Adaptive Online Learning of Quantum States

Xinyi Chen1,2, Elad Hazan1,2, Tongyang Li3,4, Zhou Lu1,2, Xinzhao Wang3,4, and Rui Yang3,4

1Department of Computer Science, Princeton University, NJ 08540, USA
2Google DeepMind Princeton, NJ 08542, USA
3Center on Frontiers of Computing Studies, Peking University, 100871 Beijing, China
4School of Computer Science, Peking University, 100871 Beijing, China

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Abstract

The problem of efficient quantum state learning, also called shadow tomography, aims to comprehend an unknown $d$-dimensional quantum state through POVMs. Yet, these states are rarely static; they evolve due to factors such as measurements, environmental noise, or inherent Hamiltonian state transitions. This paper leverages techniques from adaptive online learning to keep pace with such state changes.
The key metrics considered for learning in these mutable environments are enhanced notions of regret, specifically adaptive and dynamic regret. We present adaptive and dynamic regret bounds for online shadow tomography, which are polynomial in the number of qubits and sublinear in the number of measurements. To support our theoretical findings, we include numerical experiments that validate our proposed models.

Quantum state learning is crucial for efficiently characterizing and predicting the behavior of quantum systems. Given multiple identical copies of a quantum state, how can we determine the state through a limited number of measurements? Fully characterizing a system of $n$ qubits requires $2^n$ copies of the quantum state, making the task exponentially complex. To tackle this, methods such as shadow tomography and online learning of quantum states have been proposed, offering algorithms that scale polynomially with the number of qubits $n$.

However, current algorithms overlook an important factor in today's quantum computers: fluctuation of quantum states. Advanced quantum computers, such as those based on superconducting qubits, represent $|0\rangle$ and $|1\rangle$ using the ground and first excited states, respectively. Fluctuations must be carefully managed to ensure accurate computations. More broadly, fluctuations are common in quantum optical systems. This raises a key question: how can we learn dynamic quantum states from the perspective of machine learning? In this paper, we apply adaptive online learning techniques to tackle this challenge, achieving results that scale polynomially in the number of qubits and sublinearly in the number of measurements, under the metric of adaptive and dynamic regret.

► BibTeX data

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[1] Sitan Chen and Weiyuan Gong, "Efficient Pauli Channel Estimation with Logarithmic Quantum Memory", PRX Quantum 6 2, 020323 (2025).

[2] J Knörzer, X Liu, B F Schiffer, and J Tura, "Distributed quantum information processing: a review of recent progress", Reports on Progress in Physics 89 7, 074401 (2026).

[3] Akshay Bansal, Ian George, Soumik Ghosh, Jamie Sikora, and Alice Zheng, "Online learning of a panoply of quantum objects", Quantum Machine Intelligence 7 2, 78 (2025).

[4] Anurag Anshu and Srinivasan Arunachalam, "A survey on the complexity of learning quantum states", Nature Reviews Physics 6 1, 59 (2024).

[5] Asad Raza, Matthias C. Caro, Jens Eisert, and Sumeet Khatri, "Online learning of quantum processes", arXiv:2406.04250, (2024).

[6] Sitan Chen and Weiyuan Gong, "Efficient Pauli channel estimation with logarithmic quantum memory", arXiv:2309.14326, (2023).

[7] Steven T. Flammia and Ryan O'Donnell, "Quantum chi-squared tomography and mutual information testing", Quantum 8, 1381 (2024).

[8] Yuxuan Du, Yibo Yang, Tongliang Liu, Zhouchen Lin, Bernard Ghanem, and Dacheng Tao, "ShadowNet for Data-Centric Quantum System Learning", arXiv:2308.11290, (2023).

[9] Maxime Meyer, Soumik Adhikary, Naixu Guo, and Patrick Rebentrost, "Online Learning of Pure States is as Hard as Mixed States", arXiv:2502.00823, (2025).

[10] Naixu Guo, Feng Pan, and Patrick Rebentrost, "Estimating properties of a quantum state by importance-sampled operator shadows", arXiv:2305.09374, (2023).

[11] Wei-Fu Tseng, Kai-Chun Chen, Zi-Hong Xiao, and Yen-Huan Li, "Online Learning Quantum States with the Logarithmic Loss via VB-FTRL", arXiv:2311.04237, (2023).

[12] Taiga Hiroka and Min-Hsiu Hsieh, "Computational Complexity of Learning Efficiently Generatable Pure States", arXiv:2410.04373, (2024).

[13] Chung-En Tsai, Hao-Chung Cheng, and Yen-Huan Li, "Online Self-Concordant and Relatively Smooth Minimization, With Applications to Online Portfolio Selection and Learning Quantum States", arXiv:2210.00997, (2022).

[14] Jian-Feng Cai, Yuling Jiao, Yinan Li, Xiliang Lu, Jerry Zhijian Yang, and Juntao You, "Online Quantum State Tomography via Stochastic Gradient Descent", arXiv:2507.07601, (2025).

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