It is impossible to reach the absolute zero temperature.

The physical behavior of all materials is described by a theory called "Thermodynamics", which is based on three fundamental postulates known as the "basic laws of thermodynamics". These laws cannot be proved. They are basic assumptions that we accept in order to explain the multitude of phenomena in materials science.

The unattainability of absolute zero is one of these basic law (the third law). The other laws are the conservation of energy (the first law), and the fact that the entropy of the universe cannot decrease (the second law).

The "world record" for the lowest temperature is achieved using "laser cooling" techniques, which can bring small systems (of the size of about 1 billion atoms) down to 1 billionth degree Kelvin.

It is the heat input from the outside world (or "heat leak") into an experiment which prevents further cooling.

The third law of thermodynamic can be also related to quantum mechanics (which governs the behavior of law temperature systems).

By definition, absolute zero is the temperature at which all molecular motion ceases and no energy present. But quantum mechanics (Heisenberg Uncertainty Principle) states that even at absolute zero some energy must be present.