2026年5月26日（火）13:30- (13:30-, May 26, 2026)
理学部4号館1220教室 (Room No. 1220, Science 4th Bldg.)
Speaker: Ken M. Nakanishi (CEMS, RIKEN and Department of Physics, UTokyo)
Title: From Attention to Screening: A New LLM Paradigm Based on Absolute Relevance
Abstract:
A core limitation of standard softmax attention is that it does not provide an independently interpretable measure of query-key relevance: attention scores are unbounded, while attention weights are defined only relative to competing keys. Consequently, irrelevant keys cannot be explicitly rejected, and some attention mass is assigned even when no key is genuinely relevant. We introduce Multiscreen, a language-model architecture built around a mechanism we call screening, which enables absolute query-key relevance. Instead of redistributing attention across all keys, screening computes bounded query--key similarities and applies an explicit threshold, discarding irrelevant keys and aggregating the remaining keys without global competition. Across experiments, Multiscreen achieves comparable validation loss with roughly 30% fewer parameters than a Transformer baseline and remains stable at substantially larger learning rates. It maintains stable long-context perplexity beyond the training context and shows little degradation in retrieval performance as context length increases. Finally, Multiscreen achieves lower full-context forward-pass latency at long context lengths.
References:
[1] Ken M. Nakanishi, "Screening is Enough," arXiv:2604.01178.
2026年6月16日（火）13:30- (13:30-, June 16, 2026)
理学部4号館1220教室 (Room No. 1220, Science 4th Bldg.)
Speaker: Yuxuan GUO (Department of Physics, UTokyo)
Title: Strong-to-Weak Spontaneous Symmetry Breaking in Decohered Quantum Systems: From Foundations to Field Theory Approach
Abstract:
Quantum phases and phase transitions in open systems have attracted widespread attention in recent years. In this talk, we explore how quantum phases can be rigorously defined for mixed states. Starting from a pedagogical introduction to the foundational concepts, we introduce the classification of mixed-state phases based on the equivalence relation of two-way connectivity via local quantum channels, alongside relevant quantum informational diagnosis. In the landscape of mixed state phases, spontaneous symmetry breaking (SSB)—the cornerstone of conventional phase classification in many-body physics—manifests in uniquely rich ways. Depending on whether a symmetry is satisfied on average by the ensemble or strictly by each individual pure state component, the concept of SSB is generalized into strong to weak spontaneous symmetry breaking (SWSSB). We will discuss the appropriate order parameters designed to diagnose these distinct symmetry-breaking patterns. Finally, we will illustrate the physics of SWSSB with concrete examples, focusing on a 1+1D Luttinger liquid and a 2+1D transverse-field Ising model subjected to decoherence.
References:
[1] S. Sang, Y. Zou, and T. H. Hsieh, Mixed-State Quantum Phases: Renormalization and Quantum Error Correction, Phys. Rev. X 14, 031044 (2024).
[2] L. A. Lessa, R. Ma, J.-H. Zhang, Z. Bi, M. Cheng, and C. Wang, Strong-to-Weak Spontaneous Symmetry Breaking in Mixed Quantum States, PRX Quantum 6, 010344 (2025).
[3] Y. Guo, S. Yang, and X.-J. Yu, Quantum strong-to-weak spontaneous symmetry breaking in decohered one-dimensional critical states, PRX Quantum 6, 040311 (2025).
[4] Y.-M. Ding, Y. Guo, Z. Bi, and Z. Yan, Strong-to-Weak Spontaneous Symmetry Breaking in a 2+1 D Transverse-Field Ising Model under Decoherence, arXiv:2603.24342 (2026).