RQC Colloquium

35th RQC Colloquium

• Speaker

  Prof. Kim Yoon-Ho
  (Department of Physics, Pohang University of Science and Technology, Korea)

• Date

  16:00-17:00 (JST), May 29, 2025 (Thursday)

• Venue

  Hybrid (Zoom・Wako C00 HQ Building, 2F Large Meeting Room)

• Title

  Learning more about Quantum Systems via Weak Interactions and Measurements

• Inquiries

  rqc_colloquium_inquiry[at]ml.riken.jp

• Registration Form

  https://krs2.riken.jp/m/rqc_registration_form

Abstract
Information on the state of a quantum system is obtained through measurement. The projection postulate stipulates that a quantum system is irrecoverably collapsed into one of the eigenstates of the observable, resulting in maximum state disturbance. Such a measurement is known as a projection or von Neumann measurement. A more general quantum measurement based on a weak interaction between the quantum system and the measuring apparatus is known as a weak measurement. In this talk, I will discuss how weak interactions and measurements may be used to learn more about quantum systems than we would with projection measurements.
Unlike projection measurement, weak measurement allows minimum-disturbance measurement, in which maximal information gain is achieved by minimally disturbing a quantum state [1]. Moreover, weak measurement may be reversed through the process of reversal measurement. Such a weak-reversal measurement pair has an interesting application in quantum information, as it can negate the effect of decohierence, even protecting entanglement from highly decoherent noisy channels [2, 3]. A sequential application of weak and projection measurements leads to the weak value, which is not bounded by the eigenvalue spectrum of the associated observable. By applying multiple weak interactions sequentially, we can measure the so-called sequential weak value, and the sequential weak value of two incompatible observables is particularly important in quantum information, as it can be used to directly quantify a quantum process [4]. Also, by carefully changing the interaction strengths of the sequential weak interactions, it can be shown that the emergence of a geometric phase in quantum systems is due to quantum measurement back-action; the stronger a quantum measuremnt, the larger the accumlated geometric phase [5]. Finally, I will introduce a novel concept of the metrological weak value is valid for arbitrary interaction strengths and, therefore, can be used to measure an arbitrary interaction strength, making it an important tool in quantum metrology [6, 7].

[1]H.-T.Limetal.,PhysicalReviewLetters113,020504(2014).
[2]Y.-S.Kimetal.,NaturePhysics8,117(2012).
[3]J.-C.Leeetal.,NatureCommunications5:4522(2014).
[4]Y.Kimetal.,NatureCommunications9:192(2018).
[5]Y.-W.Choetal.,NaturePhysics15,665(2019).
[6]Y.Kim,S.-Y.Yoo,andY.-H.Kim,PhysicalReviewLetters128,040503(2022).
[7]S.-Y.Yooetal.,QuantumScienceandTechnology10,025054(2025).
Flyer: 35th RQC Colloquium Flyer