In 1993, Yakir Aharonov, Lev Vaidman and Jeeva Anandan discovered an important new method of measurement in quantum mechanics, the so-called protective measurement. Distinct from conventional measurements, protective measurement is a method for measuring the expectation value of an observable on a single quantum system. By a series of protective measurements, one can even measure the wave function of a single quantum system. In this way, theoretical analysis of protective measurement may lead to a new and deeper understanding of quantum mechanics. Moreover, its experimental realization may also be useful for quantum information technology.

This book is an anthology celebrating the 20th anniversary of the discovery of protective measurement. It begins with a clear and concise introduction to standard quantum mechanics, conventional measurement and protective measurement, and contains fourteen original essays written by physicists and philosophers of physics, including Yakir Aharonov and Lev Vaidman, the two discoverers. The topics include the fundamentals of protective measurement, its meaning and applications, and current views on the importance and implications of protective measurement. The book is accessible to graduate students in physics and chemistry. It will be of value to students and researchers with an interest in the meaning of quantum theory and especially to physicists and philosophers working on the foundations of quantum mechanics.

When I contacted potential contributors to this anthology, one of them replied, “Protective measurements are something I know nothing about.” Indeed, as one referee of this book also admitted, although protective measurement has attracted attention over the last 20 years and has raised many interesting questions, it is still an under-studied aspect of quantum mechanics. In recent years the associated field of weak measurement has seen significant increased activity, and the latest Pusey–Barrett–Rudolph theorem has also caused many people to revisit the question of the reality of the wave function.