About

I am Josep Lumbreras, a Ph.D. student at Centre for Quantum Technologies (National University of Singapore) under the supervision of Prof. Marco Tomamichel. Previously, I completed a double major in physics and mathematics and a master’s in particle physics and gravitation at the University of Barcelona.

Research interests

In general, I am interested in learning theory problems applied to quantum systems.

My main line of research lies at the intersection of Machine Learning and Quantum Information, where I aim to prove rigorous theoretical results. I am particularly interested in applying reinforcement learning to quantum settings, such as adaptive measurements and quantum control. Among the various RL frameworks, I focus especially on the multi-armed bandit problem in quantum contexts.

In addition, I work on the quantum generalization of (Partially Observable) Hidden Markov Models, exploring both their fundamental aspects and practical applications.

Publications

1. Quantum state-agnostic work extraction (almost) without dissipation
   J. Lumbreras, R.C. Cheng, Y. Hu, M. Gu, M. Tomamichel.
   [AQIS25][Quantum Resources (2025)][arXiv (2025)]

2. Learning pure quantum states (almost) without regret
   J. Lumbreras, M. Terekhov, M. Tomamichel.
   [AQIS24 (long talk)][arXiv (2024)]

3. Linear bandits with polylogarithmic minimax regret
   J. Lumbreras, M. Tomamichel.
   [COLT (2024)][arXiv (2024)]

4. Learning finitely correlated states: stability of the spectral reconstruction
   M. Fanizza, N. Galke, J. Lumbreras, C. Rouzé, A. Winter.
   [Beyond IID 2024][arXiv (2024)]

5. Quantum Theory in Finite Dimension Cannot Explain Every General Process with Finite Memory
   M. Fanizza^†, J. Lumbreras^† (co-first author), A. Winter.
   [ Communications in Mathematical Physics][TQC 23][Beyond IID 2022][arXiv (2023)]

6. Quantum contextual bandits and recommender systems for quantum data
   S. Brahmachari^†, J. Lumbreras^† (co-first author), M. Tomamichel.
   [Quantum Machine Intelligence][arXiv (2023)]

7. Multi-armed quantum bandits: Exploration versus exploitation when learning properties of quantum states
   J. Lumbreras, E. Haapasalo, M. Tomamichel.
   [Quantum][AQIS 21 (long talk)][Beyond IID 2021][QTML 21][arXiv (2022)]

8. Scaling of variational quantum circuit depth for condensed matter systems
   C. Bravo^†, J. Lumbreras^† (co-first author), L. Tagliacozzo, J. I. Latorre.
   [Quantum][arXiv (2020)]