Ultracold atoms in 1-D optical lattices: experiments towards quantum chaos and atom interferometry

Abstract

The present thesis aims at addressing two major problems using the interaction of ultracold atoms with 1-D lattice. The initial goal was to set up a cross-optical dipole to generate a Bose-einstein condensate. This would provide a basic platform in order to realize the problems of our interest. An existing experimental set up capable of producing laser-cooled atoms was used to realize the Bose-Einstein Condensation in an optical dipole trap. The problems addressed using this system are as follows: (a) Demonstration of non-exponential quantum decoherence in a Levy kicked rotor system. and (b) Realization of diffraction of an atom laser in Raman-Nath regime. A major part of this thesis was to demonstrate experimentally the non-exponential decoherence in a Levy kicked rotor system. This experiment addresses one of the fundamental problems in quantum mechanics, known as decoherence. In general, quantum systems lose coherence exponentially fast upon interacting with the environment, and this puts a limititations on some technological applications using Quantum systems, for e.g. on the Quantum gate-operation time and efficiency in Quantum information processing and precision metrology. In this work, we used an atom optics delta-kicked rotor subjected to non-stationary Levy noise in the kicking sequence and demonstrated slower than exponential decoherence manifested in the form of sub-diffusion in the mean energy growth of the rotor. To realize the system experimentally using cold atoms, we used a cold cloud of atoms having Maxwell-Boltzmann distribution in momentum space and made it to interact with a pulsed 1-D optical lattice to realize the kicking sequence. By controlling the kicking sequence, we studied the decay of coherence in the system. This work is extremely relevant in the context of understanding the dynamics in disordered nonlinear lattices and diffusion in chaotic quantum systems. This work will also be helpful in understanding fundamental problems related to quantum mechanics and non-linear dynamics of chaotic systems. Another principal objective of this thesis work was to generate a slow coherent beam of ultracold atoms (known as atom laser) from the reservoir of BEC and demonstrate the diffraction of an atom laser. We form the atom laser by outcoupling a quasi-continuous beam of coherent atoms from a reservoir of $^{87}$Rb BEC by spilling method. The flux of the outcoupled atoms is controlled to achieve a continuous wave atom laser lasting up to 400 ms. The diffraction of this atom laser in Raman-Nath regime is realized using a grating formed by a standing wave of far-detuned laser light. By controlling the interaction of the atom laser and light-grating, the strength of the diffraction into various orders is precisely controlled, and momentum up to $\pm$18$\hbar$k is imparted. Such diffraction would allow for further construction of an atom-interferometer to probe changes in physical environments continuously up to a few hundred milliseconds. This would provide a novel platform to construct atom interferometers which can provide measurement with high bandwidth and free from aliasing effects.