自旋压缩态特有的量子关联和纠缠性质对量子信息和量子测量具有重要的影响。前几年，两个实验小组在Nature杂志上发表文章，分别报道他们通过调控两分量玻色-爱因斯坦凝聚体中原子的相互作用，实现单轴扭曲压缩到-8.2dB和-2.5dB。最近，又有三个实验小组分别在Science， Nature，和Nature Physics上报导了利用旋量凝聚体原子形成的类双数态压缩了原子数涨落的量子噪声。在本项目中，我们基于利用简单的外加自旋控制脉冲成功实现了单轴扭曲到双轴扭曲转换的想法，深入研究超越量子噪声标准极限的机制和调控方法。具体包括以下内容：1）探索其它形式的脉冲序列，以进一步降低量子噪声，并同时抑制量子退相干；2）探讨粒子数损耗、原子碰撞等退相干因素对原子自旋压缩态或多体纠缠态的影响；3）研究如何通过多份量凝聚体的自旋混合产生双数态，实现SU(1,1)干涉仪，达到和双轴扭曲压缩可比甚至更高的精度

Squeezed spin states (SSSs) are entangled quantum states of a collection of spins in which correlations among individual spins reduce the quantum uncertainty of a particular spin component below the classical limit for uncorrelated particles. Research on SSSs is a topical area due to its significant applications in high-precision measurements and in quantum information science. In back-to-back publications from several years ago, reduced spin fluctuations are observed leading to spin squeezing at -8.2dB and -2.5dB, respectively, in two-component atomic condensates exhibiting one-axis-twisting(OAT) interactions. The theoretical limit of spin squeezing is 1/N^(2/3), with N the total number of atoms for the OAT model, and 1/N for two-axis-twisting (TAT) model. Subsequently, we proposed a readily implementable idea transforming the OAT model systems into effective TAT spin squeezing, leading to a Heisenberg limited noise reduction∝1/N. More recently, three groups reported new experiments observing below shot-noise limit fluctuations in atom numbers making use of paired atom states, or the so called Twin Fock like states produced from atomic spin mixing in spinor condensates. In this project, we propose to investigate schemes beyond standard quantum limit. It contains the following details: 1) Investigate other pulse sequences to further suppress quantum noise and quantum decoherence. 2) Discuss the influence of decoherence factors, including particle loss, atomic collision, on spin squeezed states or other potentially useful many body entangled states. 3) Explore how to use multi-component BEC to realize SU(1,1) interferometer to obtain sensitivities comparable to that of TAT or even higher.