Temperature doesn't really make sense for one particle. In fact any one particle will have 0 kinetic energy in some frame of reference. The third law of thermo

Temperature doesn't really make sense for one particle. In fact any one particle will have 0 kinetic energy in some frame of reference. The third law of thermodynamics actually states that there's no systematic way that you can reach 0K, although I haven't found a particularly thorough explanation why. In general it seems to boil down to the fact that no matter which parameter you try to tune there will always be some entropy remaining, so you can't reach T=0 (zero entropy) in a finite number of steps. And then you have quantum mechanical effects that prevent a particle from actually having exactly 0 kinetic energy.