This is bizarre to contemplate with everyday objects– a dog isn’t going to quantum tunnel through a fence into the neighbors’ yard (though a bad dog might classically tunnel through the dirt under the fence…). This has a surprisingly direct application to an everyday technology, though, in the form of a smoke detector. One of the two main processes used to make modern smoke detectors uses a small amount of radioactive americium to ionize air molecules, producing a steady flow of current between two plates inside the detector. When heavy smoke particles enter the region between the plates, they also get ionized by the radiation, but are too heavy to be deflected into the plates, and the current flow drops, triggering the alarm.

The particular decay channel than makes americium useful for this application is an “alpha decay,” in which a very heavy nucleus decays by spitting out a helium nucleus: two protons and two neutrons tightly bound together. This process initially seemed mysterious, because the energy of the emitted alpha particle is much lower than the energy needed to shoot an alpha particle into the nucleus. The colorful Russian theorist George Gamow realized, though, that this is exactly what you expect from quantum tunneling: you can think of the nucleus as a collection of alpha particles held inside the nucleus without enough energy to escape, but thanks to quantum tunneling they have a tiny chance of escape.

The chance is really tiny– the half-life of americium is several hundred years, and an estimate of how many times an alpha inside the nucleus would hit the barrier in an attempt to leave is something like 10²⁰per second. The process works, though, and thus quantum physics plays an essential role in protecting your stuff.