When do particles have no kinetic energy?

Answer 1

At zero degrees kelvin, absolute zero. This is also the temperature at which some elements display the Bose-Einstein condensate, theorized by Einstein but not demonstrated until decades after his death.

Answer 2

Absolute zero, or "zero Kelvins".

(Picky point: The units of some other temperature scales are the Celsius degree,

the Fahrenheit degree, the Rankine degree, etc. But on this particular absolute

scale, the unit is the "Kelvin". The Kelvin and the Celsius degree have the same

size, and the zeros of their usual scales are different by 273.15 of either one.)

Answer 3

0 Kelvin