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Tunable nonlocal valley-entangled Cooper pair splitter realized in bilayer-graphene van der Waals spin valves
摘要： We investigate the Cooper pair splitting based on crossed Andreev reflection in a two-dimensional narrow superconductor coupled to two leads, where the two leads are van der Waals (vdW) spin valves formed by inserting bilayer graphene (BLG) between two antiparallel insulating ferromagnetic layers. In this BLG-based n-type vdW spin valve/superconductor/p-type vdW spin valve junction with edge-contact geometry, when controlling the Fermi energy and the interlayer electric field in BLG independently by top- and bottom-gate voltages in the vdW spin valve region, we first demonstrate that the two electrons of perfect Cooper pair splitting can remain nonlocal valley entanglement. This effect can be explained as follows: When both the two antiparallel insulating ferromagnetic layers and the interlayer electric field are taken into account, BLG exhibits a spin-splitting but valley-degenerate band gap near the Dirac point. Thus, our setups allow the Cooper pair in the BCS scenario to serve as a natural source of valley entanglement. We further predict large oscillations of cross conductance as a function of the doping of the superconductor, the junction length, and the bias voltage. Our results may provide a tool for realizing fast nonlocal switching of the valley-entanglement state.
关键词： valley entanglement,crossed Andreev reflection,Cooper pair splitting,van der Waals spin valves,bilayer graphene
更新于2025-09-19 17:13:59
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Single atom laser in normal-superconductor quantum dots
摘要： We study a single-level quantum dot strongly coupled to a superconducting lead and tunnel coupled to a normal electrode which can exchange energy with a single-mode resonator. We show that such a system implements a single atom laser. We employ both a semiclassical treatment and a quantum master-equation approach to characterize the properties of this laser. In particular, we ?nd that this system can be operated with ef?ciency approaching unity; that is, a single photon is emitted into the cavity for every Cooper pair participating in the charge current. We ?nd also that lasing in the proposed setup is clearly identi?able in the transport properties: in the lasing state, the electrical current through the quantum dot is pinned to the maximum value achievable in this hybrid nanostructure, and hence, the onset of lasing can be detected simply by a current measurement.
关键词： semiclassical treatment,quantum dot,Cooper pair,transport properties,superconducting lead,charge current,single atom laser,quantum master-equation approach,single-mode resonator,normal electrode
更新于2025-09-12 10:27:22
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Energy conversion from environmental fluctuations to coherent fields by Cooper-pair box quantum meta-materials
摘要： Electromagnetic waves propagating in open Cooper-Pair Boxes (CPBs) system is studied by using Maxwell-Bloch equations and Lindblad master equation. The results demonstrate an ensemble of CPBs as highly non-linear meta-material for electromagnetic waves. Incorporating the CPBs in a ring resonator or a Fabry-Perot cavity, one finds that: (1) With weak environmental couplings and CPBs in superconducting phase dominant regime, the non-linearity is enhanced and the system exhibits regular optical hysteresis. (2) With finite environmental couplings and CPBs in charge dominant regime, the Josephson effect and environmental effect can constructively interplay to produce a gain. In the later case, the electromagnetic field would be amplified by the CPB medium, indicating energy conversion from the environment to coherent fields mediated by CPBs.
关键词： environmental effect,Maxwell-Bloch equations,optical hysteresis,Josephson effect,electromagnetic waves,Lindblad master equation,quantum meta-materials,Cooper-Pair Boxes
更新于2025-09-09 09:28:46