Quantum trajectories, new insights and applications

Speaker: Klaus Mølmer, Niels Bohr Institute, University of Copenhagen
Series: Cavendish Quantum Colloquium
Note: Note unusual week

From the early days of quantum mechanics until today, physicists have discussed the role played by probabilities and the nature of the collapse of the state of quantum systems subject to measurement. As long as experiments dealt only with large ensembles of particles, questions about the random nature of single hypothetical measurements were mostly of philosophical interest and were sometimes even discouraged with the advice to “shut up and calculate”. From about 1980 and onwards, a variety of single quantum systems have become routinely subject to experimental investigation in the laboratory, and dynamical quantum trajectory methods in the form of stochastic master equations were developed to describe the behavior of monitored systems. With these approaches we can “shut up and simulate” fluctuating signal records. While they were not initially aimed at foundational questions, the quantum trajectory descriptions provide insights in almost every potential paradox in the quantum description. They do not settle any debate or dismiss any interpretation of quantum theory, but they emphasize differences between them very clearly and they show their individual advantages and shortcomings in different applications. In the talk, I shall review the quantum trajectory theory, and I shall show recent examples of how they pose questions and provide answers that even extend the concept of quantum states. These extensions have practical applications in experimental studies of quantum dynamics and in the use of quantum systems for sensing and metrology.