Swapping Kondo resonances in coupled double quantum dots

Abstract

Strong electron and spin correlations are studied in parallel-coupled double quantum dots with interdot spin superexchange J. In the Kondo regime with degenerate dot energy levels, a coherent transport occurs at zero temperature, where two entangled (bonding and antibonding) resonances are formed near the Fermi energy. When increasing J or the dot-lead parallel-coupling asymmetry ratio Γ2/Γ1, a swap between two entangled resonances occurs and the line shapes of the linear conductance are interchanged. The zero-bias differential conductance shows a peak at the critical values. Such a peculiar effect with the virtue of many-body coherence may be useful in future quantum computing.A large number of proposals have been made to materialize quantum bits (qubits) and quantum computing. Among these proposals, coupled quantum dot (QD) systems are particularly attractive.[1] The spin degree of freedom of the localized electrons on the dots is considered as a qubit due to the comparatively long coherence time. A key challenge is the construction of coupled double QD (DQD) to perform a swap operation, i.e. exchanging the electron spin states on the two dots. When the square root of a swap operation is combined with other isolated qubit rotations, a quantum controlled-NOT gate can be built and any quantum algorithms can be implemented.[2] In the coupled DQD, two local electrons form a singlet state. It has been proposed that the swap operation can be realized by tuning the time-dependent interdot spin superexchange (ISS) J(t) from positive to negative, flopping the singlet and triplet states. [1,3] In this Letter, we propose a simple and reliable mechanism to perform such a swap process in a parallel-coupled DQD at low temperatures. It has been well-established that under the Coulomb blockade with odd number electrons on a single QD, a quantum coherent many-body (Kondo) resonance is formed near the Fermi energy in the dot density of states (DOS) [4]. The Kondo effect for even number electrons on a single multilevel dot and possible phase transitions between singlet and triplet states have been considered for both "vertical" [5] and "lateral" [6,7,8,9] configurations. For the coupled DQD with degenerate energy levels, the Kondo behavior and the ISS interplay and strongly compete, as seen in the bulk two-impurity Kondo problem,[10] and a question arises whether there exists a spin entangled state composed of the coherent Kondo resonances.[11] Previous experimental and theoretical studies have mainly focused on the serial-coupled DQD [12,13,14,15,16,17,18,19], except for Ref. 9, 19. However, it has been recently realized that the serial geometry is unsuitable for studying this competition experimentally and a direct evidence for observing spin entanglement between the dot local electrons could be sought in the parallel coupled configuration. [20] It has thus motivated us to investigate whether and how this competition manifests itself in the coherent transport through DQD in the parallel configuration.For a degenerate DQD with ...

Characterization of platinum nitride from first-principles calculations

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Tunable spin-polarized transport through a side-gated double quantum dot molecular junction in the Coulomb blockade regime

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The role of the indirect tunneling processes and asymmetry in couplings in orbital Kondo transport through double quantum dots

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