Production Of Metrologically Useful Entangled Atomic Condensates And Their Interferometric Measurements

Interferometry is a paradigm for most precision measurements.Using N uncorrelated particles,the achievable precision of an interferometer is bounded by the standard quantum limit(SQL),1/√N,due to discrete nature of individual measurement.However,this “classical” bound is not the fundamental limit and the aim of quantum metrology is to overcome the SQL by employing quantum entangled states,such as squeezed light,atomic spin squeezed state and Dicke state.The main focus of this thesis is the generation of metrologically useful entangled atomic states and the subsequent demonstrations of quantum enhanced measurement precision using the prepared states.In the first part,we report on the generation of a twin-Fock state in a near deterministic way,by driving a spin-1 Bose-Einstein condensate(BEC)undergoing spin mixing through two consecutive quantum phase transitions.Compared with the previous experiments for generating twin-Fock states by spin dynamics,our method provides a much improved conversion efficiency(96%)(which is defined as the ratio between the number of atoms transferred to entangled states and the total atom number in the initial uncorrelated state)and smaller fluctuations(2%)on the total atom number of the twin-Fock state.By measuring the number squeezing parameter(10.7 dB)and the normalized collective spin length(0.99),we infer an entanglement breadth of 910 atoms for prepared samples.In the second work,we prepare the balanced spin-1 Dicke state for the first time by driving a spin-1 BEC through one quantum phase transition.In contrast with the spin-1/2 entangled states,the spin-1 Dicke state is a three-mode entangled state,which promises an even better measurement precision by involving all three modes for interferometry.The experimentally prepared state is characterized to lie very close to the ideal balanced spin-1 Dicke state.Further more,using the prepared state,we demonstrate a three-mode interferometry with a precision of 2.42 dB beyond the three-mode SQL and 8.44 dB beyond the two-mode SQL.The measured sensitivity is mainly limited by the atom detection noise.Our experiment results highlight the power of quantum phase transitions for deterministic generation of metrologically useful entangled states.The prepared states constitute the ground states of the system and thus are stable,making them more advantageous for practical applications.In addition,the first creation of the blanced spin-1 Dicke state provides new possibilities for quantum metrology.