P8: Controlled single-spin sources with time-dependent driving

 

Project Description

Diagram Copyright: Splettstoesser

In this project, controlled spin- and pseudospin emission from time-dependently driven single- and few-electron devices will be investigated. The project addresses two types of experimentally relevant model systems: (i) superconducting turnstiles and (ii) quantum dot pumps. Going beyond previous proposals [1,2,3], this project focuses on coherent, higher-order tunneling processes and their impact on the precision of these emitters, as well as their opportunities for new types of emission processes. The combined application of perturbative analytic techniques and time-dependent numerical renormalization group (see [4] for a comprehensive study of the accuracy of this approach) aims at an understanding of the dominant tunneling processes up to a full picture of strong coherent coupling in all parts of the project.

In a previous RTG work by N. Dittmann, we have proposed a superconducting turnstile, consisting of a superconducting island spin-split by a nearby ferromagnetic insulator layer and tunnel-coupled to superconducting contacts, as a clocked single-spin source [1]; see the Figure above. The ferromagnetic insulator layer, however, seems to even provide highly enhanced precision when the device is operated as a charge turnstile. A detailed analysis of higher order (Andreev- and co-tunneling) processes in the spin- and charge turnstile current is the starting point of the present project. The type of pro-cesses involved is expected to differ when replacing the superconducting island by a (possibly multi-level) quantum dot [5,6] and we will compare the quality of this device as single charge- or spin-emitter to the fully superconducting turnstile.

In time-dependently driven quantum dots in normal conducting devices, the complex internal degrees of freedom of the dot (e.g. via spin-orbit coupling and Dzyaloshinskii-Moriya interaction) and strong spin polarization effects in Kondo correlated quantum dots can also be ex-ploited for spin pumping by external gating [7]. Beyond this, we will, in the second half of this project, particularly focus on the emission of pseudo-spins from multi-level quantum dots, encoded, for example, in singlet- and triplet states. This can be realized via higher-order tun-neling events only, and is of relevance for the possible initialization of flying qubits.

[1] N. Dittmann, J. Splettstoesser, and F. Giazotto, New J. Phys. 18, 083019 (2016)
[2] R.-P. Riwar and J. Splettstoesser, Phys. Rev. B 82, 205308 (2010)
[3] D. M. T. van Zanten et al., Phys. Rev. Lett. 116, 166801 (2016)
[4] H. T. M. Nghiem, D. M. Kennes, C. Klöckner, V. Meden, and T. A. Costi, Phys. Rev. B 93, 165130 (2016)
[5] H. T. M. Nghiem and T. A. Costi, Phys. Rev. Lett. 119, 156601 (2017)
[6] H. T. M. Nghiem and T. A. Costi, submitted to Phys. Rev. B, arXiv:1803.04098
[7] H. T. M. Nghiem and T. A. Costi, Phys. Rev. B 90, 035129 (2014)