Seminar of Department of Theoretical Physics - Hamed Mohammady (19.11.2024)
Tuesday 19.11.2024 at 14:00, Lecture room F2/125
Hamed Mohammady:
Physical constraints on the quantum measurement process
Abstract:
In the general framework of operational quantum physics, the observables of a quantum system are represented by normalised positive operator valued measures (POVMs) which, together with states, produce the probability distributions obtained when measuring a system. Similarly, the so-called state-update rule is characterised by instruments, which transform the system to be measured in such a way so as to be compatible with the observed measurement statistics. Instruments that reveal non-trivial information about the measured system necessarily disturb at least some property of the system: there is no information without disturbance. However, quantum theory allows for some observables to be measured in a "minimally invasive" fashion, so that at least the measured observable itself is not disturbed by the act of measurement. Such measurements are referred to as "measurements of the first kind", which include the standard formulation of measurements in textbook quantum mechanics. But assuming the universal validity of quantum theory, a quantum system of interest is measured when it undergoes an appropriate interaction with another quantum system playing the role of a measuring apparatus. It stands to reason, therefore, that the measurement process must be compatible with other known physical laws aside from quantum theory: for example, conservation laws and the laws of thermodynamics. In this talk, I will discuss how conservation laws, as well as the second and third laws of thermodynamics, impose fundamental constraints on the observables that can be measured, and their commensurate disturbance, in practice.