Decoherence of an electronic and macroscopic superposition state by interaction with a superconductor

分析从量子力学到经典系统的转变是现代物理学基础的一部分。这种量子特性的损失可以用退相干理论来描述。它具有基础和技术相关性，例如用于实现混合量子系统。本课题将实现双棱镜干涉仪中宏观和电子叠加态通过库仑相互作用耦合到超导环境的模型系统。因此，电子波被一根细的带静电的导线分离并相干地结合在一起。在部分波重叠和干扰之前，它们以几十微米的可变距离穿过超导表面。这导致了超导体中电子物质波和库珀对之间的库仑相互作用。它对叠加态的退相干有直接影响，可以通过干涉图中的对比度损失来测量。在计划中的项目中，由于施加了超导体，电阻在表面完全消失，这与以前在半导体表面附近进行的电子退相干实验相反。对于半导体来说，由于在电子路径下形成的像电荷，正是这种电阻导致了向环境传递的路径信息，从而导致了退相干。各种理论模型都假定像电荷的摩擦是耗散项，这是材料中电阻的结果。因此，随着电子路径之间距离的增加和与表面距离的减小，可以观察到对比度的逐渐丧失。然而，超导体之上的电子叠加态的性质不能用以前的理论来描述。本项目将对其进行详细的实验研究。此外，根据这种电子叠加态在金属和可变掺杂半导体表面上的退相干性，将关注开放性问题。以往的预测只能描述在某一半导体上测量到的退相干分布，而不能描述退相干的强度。它将在电阻率可变化达9个数量级的表面上以电流方法测量。理论上假定退相干也取决于表面的温度。因此，在表面温度在4 ~ 500 K之间时，可以测量叠加电子态的退相干。然后将结果与当前的退相干模型进行比较。目的是发展一个正确的描述在导电和超导表面附近的退相干机制。

The analysis of the transition from a quantum mechanical to a classical system is part of the foundations of modern physics. This loss of quantum properties is described by the theory of decoherence. It is of fundamental and technical relevance, e.g. for the implementation of hybride quantum systems. In this project a model system will be realized, where a macroscopic and electronic superposition state in a biprism interferometer couples to a superconducting environment by Coulomb interaction. Thereby an electron wave gets separated and combined coherently by a thin electrostatically charged wire. Before the partial waves overlap and interfere, they traverse a superconducting surface in a variable distance of a few ten micrometers. This results in a Coulomb interaction between the electronic matter wave and the Cooper pairs in the superconductor. It has a direct influence to decoherence of the superposition state that can be measured by a contrast loss in the interference pattern. In the planned project the electric resistance disappears completely in the surface because of the applied superconductor, in contrary to former decoherence experiments with electrons nearby semiconducting surfaces. It was shown for semiconductors that due to the formation of image charges below the electron paths, exactly this resistance leads to a which-path-information to the environment and consequently to decoherence. Various theoretical models assume a dissipative term for the friction of the image charge, as a consequence of the electric resistance in the material. A gradual loss of contrast is therefore observed with increasing distance between the electron pathways and with decreasing distance to the surface. However, the properties of the electronic superposition state above the superconductor cannot be described with previous theories. It will be studied experimentally in detail within this project.Furthermore open questions will be concerned according decoherence of such an electronic superposition state above metallic and variable doped semiconducting surfaces. Previous predictions could only provide a description of the measured decoherence distribution above a certain semiconductor but not for the strength of decoherence. It will be measured in the current approach above surfaces with electric resistivities that can be varied by up to nine orders of magnitude. It is theoretically assumed, that decoherence depends also on the temperature of the surface. Therefore decoherence of the superposed electronic state will be measured with surface temperatures between 4 and 500 K. The results will then be compared to current decoherence models. The aim is to develop thereby a correct description of the decoherence mechanisms near conducting and superconducting surfaces.