Supervised learning with quantum-enhanced feature spaces, Vojtěch Havlíček, Antonio D. Córcoles, Kristan Temme, Aram W. Harrow, Abhinav Kandala, Jerry M. Chow, Jay M. Gambetta, 2019Nature, Vol. 567 (Springer Nature)DOI: 10.1038/s41586-019-0980-2 - This foundational paper introduces quantum kernel methods and proposes specific quantum feature map circuits, including those utilizing ZZ-interactions for encoding data into higher-order polynomial features.
Power of data in quantum machine learning, Hsin-Yuan Huang, Michael Broughton, Masoud Mohseni, Ryan Babbush, Sergio Boixo, Hartmut Neven, Jarrod R. McClean, 2021Nature Communications, Vol. 12DOI: 10.1038/s41467-021-22539-9 - This work investigates the trainability of quantum machine learning models, addressing the 'kernel concentration' problem, where quantum kernels can become untrainable for large numbers of qubits, a consideration for higher-order feature maps.
Quantum Machine Learning in Feature Hilbert Spaces, Maria Schuld, Nathan Killoran, 2019Physical Review Letters, Vol. 122 (American Physical Society)DOI: 10.1103/PhysRevLett.122.040504 - This paper provides a theoretical framework for quantum machine learning by embedding classical data into a quantum Hilbert space and defines quantum kernels based on inner products in this space.
ZZFeatureMap, Qiskit Development Team, 2023 (IBM Corp.) - Provides practical details and an implementation of a widely used quantum feature map that creates second-order polynomial features through ZZ-interactions, matching the example discussed in the section.