We study the J1-J1'-J2 quantum spin model on the two-dimensional square lattice using the coupled cluster method. We compare and contrast the influence of the interchain coupling J1' on the zero-temperature phase diagrams for the two spin values s = 1/2 and s = 1. Our most important result for the s = 1/2 case is the predicted existence of a quantum triple point (QTP) at (J1' ≈ 0.60 ± 0.03, J2 ≈ 0.33 ± 0.02) when J1 = 1. Below the QTP (J1'/J1 ≤ 0.60) we predict a second-order phase transition between the quasi-classical Neel and stripe-ordered phases, whereas the corresponding classical model, which contains only these two phases for all spin values s, yields a first-order transition. Above the QTP (J1'/J1 ≥ 0.60) an intermediate disordered phase emerges, which has no classical counterpart. By contrast, the situation for s = 1 is qualitatively different. Instead of a QTP where three phases co-exist, we now predict a quantum tricritical point at (J1' ≈ 0.66 ± 0.03, J2 ≈ 0.35 ± 0.02) when J1 = 1, where a line of second-order phase transitions between the quasi classical Neel and stripe-ordered phases (for J1'/J1 ≤ 0.66) meets a line of first-order phase transitions between the same two states (for J1'/J1 ≥ 0.66). Surprisingly, we find no evidence at all for any intermediate disordered phase in the s = 1 case.
Two entangled particles in threedimensional Hilbert space (per particle) are considered in an EPR-type arrangement. On each side the Kochen-Specker observables
{J12,J22,J32} and
{Jˉ12,Jˉ22,J32} with
[J12,Jˉ12]=0 are measured. The outcomes of measurements of
J32 (via
J12,J22) and
J32 (via
Jˉ12,Jˉ22) are compared. Although formally
J32 is associated with the same projection operator, a strong form of quantum contextuality states that an outcome depends on the complete disposition of the measurement apparatus, in particular whether
J12 or
Jˉ12 is measured alongside. It is argued that in this case it is impossible to measure contextuality directly, a necessary condition being a non-operational counterfactuality of the argument.
通过理论计算与量子计算模拟机模拟两种方法研究了波-粒量子行走的演化过程与性质。通过量子控制操作使量子行走中的行走者处于相对相位的波-粒相干叠加态。利用后选择操作实现了量子行走以相干和混合两种不同方式,从多路径相干的波的状态到无相干的粒子的状态的连续调控。由于量子的干涉性,相干和混合两种方式存在本质的区别,通过位置方差对两种方式的具体特征进行刻画。最后通过量子计算模拟机模拟了波-粒量子行走的演化过程。当行走者处于波-粒相干态时,通过一次测量能够同时观测到两个完全不同的性质。通过调节波-粒相干态的相对相位,可以实现对行走者扩散速率的控制。
The surface scaling behavior of classical isotropic Heisenberg magnets is investigated by Monte Carlo methods in
d=3 dimensions for various values of the surface-to-bulk coupling ratio
J1/J. For
J1/J< 1.0, critical behavior according to the ordinary surface universality class is found. Estimates for magnetic surface exponents are presented and compared to older estimates and their theoretical counterparts. For
J1/J> 2.0, scaling is still valid with effective exponents which depend on
J1/J. The surface structure factor
S1(p,L) is investigated at bulk criticality as a function of the momentum transfer p parallel to the surface and the system size L. For
J1/J< 1.0 and
J1/J> 2.0 the full p dependence of
S1(p,L) can be captured by generalized shape functions to a remarkable accuracy. Profiles of the magnetization and the energy density also confirm scaling, where for
J1/J< 1.0 the ordinary surface universality class is recovered and for
J1/J> 2.0, scaling with
J1/J dependent exponents is found. For
J1/J=1.5 the system displays a striking crossover behavior from spurious long-range surface order to the ordinary surface universality class. For
J1/J> 2.0 the effective scaling laws must be interpreted as nonasymptotic and the value
J1/J=1.5 marks a crossover regime, in which the crossover from the nonasymptotic to the asymptotic (ordinary) surface scaling behavior can be resolved within numerically attainable system sizes.