A research team at POSTECH led by Prof. Gil-Ho Lee and Prof. Hu-Jong Lee, together with a graduate research associate Geon-Hyoung Park, in Department of Physics, reported achievement of highly efficient electronic quantum entanglement in Cooper pair splitter devices composed of two atomically stacked bilayer graphene (BLG) sheets, each being an atomic bilayer of graphite, which were commonly contacted by a superconducting lead. The paper was published in online issue of Nano Letters on the 18th of November, 2019.

The quantum entanglement terms the condition of a pair of particles where the quantum states of particles are perfectly correlated and not separable, even when the two particles are spatially far apart. Thus, if the spin state of two particles are entangled, measurement of the spin state of one particle along an axis immediately determines the spin state of the other particle along the axis, so-called ‘spooky action at a distance’ that Albert Einstein referred to. Ever since the concept was first proposed by Einstein, Podolsky, and Rosen in 1935 [Phys. Rev. 47, 777 (1935)], it was successfully demonstrated in measurements using pairs of linearly polarized photons. This earlier demonstration was followed by tremendous efforts for realizing the solid-state version of electronic entangler devices to date, as they are the key ingredients for quantum computation and quantum communications. A superconductor, with its intrinsic strong quantum coherence of the electronic state, is one of the most promising sources of entangled electronic state by splitting superconducting Cooper pairs. However, the efficiency of Cooper pair splitting has largely been limited by competing processes; local Andreev reflection and elastic cotunneling.

The POSTECH research team effectively suppressed the competing processes by tuning the carrier energy level, either electron or hole, in a BLG sheet to the Dirac point of another BLG sheet and observed the crossed Andreev reflection, which enormously enhanced the relative efficiency of the Cooper pair splitting or quantum entanglement. Simultaneously, they maximized the quantum entanglement by separating the two stacked BLG sheets by insulating and graphite screening layers with total thickness of only ~10 nm, which is much shorter than the superconducting coherence length. Thus, they ingeniously took advantage of the exotic band structure and the extreme thinness of graphene in achieving their goal.

Prof. Hu-Jong Lee expresses high confidence that this study would stand as a milestone achievement in graphene research, both fundamental and quantum device applications, and will attract great interest from the communities of graphene studies and quantum entangler research.

This work was supported by National Research Foundation of Korea (NRF) through the SRC Center for Topological Matter (grant no. 2018R1A5A6075964 for H.-J.L.), the Samsung Science and Technology Foundation (project no. SSTF-BA1702-05 for G.-H.L.).