# IQI Weekly Seminar

Tuesday, April 12, 2016
3:00pm to 4:00pm
Annenberg 107
Quantum thermodynamics for very small machines
Stephanie Wehner, QuTech, University of Delft,

The laws of thermodynamics as we know them have originally been conceived to study the properties of large machines such as steam engines. Over time, statistics helped us justify these laws, where the law of large numbers allowed us to make statements about machines that indeed consist of a large number of particles. When machines become extremely small, however, not only are we unable to use such statistical methods, but quantum mechanical effects become relevant. What can we say about thermodynamics in such regimes?

We start by briefly reviewing a useful tool, a second law that is valid for even the smallest quantum machines. We will then proceed to apply this tool to the study of the maximum efficiency of a heat engine, known as the Carnot efficiency. For large machines, this efficiency famously only depends on the temperatures on the heat baths we use . In contrast, we will see that at the nano scale, Carnot's law needs to be revised in the sense that more information about the bath other than its temperature is required to decide whether maximum efficiency can be achieved. In particular, we derive new fundamental limitations of the efficiency of heat engines at the nano and quantumscale that show that the Carnot efficiency can only be achieved under special circumstances, and we derive a new maximum efficiency for others.

We conclude by discussing some of the many open challenges in quantum thermodynamics.