Seminar

Monday, April 13, 2026, from 2:45 PM

Room 826, West Building 2

Philip Taranto (The University of Manchester)　

Higher-Order Quantum Operations

Quantum operations form a fundamental pillar of quantum theory and play a central role in quantum technology. Traditionally, quantum operations were regarded solely as devices for transforming quantum states, such as quantum communication channels between distant parties or quantum gate elements in a quantum circuit. However, quantum operations themselves can also undergo transformations and thus be considered inputs to higher-order quantum operations (HOQOs). From a broad perspective, HOQOs are arbitrary transformations between quantum objects, beyond states and operations: For instance, transformations between unitary gates, quantum channels, measurements, quantum circuits, multi-time processes, and others.
The HOQOs approach has led to powerful mathematical methods and a solid framework for analysing quantum circuits and addressing questions such as estimating, discriminating, and learning quantum operations and quantum measurements, analysing and formalising non-Markovian processes and quantum memory. Beyond its practical applications, HOQOs also provide a framework for addressing foundational questions in quantum theory. Quantum operations exhibit a richer structure than quantum states, particularly due to their functional nature, which involves a clear distinction between input and output. Consequently, when considering transformations between two or more operations, questions regarding causality naturally arise. This opens the possibility for quantum processes with indefinite causality and raises fundamental questions about how causality should be understood in quantum theory.
This talk will provide a guide to HOQOs, covering the mathematical requirements and central concepts and definitions. The methods are followed by various examples that aid in the comprehension of core ideas and illustrate their pivotal role in various applications. After an initial pedagogical part, this work illustrates the power and versatility of the HOQO approach by presenting known applications to multiple branches of quantum theory, ranging from applications to foundations.

Monday, March 23, 2026, from 2:40 PM

Room 826, West Building 2

M. Hamed Mohammady (Research Centre for Quantum Information, Institute of Physics, Slovak Academy of Sciences)　

Wigner-Araki-Yanase theorems for general quantum measurements

According to the unitary interaction based model of quantum measurement dating back to von Neumann, a sharp (projective) observable is measured in a quantum system by a unitary premeasurement interaction between the system being measured and a quantum measuring apparatus, followed by objectification of the apparatus with respect to a pointer observable. In many physically relevant scenarios, however, the premeasurement interaction is constrained by an additive conservation law. That is, the interaction conserves some total quantity of the system and apparatus such as energy, charge, or angular momentum. The Wigner-Araki-Yanase (WAY) theorem states that in such a case, if either (i) the measurement is “repeatable”, i.e., repeated measurements are guaranteed to produce the same outcome, or (ii) the pointer observable commutes with the apparatus part of the conserved quantity, referred to as the “Yanase condition”, then the observable measured in the system must commute with the system part of the conserved quantity. However, provided that the apparatus is prepared in a state with sufficiently large coherence or asymmetry with respect to the conserved quantity, then approximately accurate and repeatable measurements of observables not commuting with the conserved quantity are not ruled out. In this talk, I will present several extensions of the WAY theorem for general quantum observables, represented as positive operator valued measures (POVMs), and general premeasurement interactions, represented as completely positive trace preserving maps. In particular, I will show that the strict impossibility part of the WAY theorem is more properly understood as not pertaining to sharpness of the measured observable, but rather as its “definitiveness”, i.e., the property that for some states it is possible to predict with certainty that a given outcome of measurement obtains, or does not obtain. 

Tuesday, October 14, 2025, from 2:40 PM

Room 826, West Building 2 (4th Conference Room)

Seok Hyung Lie（Ulsan National Institute of Science and Technology）　

Thermal operations from informational equilibrium

Thermal operations are quantum channels that have taken a prominent role in deriving fundamental thermodynamic limitations in quantum systems. In this talk, we show that these channels are uniquely characterized by a purely quantum information theoretic property: They admit a dilation into a unitary process that leaves the environment invariant when applied to the equilibrium state. In other words, they are the only channels that preserve equilibrium between system and environment. Extending this perspective, we explore an information theoretic idealization of heat bath behavior, by considering channels where the environment remains locally invariant for every initial state of the system. These are known as catalytic channels. We show that catalytic channels provide a refined hierarchy of Gibbs-preserving maps for fully-degenerate Hamiltonians, and are closely related to dual unitary quantum circuits.

Friday, September 12, 2025, 3:00 PM

Room 710, West Building 2

Shunsuke Kamimura（NEC）

ボソニック量子回路における一次相転移の理論

ジョセフソン接合によって構成される超伝導量子干渉計(Superconducting QUantum Interference Device, SQUID)や、その一般化である超伝導非線形非対称誘導素子(Superconducting Nonlinear Asymmetric Inductive eLement, SNAIL)は、超伝導検出器やマイクロ波増幅器に用いられ、また超伝導量子コンピュータの研究においても欠かすことのできない基本構成要素である。特に、多準位性が無視できない超伝導量子回路である「ボソニック量子回路」においてSNAILを用いることにより、連続変数万能量子計算を実現できることが知られており、このような非線形性の強い超伝導量子回路を調べることは基礎理論的にも重要である。

本セミナーでは、ジョセフソン接合の量子化や超伝導回路の量子力学的取り扱いについて、（発表内容に関連する事項に限って）可能な限り詳しく説明することから始める。超伝導量子回路は一見特殊な物理系であるが、その設計自由度の高さから、実際に実験室で扱える量子系として様々な可能性を秘めた系である。特に、本発表の主題であるSNAILについて、最新の実験報告などを交えて解説する。

その後、SNAILを含むボソニック量子回路によって、$p$スピン模型と呼ばれるある種の全結合量子スピン模型や、そこにおいて実現される一次相転移を模擬できることを議論する。$p$スピン模型とは、主に量子スピン系や量子アニーリングの文脈において調べられている模型であり、$p$体全結合相互作用を持つ模型である。ボソンをフェルミオンに（またはフェルミオンをボソンに）変換する一手法であるHolstein–Primakoff (HP) 変換を用いることにより、SNAILを含むボソニック量子回路は$p=3$の$p$スピン模型に対応付けられることを示す。ボソニック量子回路の基底状態は、駆動マイクロ波周波数がある値を超えたところで真空状態から有限光子数状態へと変化するが、これは$p$スピン模型の基底状態において磁化が不連続に変化する一次相転移に対応する。HP変換ではボソンの光子数に上限を設けるが、このカットオフ光子数はHP変換後のスピン模型におけるスピン数に対応する。

本発表の手法によって、$p$スピン模型の相転移を実験的に調べることが可能となる。また、一次相転移の急峻な振る舞いを用いる量子センシングやマイクロ波増幅などの量子技術への応用も期待される。