Humanity stands at the brink of two groundbreaking revolutions: one involving the emergence of 2-dimensional supermaterials like graphene, celebrated for their extraordinary properties, and the other, the advent of quantum computers, poised to outshine traditional computers in processing power.
Understanding materials like graphene, composed of single atomic layers, opens the door to meticulous exploration of their atomic-level properties. This extends to the behavior of electrons in the proximity of superconductors – substances capable of conducting electricity without any energy loss when chilled to near absolute zero.
In scenarios where a superconductor is situated between metallic materials, an intriguing phenomenon known as “crossed Andreev reflection” can occur. In an s-wave superconductor junction, this reflection typically induces correlated electrons with opposite spins, which is crucial for harnessing entanglement – a pivotal quantum phenomenon essential for quantum computing.
In a recent publication in The European Physical Journal B, Rui Shen, hailing from the National Laboratory of Solid State Microstructures and the School of Physics at Nanjing University, along with fellow researchers, delved into the theoretical assessment of nonlocal transport and crossed Andreev reflection. Their focus was on a ferromagnetic s-wave superconductor junction comprised of gapped graphene lattices.
Shen elaborates on their findings, highlighting the creation of a staggered potential by growing ferromagnetic graphene on a boron nitride substrate. This leads to symmetry breaking and the emergence of fully spin-polarized electron states. By skillfully adjusting the Fermi level through gate voltage or doping, they ensured it crossed only one conduction band in the left lead and one valence band in the right lead.
Their work predicted the presence of a pure equal-spin crossed Andreev reflection signal when the ferromagnetic exchange fields in the two leads were in antiparallel configuration, while a pure opposite-spin CAR signal emerged in the parallel configuration.
Importantly, they demonstrated the ease with which equal-spin and opposite-spin correlations could be toggled by swapping the exchange fields. Notably, the pure equal-spin signal was conspicuously absent in gapless graphene junctions. Shen emphasizes that this achievement extends not only to the Dirac point but also over a wide voltage range, implying highly efficient nonlocal splitting of Cooper pairs with spin-triplet or spin-singlet pairing correlations.
Shen concludes by highlighting the significance of their findings, particularly in the realm of quantum science. The time-reversed process of crossed Andreev reflection, known as Cooper pair splitting, offers a powerful avenue to generate entangled states within quantum systems, promising applications in quantum communication and quantum computation.