# Finding particles in the classically forbidden regions [duplicate]

Have particles ever been found in the classically forbidden regions of potentials?

For example, in a square well: has an experiment been able to find an electron outside the rectangular well (i.e. in the exponential fall-off regions) ?

And more importantly, has anyone ever observed a particle while tunnelling?

There are numerous applications of quantum tunnelling. A few that pop in my mind right now are:

The answer is unfortunately no. This is impossible as particles are quantum objects they do not have the well defined trajectories we are used to from Classical Mechanics. A particle has a probability of being in a specific place at a particular time, and this probabiliy is described by the square of its wavefunction, i.e $|\psi(x, t)|^2$. The same applies to quantum tunneling. A particle has a certain probability of being observed inside (or outside) the classically forbidden region, and any measurements we make will only either observe a particle there or they will not observe it there. You simply cannot follow a particle's trajectory because quite frankly such a thing does not exist in Quantum Mechanics.