start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_L end_POSTSUPERSCRIPT(1)
U2subscript𝑈2\displaystyle U_{2}italic_U start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT=u2Tu2Babsentsuperscriptsubscript𝑢2𝑇superscriptsubscript𝑢2𝐵\displaystyle=u_{2}^{T}-u_{2}^{B}= italic_u start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_T end_POSTSUPERSCRIPT - italic_u start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_B end_POSTSUPERSCRIPT
U3subscript𝑈3\displaystyle U_{3}italic_U start_POSTSUBSCRIPT 3 end_POSTSUBSCRIPT=u2Ru2Labsentsuperscriptsubscript𝑢2𝑅superscriptsubscript𝑢2𝐿\displaystyle=u_{2}^{R}-u_{2}^{L}= italic_u start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_R end_POSTSUPERSCRIPT - italic_u start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_L end_POSTSUPERSCRIPT
U4subscript𝑈4\displaystyle U_{4}italic_U start_POSTSUBSCRIPT 4 end_POSTSUBSCRIPT=u1Tu1B,absentsuperscriptsubscript𝑢1𝑇superscriptsubscript𝑢1𝐵\displaystyle=u_{1}^{T}-u_{1}^{B},= italic_u start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_T end_POSTSUPERSCRIPT - italic_u start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_B end_POSTSUPERSCRIPT ,

where directions 1111 and 2222 are the horizontal and vertical directions in Figure1. The hom*ogenized strains are then obtained as

ε11subscript𝜀11\displaystyle\varepsilon_{11}italic_ε start_POSTSUBSCRIPT 11 end_POSTSUBSCRIPT=U1D1absentsubscript𝑈1subscript𝐷1\displaystyle=\frac{U_{1}}{D_{1}}= divide start_ARG italic_U start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT end_ARG start_ARG italic_D start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT end_ARG(2)
ε22subscript𝜀22\displaystyle\varepsilon_{22}italic_ε start_POSTSUBSCRIPT 22 end_POSTSUBSCRIPT=U2D2absentsubscript𝑈2subscript𝐷2\displaystyle=\frac{U_{2}}{D_{2}}= divide start_ARG italic_U start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT end_ARG start_ARG italic_D start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT end_ARG
γ12subscript𝛾12\displaystyle\gamma_{12}italic_γ start_POSTSUBSCRIPT 12 end_POSTSUBSCRIPT=U3D1+U4D2,absentsubscript𝑈3subscript𝐷1subscript𝑈4subscript𝐷2\displaystyle=\frac{U_{3}}{D_{1}}+\frac{U_{4}}{D_{2}},= divide start_ARG italic_U start_POSTSUBSCRIPT 3 end_POSTSUBSCRIPT end_ARG start_ARG italic_D start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT end_ARG + divide start_ARG italic_U start_POSTSUBSCRIPT 4 end_POSTSUBSCRIPT end_ARG start_ARG italic_D start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT end_ARG ,

where D1subscript𝐷1D_{1}italic_D start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT and D2subscript𝐷2D_{2}italic_D start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT are the planar dimensions of the plate.

Predicting Open-Hole Laminates Failure Using Support Vector Machines With Classical and Quantum Kernels (2)

As mentioned, the input space was sampled through nonlinear incremental-iterative finite element analyses. Figure2 illustrates the sampling strategy used in this work in the simplified case of two-dimensional input. We refer to this technique as radial sampling, due to the fact that the design of experiments (DoE) does not directly affect all the points in this input space, but only the ones on the boundary. On the other hand, all the intermediate points are generated internally by the FE solver and they correspond to the hom*ogenized strain values at every time increment111Of course, the user maintains a certain control on the inner samples values, by the choice of initial, minimum and maximum time steps.. For this work, we chose the sampling space to be the hypercube [102, 102]3superscriptsuperscript102superscript102tensor-productabsent3\left[-10^{-2},\,10^{-2}\right]^{\otimes 3}[ - 10 start_POSTSUPERSCRIPT - 2 end_POSTSUPERSCRIPT , 10 start_POSTSUPERSCRIPT - 2 end_POSTSUPERSCRIPT ] start_POSTSUPERSCRIPT ⊗ 3 end_POSTSUPERSCRIPT in 3superscript3\mathbb{R}^{3}blackboard_R start_POSTSUPERSCRIPT 3 end_POSTSUPERSCRIPT, meaning that all three components of the applied strains vector have the same bounds.

2.1 Labeling criterion

Each strain sample was assigned a label based on an ultimate failure criterion. In particular, we defined failure by the loss of stiffness of the laminate for given a user-defined threshold.

From the results of the FE analyses with periodic boundary conditions, one obtains the reaction forces F1subscript𝐹1F_{1}italic_F start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT, F2subscript𝐹2F_{2}italic_F start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT, F3subscript𝐹3F_{3}italic_F start_POSTSUBSCRIPT 3 end_POSTSUBSCRIPT and F4subscript𝐹4F_{4}italic_F start_POSTSUBSCRIPT 4 end_POSTSUBSCRIPT conjugate to the degrees of freedom in Equation1. These provide the hom*ogenized stresses, which can then be derived via the Hill-Mandel principle of energy balance as

σ11subscript𝜎11\displaystyle\sigma_{11}italic_σ start_POSTSUBSCRIPT 11 end_POSTSUBSCRIPT=F1tD2absentsubscript𝐹1𝑡subscript𝐷2\displaystyle=\frac{F_{1}}{tD_{2}}= divide start_ARG italic_F start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT end_ARG start_ARG italic_t italic_D start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT end_ARG(3)
σ22subscript𝜎22\displaystyle\sigma_{22}italic_σ start_POSTSUBSCRIPT 22 end_POSTSUBSCRIPT=F2tD1absentsubscript𝐹2𝑡subscript𝐷1\displaystyle=\frac{F_{2}}{tD_{1}}= divide start_ARG italic_F start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT end_ARG start_ARG italic_t italic_D start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT end_ARG
σ12subscript𝜎12\displaystyle\sigma_{12}italic_σ start_POSTSUBSCRIPT 12 end_POSTSUBSCRIPT=F3U3+F4U4γ12tD1D2,absentsubscript𝐹3subscript𝑈3subscript𝐹4subscript𝑈4subscript𝛾12𝑡subscript𝐷1subscript𝐷2\displaystyle=\frac{F_{3}U_{3}+F_{4}U_{4}}{\gamma_{12}tD_{1}D_{2}},= divide start_ARG italic_F start_POSTSUBSCRIPT 3 end_POSTSUBSCRIPT italic_U start_POSTSUBSCRIPT 3 end_POSTSUBSCRIPT + italic_F start_POSTSUBSCRIPT 4 end_POSTSUBSCRIPT italic_U start_POSTSUBSCRIPT 4 end_POSTSUBSCRIPT end_ARG start_ARG italic_γ start_POSTSUBSCRIPT 12 end_POSTSUBSCRIPT italic_t italic_D start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT italic_D start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT end_ARG ,

where t𝑡titalic_t is the thickness of the plate.

The laminate stiffness in the two axial directions and in shear can thus be defined at every timestep t𝑡titalic_t as

E1(t)superscriptsubscript𝐸1𝑡\displaystyle E_{1}^{(t)}italic_E start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( italic_t ) end_POSTSUPERSCRIPT=σ11(t)ε11(t)absentsuperscriptsubscript𝜎11𝑡superscriptsubscript𝜀11𝑡\displaystyle=\frac{\sigma_{11}^{(t)}}{\varepsilon_{11}^{(t)}}= divide start_ARG italic_σ start_POSTSUBSCRIPT 11 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( italic_t ) end_POSTSUPERSCRIPT end_ARG start_ARG italic_ε start_POSTSUBSCRIPT 11 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( italic_t ) end_POSTSUPERSCRIPT end_ARG(4)
E2(t)superscriptsubscript𝐸2𝑡\displaystyle E_{2}^{(t)}italic_E start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( italic_t ) end_POSTSUPERSCRIPT=σ22(t)ε22(t)absentsuperscriptsubscript𝜎22𝑡superscriptsubscript𝜀22𝑡\displaystyle=\frac{\sigma_{22}^{(t)}}{\varepsilon_{22}^{(t)}}= divide start_ARG italic_σ start_POSTSUBSCRIPT 22 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( italic_t ) end_POSTSUPERSCRIPT end_ARG start_ARG italic_ε start_POSTSUBSCRIPT 22 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( italic_t ) end_POSTSUPERSCRIPT end_ARG
G12(t)superscriptsubscript𝐺12𝑡\displaystyle G_{12}^{(t)}italic_G start_POSTSUBSCRIPT 12 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( italic_t ) end_POSTSUPERSCRIPT=σ12(t)ε12(t).absentsuperscriptsubscript𝜎12𝑡superscriptsubscript𝜀12𝑡\displaystyle=\frac{\sigma_{12}^{(t)}}{\varepsilon_{12}^{(t)}}.= divide start_ARG italic_σ start_POSTSUBSCRIPT 12 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( italic_t ) end_POSTSUPERSCRIPT end_ARG start_ARG italic_ε start_POSTSUBSCRIPT 12 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( italic_t ) end_POSTSUPERSCRIPT end_ARG .

The stiffness degradation dSsubscript𝑑𝑆d_{S}italic_d start_POSTSUBSCRIPT italic_S end_POSTSUBSCRIPT is defined as the minimum ratio between the instantaneous stiffness and the corresponding stiffness measure in the linear elastic region,

dS(t)=min{E1(t)E1(0),E2(t)E2(0),G12(t)G12(0)}superscriptsubscript𝑑𝑆𝑡superscriptsubscript𝐸1𝑡superscriptsubscript𝐸10superscriptsubscript𝐸2𝑡superscriptsubscript𝐸20superscriptsubscript𝐺12𝑡superscriptsubscript𝐺120d_{S}^{(t)}=\min\left\{\frac{E_{1}^{(t)}}{E_{1}^{(0)}},\,\frac{E_{2}^{(t)}}{E_%{2}^{(0)}},\,\frac{G_{12}^{(t)}}{G_{12}^{(0)}}\right\}italic_d start_POSTSUBSCRIPT italic_S end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( italic_t ) end_POSTSUPERSCRIPT = roman_min { divide start_ARG italic_E start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( italic_t ) end_POSTSUPERSCRIPT end_ARG start_ARG italic_E start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( 0 ) end_POSTSUPERSCRIPT end_ARG , divide start_ARG italic_E start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( italic_t ) end_POSTSUPERSCRIPT end_ARG start_ARG italic_E start_POSTSUBSCRIPT 2 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( 0 ) end_POSTSUPERSCRIPT end_ARG , divide start_ARG italic_G start_POSTSUBSCRIPT 12 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( italic_t ) end_POSTSUPERSCRIPT end_ARG start_ARG italic_G start_POSTSUBSCRIPT 12 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT ( 0 ) end_POSTSUPERSCRIPT end_ARG }(5)

Therefore, given M𝑀Mitalic_M the total number of samples, every sample 𝜺(m)superscript𝜺𝑚\bm{\varepsilon}^{(m)}bold_italic_ε start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT (m=1,,M𝑚1𝑀m=1,\dots,Mitalic_m = 1 , … , italic_M) is assinged a label y(m)=1superscript𝑦𝑚1y^{(m)}=-1italic_y start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT = - 1 if d(m)<d¯Ssuperscript𝑑𝑚subscript¯𝑑𝑆d^{(m)}<\bar{d}_{S}italic_d start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT < over¯ start_ARG italic_d end_ARG start_POSTSUBSCRIPT italic_S end_POSTSUBSCRIPT and y(m)=+1superscript𝑦𝑚1y^{(m)}=+1italic_y start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT = + 1 otherwise.

3 Methodology

Predicting Open-Hole Laminates Failure Using Support Vector Machines With Classical and Quantum Kernels (3)

As already mentioned, we solve the ultimate failure binary classification problem using the SVM algorithm [21]. This consists in the following quadratic optimization problem in dual form

max𝜶subscript𝜶\displaystyle\max_{\bm{\alpha}}roman_max start_POSTSUBSCRIPT bold_italic_α end_POSTSUBSCRIPTm=1Mα(m)12m,m=1My(m)y(m)α(m)α(m)κ(𝒙(m),𝒙(m))superscriptsubscript𝑚1𝑀superscript𝛼𝑚12superscriptsubscript𝑚superscript𝑚1𝑀superscript𝑦𝑚superscript𝑦superscript𝑚superscript𝛼𝑚superscript𝛼superscript𝑚𝜅superscript𝒙𝑚superscript𝒙superscript𝑚\displaystyle\quad\sum_{m=1}^{M}\alpha^{(m)}-\frac{1}{2}\sum_{m,m^{\prime}=1}^%{M}y^{(m)}y^{(m^{\prime})}\alpha^{(m)}\alpha^{(m^{\prime})}\kappa\left(\bm{x}^%{(m)},\,\bm{x}^{(m^{\prime})}\right)∑ start_POSTSUBSCRIPT italic_m = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_M end_POSTSUPERSCRIPT italic_α start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT - divide start_ARG 1 end_ARG start_ARG 2 end_ARG ∑ start_POSTSUBSCRIPT italic_m , italic_m start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_M end_POSTSUPERSCRIPT italic_y start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT italic_y start_POSTSUPERSCRIPT ( italic_m start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) end_POSTSUPERSCRIPT italic_α start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT italic_α start_POSTSUPERSCRIPT ( italic_m start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) end_POSTSUPERSCRIPT italic_κ ( bold_italic_x start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT , bold_italic_x start_POSTSUPERSCRIPT ( italic_m start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) end_POSTSUPERSCRIPT )(6)
s.t.formulae-sequencest\displaystyle\mathrm{s.t.}roman_s . roman_t .0α(m)C,m=1,,Mformulae-sequence0superscript𝛼𝑚𝐶𝑚1𝑀\displaystyle\quad 0\leq\alpha^{(m)}\leq C,\quad m=1,\,\dots,\,M0 ≤ italic_α start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT ≤ italic_C , italic_m = 1 , … , italic_M
m=1Mα(m)y(m)=0,superscriptsubscript𝑚1𝑀superscript𝛼𝑚superscript𝑦𝑚0\displaystyle\quad\sum_{m=1}^{M}\alpha^{(m)}y^{(m)}=0,∑ start_POSTSUBSCRIPT italic_m = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_M end_POSTSUPERSCRIPT italic_α start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT italic_y start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT = 0 ,

where 𝒙(m)=𝜺(m)superscript𝒙𝑚superscript𝜺𝑚\bm{x}^{(m)}=\bm{\varepsilon}^{(m)}bold_italic_x start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT = bold_italic_ε start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT, y(m)=±1superscript𝑦𝑚plus-or-minus1y^{(m)}=\pm 1italic_y start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT = ± 1 are the labels, respectively non-failed and failed, α(m)superscript𝛼𝑚\alpha^{(m)}italic_α start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT are the Lagrange multipliers and C𝐶Citalic_C is the slack penalty. The kernel function κ(,)𝜅\kappa\left(\cdot,\,\cdot\right)italic_κ ( ⋅ , ⋅ ) is a similarity metric between two samples in a higher-dimensional feature space. More details on the SVM algorithm are left to AppendixB.

The performance of the dual SVM depends on the choice of its hyperparameters, namely the kernel function κ𝜅\kappaitalic_κ and slack penalty C𝐶Citalic_C. To restrict the search space, the kernel function is generally parametrized via one or more parameters 𝜽𝜽\bm{\theta}bold_italic_θ and the standard practice is to perform a grid-search cross-validation procedure in the (𝜽,C)𝜽𝐶\left(\bm{\theta},C\right)( bold_italic_θ , italic_C ) space. In this work, we use instead a mixed procedure, where the kernel function is determined by optimizing the kernel-target alignment (KTA) [26] and the slack penalty is found by grid search cross-validation. The overall methodology is illustrated in Figure3, where we refer to the two steps as kernel training and SVM selection. Once the SVM has been fully determined, it can be trained by solving Equation6 and its learning ability can be measured as the accuracy on unseen test data, for different training dataset sizes.

We compare one classical and two quantum kernels. The classical kernel is the radial basis function (RBF) kernel, defined as

κRBF(𝒙(m),𝒙(m))=exp(γ𝒙(m)𝒙(m)2).subscript𝜅RBFsuperscript𝒙𝑚superscript𝒙superscript𝑚𝛾superscriptnormsuperscript𝒙𝑚superscript𝒙superscript𝑚2\kappa_{\text{RBF}}\left(\bm{x}^{(m)},\,\bm{x}^{(m^{\prime})}\right)=\exp\left%(-\gamma\|\bm{x}^{(m)}-\bm{x}^{(m^{\prime})}\|^{2}\right).italic_κ start_POSTSUBSCRIPT RBF end_POSTSUBSCRIPT ( bold_italic_x start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT , bold_italic_x start_POSTSUPERSCRIPT ( italic_m start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) end_POSTSUPERSCRIPT ) = roman_exp ( - italic_γ ∥ bold_italic_x start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT - bold_italic_x start_POSTSUPERSCRIPT ( italic_m start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) end_POSTSUPERSCRIPT ∥ start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) .(7)

RBF is a powerful kernel which corresponds to a feature map in an infinite-dimensional feature space [27]. It induces a Gaussian similarity function, whose width is controlled by the hyperparameter γ𝛾\gammaitalic_γ.

On the other hand, the quantum kernel is defined via a quantum embedding, which is constructed via data-depending unitary transformations U(𝒙)𝑈𝒙U\left(\bm{x}\right)italic_U ( bold_italic_x ) that prepare the quantum state

|ψ(𝒙)=U(𝒙)|0.ket𝜓𝒙𝑈𝒙ket0\ket{\psi\left(\bm{x}\right)}=U\left(\bm{x}\right)\ket{0}.| start_ARG italic_ψ ( bold_italic_x ) end_ARG ⟩ = italic_U ( bold_italic_x ) | start_ARG 0 end_ARG ⟩ .(8)

Given two samples 𝒙(m)superscript𝒙𝑚\bm{x}^{(m)}bold_italic_x start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT and 𝒙(m)superscript𝒙superscript𝑚\bm{x}^{(m^{\prime})}bold_italic_x start_POSTSUPERSCRIPT ( italic_m start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) end_POSTSUPERSCRIPT, the quantum kernel is simply the inner product

κQ(𝒙(m),𝒙(m))=|ψ(𝒙(m)),ψ(𝒙(m))|2.subscript𝜅Qsuperscript𝒙𝑚superscript𝒙superscript𝑚superscript𝜓superscript𝒙𝑚𝜓superscript𝒙superscript𝑚2\kappa_{\text{Q}}\left(\bm{x}^{(m)},\,\bm{x}^{(m^{\prime})}\right)=%\absolutevalue{\langle\psi\left(\bm{x}^{(m)}\right),\,\psi\left(\bm{x}^{(m^{%\prime})}\right)\rangle}^{2}.italic_κ start_POSTSUBSCRIPT Q end_POSTSUBSCRIPT ( bold_italic_x start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT , bold_italic_x start_POSTSUPERSCRIPT ( italic_m start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) end_POSTSUPERSCRIPT ) = | start_ARG ⟨ italic_ψ ( bold_italic_x start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT ) , italic_ψ ( bold_italic_x start_POSTSUPERSCRIPT ( italic_m start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) end_POSTSUPERSCRIPT ) ⟩ end_ARG | start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT .(9)

Figure4 shows the generic quantum embedding and the two specific ones used in this work, which are the hardware efficient embedding (HE2) [28] and the instantaneous quantum polynomial (IQP) [29] one. To have more expressive feature mapping, either the width or the depth of the quantum embedding can be increased. The first one is the number of qubits, which can be even higher than the number of features in the dataset, by cyclically re-encoding the features to generate a highly nonlinear and potentially better separable feature space. Meanwhile, the embedding’s depth can be increased by repeating a base data-encoding block, such as IQP and HE2. Even in this case, re-encoding of the features may lead to a higher expressivity of the overall feature map [30]. For a short summary of relevant quantum computing concepts, we refer the reader to AppendixC.

Predicting Open-Hole Laminates Failure Using Support Vector Machines With Classical and Quantum Kernels (4)

4 Results

We tested our machine learning models on a dataset of 1960 labelled strain vectors 𝜺(m)superscript𝜺𝑚\bm{\varepsilon}^{(m)}bold_italic_ε start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT, which we obtained by uniformly sampling the hom*ogeneous strain/stress pairs from the FE simulations of the open-hole composite specimen. The input hom*ogeneous strains in both normal and shear directions were varied between 1e41e4{1}\mathrm{e}{-4}1 roman_e - 4 and 1e41e4{1}\mathrm{e}{4}1 roman_e 4 microstrains and a stiffness degradation threshold of 0.9 was used to discriminate non-failed and failed loading states.

Both classical- and quantum-kernel SVMs were implemented using different Python libraries. We used PyTorch for training the RBF kernel and PennyLane for the quantum kernels. These libraries implement automatic differentiation (AD), which allows to optimize the KTA with gradient-based methods. We also used JAX together with PennyLane to just-in-time compile the quantum kernel functions. Concerning the classification problem, we employed the SVM and grid-search cross validation routines available from the Scikit-Learn Python package.

Predicting Open-Hole Laminates Failure Using Support Vector Machines With Classical and Quantum Kernels (5)
Predicting Open-Hole Laminates Failure Using Support Vector Machines With Classical and Quantum Kernels (6)

The KTA of both RBF and quantum kernels was maximized using stochastic gradient descent and Adam parameters update [31]. Figure5 shows the kernel alignment training of the RBF kernel. Figure6 presents instead the KTAs before and after training for nine different quantum kernels with HE2 embedding. It can be seen that increasing width and depth of these kernels generally improves their KTA. A higher number of qubits means that the strain features are mapped in a higher dimensional space, which can favor separability of the classes. On the other hand, increasing the depth benefits the kernel alignment, since it results in more expressive feature maps. Also, every additional layer of the HE2 embedding doubles the number of free parameters, explaining why optimization of deeper kernels mostly leads to higher gains in KTA. However, the advantage of increasing these quantum encoding resources does not scale uniformly. Already with 6 qubits and 3 HE2 layers, the optimization only modestly improves the KTA, likely due to the vanishing KTA gradients [32].

Predicting Open-Hole Laminates Failure Using Support Vector Machines With Classical and Quantum Kernels (7)

To find the hyperparameter C𝐶Citalic_C that guarantees the highest off-training accuracy of the SVM algorithm, we used grid-search cross validation for the kernels considered. The validation accuracy values are reported in Figure7 for multiple values of C𝐶Citalic_C and γ𝛾\gammaitalic_γ. We observe that kernels with γ10𝛾10\gamma\leq 10italic_γ ≤ 10 achieve the higest scores, with the highest-KTA γ𝛾\gammaitalic_γ scoring first for the whole range of C𝐶Citalic_C values. Furthermore, the accuracy of the maximally-aligned RBF kernel increases monotonically with C𝐶Citalic_C, which suggests the usefulness of maximizing the KTA, but also that the class boundary in this feature space is densely populated and still requires a tight margin.

Predicting Open-Hole Laminates Failure Using Support Vector Machines With Classical and Quantum Kernels (8)

The same analysis was performed for all the quantum kernels considered, where we wanted to take into account the effect on accuracy of different embeddings and of maximizing the kernel-target alignment. The results are reported in Figure8, which shows accuracies roughly between 67% and 87% for all embeddings with different values of C𝐶Citalic_C. Except for IQP case, increasing C𝐶Citalic_C leads to higher accuracies, hinting to the the need of a tight bound when mapping with these embeddings, similar to the RBF kernel. Unfortunately, for C1e4𝐶1e4C\geq{1}\mathrm{e}{4}italic_C ≥ 1 roman_e 4, the optimization of the dual SVM failed to converge for the quantum kernels, likely due to numerical ill-conditioning, presumably preventing from reaching higher accuracies. In fact, we observe that the accuracies of both untrained and trained HE2 kernels monotonically increase with C𝐶Citalic_C. For the trained HE2 case, this is true regardless of the number of qubits and depth. Furthermore, Figure8 also shows that increasing the embedding resources, especially the number of qubits, pays off more when also optimizing the KTA.

Predicting Open-Hole Laminates Failure Using Support Vector Machines With Classical and Quantum Kernels (9)

Classical and quantum kernels are finally compared in Figure9, which shows how 5 different models classify a test set of strain loading data when fitted on progressively larger training sets. A similar comparison on additional classification metrics can be found in AppendixD. The RBF kernel achieves 80% accuracy with just 10% the total training set size, and with C=1e7𝐶1e7C={1}\mathrm{e}{7}italic_C = 1 roman_e 7 it reaches over 90% with just half the training points. In comparison, all quantum kernel classifiers are at least 5% less accurate than the best RBF-kernel SVM. However, especially for HE2 embeddings, the scores are similar to the C=1e4𝐶1e4C={1}\mathrm{e}{4}italic_C = 1 roman_e 4 RBF case, suggesting that RBF and HE2 kernels separate the non-failed and failed classes to a similar extent. Changing the embedding from HE2 to IQP, there is a drop in accuracy for small training set sizes, while the performance is similar when more than half the training set is used. On the other hand, the effect of training the kernel is less visible at this stage, reflecting the fact that the accuracies obtained during grid-search cross validation are alike for untrained and trained HE2.

5 Conclusion

In this paper, we proposed a methodology to build a binary classifier from finite element analyses data for the particular case of an open hole composite specimen. We studied the case of in-plane strain loading of the specimen where the objective is to correctly label strain combinations that lead to ultimate failure.

From a design of experiment point of view, we demonstrated a radial sampling strategy technique, where the choice of which simulations to make to cover the input space takes into account the incremental-iterative nature of the nonlinear FE method. We then proposed a labelling criterion of hom*oegenized strain-stress pairs based on residual in-plane stiffness.

For classification of the labelled data, we used the kernel-based SVMs, which also allowed us to compare the performance of the recently proposed quantum kernels against the more traditional RBF. Furthermore, we employed kernel-target alignment to improve class separbility of both RBF and the HE2 embedding kernel.

For all the kernel examined, the corresponding SVMs separate non-failed and failed loading states with good accuracy. The RBF-based model classify more accurately than its quantum counterparts, although this likely happens due to numerical ill-conditioning in the current quantum SVM implementation. These numerical issues can likely be fixed by studying the dual SVM problem for the problematic instances, which will be the subject of future work.

Regarding kernel alignment, optimizing the KTA is shown to be powerful for RBF, since the SVM for the trained kernel outperforms the other RBF-based models in terms of accuracy. Aligning quantum kernels for this dataset also helps them to better separate the two classes, but for simple architectures the improvement is moderate, while more complex embeddings only reach the scores of the more simple ones after they have been aligned. Furthermore, one should remember that optimizing quantum kernels is almost always more computationally involved than for RBF, as the formers can have highly parametrized embeddings, while RBF is completely defined by the single parameter γ𝛾\gammaitalic_γ.

Extensions of this work can go in many directions. From the point of view of the problem, it would be interesting to increase the number of degrees of freedom, by allowing the notch radius or the lamination sequence to also change. The latter could be written in terms of lamination parameters [33] to have a continuos representation.

In terms of algorithms, both classical and quantum kernels can be explored further. RBF is the most popular choice for classical kernels, but certainly not the only one. Due to Mercer’s condition, any function which defines a positive semi-definite kernel matrix is a valid kernel function [27]. Obviously, the design space is vast, but automated procedures help reduce the search for instance by exploring combinations of only a fixed set of standard kernel functions.

On the other hand, the freedom of designing and parametrizing quantum embedding circuit also makes the choice of a quantum kernel nontrivial. Within the limits of classical simulation of quantum circuits, one could experiment with increasing number of qubits or different layering strategies, for instance the one proposed in [34] for the task of satellite image classification. From an optimization point of view, a recent technique has been proposed to maximize the quantum kernel alignment KTA and solving the SVM in a single optmization loop [35], which would of course greatly reduce the computational cost. Nevertheless, to truly understand a potential competitiveness of quantum kernels, it is probably most important to remove layers of simulation and study the effects of statistical and hardware noise on SVM convergence and accuracy.

Appendix A Open hole specimen features and finite element model details

A.1 Geometry and material properties

Predicting Open-Hole Laminates Failure Using Support Vector Machines With Classical and Quantum Kernels (10)

The plate’s hole has a 6 mm diameter and the in-plane dimensions are both 5 times the hole diameter. The ply material is IM7/8552 prepreg (carbon fibres and epoxy matrix) and each ply has t=0.125𝑡0.125t=0.125italic_t = 0.125 mm thickness. We considered the lamination sequence [45/90/45/0]S\left[45/90/-45/0\right]_{S}[ 45 / 90 / - 45 / 0 ] start_POSTSUBSCRIPT italic_S end_POSTSUBSCRIPT for a total of 8 plies and 1 mm laminate thickness.

A.2 Details of the FE models

All finite element models were done using the Abaqus finite element code [25] and Python scripting was used to automatically generate a different FE models for each of the strain loading combinations [36, 37].

The meshed part is illustrated in Figure10, which shows that a radial mesh was obtained by seeding the hole edge 4 times as much as the outer edges. Since no delaminations were expected due to the absence of ply blocks, the elements were chosen to be S4 shells elements of the Abaqus Standard Element Library [38], whose in-plane and bending behaviour are described by the classical lamination theory (CLT), once the stacking sequence and ply thicknesses are specified.

Damage initiation was modeled with the Hashin criterion, while damage evolution was represented in a smeared crack fashion. For this purpose, the cohesive law available in Abaqus [38] was employed to model the stiffness degradation due to matrix and fiber tensile and compressive failure.

Appendix B Support vector machines, kernel methods and kernel-target alignment

B.1 Primal SVM

The SVM is the linear decision model

y=𝒘𝒙+b𝑦superscript𝒘top𝒙𝑏y=\bm{w}^{\top}\bm{x}+bitalic_y = bold_italic_w start_POSTSUPERSCRIPT ⊤ end_POSTSUPERSCRIPT bold_italic_x + italic_b(10)

which assigns labels through the sign function

sgn(y)=sgn(𝒘𝒙+b).sgn𝑦sgnsuperscript𝒘top𝒙𝑏\mathrm{sgn}(y)=\mathrm{sgn}\left(\bm{w}^{\top}\bm{x}+b\right).roman_sgn ( italic_y ) = roman_sgn ( bold_italic_w start_POSTSUPERSCRIPT ⊤ end_POSTSUPERSCRIPT bold_italic_x + italic_b ) .(11)

In Equations10 and11, 𝒘𝒘\bm{w}bold_italic_w is the vector normal to the decision hyperplane and b𝑏bitalic_b is the intercept.

The optimal hyperplane is found by maximizing the geometric margin of the dataset, which can be proved to be

γ=1𝒘.superscript𝛾1norm𝒘\gamma^{*}=\frac{1}{\|\bm{w}\|}.italic_γ start_POSTSUPERSCRIPT ∗ end_POSTSUPERSCRIPT = divide start_ARG 1 end_ARG start_ARG ∥ bold_italic_w ∥ end_ARG .(12)

By minimizing the squared norm 𝒘2superscriptnorm𝒘2\|\bm{w}\|^{2}∥ bold_italic_w ∥ start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT one obtaines the primal optimization problem of the SVM,

min𝒘,bsubscript𝒘𝑏\displaystyle\min_{\bm{w},\,b}roman_min start_POSTSUBSCRIPT bold_italic_w , italic_b end_POSTSUBSCRIPT12𝒘212superscriptnorm𝒘2\displaystyle\quad\frac{1}{2}\|\bm{w}\|^{2}divide start_ARG 1 end_ARG start_ARG 2 end_ARG ∥ bold_italic_w ∥ start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT(13)
s.t.formulae-sequencest\displaystyle\mathrm{s.t.}roman_s . roman_t .y(m)(𝒘𝒙(m)+b)1m=1,,M,formulae-sequencesuperscript𝑦𝑚superscript𝒘topsuperscript𝒙𝑚𝑏1𝑚1𝑀\displaystyle\quad y^{(m)}\left(\bm{w}^{\top}\bm{x}^{(m)}+b\right)\geq 1\quad m%=1,\,\dots,\,M,italic_y start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT ( bold_italic_w start_POSTSUPERSCRIPT ⊤ end_POSTSUPERSCRIPT bold_italic_x start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT + italic_b ) ≥ 1 italic_m = 1 , … , italic_M ,

where m𝑚mitalic_m identifies the sample and M𝑀Mitalic_M is the total number of training samples.

Equation13 enforces exact separability, which can lead to overfitting. A way to improve generalization is the so-called soft margin SVM, which modifies Equation13 by introducing the constraints slack variables ξ(m)superscript𝜉𝑚\xi^{(m)}italic_ξ start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT and the penalty constant C𝐶Citalic_C,

min𝒘,bsubscript𝒘𝑏\displaystyle\min_{\bm{w},\,b}roman_min start_POSTSUBSCRIPT bold_italic_w , italic_b end_POSTSUBSCRIPT12𝒘2+Cm=1Mξ(m)12superscriptnorm𝒘2𝐶superscriptsubscript𝑚1𝑀superscript𝜉𝑚\displaystyle\quad\frac{1}{2}\|\bm{w}\|^{2}+C\sum_{m=1}^{M}\xi^{(m)}divide start_ARG 1 end_ARG start_ARG 2 end_ARG ∥ bold_italic_w ∥ start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT + italic_C ∑ start_POSTSUBSCRIPT italic_m = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_M end_POSTSUPERSCRIPT italic_ξ start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT(14)
s.t.formulae-sequencest\displaystyle\mathrm{s.t.}roman_s . roman_t .y(m)(𝒘𝜺(m)+b)1ξ(m)m=1,,Mformulae-sequencesuperscript𝑦𝑚superscript𝒘topsuperscript𝜺𝑚𝑏1superscript𝜉𝑚𝑚1𝑀\displaystyle\quad y^{(m)}\left(\bm{w}^{\top}\bm{\varepsilon}^{(m)}+b\right)%\geq 1-\xi^{(m)}\quad m=1,\,\dots,\,Mitalic_y start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT ( bold_italic_w start_POSTSUPERSCRIPT ⊤ end_POSTSUPERSCRIPT bold_italic_ε start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT + italic_b ) ≥ 1 - italic_ξ start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT italic_m = 1 , … , italic_M
ξ(m)0.superscript𝜉𝑚0\displaystyle\quad\xi^{(m)}\geq 0.italic_ξ start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT ≥ 0 .

B.2 Dual SVM and kernels

By introducing the Lagrange multipliers α(m)superscript𝛼𝑚\alpha^{(m)}italic_α start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT and β(m)superscript𝛽𝑚\beta^{(m)}italic_β start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT, one can write the Lagrangian of the SVM optimization problem,

(𝒘,b,𝝃,𝜶)=𝒘𝑏𝝃𝜶absent\displaystyle\mathcal{L}\left(\bm{w},b,\bm{\xi},\bm{\alpha}\right)=caligraphic_L ( bold_italic_w , italic_b , bold_italic_ξ , bold_italic_α ) =12𝒘,𝒘+Cm=1Mξ(m)12𝒘𝒘𝐶superscriptsubscript𝑚1𝑀superscript𝜉𝑚\displaystyle\frac{1}{2}\langle\bm{w},\,\bm{w}\rangle+C\sum_{m=1}^{M}\xi^{(m)}divide start_ARG 1 end_ARG start_ARG 2 end_ARG ⟨ bold_italic_w , bold_italic_w ⟩ + italic_C ∑ start_POSTSUBSCRIPT italic_m = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_M end_POSTSUPERSCRIPT italic_ξ start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT(15)
m=1Mα(m)(y(m)(𝒘𝒙(m)+b)1+ξ(m))m=1Mβ(m)ξ(m).superscriptsubscript𝑚1𝑀superscript𝛼𝑚superscript𝑦𝑚superscript𝒘topsuperscript𝒙𝑚𝑏1superscript𝜉𝑚superscriptsubscript𝑚1𝑀superscript𝛽𝑚superscript𝜉𝑚\displaystyle-\sum_{m=1}^{M}\alpha^{(m)}\left(y^{(m)}\left(\bm{w}^{\top}\bm{x}%^{(m)}+b\right)-1+\xi^{(m)}\right)-\sum_{m=1}^{M}\beta^{(m)}\xi^{(m)}.- ∑ start_POSTSUBSCRIPT italic_m = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_M end_POSTSUPERSCRIPT italic_α start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT ( italic_y start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT ( bold_italic_w start_POSTSUPERSCRIPT ⊤ end_POSTSUPERSCRIPT bold_italic_x start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT + italic_b ) - 1 + italic_ξ start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT ) - ∑ start_POSTSUBSCRIPT italic_m = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_M end_POSTSUPERSCRIPT italic_β start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT italic_ξ start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT .

The dual soft-margin SVM is obtained by setting all the derivatives of the Lagrangian in Equation15 equal to zero,

max𝜶subscript𝜶\displaystyle\max_{\bm{\alpha}}roman_max start_POSTSUBSCRIPT bold_italic_α end_POSTSUBSCRIPTm=1Mα(m)12m,m=1My(m)y(m)α(m)α(m)𝒙(m),𝒙(m)superscriptsubscript𝑚1𝑀superscript𝛼𝑚12superscriptsubscript𝑚superscript𝑚1𝑀superscript𝑦𝑚superscript𝑦superscript𝑚superscript𝛼𝑚superscript𝛼superscript𝑚superscript𝒙𝑚superscript𝒙superscript𝑚\displaystyle\quad\sum_{m=1}^{M}\alpha^{(m)}-\frac{1}{2}\sum_{m,m^{\prime}=1}^%{M}y^{(m)}y^{(m^{\prime})}\alpha^{(m)}\alpha^{(m^{\prime})}\langle\bm{x}^{(m)}%,\,\bm{x}^{(m^{\prime})}\rangle∑ start_POSTSUBSCRIPT italic_m = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_M end_POSTSUPERSCRIPT italic_α start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT - divide start_ARG 1 end_ARG start_ARG 2 end_ARG ∑ start_POSTSUBSCRIPT italic_m , italic_m start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_M end_POSTSUPERSCRIPT italic_y start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT italic_y start_POSTSUPERSCRIPT ( italic_m start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) end_POSTSUPERSCRIPT italic_α start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT italic_α start_POSTSUPERSCRIPT ( italic_m start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) end_POSTSUPERSCRIPT ⟨ bold_italic_x start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT , bold_italic_x start_POSTSUPERSCRIPT ( italic_m start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) end_POSTSUPERSCRIPT ⟩(16)
s.t.formulae-sequencest\displaystyle\mathrm{s.t.}roman_s . roman_t .0α(m)+β(m)C,m=1,,Mformulae-sequence0superscript𝛼𝑚superscript𝛽𝑚𝐶𝑚1𝑀\displaystyle\quad 0\leq\alpha^{(m)}+\beta^{(m)}\leq C,\quad m=1,\,\dots,\,M0 ≤ italic_α start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT + italic_β start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT ≤ italic_C , italic_m = 1 , … , italic_M
m=1Mα(m)y(m)=0.superscriptsubscript𝑚1𝑀superscript𝛼𝑚superscript𝑦𝑚0\displaystyle\quad\sum_{m=1}^{M}\alpha^{(m)}y^{(m)}=0.∑ start_POSTSUBSCRIPT italic_m = 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_M end_POSTSUPERSCRIPT italic_α start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT italic_y start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT = 0 .

Equation16 is still a linear model in the original feature space. However, by introducing a feature map

ϕ:𝒙ϕ(𝒙):italic-ϕ𝒙italic-ϕ𝒙\phi:\bm{x}\longrightarrow\phi(\bm{x})italic_ϕ : bold_italic_x ⟶ italic_ϕ ( bold_italic_x )(17)

we can map the features nonlinearly and potentially to a manifold where they are more easily separable. Furthermore, replacing 𝒙𝒙\bm{x}bold_italic_x with ϕ(𝒙)italic-ϕ𝒙\phi(\bm{x})italic_ϕ ( bold_italic_x ) in Equation16, we see that the mapped features only appear in the inner product

κ(𝒙(m),𝒙(m))=ϕ(𝒙(m)),ϕ(𝒙(m)),𝜅superscript𝒙𝑚superscript𝒙superscript𝑚italic-ϕsuperscript𝒙𝑚italic-ϕsuperscript𝒙superscript𝑚\kappa\left(\bm{x}^{(m)},\,\bm{x}^{(m^{\prime})}\right)=\langle\phi(\bm{x}^{(m%)}),\,\phi(\bm{x}^{(m^{\prime})})\rangle,italic_κ ( bold_italic_x start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT , bold_italic_x start_POSTSUPERSCRIPT ( italic_m start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) end_POSTSUPERSCRIPT ) = ⟨ italic_ϕ ( bold_italic_x start_POSTSUPERSCRIPT ( italic_m ) end_POSTSUPERSCRIPT ) , italic_ϕ ( bold_italic_x start_POSTSUPERSCRIPT ( italic_m start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) end_POSTSUPERSCRIPT ) ⟩ ,(18)

which is known as the kernel of the feature map. The advantage of having only inner product of features (kernel trick) is the possibility of classifying in nonlinear feature spaces without having to compute the feature map explicitly.

The kernels mostly used in machine learning are the polynomial, Gaussian and sigmoid kernels

κ(𝒙,𝒙)={(γ𝒙𝒙+c0)d(polynomial)exp(γ𝒙𝒙2)(Gaussian)tanh(γ𝒙𝒙+c0)(sigmoid)𝜅𝒙superscript𝒙casessuperscript𝛾superscript𝒙topsuperscript𝒙subscript𝑐0𝑑polynomial𝛾superscriptnorm𝒙superscript𝒙2Gaussian𝛾superscript𝒙topsuperscript𝒙subscript𝑐0sigmoid\kappa\left(\bm{x},\,\bm{x}^{\prime}\right)=\begin{cases}(\gamma\bm{x}^{\top}%\bm{x}^{\prime}+c_{0})^{d}&\quad\mathrm{(polynomial)}\\\exp\left(-\gamma\|\bm{x}-\bm{x}^{\prime}\|^{2}\right)&\quad\mathrm{(Gaussian)%}\\\tanh\left(\gamma\bm{x}^{\top}\bm{x}^{\prime}+c_{0}\right)&\quad\mathrm{(%sigmoid)}\end{cases}italic_κ ( bold_italic_x , bold_italic_x start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) = { start_ROW start_CELL ( italic_γ bold_italic_x start_POSTSUPERSCRIPT ⊤ end_POSTSUPERSCRIPT bold_italic_x start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT + italic_c start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT ) start_POSTSUPERSCRIPT italic_d end_POSTSUPERSCRIPT end_CELL start_CELL ( roman_polynomial ) end_CELL end_ROW start_ROW start_CELL roman_exp ( - italic_γ ∥ bold_italic_x - bold_italic_x start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ∥ start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ) end_CELL start_CELL ( roman_Gaussian ) end_CELL end_ROW start_ROW start_CELL roman_tanh ( italic_γ bold_italic_x start_POSTSUPERSCRIPT ⊤ end_POSTSUPERSCRIPT bold_italic_x start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT + italic_c start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT ) end_CELL start_CELL ( roman_sigmoid ) end_CELL end_ROW(19)

B.3 Kernel-target alignment

The alignment between two kernels is defined as

A(K(1),K(2))𝐴superscript𝐾1superscript𝐾2\displaystyle A\left(K^{(1)},\,K^{(2)}\right)italic_A ( italic_K start_POSTSUPERSCRIPT ( 1 ) end_POSTSUPERSCRIPT , italic_K start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT )=K(1),K(2)FK(1),K(1)FK(2),K(2)Fabsentsubscriptsuperscript𝐾1superscript𝐾2𝐹subscriptsuperscript𝐾1superscript𝐾1𝐹subscriptsuperscript𝐾2superscript𝐾2𝐹\displaystyle=\frac{\langle K^{(1)},\,K^{(2)}\rangle_{F}}{\sqrt{\langle K^{(1)%},\,K^{(1)}\rangle_{F}\langle K^{(2)},\,K^{(2)}\rangle_{F}}}= divide start_ARG ⟨ italic_K start_POSTSUPERSCRIPT ( 1 ) end_POSTSUPERSCRIPT , italic_K start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT ⟩ start_POSTSUBSCRIPT italic_F end_POSTSUBSCRIPT end_ARG start_ARG square-root start_ARG ⟨ italic_K start_POSTSUPERSCRIPT ( 1 ) end_POSTSUPERSCRIPT , italic_K start_POSTSUPERSCRIPT ( 1 ) end_POSTSUPERSCRIPT ⟩ start_POSTSUBSCRIPT italic_F end_POSTSUBSCRIPT ⟨ italic_K start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT , italic_K start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT ⟩ start_POSTSUBSCRIPT italic_F end_POSTSUBSCRIPT end_ARG end_ARG(20)
=K(1),K(2)FK(1)FK(2)F,absentsubscriptsuperscript𝐾1superscript𝐾2𝐹subscriptnormsuperscript𝐾1𝐹subscriptnormsuperscript𝐾2𝐹\displaystyle=\frac{\langle K^{(1)},\,K^{(2)}\rangle_{F}}{\|K^{(1)}\|_{F}\|K^{%(2)}\|_{F}},= divide start_ARG ⟨ italic_K start_POSTSUPERSCRIPT ( 1 ) end_POSTSUPERSCRIPT , italic_K start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT ⟩ start_POSTSUBSCRIPT italic_F end_POSTSUBSCRIPT end_ARG start_ARG ∥ italic_K start_POSTSUPERSCRIPT ( 1 ) end_POSTSUPERSCRIPT ∥ start_POSTSUBSCRIPT italic_F end_POSTSUBSCRIPT ∥ italic_K start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT ∥ start_POSTSUBSCRIPT italic_F end_POSTSUBSCRIPT end_ARG ,

where K𝐾Kitalic_K is the kernel matrix, obtained by taking the kernel of all pairs of features, and

K(1),K(2)F=tr(K(1)K(2)).subscriptsuperscript𝐾1superscript𝐾2𝐹tracesuperscript𝐾limit-from1topsuperscript𝐾2\langle K^{(1)},\,K^{(2)}\rangle_{F}=\tr\left(K^{(1)\top}K^{(2)}\right).⟨ italic_K start_POSTSUPERSCRIPT ( 1 ) end_POSTSUPERSCRIPT , italic_K start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT ⟩ start_POSTSUBSCRIPT italic_F end_POSTSUBSCRIPT = roman_tr ( italic_K start_POSTSUPERSCRIPT ( 1 ) ⊤ end_POSTSUPERSCRIPT italic_K start_POSTSUPERSCRIPT ( 2 ) end_POSTSUPERSCRIPT ) .

The alignment between two kernels is always lesser or equal to 1, where 1 corresponds to perfect alignment.

Assume a kernel κ𝜽subscript𝜅𝜽\kappa_{\bm{\theta}}italic_κ start_POSTSUBSCRIPT bold_italic_θ end_POSTSUBSCRIPT, parametrized by 𝜽𝜽\bm{\theta}bold_italic_θ and define the target kernel matrix as

K=𝐲𝐲.superscript𝐾superscript𝐲𝐲topK^{*}=\mathbf{y}\mathbf{y}^{\top}.italic_K start_POSTSUPERSCRIPT ∗ end_POSTSUPERSCRIPT = bold_yy start_POSTSUPERSCRIPT ⊤ end_POSTSUPERSCRIPT .(21)

The kernel-target alignment (KTA) of κ𝜽subscript𝜅𝜽\kappa_{\bm{\theta}}italic_κ start_POSTSUBSCRIPT bold_italic_θ end_POSTSUBSCRIPT is the alignment between the chosen kernel and the target,

A(K𝜽,K)𝐴subscript𝐾𝜽superscript𝐾\displaystyle A\left(K_{\bm{\theta}},\,K^{*}\right)italic_A ( italic_K start_POSTSUBSCRIPT bold_italic_θ end_POSTSUBSCRIPT , italic_K start_POSTSUPERSCRIPT ∗ end_POSTSUPERSCRIPT )=K𝜽,KFK𝜽FKFabsentsubscriptsubscript𝐾𝜽superscript𝐾𝐹subscriptnormsubscript𝐾𝜽𝐹subscriptnormsuperscript𝐾𝐹\displaystyle=\frac{\langle K_{\bm{\theta}},\,K^{*}\rangle_{F}}{\|K_{\bm{%\theta}}\|_{F}\|K^{*}\|_{F}}= divide start_ARG ⟨ italic_K start_POSTSUBSCRIPT bold_italic_θ end_POSTSUBSCRIPT , italic_K start_POSTSUPERSCRIPT ∗ end_POSTSUPERSCRIPT ⟩ start_POSTSUBSCRIPT italic_F end_POSTSUBSCRIPT end_ARG start_ARG ∥ italic_K start_POSTSUBSCRIPT bold_italic_θ end_POSTSUBSCRIPT ∥ start_POSTSUBSCRIPT italic_F end_POSTSUBSCRIPT ∥ italic_K start_POSTSUPERSCRIPT ∗ end_POSTSUPERSCRIPT ∥ start_POSTSUBSCRIPT italic_F end_POSTSUBSCRIPT end_ARG(22)
=K𝜽,KFMK𝜽F,absentsubscriptsubscript𝐾𝜽superscript𝐾𝐹𝑀subscriptnormsubscript𝐾𝜽𝐹\displaystyle=\frac{\langle K_{\bm{\theta}},\,K^{*}\rangle_{F}}{M\|K_{\bm{%\theta}}\|_{F}},= divide start_ARG ⟨ italic_K start_POSTSUBSCRIPT bold_italic_θ end_POSTSUBSCRIPT , italic_K start_POSTSUPERSCRIPT ∗ end_POSTSUPERSCRIPT ⟩ start_POSTSUBSCRIPT italic_F end_POSTSUBSCRIPT end_ARG start_ARG italic_M ∥ italic_K start_POSTSUBSCRIPT bold_italic_θ end_POSTSUBSCRIPT ∥ start_POSTSUBSCRIPT italic_F end_POSTSUBSCRIPT end_ARG ,

where K𝜽subscript𝐾𝜽K_{\bm{\theta}}italic_K start_POSTSUBSCRIPT bold_italic_θ end_POSTSUBSCRIPT is the kernel matrix of κ𝜽subscript𝜅𝜽\kappa_{\bm{\theta}}italic_κ start_POSTSUBSCRIPT bold_italic_θ end_POSTSUBSCRIPT.

The KTA enjoys theoretical properties such as concentration around its expected value and generalisation [26] and therefore it is indicative of the ability of a kernel to separate classes of data.

Appendix C Quantum computing notions

C.1 Quantum states

The basic logical unit in quantum computing is the qubit. Mathematically speaking, this is a unit-norm vector in the complex 2-dimensional space 2superscript2\mathbb{C}^{2}blackboard_C start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT defined as a linear combination of two orthogonal basis states, |0ket0\ket{0}| start_ARG 0 end_ARG ⟩ and |1ket1\ket{1}| start_ARG 1 end_ARG ⟩, i.e.

|ψ=ψ0|0+ψ1|1,ψ0,ψ1,formulae-sequenceket𝜓subscript𝜓0ket0subscript𝜓1ket1subscript𝜓0subscript𝜓1\ket{\psi}=\psi_{0}\ket{0}+\psi_{1}\ket{1},\quad\psi_{0},\,\psi_{1}\in\mathbb{%C},| start_ARG italic_ψ end_ARG ⟩ = italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT | start_ARG 0 end_ARG ⟩ + italic_ψ start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT | start_ARG 1 end_ARG ⟩ , italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT , italic_ψ start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT ∈ blackboard_C ,(23)

where the |ket\ket{\cdot}| start_ARG ⋅ end_ARG ⟩ notation is used to indicate unit vectors.

As opposed to classical bits, Equation23 shows that a single qubit can be in any complex superposition of the two basis states. However, reading of a quantum state can only happen through a measurement, which will make the qubit collapse to one of the two basis states, |0ket0\ket{0}| start_ARG 0 end_ARG ⟩ or |1ket1\ket{1}| start_ARG 1 end_ARG ⟩. More specificallly, the qubit is measured as |0ket0\ket{0}| start_ARG 0 end_ARG ⟩ with probability p0=ψ02subscript𝑝0superscriptsubscript𝜓02p_{0}=\psi_{0}^{2}italic_p start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT = italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT and as |1ket1\ket{1}| start_ARG 1 end_ARG ⟩ with probability p1=ψ12subscript𝑝1superscriptsubscript𝜓12p_{1}=\psi_{1}^{2}italic_p start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT = italic_ψ start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT. Since these are the only two possible outcomes, it must be that p0+p1=ψ02+ψ12=1subscript𝑝0subscript𝑝1superscriptsubscript𝜓02superscriptsubscript𝜓121p_{0}+p_{1}=\psi_{0}^{2}+\psi_{1}^{2}=1italic_p start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT + italic_p start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT = italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT + italic_ψ start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT = 1, which explains the unitary norm of the qubit.

Similarly, a states of n𝑛nitalic_n qubits is defined as a superpositions of basis states that correspond to bitstrings, that is

|ψ=ψ0|00+ψ1|01++ψN1|11ψ0,,ψN1,formulae-sequenceket𝜓subscript𝜓0ket00subscript𝜓1ket01subscript𝜓𝑁1ket11subscript𝜓0subscript𝜓𝑁1\ket{\psi}=\psi_{0}\ket{0\dots 0}+\psi_{1}\ket{0\dots 1}+\dots+\psi_{N-1}\ket{%1\dots 1}\quad\psi_{0},\,\dots,\,\psi_{N-1}\in\mathbb{C},| start_ARG italic_ψ end_ARG ⟩ = italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT | start_ARG 0 … 0 end_ARG ⟩ + italic_ψ start_POSTSUBSCRIPT 1 end_POSTSUBSCRIPT | start_ARG 0 … 1 end_ARG ⟩ + ⋯ + italic_ψ start_POSTSUBSCRIPT italic_N - 1 end_POSTSUBSCRIPT | start_ARG 1 … 1 end_ARG ⟩ italic_ψ start_POSTSUBSCRIPT 0 end_POSTSUBSCRIPT , … , italic_ψ start_POSTSUBSCRIPT italic_N - 1 end_POSTSUBSCRIPT ∈ blackboard_C ,(24)

where N=2n𝑁superscript2𝑛N=2^{n}italic_N = 2 start_POSTSUPERSCRIPT italic_n end_POSTSUPERSCRIPT.

The exponential relation between the number of qubits and the number of possible bitstrings speaks for the potential advantage of quantum superposition, which allows multiple classical information states to be processed simultaneously through a quantum algorithm. Quantum superposition is at the heart of fundamental algorithms with proved complexity improvement such as quantum integer factoring [39] and quantum database search [40].

Nevertheless, the quantum state is inaccessible as readable information and measurement will collapse the wavefunction to only one of the 2nsuperscript2𝑛2^{n}2 start_POSTSUPERSCRIPT italic_n end_POSTSUPERSCRIPT basis states. Similarly to the single-qubit case, the basis state |iket𝑖\ket{i}| start_ARG italic_i end_ARG ⟩ has probability pi=ψi2subscript𝑝𝑖superscriptsubscript𝜓𝑖2p_{i}=\psi_{i}^{2}italic_p start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT = italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT of being measured and

i=0N1pi=i=0N1ψi2=1.superscriptsubscript𝑖0𝑁1subscript𝑝𝑖superscriptsubscript𝑖0𝑁1superscriptsubscript𝜓𝑖21\sum_{i=0}^{N-1}p_{i}=\sum_{i=0}^{N-1}\psi_{i}^{2}=1.∑ start_POSTSUBSCRIPT italic_i = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_N - 1 end_POSTSUPERSCRIPT italic_p start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT = ∑ start_POSTSUBSCRIPT italic_i = 0 end_POSTSUBSCRIPT start_POSTSUPERSCRIPT italic_N - 1 end_POSTSUPERSCRIPT italic_ψ start_POSTSUBSCRIPT italic_i end_POSTSUBSCRIPT start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT = 1 .(25)

Quantum states can be prepared by applying unitary transformations to a reference state, such as the all-zero state,

|ψ=U|0n,ket𝜓𝑈superscriptket0tensor-productabsent𝑛\ket{\psi}=U\ket{0}^{\otimes n},| start_ARG italic_ψ end_ARG ⟩ = italic_U | start_ARG 0 end_ARG ⟩ start_POSTSUPERSCRIPT ⊗ italic_n end_POSTSUPERSCRIPT ,(26)

where U𝑈Uitalic_U is the generic unitary transformation.

Predicting Open-Hole Laminates Failure Using Support Vector Machines With Classical and Quantum Kernels (11)

Figure11 shows a unitary operation as a quantum circuit, i.e. a sequence of single- and two-qubit operations. Here, Hadamard gates H𝐻Hitalic_H are first applied to every qubit, where

H|0𝐻ket0\displaystyle H\ket{0}italic_H | start_ARG 0 end_ARG ⟩=12(|0+|1)absent12ket0ket1\displaystyle=\frac{1}{\sqrt{2}}\left(\ket{0}+\ket{1}\right)= divide start_ARG 1 end_ARG start_ARG square-root start_ARG 2 end_ARG end_ARG ( | start_ARG 0 end_ARG ⟩ + | start_ARG 1 end_ARG ⟩ )(27)
H|1𝐻ket1\displaystyle H\ket{1}italic_H | start_ARG 1 end_ARG ⟩=12(|0|1).absent12ket0ket1\displaystyle=\frac{1}{\sqrt{2}}\left(\ket{0}-\ket{1}\right).= divide start_ARG 1 end_ARG start_ARG square-root start_ARG 2 end_ARG end_ARG ( | start_ARG 0 end_ARG ⟩ - | start_ARG 1 end_ARG ⟩ ) .

This first layer of Hadamard gates creates the uniform superposition state

12n(|00,|01,,|11),1superscript2𝑛ket00ket01ket11\frac{1}{\sqrt{2^{n}}}\left(\ket{0\dots 0},\,\ket{0\dots 1},\,\dots,\,\ket{1%\dots 1}\right),divide start_ARG 1 end_ARG start_ARG square-root start_ARG 2 start_POSTSUPERSCRIPT italic_n end_POSTSUPERSCRIPT end_ARG end_ARG ( | start_ARG 0 … 0 end_ARG ⟩ , | start_ARG 0 … 1 end_ARG ⟩ , … , | start_ARG 1 … 1 end_ARG ⟩ ) ,(28)

where each basis state can be sampled with the equal probability 1/2n1superscript2𝑛1/2^{n}1 / 2 start_POSTSUPERSCRIPT italic_n end_POSTSUPERSCRIPT. This is often a starting point state in many quantum algorithms.

Following, CNOT gates act between neighbouring couples of qubits as

CNOT|00CNOTket00\displaystyle\mathrm{CNOT}\ket{00}roman_CNOT | start_ARG 00 end_ARG ⟩=|00absentket00\displaystyle=\ket{00}= | start_ARG 00 end_ARG ⟩(29)
CNOT|01CNOTket01\displaystyle\mathrm{CNOT}\ket{01}roman_CNOT | start_ARG 01 end_ARG ⟩=|01absentket01\displaystyle=\ket{01}= | start_ARG 01 end_ARG ⟩
CNOT|10CNOTket10\displaystyle\mathrm{CNOT}\ket{10}roman_CNOT | start_ARG 10 end_ARG ⟩=|11absentket11\displaystyle=\ket{11}= | start_ARG 11 end_ARG ⟩
CNOT|11CNOTket11\displaystyle\mathrm{CNOT}\ket{11}roman_CNOT | start_ARG 11 end_ARG ⟩=|10.absentket10\displaystyle=\ket{10}.= | start_ARG 10 end_ARG ⟩ .

CNOT gates are used to set qubits in an entangled state, a condition in which any operation on any of the qubits affects also the rest of the state. In particular, the series of CNOT gates creates one of the maximally entangled Greenberger-Horne-Zeilinger (GHZ) states [41], specifically

|GHZ=12(|0000+|1111).ketGHZ12ket0000ket1111\ket{\mathrm{GHZ}}=\frac{1}{\sqrt{2}}\left(\ket{0000}+\ket{1111}\right).| start_ARG roman_GHZ end_ARG ⟩ = divide start_ARG 1 end_ARG start_ARG square-root start_ARG 2 end_ARG end_ARG ( | start_ARG 0000 end_ARG ⟩ + | start_ARG 1111 end_ARG ⟩ ) .(30)

C.2 Quantum embedding

State preparation can be used to embed classical data into quantum states, by mapping the features to a unitary transformation.

|ϕ(𝒙)=U(𝒙)|0,ketitalic-ϕ𝒙𝑈𝒙ket0\ket{\phi\left(\bm{x}\right)}=U(\bm{x})\ket{0},| start_ARG italic_ϕ ( bold_italic_x ) end_ARG ⟩ = italic_U ( bold_italic_x ) | start_ARG 0 end_ARG ⟩ ,(31)

A complete review of the different types of quantum embeddings is beyond the current scope and the interested reader is pointed to [42] for a critical overeview.

C.3 Quantum kernels

Quantum embeddings are effectively feature maps in the Hilbert space 2nsuperscriptsuperscript2𝑛\mathcal{H}\subseteq\mathbb{C}^{2^{n}}caligraphic_H ⊆ blackboard_C start_POSTSUPERSCRIPT 2 start_POSTSUPERSCRIPT italic_n end_POSTSUPERSCRIPT end_POSTSUPERSCRIPT. The kernel associated with it computes the overlap between quantum feature vectors in \mathcal{H}caligraphic_H, that is

κ(𝜺,𝜺)=|ϕ(𝜺)|ϕ(𝜺)|2,𝜅𝜺superscript𝜺superscriptinner-productitalic-ϕ𝜺italic-ϕsuperscript𝜺2\kappa\left(\bm{\varepsilon},\bm{\varepsilon}^{\prime}\right)=\absolutevalue{%\innerproduct{\phi\left(\bm{\varepsilon}\right)}{\phi\left(\bm{\varepsilon}^{%\prime}\right)}}^{2},italic_κ ( bold_italic_ε , bold_italic_ε start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) = | start_ARG ⟨ start_ARG italic_ϕ ( bold_italic_ε ) end_ARG | start_ARG italic_ϕ ( bold_italic_ε start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) end_ARG ⟩ end_ARG | start_POSTSUPERSCRIPT 2 end_POSTSUPERSCRIPT ,(32)

where the braket notation |inner-product\innerproduct{\cdot}{\cdot}⟨ start_ARG ⋅ end_ARG | start_ARG ⋅ end_ARG ⟩ indicates the inner product between two vectors in \mathcal{H}caligraphic_H.

By introducing Equation31 in Equation32, the quantum kernel can be rewritten as

κ(𝒙,𝒙)=0|U(𝒙)U(𝒙)|0,𝜅𝒙superscript𝒙expectation-valuesuperscript𝑈superscript𝒙𝑈𝒙00\kappa\left(\bm{x},\,\bm{x}^{\prime}\right)=\matrixelement{0}{U^{\dagger}\left%(\bm{x}^{\prime}\right)U\left(\bm{x}\right)}{0},italic_κ ( bold_italic_x , bold_italic_x start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) = ⟨ start_ARG 0 end_ARG | start_ARG italic_U start_POSTSUPERSCRIPT † end_POSTSUPERSCRIPT ( bold_italic_x start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT ) italic_U ( bold_italic_x ) end_ARG | start_ARG 0 end_ARG ⟩ ,(33)

which shows that the quantum kernel can be computed as the probability of the all-zeros state, after applying the direct embedding for 𝒙𝒙\bm{x}bold_italic_x and the reversed embedding for 𝒙superscript𝒙\bm{x}^{\prime}bold_italic_x start_POSTSUPERSCRIPT ′ end_POSTSUPERSCRIPT.

Appendix D Classical and quantum SVM comparison on different classification metrics

Ntrainsubscript𝑁trainN_{\mathrm{train}}italic_N start_POSTSUBSCRIPT roman_train end_POSTSUBSCRIPTAccuracyJaccard IndexPrecisionRecallSpecificity
RBF kernel
1560.6940.6270.7840.7590.795
3130.7500.7080.8350.8240.838
4700.7880.7510.8860.8320.895
6270.7880.7500.8930.8250.903
7840.8380.8130.9390.8590.944
9400.8240.7970.9150.8610.921
10970.8480.8270.9380.8740.943
12540.8780.8660.9430.9130.945
14110.8730.8600.9370.9120.939
15680.8820.8690.9550.9060.958
HE2W6D3 kernel
1560.6990.6260.8040.7390.823
3130.7310.6750.8350.7800.846
4700.7560.7080.8580.8020.869
6270.7790.7410.8780.8260.887
7840.7950.7620.8920.8390.900
9400.7940.7570.8970.8290.907
10970.8050.7730.9020.8440.910
12540.7970.7650.8840.8490.891
14110.8140.7850.9110.8510.917
15680.8180.7900.9120.8550.919

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