Bristol Benjamin Meaker Distinguished Visiting Professor Lev Vaidman, Tel Aviv University, Israel

Research on the foundations of quantum mechanics and quantum information

1 May - 31 July 2024

B‌iography

Professor Vaidman is one of the leading physicists in the field of foundations of quantum mechanics. He not only gave significant inputs for the understanding of quantum mechanics, but proposed several applied concepts in quantum optics and quantum information which became the basis of practical implementations in laboratories around the world. One of his many seminal contributions is the extension of the fundamental concept of quantum teleportation to continuous variables, which allowed to apply teleportation to much more complex and general systems. Furthermore, he contributed an essential and practically useful element to the toolbox of quantum cryptography. He derived a unique quantum key distribution protocol which, in contrast to all other previous protocols, relies on orthogonal quantum states while still retaining a fundamentally secure status. Recently, he himself became involved with experimental realizations of his ideas, collaborating directly with several experimental groups. In particular, he led experimental demonstrations of his widely discussed novel approach to analysis  of the past of quantum particles. By developing new concepts and types of quantum measurements – nonlocal, weak, protective and interaction-free – Lev Vaidman shaped the course of the debate about the status of properties of physical systems and the status of measurements in quantum mechanics. Lev Vaidman initiated arXiv of quantum physics which revolutionized the way the science is developed and today he is a Chief Editor of the MDPI journal Quantum Reports, managing editor of the Springer journal Quantum Studies and involved in editorial work in other journals of quantum physics. He published more than hundred seventy papers including about twenty publications in Physical Review Letters. Lev Vaidman is a professor of physics and holds the Alex Maguy-Glass Chair in Physics of Complex Systems at Tel Aviv University.

Research Summary

Profeesor Vaidman is one of the main developers of the two-state vector formalism of quantum mechanics originated by Yakir Aharonov who was both Sandu Popescu's and Vaidman’s Ph.D. advisor. According to this approach a quantum system between two measurements is described by both forward and backward evolving quantum states. The main concept of the formalism, the weak value, continues to be in the front of conceptual and practical development of quantum mechanics, revolutionising foundations as well as  technological applications of quantum mechanics. We plan to develop the formalism focusing on the following questions:

Characterization of  pre and postselected systems  with separate pre and postselection of different degrees of freedom.
Analysis of the modification of effective interaction between two systems which are both pre and postselected.
Adding entanglement between the two systems in their pre and postselection.
Development of the concept of robust weak value which will help understand better the reality of the past of a quantum particle between two measurements.
Analysis of the hot controversial topic of counterfactual quantum communication.

These subjects are also in the field of interest of the philosopher Professor James Ladyman who will take part in the project.

We plan to clarify Professor Vaidman's far reaching claims about the locality of the Aharonov-Bohm effect which are at the center of Professor Popescu's research and touch upon the expertise of Sir Michael Berry.

Professor Vaidman continues the tradition of Aharonov in developing physics through paradoxes. He teaches a graduate course on paradoxes of quantum mechanics in Tel Aviv university. We plan to organise such a mini course in Bristol which will deepen the understanding of quantum mechanics of our students.

Professor Vaidman is hosted by Professor Sandu Popescu in the School of Physics.

Planned lectures and seminars include:

• Open lecture
  
  From quantum miracles to many worlds
  
  Action at a distance is a miracle. Quantum phenomena such as interaction-free measurements, teleportation, Bell-type correlations apparently require such miracles. Accepting the many-worlds interpretation of quantum mechanics avoids the necessity of action at a distance and thus removes paradoxes from quantum theory.
  
  
• School lecture (aimed at graduate students and staff in the School of Physics and Department of Philosophy)
  
  The past of a quantum particle
  
  Textbooks of quantum mechanics lack the concept of the past of quantum particles. A few years ago, I proposed to define the past of a quantum particle according to the trace it leaves. While in many cases this definition provides a reasonable description, for a nested Mach-Zehnder interferometer it leads to a picture seemingly contradicting common sense: the particle leaves a trace in a place through which it could not pass. I will discuss recent theoretical and experimental developments.
  
  
• School of Physics lecture (graduate students welcome to attend)
  
  Weak value: a property of a single pre and postselected system
  
  The weak value of a quantum variable was introduced by Aharonov, Albert and Vaidman in 1988 as an outcome of a standard quantum measurement with weakened coupling performed on an ensemble of pre and post-selected systems. In this lecture it will be argued that it has a deeper meaning as a robust property of a single pre and post-selected system. Experimental demonstrations supporting this claim and an application of a new paradigm of the past of a quantum particle as the weak value of a projection for a novel alignment method will be reported.