摘要：
Due to its high sensitivity to weak forces, the classical torsion pendulum was widely employed in the fields of precision gravitational measurement, which can be evaluated by testing of Newtonian inverse square law (ISL). In order to get much higher precision for the tests of ISL at short ranges, such as at millimeter and/or sub-millimeter level, the torsion pendulum cannot work precisely in free oscillation motion due to the electrostatic disturbance. This paper presents a method using proportional-integral-differential (PID) technology to make pendulum to keep quasi-static. A closed-loop system with PID controller is realized and calibration results show the system has the potential to get the precision of test of the ISL violation at the order of 10
<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−16</sup>
N·m at millimeter range.
摘要：
The ground state of interacting spin chains in external magnetic fields can undergo a quantum phase transition (QPT) characterized by dramatic changes at a critical value of the magnetic field. In this paper, we use Bell-type inequalities to study the multipartite correlations (including multipartite entanglement and multipartite nonlocality in an n-spin subsystem) in the QPT of an infinite XY chain. An efficient numerical optimization procedure is proposed to figure out the violation measure Mn of the inequalities. For n≤7, the magnetic-field ( λ) dependence of Mn is studied. We find the derivative of Mn is divergent exactly at the QPT point λc=1 for any n. In addition, with the increase of n, Mn converges quickly for λ<λc and converges very slowly for λ>λc, which can be regarded as another signal for the QPT. Furthermore, in the vicinity of λc, high-order Bell-type inequalities will be violated as long as n is large enough. This indicates that high-level multipartite correlation will be present when the system is in the vicinity of the QPT point. Nevertheless, genuine n-partite entanglement or genuine n-partite nonlocality is not observed in the QPT.
摘要：
In this paper, using the infinite time-evolving block decimation (iTEBD) algorithm and Bell-type inequalities, we investigate multipartite quantum nonlocality in an infinite one-dimensional quantum spin- 12 XXZ system. High hierarchy of multipartite nonlocality can be observed in the gapless phase of the model, while only the lowest hierarchy of multipartite nonlocality is observed in most regions of the gapped antiferromagnetic phase. Thereby, Bell-type inequalities disclose different correlation structures in the two phases of the system. Furthermore, at the infinite-order quantum phase transition (QPT, or Kosterlitz-Thouless QPT) point of the model, the correlation measures always show a local minimum value, regardless of the length of the subchains. It indicates that relatively low hierarchy of multipartite nonlocality would be observed at the infinite-order QPT point in a Bell-type experiment. The result is in contrast to the existing results of the second-order QPT in the one-dimensional XY model, where multipartite nonlocality with the highest hierarchy has been observed. Thus, multipartite nonlocality provides an alternative perspective to distinguish between these two kinds of QPTs. Reliable clues for the existence of tripartite quantum entanglement have also been found.
