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Quantum Information and Computation

Current noise cross correlations and dynamical spin entanglement in coupled quantum dots

Hai‐Feng Lü, Yong Guo, Lian-Fu Wei

Published Jul 1, 2011• 0 citations
Publisher

Abstract

The authors investigate current noise cross correlations (CCs) through coupled double quantum dots under transport conditions. It is demonstrated that positive CCs relate to either thermal fluctuations at nonzero temperature or quantum tunneling processes at zero temperature. These measurable CCs could service as the sensitive indicators of entanglements between electronic spins in double dots coupled by exchange interactions.

DOI
10.26421/qic11.7-8-5

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The authors investigate current noise cross correlations (CCs) through coupled double quantum dots under transport conditions. It is demonstrated that positive CCs relate to either thermal fluctuations at nonzero temperature or quantum tunneling processes at zero temperature. These measurable CCs could service as the sensitive indicators of entanglements between electronic spins in double dots coupled by exchange interactions.
Cross-correlated charge noise in silicon quantum dots has emerged as an important issue for fault-tolerant quantum computation, as it has been suggested to both limit quantum error correction performance and reveal unique information about noise sources. We develop a method to characterize charge noise cross-correlations in a semiconductor qubit device based solely on quantum-dot transport current. By dynamically switching the site-selective sensitivity of transport current through a double quantum dot to charge noise, we extract potential fluctuations at individual dots without requiring additional sensing devices or spin operations. Based on the obtained power spectral densities, we discuss the coupling of a dominant two-level fluctuator to individual dots. We furthermore verify the consistency of the measurement protocol by applying artificial noise via gate electrodes and investigate the temperature dependence of charge noise correlations. The method offers a convenient and versatile alternative approach that substantially facilitates the assessment of charge noise correlations in diverse semiconductor qubit devices.
Abstract Entanglement of electrons is studied by means of current–current correlations in two Cooper pair splitter devices: with one and two proximized quantum dots (1QD and 2QD), in presence of intra- and inter-dot Coulomb interactions, and weakly coupled with metallic electrodes. The 1QD system, where Cooper pairs can be transmitted to the same or split to different normal electrodes, is contrasted with the 2QD device, where double occupancy of a single quantum dot is forbidden and transport is only through an inter-dot singlet due to non-local crossed Andreev reflection processes deep in the superconducting energy gap. Separating the current correlation function into components for partial currents of electrons and holes through various Andreev bound states, one can see bunching and antibunching of split particles: inter-level components between electron and hole currents flowing to different electrodes are positive, while intra-level electron–electron or hole–hole components are negative, respectively. Spectral decomposition of the frequency-dependent current cross-correlation is performed to get better insight into mechanisms of entanglement and dynamics of split Cooper pairs, and to extract various charge fluctuation processes with different relaxation times, related to electron and hole currents flowing through the Andreev bound states. Only low frequency polarization fluctuations are seen in the current cross-correlations, while various negative and positive high frequency (charge fluctuations) components compensate each other in the symmetric system.
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We have recently developed methods for calculating the statistics of a single counted quantity in a stochastic transport process described by a non‐Markovian generalized master equation. Here, we expand on these ideas and extend our formalism to several counted quantities, allowing us to study cross‐correlations between different variables. As an illustrative example, we consider charge transport through a double quantum dot capacitively coupled to a quantum point contact. We calculate the cross‐correlations between the current through the double quantum dot and the current through the quantum point contact, and demonstrate the sensitivity of this cross‐correlator to measurement quantum back‐action.
This thesis addresses two different topics related to the physics of nanoscale systems. The first topic concerns quantum correlations and entanglement between electrons in solid-state systems, with a focus on how to generate electronic orbital entanglement on a sub-decoherence time scale and how to achieve experimentally more feasible entanglement detection schemes. The second topic concerns heat transport and temperature fluctuations in nanoscale systems, with a focus on how to utilize temperature fluctuations for calorimetric detection of single particles. The thesis comprises five papers.In Paper I, we propose a quantum dot system to generate and detect, using cotunneling processes, orbitally entangled pairs of electrons on a sub-decoherence time scale.In Paper II, we investigate, by applying an entanglement witness, the minimal number of zero-frequency current cross-correlation measurements needed to detect bipartite entanglement between two flying qubits.In Paper III, we consider energy and temperature fluctuations, and the influence of charging effects, in a metallic island tunnel coupled to a normal metallic lead, the so-called single electron box.In Paper IV, we investigate nanoscale quantum calorimetry and propose a setup consisting of a metallic island and a superconducting lead to realize a nanoscale calorimeter that may probe the energies of tunneling electrons.In Paper V, we investigate photon transport statistics of a microwave cavity, including the short-time statistics of single photon emissions and the long-time statistics of heat transport through the cavity.