We study the quantum phase transition from a superfluid to a Mott insulator of ultracold atoms in a three-dimensional optical lattice with adjustable filling factors. Based on the density-adjustable Bose–Einstein condensate we prepared, the excitation spectrum in the superfluid and the Mott insulator regime is measured with different ensemble-averaged filling factors. We show that for the superfluid phase, the center of the excitation spectrum is positively correlated with the ensemble-averaged filling factor, indicating a higher sound speed of the system. For the Mott insulator phase, the discrete feature of the excitation spectrum becomes less pronounced as the ensemble-averaged filling factor increases, implying that it is harder for the system to enter the Mott insulator regime with higher filling factors. The ability to manipulate the filling factor affords further potential in performing quantum simulation with cold atoms trapped in optical lattices.

我们研究了具有可调填充因子的三维光学晶格中从超流体到超冷原子的莫特绝缘子的量子相变。根据我们准备的密度可调的玻色-爱因斯坦冷凝物，用不同的集合平均填充因子测量了超流体和莫特绝缘子体系中的激发光谱。我们显示出，对于超流体相，激发谱的中心与整体平均填充因子呈正相关，表明系统的声速更高。对于莫特绝缘子相，随着整体平均填充因子的增加，激发光谱的离散特征变得不那么明显，这意味着系统很难以更高的填充因子进入莫特绝缘子体系。