Atomic Tunneling off a STM/AFM Tip

Mark Stiles' Phonons give an Ohmic Coupling. (Louis; 73)

I was writing my thesis, coupling phonons to quantum tunneling, just when Leggett and Caldeira came out with their ``macroscopic quantum tunneling'' ideas. They coupled harmonic oscillators (phonons) to quantum tunneling, but made the coupling to low--frequency phonons much larger than I could get from my microscopic models. By doing so, their model developed lots of bizarre behavior: later work showed that as the coupling was increased, there was a phase transition at which the tunneling stopped (the environment ``collapsed the wavefunction'', keeping the particle in one of the two potential wells). Around this time, Mark Stiles was writing his thesis on helium scattering off surfaces, and I was pestering him about his phonon coupling: his helium atoms coupled just like Leggett's heat bath, which seemed obviously unphysical and likely to lead to weird answers... Finally a couple of years ago, in a conversation with Risto Nieminen in Helsinki, I finally understood: atoms hitting from above could transfer a net momentum (force monopole), where my atoms tunneling inside the material could not (force dipole). This implied that an atom tunneling to and fro from a scanning tunneling microscope (STM) tip would be a microscopic system where phonons would give Tony's weird effects. Ard Louis transformed this idea into a proposal for an experiment, which although ambitious (a low--temperature AFM/STM connected via strip lines to a picosecond laser) should be able to see the phase transition in the quantum coherence predicted by theory. Schrödinger's cat now does not need to couple to a metal to have its wavefunction collapsed: it can just brush up against the surface of the box.

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