A single bosonic Josephson junction has been experimentally realized using two weakly coupled Bose-Einstein condensates in a double-well potential. The study reports the first observation of nonlinear tunneling and macroscopic quantum self-trapping in such a system. The density distribution of the condensates was measured in situ, and the relative phase between the two condensates was deduced from interference fringes. The results confirm the nonlinear generalization of tunneling oscillations in superconducting and superfluid Josephson junctions, as well as the occurrence of macroscopic quantum self-trapping, which inhibits large amplitude tunneling oscillations. Tunneling through a barrier is a fundamental quantum phenomenon. The Josephson effect, based on tunneling, has been observed in various systems, including superconductors and superfluid helium. In this experiment, a bosonic Josephson junction was created using two weakly coupled Bose-Einstein condensates in a macroscopic double-well potential. The interaction between the tunneling particles plays a crucial role, leading to new dynamical regimes. Anharmonic Josephson oscillations are predicted when the initial population imbalance is below a critical value. Above this threshold, large amplitude oscillations are inhibited due to macroscopic quantum self-trapping. The experiment involved creating Bose-Einstein condensates of rubidium atoms and adiabatically ramping up a periodic potential to form a double-well structure. The initial population imbalance was controlled by adjusting the asymmetry of the potential. The dynamics were observed by measuring the time evolution of the atomic density distribution and the relative phase between the condensates. The results showed that when the initial population imbalance was below the critical value, Josephson oscillations occurred. When it was above the threshold, macroscopic quantum self-trapping was observed, with the population imbalance remaining constant and the phase increasing monotonically. The experimental setup and results were compared with theoretical models, showing excellent agreement. The study provides insights into the dynamics of Josephson junctions and highlights the importance of atom-atom interactions in such systems. The findings have implications for quantum optics, including the generation of squeezed atomic states and entangled number states, as well as applications in atom interferometry. The detailed investigation of self-trapping could also test the validity of mean field descriptions in strongly nonlinear regimes.A single bosonic Josephson junction has been experimentally realized using two weakly coupled Bose-Einstein condensates in a double-well potential. The study reports the first observation of nonlinear tunneling and macroscopic quantum self-trapping in such a system. The density distribution of the condensates was measured in situ, and the relative phase between the two condensates was deduced from interference fringes. The results confirm the nonlinear generalization of tunneling oscillations in superconducting and superfluid Josephson junctions, as well as the occurrence of macroscopic quantum self-trapping, which inhibits large amplitude tunneling oscillations. Tunneling through a barrier is a fundamental quantum phenomenon. The Josephson effect, based on tunneling, has been observed in various systems, including superconductors and superfluid helium. In this experiment, a bosonic Josephson junction was created using two weakly coupled Bose-Einstein condensates in a macroscopic double-well potential. The interaction between the tunneling particles plays a crucial role, leading to new dynamical regimes. Anharmonic Josephson oscillations are predicted when the initial population imbalance is below a critical value. Above this threshold, large amplitude oscillations are inhibited due to macroscopic quantum self-trapping. The experiment involved creating Bose-Einstein condensates of rubidium atoms and adiabatically ramping up a periodic potential to form a double-well structure. The initial population imbalance was controlled by adjusting the asymmetry of the potential. The dynamics were observed by measuring the time evolution of the atomic density distribution and the relative phase between the condensates. The results showed that when the initial population imbalance was below the critical value, Josephson oscillations occurred. When it was above the threshold, macroscopic quantum self-trapping was observed, with the population imbalance remaining constant and the phase increasing monotonically. The experimental setup and results were compared with theoretical models, showing excellent agreement. The study provides insights into the dynamics of Josephson junctions and highlights the importance of atom-atom interactions in such systems. The findings have implications for quantum optics, including the generation of squeezed atomic states and entangled number states, as well as applications in atom interferometry. The detailed investigation of self-trapping could also test the validity of mean field descriptions in strongly nonlinear regimes.