system
Given the following post, respond as if you're commenting on Reddit:
human
How are extremely low temperatures measured?

Such as when MIT cooled molecules in a gas of sodium potassium to a temperature of 500nK.

Non-contact measurements systems like lasers would impart heat, so what is used?
gpt
The most common technique for cold atoms/molecules is called time of flight imaging. For a gas of molecules, the kinetic energy distribution is a function of temperature. The gas is confined to a small space, then it is released for a set amount of time, and then a picture is taken (with resonant laser light). Based on how far the molecules have traveled, we can infer their velocities and thus the temperature.