Hofstetter L, Csonka S, Nygård J, Schönenberger C
Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland.
Nature. 2009 Oct 15;461(7266):960-3. doi: 10.1038/nature08432.
Non-locality is a fundamental property of quantum mechanics that manifests itself as correlations between spatially separated parts of a quantum system. A fundamental route for the exploration of such phenomena is the generation of Einstein-Podolsky-Rosen (EPR) pairs of quantum-entangled objects for the test of so-called Bell inequalities. Whereas such experimental tests of non-locality have been successfully conducted with pairwise entangled photons, it has not yet been possible to realize an electronic analogue of it in the solid state, where spin-1/2 mobile electrons are the natural quantum objects. The difficulty stems from the fact that electrons are immersed in a macroscopic ground state-the Fermi sea-which prevents the straightforward generation and splitting of entangled pairs of electrons on demand. A superconductor, however, could act as a source of EPR pairs of electrons, because its ground-state is composed of Cooper pairs in a spin-singlet state. These Cooper pairs can be extracted from a superconductor by tunnelling, but, to obtain an efficient EPR source of entangled electrons, the splitting of the Cooper pairs into separate electrons has to be enforced. This can be achieved by having the electrons 'repel' each other by Coulomb interaction. Controlled Cooper pair splitting can thereby be realized by coupling of the superconductor to two normal metal drain contacts by means of individually tunable quantum dots. Here we demonstrate the first experimental realization of such a tunable Cooper pair splitter, which shows a surprisingly high efficiency. Our findings open a route towards a first test of the EPR paradox and Bell inequalities in the solid state.
非定域性是量子力学的一个基本特性，它表现为量子系统空间分离部分之间的相关性。探索此类现象的一条基本途径是生成爱因斯坦-波多尔斯基-罗森（EPR）对量子纠缠物体，用于检验所谓的贝尔不等式。虽然这种非定域性的实验测试已成功地用成对纠缠光子进行，但在固态中尚未实现其电子类似物，在固态中自旋1/2的移动电子是天然的量子物体。困难源于电子沉浸在宏观基态——费米海中，这阻止了按需直接生成和分裂纠缠电子对。然而，超导体可以作为电子EPR对的源，因为其基态由自旋单态的库珀对组成。这些库珀对可以通过隧穿从超导体中提取出来，但是，为了获得一个有效的纠缠电子EPR源，必须强制将库珀对分裂成单独的电子。这可以通过让电子通过库仑相互作用“相互排斥”来实现。通过借助单独可调谐的量子点将超导体耦合到两个正常金属漏极接触，可以实现可控的库珀对分裂。在这里，我们展示了这种可调谐库珀对分裂器首个实验实现，其显示出惊人的高效率。我们的发现为在固态中首次测试EPR佯谬和贝尔不等式开辟了一条途径。
