  In the usual quantum tunneling, a low-energy quantum particle penetrates
across a physical barrier of higher potential energy, by traversing a
classically forbidden region, and finally escapes into another region. In an
analogous scenario, a classical particle inside a closed regular region in the
phase space is dynamically bound from escaping to other regions of the phase
space. Here, the physical potential barrier is replaced by dynamical barriers
which separate different regions of the phase space. However, in the quantum
regime, the system can overcome such dynamical barriers and escape through
them, giving rise to dynamical tunneling. In chaotic Hamiltonian systems,
dynamical tunneling refers to quantum tunneling between states whose classical
limit correspond to symmetry-related regular regions separated by a chaotic
zone between which any classical transport is prohibited. Here, an experimental
realization of dynamical tunneling in spin systems is reported using nuclear
magnetic resonance (NMR) architecture. In particular, dynamical tunneling in
quantum kicked tops of spin-1 and spin-3/2 systems using two- and three-qubit
NMR registers is investigated. By extracting time-dependent expectation values
of the angular momentum operator components, size-dependent tunneling behaviour
for various initial states is systematically investigated. Further, by
monitoring the adverse effects of dephasing noise on the tunneling
oscillations, we assert the importance of quantum coherence in enabling
dynamical tunneling.
