Detection of Particles Under Potential Barrier
  We introduce a model detector which registers the passage of a particle through the detector location, without substantially perturbing the particle wave function. (The exact time of passage is not determined in such measurements.) We then show that our detector can operate in a classically forbidden region and register particles passing through a certain point under a potential barrier. We show that it should be possible to observe the particle's track under the barrier. 
Alexander Vilenkin and Serge Winitzki