YMSC-BIMSA Quantum Information Seminar

Can we compute on small quantum processors? In this talk, I explore the extent to which noise presents a barrier to this goal by quickly drowning out the information in a quantum computation. Noise is a tough adversary: we show that a large class of error mitigation algorithms – proposals to “undo” the effects of quantum noise through mostly classical post-processing - can never scale up. Then we’ll journey into the wild west of non-unital noise, a theoretically understudied class of noise (including damping and photon loss) that predominates on certain physical platforms, and study the task of estimating expectation values in its presence. Having presented some no-go theorems about inference in the presence of noise, I’ll attempt to make lemons out of lemonade by turning to cryptography, where I will propose a new hardness conjecture, based on the task of decoding random stabilizer codes.

Brief bio: Yihui Quek is a postdoctoral fellow at MIT, working with Peter Shor and Aram Harrow, and in August 2025, she will start as an Assistant Professor at EPFL in Switzerland. In the last three years, she has also been a visitor at the Simons Institute for the Theory of Computing,a Harvard Quantum Initiative research fellow at Harvard University and a Alexander von Humboldt fellow at the Free University of Berlin. She is driven by the question: “What can theoretical computer science teach us about physics in the age of supercomputers?” To the question of whether she is a physicist or a computer scientist, no computationally bounded algorithm can have a significant advantage over random guessing.

We investigate what quantum advantages can be obtained in multipartite non-cooperative games by studying how different types of quantum resources can lead to new Nash equilibria and improve social welfare – a measure of the quality of an equilibrium. Two different quantum settings are analyzed: a first, in which players are given direct access to an entangled quantum state, and a second, which we introduce here, in which they are only given classical advice obtained from quantum devices. For a given game G, these two settings give rise to different equilibria characterized by the sets of equilibrium correlations Qcorr(G) and Q(G), respectively. We show that Q(G) ⊆ Qcorr(G), and by exploiting the self-testing property of some correlations, that the inclusion is strict for some games G. We make use of SDP optimization techniques to study how these quantum resources can improve social welfare, obtaining upper and lower bounds on the social welfare reachable in each setting. We investigate, for several games involving conflicting interests, how the social welfare depends on the bias of the game and improve upon a separation that was previously obtained using pseudo-telepathic solutions.

For the brief bio:

Pierre Pocreau, I am a Ph.D. candidate at the Université Grenoble Alpes, working under the supervision of Alastair Abbott and Mehdi Mhalla. My research focuses on quantum foundations, particularly on the computational power of causal indefiniteness, and on quantum correlations and their role in game theory.

In this talk, we will begin with a brief mathematical introduction to quantum Markovian semigroups and the GKsl equation [BPW19]. Next, we will discuss a Hamiltonian-simulation-based quantum implementation for simulating Lindblad dynamics [DLL24]. Finally, we will present recent examples of the algorithmic application of Lindblad equations to state preparation problems in quantum physics and quantum chemistry [CKG23, DCL24, LZL24].

References.

[BPW19] D. Bahns, A. Pohl, and l. Wit., Open Quantum systems: A Mathe-matical Perspective. Springer International Publishing, 2019

[CKG23] Chi-Fang Chen, Michael 」 Kastoryano, and Andr’as Gily’en. An efficient and exact noncommutative quantum Gibbs sampler. arXiv preprintarXiv:2311.09207,2023.

[DCL24] Zhiyan Ding, chi-fang chen, and lin Lin. single-ancila ground state preparation via lindbladians. Phys. Rev. Res 6:033147, 2024.

[DLL24] Zhiyan Ding, Xiantao li, and Lin lin. simulating open quantum systems using hamiltonian simulations. PRX Quantum, 5:020332, 2024

[LZL24] Hao-En li, Yongtao Zhan, and Lin Lin. Dissipative ground state preparation in ab initio electronic structure theory. arXiv preprint arXiv:2411.014702024.

Reliable quantum computation relies on the utilization of logical qubits, making it imperative to demonstrate their superior performance over physical qubits. Dynamical decoupling emerges as an effective approach to protecting logical qubits against environmental noise. Recent progress in this area includes using dynamical decoupling to generate decoherence-free subspaces (DFS) and subsystems (NS), where logical qubits can be stored. Here, we propose and experimentally demonstrate a scheme for protecting logical qubits by implementing iSWAP gates on nearest-neighbor superconducting transmons. Our experiments reveal that the decoherence time of a logical qubit is extended by up to 365% compared to the best-performing physical qubit. To the best of our knowledge, we demonstrate for the first time that multiple logical qubits outperform their physical counterparts in superconducting qubits. Given its measurement-free nature, our scheme holds promise as a component for future fault-tolerant quantum computation.

Speaker Intro:

Jiang Zhang is an assistant professor in the quantum algorithm group of Beijing Academy of Quantum Information Sciences (BAQIS). He received his Ph.D degree from Shandong University in 2015. Then he worked at Beijing Computational Science Research Center (CSRC) and Tsinghua University as a postdoctoral fellow. Before joining BAQIS in 2020, he was an assistant professor at Pengcheng Laboratory (PCL) in Shenzhen. His research interests lie in holonomic quantum computation, dynamical decoupling, quantum error correction, and quantum algorithms.

张江，现任北京量子信息科学研究院副研究员，于2015年在山东大学物理系获得博士学位，2015至2019年先后在北京计算科学研究中心和清华大学物理系从事博士后工作。2019年10月加入鹏城国家实验室量子计算研究中心，获得深圳市后备人才计划。2020年11月起在北京量子信息科学研究院算法团队工作，主要研究方向为几何量子计算、量子噪声的抑制和量子纠错。目前已主持两项国家级基金，发表文章十余篇，包括Physics Review Letters一篇、长篇综述Physics Reports一篇。

Understanding causal relationships is an everlasting theme of scientific research. To interpret correlations between observed variables, a natural thought suggests causal explanations. In classical causal theories, all the variables, including both observed and latent ones, are modeled as random variables. However, as shown by the renowned Bell’s theorem, the classical formulation is incomplete when the latent variables enjoy a quantum nature. In this talk, I will present a new causal model formulated solely from the concept of independence. At a high level, we treat every causal structure as a low-dimensional projection from a high-dimensional Bell-type structure, where valid correlations among observed variables are projected from correlations in the Bell-type structure subjected to the independence conditions. We prove this formulation comprises all the predictions in generalized probabilistic theories. From a mathematical perspective, it reproduces the nested Markov model, an algebraically complete causal model defined by all the equality constraints within a causal structure. Moreover, we show inequality constraints may emerge from the projection of the equality constraints of independence. Nevertheless, we also present causal structures where “physical operations” pose stringently additional inequality constraints. Considering the undebatable role of independence in physics and algebra, we suggest the new model supersets any physically motivated causal theory. If time allows, I will also discuss applications of the new model in causal inference and the discovery of non-classical causal structures.

Bio:

Xingjian Zhang is now a postdoctoral fellow at the Department of Computer Science, University of Hong Kong. He graduated from the Institute for Interdisciplinary Information Sciences (IIIS) at Tsinghua University in 2023 and obtained a PhD degree in physics. Xingjian Zhang is mainly dedicated to the theoretical study of quantum foundation and quantum communication. Thus far, Xingjian Zhang has published research papers in top journals, including PRL, PNAS, PRX Quantum, and Quantum, and has presented contributed talks at top conferences of quantum information, including TQC and AQIS. Based on his experiences, Xingjian Zhang participated in the ITU Standardization of ‘Quantum information technology for networks’ (QIT4N-I-018, focus group) and served as a reviewer for journals and conferences, including Nat. Commun., Phys. Rev. Lett., Sci. Adv., IEEE Trans. Inf. Theor., SODA, QIP, and TQC.

Floquet codes and dynamical codes offer a new avenue for quantum error correcting codes. In this talk, I will discuss a general formalism for analyzing the error correcting properties of these codes. Specifically, we extend the notion of distance of static stabilizer codes and subsystem codes to the unmasked distance of dynamical codes, and we develop an algorithm that determines what syndrome information can be learnt given an arbitrary dynamical code and use this to obtain the code’s unmasked distance. Further, we use the tools developed for the algorithm to reveal the structure of a generic Floquet code. Based on joint work with Daniel Gottesman.

Bio: Xiaozhen Fu is a doctoral student at the University of Maryland, College Park. Her research interests involve quantum error correction and fault tolerance.

• Insights on Analysis Meeting Topology in Quantum Field Theory

Quantum gravity in 4D asymptotically flat spacetimes features spontaneous symmetry breaking due to soft radiation hair, intimately tied to the proliferation of IR divergences. A holographic description via a putative 2D CFT is expected free of such redundancies. In this talk, we address this issue by initiating the study of Quantum Error Correction in Celestial CFT (CCFT). We start by constructing a toy model with finite degrees of freedom by revisiting noncommutative geometry in Kleinian hyperkähler spacetimes. The model obeys a Wick algebra that renormalizes in the radial direction and admits an isometric embedding à la Gottesman-Kitaev-Preskill. Then we promote qubits to qunits and construct a toy model of CCFT from the perspective of quantum error-correcting codes. In our code, the hard states with quantized BMS soft hair form the logical subspace. This allows us to reverse errors induced by soft radiation. Technically, the construction relies on the recently studied \(w_{1+\infty}\) hierarchy of soft currents and its realization in twistor space.

Biography:

Dr. Yangrui Hu is a Postdoctoral Fellow at the University of Waterloo in Canada. She earned her Ph.D. in physics from Brown University in 2022 and was a postdoctoral fellow at the Perimeter Institute from 2022 to 2024. Her research interests center on theoretical physics, specifically in the areas of AI+quantum, quantum information, and high-energy theory.

Quantum walks display very different behavior from classical random walks and are interesting technique for designing quantum algorithms. This talk will cover some basic concepts and algorithmic design of quantum mean estimator and quantum walks, with applications in quantum speedups of Monte Carlo methods and Markov Chains.

报告人简介：刘锦鹏，清华大学数学科学中心助理教授，2022-2024年在麻省理工和伯克利任博士后，2022年博士毕业于马里兰大学，2017年本科毕业于北航-中科院华罗庚班。研究方向为量子科学计算与量子科学智能，发表PNAS、Nat.Commun. 、PRL、CMP、JCP、Quantum等期刊和NeurIPS、QIP、TQC等会议，担任量子信息权威期刊Quantum的编委。

Quantum telepathy is the phenomenon where two non-communicating parties can exhibit correlated behaviors that are impossible to achieve using classical mechanics. This is also known as Bell inequality violation and is made possible by quantum entanglement. In this work, we present a conceptual framework for applying quantum telepathy to real-world problems. In general, the problems involve coordinating decisions given a set of observations without being able to communicate. We argue this inability is actually quite prevalent in the modern era where the decision-making timescales of computer processors are so short that the speed of light delay is actually quite appreciable in comparison. We highlight the example of high-frequency trading (HFT), where trades are made at microsecond timescales, but the speed of light delay between different exchanges can range from the order of 100 microseconds to 10 milliseconds. Due to the maturity of Bell inequality violation experiments, experimental realization of quantum telepathy schemes that can attain a quantum advantage for real-world problems is already almost immediately possible. We demonstrate this by conducting a case study for a concrete HFT scenario that gives rise to a generalization of the CHSH game and evaluate different possible physical implementations for achieving a quantum advantage. It is well known that Bell inequality violation is a rigorous mathematical proof of a quantum advantage over any classical strategy and does not need any complexity-theoretic assumptions such as BQP≠BPP. Moreover, fault tolerance is not necessary to realize a quantum advantage: for example, violating the CHSH inequality only requires single-qubit gates applied on two entangled physical qubits.

Bio: Dawei (David) Ding obtained his PhD in applied physics from Stanford University, where he made key contributions to feedback-assisted communication over quantum channels and quantum chaos. He then worked as a quantum scientist at Alibaba Quantum Laboratory, first in the Design Automation Division and then the Quantum Computer Systems Division. His research focuses on understanding the low-level physics of quantum computing devices and determining how to best use them for computational tasks, thereby taking a bottom-up approach to quantum computing. The theoretical tools thus developed have been adopted by leading hardware teams around the world. He also recently proposed a new type of quantum technology: quantum telepathy. Using quantum entanglement, multiple parties can coordinate decisions faster than light. Applications include high frequency trading, distributed computing, and computer architecture.