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QTML2021 UAP Quantum Feature Map

This document discusses the universal approximation property (UAP) of quantum neural networks (QNNs) using quantum feature maps, highlighting their potential advantages over classical neural networks. It explores the expressivity of QNNs, methods for achieving UAP through suitable observables and activation functions, and the implications of using quantum circuits for approximation. The authors suggest QNNs can approximate continuous functions effectively and pose various research questions related to approximation performance, entanglement, and the impact of network architecture.

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Universal Approximation Property via Quantum Feature Maps Quoc Hoan Tran*,Takahiro Goto*, and Kohei Nakajima Physical Intelligence Lab, UTokyo QTML2021 arXiv:2009.00298 Phys. Rev. Lett. 127, 090506 (2021) (*) Equal contribution
The future of intelligent computations 2/20 Bits Qubits Neurons Quantum mind Quantum computing High Efficiency + Fragile Neural networks Versatile + Adaptive + Robust Two rising fields can be complementary Converge to an adaptive platform: Quantum Neural Networks
QNN via Quantum Feature Map[1,2] 3/20 [1] V. Havlíček et al., Supervised Learning with Quantum-Enhanced Feature Spaces, Nature 567, 209 (2019) [2] M. Schuld and N. Killoran, Quantum Machine Learning in Feature Hilbert Spaces, Phys. Rev. Lett. 122, 040504 (2019) [3] Y. Liu et al., A Rigorous and Robust Quantum Speed-Up in Supervised Machine Learning, Nat. Phys. 17, 1013 (2021) Input Space 𝒳 Quantum Hilbert space 𝒙 |Ψ 𝒙 ⟩ Access via Measurements or Compute Kernel Or at least the same expressivity as classical 𝜅 𝒙, 𝒙′ = ⟨Ψ(𝒙)|Ψ(𝒙′)⟩ Heuristic Quantum Circuit Learning (K. Mitarai, M. Negoro,M. Kitagawa, and K. Fujii, Phys. Rev.A 98, 032309) ◼ For a special problem, a formal quantum advantage[3] can be proven (but is neither “near-term” nor practical) ◼ We expect quantum feature maps to be more expressive than classical counter parts
Universal Approximation Property (UAP) 4/20 𝑥1 𝑥2 𝑤1 𝑤2 𝑤𝐾 𝑦 ◼ Feed-forward NN with one hidden layer can approximate any continuous function* on a closed and bounded subset of ℝ𝑑 to arbitrary 𝜀 (Hornik+1989, Cybenko+1989) * This belongs to a larger class of any Borel measurable function (beyond this talk) ➢ Given enough hidden units with any Squashing Activation Functions (Step, Sigmoid,Tanh, Relu) ➢ Extensive extensions to non-continuous activation function, resource evaluation
UAP of Quantum ML models 5/20 Data Re-uploading Adrián Pérez-Salinas et al., Data re-uploading for a universal quantum classifier, Quantum 4, 226 (2020) Expressive power via a partial Fourier series M, Schuld, R. Sweke and J. J. Meyer, Effect of data encoding on the expressive power of variational quantum-machine-learning models, Phys. Rev.A 103, 032430 (2021) 𝑓𝜽 𝒙 = ෍ 𝜔∈Ω 𝑐𝜔 𝜽 𝑒𝑖𝜔𝒙 Our simple but clear questions ◼ UAP in the language of observables and quantum feature maps ◼ How well the approximation could perform? ◼ How many parameters it needs (vs. classical NNs)?
QNN via Quantum Feature Map ◼ Choosing W is equivalent to selecting suitable observables 6/20 ◼ The output is a linear expansion of the basis functions with a suitable set of observables 𝜓𝑖 𝒙 = Ψ 𝒙 𝑂𝑖 Ψ 𝒙 𝑓 𝒙; Ψ, 𝑶, 𝒘 = ෍ 𝑖=1 𝐾 𝑤𝑖𝜓𝑖 𝒙 [1]V. Havlíček et al., Supervised Learning with Quantum-Enhanced Feature Spaces, Nature 567, 209 (2019) 𝑤𝛽 𝜽 = Tr 𝒲† 𝜽 𝑍⊗𝑁𝒲 𝜽 𝑃𝛽 𝑃𝛽 ∈ 𝐼, 𝑋, 𝑌, 𝑍 ⊗𝑁 𝜓𝛽 𝒙 = Tr Ψ 𝒙 Ψ 𝒙 𝑃𝛽 Pauli-group Basis functions (expectation) E.g., Decision function[1] sign 1 2𝑁 ෍ 𝛽 𝑤𝛽 𝜽 𝜓𝛽 𝒙 + 𝑏
UAP via Quantum Feature Map 𝑓 𝒙; Ψ, 𝑶, 𝒘 = ෍ 𝑖=1 𝐾 𝑤𝑖𝜓𝑖 𝒙 ≈ 𝑔 𝒙 ? Given a compact set 𝒳 ∈ ℝ𝑑 , for any continuous function 𝑔: 𝒳 → ℝ, can we find 𝚿, 𝑶, 𝒘 s.t 7/20 UAP. Let 𝒢 be a space of continuous functions 𝑔: 𝒳 → ℝ.The quantum feature framework ℱ = {𝑓 𝒙; Ψ, 𝑶, 𝒘 } has the UAP w.r.t. 𝒢 and a norm ⋅ if given any 𝑔 ∈ 𝒢; then for any 𝜀, there exists 𝑓 ∈ ℱ s.t. 𝑓 − 𝑔 < 𝜀 Classification capability. For arbitrary disjoint regions 𝒦1, 𝒦2, … , 𝒦𝑛 ⊂ 𝒳, there exists 𝑓 ∈ ℱ s.t. 𝑓 can separate these regions (can be induced from UAP) 𝒳 ℝ 𝒦1 𝒦2 𝑓 ∈ ℱ
UAP - Scenarios Parallel Sequential ◼ Encode the “nonlinear” property in the basis functions for UAP 1. Using appropriate observables to construct a polynomial approximation 2. Using appropriate activation functions in pre-processing 8/20
Parallel scenario Approach 1. Produce the “nonlinear” by suitable observables ◼ Basis functions can be polynomials if we choose the correlated measurement operators ◼ Then we apply the Weierstrass’s polynomial approximation theorem 𝑈𝑁 𝒙 = 𝑉1 𝒙 ⊗ ⋯ ⊗ 𝑉𝑁 𝒙 𝑉 𝑗 𝒙 = 𝑒−𝑗𝜃𝑗 𝒙 𝑌 𝜃𝑗 𝒙 = arccos( 𝑥𝑘) for 1 ≤ 𝑘 ≤ 𝑑, 𝑗 ≡ 𝑘(mod 𝑑) 𝑂𝒂 = 𝑍𝑎1 ⊗ ⋯ ⊗ 𝑍𝑎𝑁, 𝒂 ∈ 0,1 𝑁 Observables Basis functions 𝜓𝒂 𝑥 = Ψ𝑁 𝒙 𝑂𝒂 Ψ𝑁 𝒙 = ෑ 𝑖=1 𝑁 2𝑥[𝑖] − 1 𝑎𝑖 Feature map: Ψ𝑁(𝒙) = 𝑈𝑁 𝒙 0 ⊗𝑁 1 ≤ 𝑖 ≤ 𝑑 𝑖 ≡ 𝑖 (𝑚𝑜𝑑 𝑑) 9/20
Parallel scenario Approach 1. Produce the “nonlinear” by suitable observables Lemma 1. Consider a polynomial 𝑃(𝒙) of the input 𝒙 = 𝑥1, 𝑥2, … , 𝑥𝑑 ∈ 0,1 𝑑 where the degree of 𝑥𝑗 in 𝑃(𝒙) is less than or equal to 𝑁+(𝑑−𝑗) 𝑑 for 𝑗 = 1, … , 𝑑 𝑁 ≥ 𝑑 ; then there exists a collection of output weights {𝑤𝒂 ∈ 𝑅|𝒂 ∈ 0,1 𝑁} s.t. ෍ 𝒂∈ 0,1 𝑁 𝑤𝒂 𝜓𝒂 𝒙 = 𝑃(𝒙) Result 1. (UAP in the parallel scenario). For any continuous function 𝑔: 𝒳 → 𝑅 lim 𝑁→∞ min 𝒘 ෍ 𝒂∈ 0,1 𝑁 𝑤𝒂𝜓𝒂 𝒙 − 𝑔(𝒙) ∞ = 0 ※The number of observables ≤ 2 + 𝑁−1 𝑑 𝑑 10/20
Parallel scenario Approach 2. Activation function in pre-processing (straight-forward) [Huang+, Neurocomputing, 2007] 𝜎: 𝑅 → [−1,1] is nonconstant piecewise continuous function such that 𝜎(𝒂 ⋅ 𝒙 + 𝒃) is dense in 𝐿2(𝑋).Then, for any continuous function 𝑔: 𝑋 → 𝑅 and any function sequence 𝜎𝑗 𝒙 = 𝜎 𝒂𝑗 ⋅ 𝒙 + 𝑏𝑗 where 𝒂𝑗, 𝑏𝑗 are randomly generated based on any continuous sampling distribution, lim 𝑁→∞ min 𝒘 ෍ 𝑗 𝑁 𝑤𝑗𝜎𝑗 𝒙 − 𝑔(𝒙) 𝐿2 = 0 We obtain UAP if we can implement the squashing activation function in pre-processing process (Result 2) • 𝑈𝑁 𝒙 = 𝑉1 𝒙 ⊗ ⋯ ⊗ 𝑉𝑁 𝒙 • 𝑉 𝑗 𝒙 = 𝑒−𝑗𝜃𝑗 𝒙 𝑌 • 𝜃𝑗 𝒙 = arccos( 1+𝜎(𝒂𝑗⋅𝒙+𝑏𝑗) 2 ) 𝜓𝑗 𝑥 = Ψ𝑁 𝒙 𝑍𝑗 Ψ𝑁 𝒙 = ⟨0|𝑒𝑖𝜃𝑗𝑌 𝑍𝑒−𝑖𝜃𝑗𝑌 |0⟩ = 2 cos2(𝜃𝑗) − 1 = 𝜎 𝒂𝑗 ⋅ 𝒙 + 𝑏𝑗 = 𝜎𝑗(𝒙) 11/20
Sequential scenario with a single qubit Finite input set 𝒳 = {𝒙1, 𝒙2, … , 𝒙𝑀} 𝑉 𝒙 = 𝑒−𝜋𝑖𝜃 𝒙 𝑌 → 𝑉𝑛 𝒙 = 𝑒−𝜋𝑖𝑛𝜃 𝒙 𝑌 Basis functions with the Pauli-Z 𝜓𝑛 𝒙 = 0 𝑉𝑛 † 𝒙 𝑍𝑉𝑛 𝒙 0 = 2 cos2 (𝜋𝑛𝜃(𝒙)) = cos 2𝜋𝑛𝜃 𝒙 = cos(2𝜋{𝑛𝜃 𝒙 }) We can use the ergodic theory of the dynamical system on M- dimensional torus Lemma 3. If real numbers 1, 𝑎1, … , 𝑎𝑀 are linearly independent over ℚ, then 𝑛𝑎1 , … , 𝑛𝑎𝑀 𝑛∈ℕ is dense in 0, 1 𝑀 . Result 3. If 1, 𝜃(𝒙1), … , 𝜃(𝒙𝑀) are linearly independent over ℚ, then for any function 𝑔: 𝒳 → ℝ and for any 𝜀 > 0, there exists 𝑛 ∈ ℕ such that 𝑓 𝑛 𝑥 − 𝑔 𝑥 < 𝜀 for ∀𝑥 ∈ 𝒳 12/20
UAP – Approximation Rate Classical UAP of DNN inf 𝐰 𝑓 𝒘 − 𝑔 = 𝑂(𝐾−𝛽/𝑑 ) [1] Hrushiksh M Mhaskar. Neural networks for optimal approximation of smooth and analytic functions. Neural Computation, 8(1):164-167, 1996 Optimal rate[3] [2] Dmitry Yarotsky. Error bounds for approximations with deep Relu networks. Neural Networks, 94:103-114, 2017 [3] Ronald A DeVore. Optimal nonlinear approximation. Manuscripta Mathematica, 63(4):469-478, 1989. 𝐾 𝜀 𝑂(𝐾−𝛼1) 𝑂(𝐾−𝛼2) 𝛼1 < 𝛼2 13/20 The following statement[1,2] holds for continuous activation function (with 𝐿 = 2), and ReLU (with 𝐿 = 𝑂(𝛽log 𝐾 𝑑 )) ➢ 𝑑: input dimension ➢ 𝐾: number of network parameters ➢ 𝛽: derivative order of g ➢ 𝐿: number of layers. How to describe relative good and bad UAP? Approximation Rate = Decay rate of appr. error vs. (better rate)
UAP – Approximation Rate Approximation Rate = Decay rate of appr. error vs. ➢ Number K of observables ➢ Input dimension d ➢ Number N of qubits 𝐾 𝜀 𝑂(𝐾−1/3𝑑) 𝑂(𝐾−1/𝑑) 14/20 In the parallel scenario: 𝐾 = 𝑂 𝑁𝑑 → 𝜀 = 𝑂 𝐾−1/𝑑 Result 4. If 𝑋 = 0,1 𝑑 and the target function 𝑔: 𝑋 → 𝑅 is Lipschitz continuous w.r.t. the Euclidean norm, we can construct an explicit form of the approximator to 𝑔 in the parallel scenario by 𝑁 qubits with the error 𝜀 = 𝑂(𝑑7/6𝑁−1/3). Furthermore, there exist an approximator with 𝜀 = 𝑂(𝑑3/2𝑁−1) (optimal rate)
Sketch of the proof Use multivariate Bernstein polynomials for approximating continuous function g Number of qubits 𝑁 = 𝑛𝑑 (𝑑: input dimension) How to create these terms by observables? 15/20
Consider the operators for each 𝒑 = (𝑝1, … , 𝑝𝑑) 16/20
We choose the following observables Basis functions 17/20
Approximate rate in parallel scenario We evaluate the approximation rate via the approximation of multivariate Bernstein polynomials to a continuous function 18/20
Approximate rate in parallel scenario We use the Jackson theorem in higher dimension of the quantitative information on the degree of polynomial approximation (D. Newman+1964) Bernstein basis polynomials form a basis of the linear space of 𝑑–variate polynomials of degree less than or equal to 𝑁 (but we do not know the explicit form) 19/20
Summary ◼ UAP via simple quantum feature maps with observables ◼ Other questions: ◼ Evaluate how well the approximation could perform (vs. classical scenario) ➢ UAP with other properties of the target function: non continuous, varied smoothness from place to place ➢ Entanglement and UAP ➢ Approximation rate with the number of layers (such as in data re-uploading scheme) Thank you for listening! 20/20 ◼ Require a large number of qubits