Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11674024, 11875086) and the Natural Science Foundation of Beijing, China (Grant No. 2192049)
Received Date:22 October 2020
Accepted Date:16 November 2020
Available Online:23 March 2021
Published Online:05 April 2021
Abstract:In an isolated two-body composite system, the discussion of how the change of one body affects the state of the other will help to know the relation of a single particle's mixed and pure states. Given 5 one-dimensional hydrogen-like atoms models, each Coulomb interaction potential keeps invariant, while the masses of the nuclei are different. These two-body composite systems stay in their respective entangled state, each electron stays in a mixed state. If we suppose a one-dimensional hydrogen atom model having infinite nuclear mass, the electron stays in a pure state. We select position representation. The wave function of the ground state of the atom has the form of the square root of a δ function. To avoid calculation trouble of δ function, we select the first excited state and the superposed state of the first and the second excited states. We compare the two pure states, the first excited state and the superposed state, with those corresponding mixed states by fidelity and l1 norm coherence, and calculate the purities of the mixed states. The summations become integrations due to the position variable having a continuous eigenvalue spectrum. We investigate the changes in these quantities with the increase of the nuclear mass. The results show that the purities of the mixed states and the fidelities increase with the nuclear mass increasing. However, the coherences of the mixed states decrease with the nuclear mass increasing. This can be explained as that a mixed state with non-zero coherence may approach to a pure eigenstate, while the latter has zero coherence in the eigenspace. These mean that the greater a nuclear mass is, the closer the mixed state approaches to the corresponding pure state. Therefore, the two pure states are the approximations of the corresponding mixed states. The entangled state of the electron and the nucleus is related with the nuclear mass and the Coulomb interaction potential. So, each electron mixed state and its coherence are related with the nucleus and their Coulomb interaction potential. If the nuclear mass is great, the nucleus is approximately motionless or its state is approximately unchanged, and the Coulomb interaction potential approximates to the external Coulomb potential of the electron. The electron approximately stays in a pure state. The state and its coherence are related with the nucleus and the Coulomb interaction. From other point of view, the entangled state of the nucleus and the electron approximates to the product state of the unchanged nucleus state and the electron state. In this case, an electron mixed state approximates to its corresponding pure state, and then these states and their coherences are all related with the nucleus and the Coulomb interaction. Keywords:mixed state/ fidelity/ coherence/ continuous variable
按照上述$ \hbar = 1 $, $ e = 1 $, $m_{\rm e} = 1$的设定, 当积分区间取$ \left[-20, 20\right] $范围内时, 第一激发态和叠加态偶宇称波函数所表示的概率非常接近1, 所以电子的积分区间取$ \left[-20, 20\right] $. 根据两体问题的运动关系, 相应的原子核的积分区间取为$\left[\dfrac{-20}{{m}_{\rm p}}, \dfrac{20}{{m}_{\rm p}}\right]$, 其中$ {m}_{\rm p} $是原子核的质量. 考虑5个一维类氢原子系统, 分别计算了原子核质量取不同值时,电子混合态的纯度的变化趋势, 即$ {\rm{tr}}\left(\rho_{1}^{(1)}\right)^{2} $ 和$ {\rm{tr}}\left(\rho_{12}^{(1)}\right)^{2} $的变化趋势. 图1为纯度变化趋势图, 可以看出, 原子核的质量越大, 电子混合态的纯度越高, 并非常接近1. 对于氕、氘和氚原子, 电子混合态都近似于某个纯态. 反之, 说明一个纯态是由混合态近似而来的. 图 1 纯度变化趋势图, 横坐标表示原子核质量和电子质量之比 (a) 第一激发态; (b) 叠加态 Figure1. Purity vs. the ratio of mass between nucleus and electron: (a) The first excited state; (b) the superposition of the first and the second excited states
两个保真度变化趋势如图2所示. 从图2可以看出, 原子核的质量越大, 保真度越大, 电子混合态与相应的纯态也越接近. 在氕、氘和氚原子情形, 保真度都接近1, 说明电子混合态都近似于纯态. 反之, 说明这两个纯态都是由相应混合态近似而来的. 图 2 保真度变化趋势图, 横坐标表示原子核质量和电子质量之比 (a) 第一激发态; (b) 叠加态 Figure2. Fidelity vs. the ratio of mass between nucleus and electron: (a) the first excited state; (b) the superposition of the first and the second excited states
分别计算$ \psi_{1}^{+}(x) $和$ \psi_{12}^{+}(x) $以及$ \rho_{1}^{(1)} $和$ \rho_{12}^{(1)} $的相干性, 并得到了混合态的相干性随原子核质量变化的趋势图, 如图3所示. 从图3可以看出, 原子核的质量越大, 电子混合态的相干性与相应纯态的相干性越接近. 也说明氕、氘和氚原子中, 电子混合态的相干性都近似于相应纯态的相干性. 图 3 相干性变化趋势图, 横坐标表示原子核质量和电子质量之比 (a)第一激发态; (b)叠加态 Figure3. Coherence vs. the ratio of mass between nucleus and electron: (a) The first excited state; (b) the superposition of the first and the second excited states