You are cordially invited to the SCL seminar of the Center for the Study of Complex Systems, which will be held on **Wednesday**, **16** **October** **2019** **at** **14:00** in the library reading room “Dr. Dragan Popović” of the Institute of Physics Belgrade. The talk entitled

Time and clocks in quantum theory

will be given by Dr. Sandra Stupar (ETH Zurich, Switzerland).

**ABSTRACT:**

Time in classical theory is absolute and global, and usually taken as a time parameter. On the other hand, in quantum mechanics there is no time operator, and quantum time as such does not exist, hence the classical time parameter is used in quantum evolution equations. But where does this notion of classical time physically arise from? And would it be possible to introduce very precise and well-synchronised quantum clocks in a similar way as classical clocks are used? In this talk we will start from the assumption that the only observable time is the one given by clocks, and hence to learn more about quantum time we need to build good quantum clocks and try to synchronize them. We will define clocks as quantum systems that need to provide a time reference, such as a sequence of ticks. Further, we will attempt to find limits on their synchronization, since it is well known that quantum systems interact with their environment thereby getting disturbed. What we will find out is that the performance of the quantum clocks we define depends crucially on their dimension (number of the distinguishable states available), and that the quantum clocks of a certain dimension could possibly perform better than the classical ones with the same state space size. The limitations to synchronizing local quantum clocks suggest that global time in quantum mechanics might not be a physically grounded notion. We will also further briefly discuss what limitations would be imposed on the evolution of our Universe, if it itself was/contained a quantum clock providing ticks that we observe as time.