Magnetic Field Tuning and Quantum Interference in a Cooper Pair Splitter

Abstract

Cooper pair splitting (CPS) is a process in which the electrons of the naturally occurring spin-singlet pairs in a superconductor are spatially separated using two quantum dots. Here, we investigate the evolution of the conductance correlations in an InAs CPS device in the presence of an external magnetic field. In our experiments the gate dependence of the signal that depends on both quantum dots continuously evolves from a slightly asymmetric Lorentzian to a strongly asymmetric Fano-type resonance with increasing field. These experiments can be understood in a simple three-site model, which shows that the nonlocal CPS leads to symmetric line shapes, while the local transport processes can exhibit an asymmetric shape due to quantum interference. These findings demonstrate that the electrons from a Cooper pair splitter can propagate coherently after their emission from the superconductor and how a magnetic field can be used to optimize the performance of a CPS device. In addition, the model calculations suggest that the estimate of the CPS efficiency in the experiments is a lower bound for the actual efficiency. In the Cooper pair splitting (CPS) process the electrons of the Cooper pairs in a superconductor are separated spatially using two quantum dots (QDs) coupled in parallel to a central superconductor contact (S) in a three-terminal geometry [1-3], see Fig. 1(a). The Coulomb repulsion on the QDs and the quasiparticle energy gap of the superconductor enforce the electrons to separate into different normal metal electrodes (N1 and N2). Since Cooper pairs are spin-singlet states, such devices could serve as a source of nonlocal spin entangled electron pairs. Similar geometries are also relevant in the search for Majorana bound states [4] in local S-N junction experiments and in threeterminal devices, where an increase in CPS efficiency might serve as a signature of the elusive exotic states [5].In a series of recent experiments on semiconducting nanowires (NWs) [6][7][8][9], carbon nanotubes [10-12], and graphene [13], CPS was demonstrated by positive conductance correlations between the currents from S into N1 and N2. In these experiments, external magnetic fields were solely used to suppress the superconductivity for control experiments, but not as a parameter to tune CPS. In addition, most experiments were interpreted in terms of an incoherent picture with independent transport mechanisms, only coupled by the QD dynamics [9,11].Here, we report experiments in a NW-based Cooper pair splitter with a Nb superconducting electrode. The large critical magnetic field of Nb allows us to explore CPS up to ∼1 T. We find that the conductance correlations can not only manifest as symmetric peaks and dips in the gate dependence, but also as strongly asymmetric shapes reminiscent of Fano resonances. We interpret the experimental results in a minimal model that incorporates the superconducting proximity effect, the tunnel coupling between the QDs, and quantum interference [see Fig. 1(b)]. Interference results in asymmetric...

Andreev bound states probed in three-terminal quantum dots

“…Three-terminal ABS devices allow for new transport mechanisms, so that nonlocal processes like Cooper-pair splitting [38][39][40][41] compete with local mechanisms like Andreev tunneling [42]. Such mechanisms are expected to result in new effects like the triplet blockade [26], characteristic * andreas.baumgartner@unibas.ch patterns in the electrical conductance [25,28], and other nonlocal effects [29,30].…”

Current cross-correlations and waiting time distributions in Andreev transport through Cooper pair splitters based on a triple quantum dot system

“…Indeed, both the experimental results and theoretical predictions suggest an important role of quantum interference, which are generally present in CPS systems. 35 To capture and understand the role of quantum interference, a three-site model has been put forward, where the central part is coupled to superconductor, while the left and right arms of the splitter are modeled by remaining two separate sites. [35][36][37] The transport properties of such three-site models are however still rather unexplored.…”

Current cross-correlations in double quantum dot based Cooper pair splitters with ferromagnetic leads

“…One of very promising applications of such hybrid systems is the possibility to realize a Cooper pair splitter (CPS) [19,20,21,22,23,24,25,26,27,28,29,30]. In Cooper pair splitting devices the electrons forming a Cooper pair tunnel from superconductor into two separate normal leads, while the tunability of quantum dots embedded in the arms of a CPS enables controlling of the splitting process [20,21,22].…”