Why we can't use quantum tunnelling still in nuclear fusion where sun uses this trick of nature easily? Sun haven't enough temperature to break down coulomb barrier between two protons to be fussed.But yet nuclear fusion occurs through quantum tunnelling process.So,why we can't use quantum tunnelling in our nuclear fusion technology same as sun do still now?
 A: Here is what tunneling is about, simplified:


According to classical physics, a particle of energy E less than the height U0 of a barrier could not penetrate - the region inside the barrier is classically forbidden. But the wavefunction associated with a free particle must be continuous at the barrier and will show an exponential decay inside the barrier. The wavefunction must also be continuous on the far side of the barrier, so there is a finite probability that the particle will tunnel through the barrier.
As a particle approaches the barrier, it is described by a free particle wavefunction. When it reaches the barrier, it must satisfy the Schrodinger equation in the form .

This is a very simple picture of a free particle approaching a barrier, and nuclei are strong potential wells, so the wavefunctions have to be calculated or at least estimated.  Yes, theorists have done the calculations:
For example here 

arXiv:nucl-th/9708036v1  19 Aug 1997
  Quantum Tunneling in Nuclear Fusion 

where the theory is developed in chapter:

III MULTIDIMENSIONAL QUANTUM TUNNELING IN NUCLEAR PHYSICS

Actually three different calculations are offered.
So the answer is that tunneling calculations are used for fusion processes. 
