Abstract

I will describe the qualitative features of our three-decades-long experimental and theoretical quest to identify emergent phenomena within the quantum dynamics of spin and charge excitations. A quantum phase transition occurs as an analytical discontinuity of a physical observable, as was observed in our work on the Anderson metal-insulator transition in 1D polymers. However, the clearest phase transition manifested when we observed two interacting nuclear spins undergoing Rabi oscillations in the presence of a spin environment. This, according to the Fermi Golden Rule, results in an imaginary energy in a 2x2 non-Hermitian effective Hamiltonian. The oscillations became a purely exponential decay when their coupling strength fell below a critical value, a form of Quantum Zeno effect. Our first experimental hint that many-body interactions could lead to irreversible dynamics, appeared when we confronted insurmountable limitations in performing a perfect time-reversal procedure, as discussed by Boltzmann and Loschmidt, even in a fairly well-controlled setting of nuclear spins. After a decade of work, we experimentally observed a phase transition to an intrinsically irreversible regime in the thermodynamic limit. However, its analytical proof has eluded us so far. More recently, we observed a striking universal stability of diffusive 1D systems to decoherence. This gives a new light to the “poised realm” hypothesis, promoted even for biological systems, that being at the edge of chaos is favorable to charge or excitonic transport – as pointed out by R. Laughlin, classical chaos can lead to a form of quantum dynamics extremely robust against environmental noise.