| 1. |
- Josefsson, Martin, et al.
(author)
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A quantum-dot heat engine operating close to the thermodynamic efficiency limits
- 2018
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In: Nature Nanotechnology. - : Springer Science and Business Media LLC. - 1748-3387 .- 1748-3395. ; 13:10, s. 920-924
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Journal article (peer-reviewed)abstract
- Cyclical heat engines are a paradigm of classical thermodynamics, but are impractical for miniaturization because they rely on moving parts. A more recent concept is particle-exchange (PE) heat engines, which uses energy filtering to control a thermally driven particle flow between two heat reservoirs1,2. As they do not require moving parts and can be realized in solid-state materials, they are suitable for low-power applications and miniaturization. It was predicted that PE engines could reach the same thermodynamically ideal efficiency limits as those accessible to cyclical engines3–6, but this prediction has not been verified experimentally. Here, we demonstrate a PE heat engine based on a quantum dot (QD) embedded into a semiconductor nanowire. We directly measure the engine’s steady-state electric power output and combine it with the calculated electronic heat flow to determine the electronic efficiency η. We find that at the maximum power conditions, η is in agreement with the Curzon–Ahlborn efficiency6–9 and that the overall maximum η is in excess of 70% of the Carnot efficiency while maintaining a finite power output. Our results demonstrate that thermoelectric power conversion can, in principle, be achieved close to the thermodynamic limits, with direct relevance for future hot-carrier photovoltaics10, on-chip coolers or energy harvesters for quantum technologies.
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| 2. |
- Josefsson, Martin, et al.
(author)
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Double quantum-dot engine fueled by entanglement between electron spins
- 2020
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In: Physical Review B. - 2469-9950. ; 101:8
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Journal article (peer-reviewed)abstract
- The laws of thermodynamics allow work extraction from a single heat bath provided that the entropy decrease of the bath is compensated for by another part of the system. We propose a thermodynamic quantum engine that exploits this principle and consists of two electrons on a double quantum dot (QD). The engine is fueled by providing it with singlet spin states, where the electron spins on different QDs are maximally entangled, and its operation involves only changing the tunnel coupling between the QDs. Work can be extracted since the entropy of an entangled singlet is lower than that of a thermal (mixed) state, although they look identical when measuring on a single QD. We show that the engine is an optimal thermodynamic engine in the long-time limit. In addition, we include a microscopic description of the bath and analyze the engine's finite-time performance using experimentally relevant parameters.
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| 3. |
- Josefsson, Martin, et al.
(author)
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Optimal power and efficiency of single quantum dot heat engines : Theory and experiment
- 2019
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In: Physical Review B. - 2469-9950. ; 99:23
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Journal article (peer-reviewed)abstract
- Quantum dots (QDs) can serve as near perfect energy filters and are therefore of significant interest for the study of thermoelectric energy conversion close to thermodynamic efficiency limits. Indeed, recent experiments in [Nat. Nano. 13, 920 (2018)1748-338710.1038/s41565-018-0200-5] realized a QD heat engine with performance near these limits and in excellent agreement with theoretical predictions. However, these experiments also highlighted a need for more theory to help guide and understand the practical optimization of QD heat engines, in particular regarding the role of tunnel couplings on the performance at maximum power and efficiency for QDs that couple seemingly weakly to electronic reservoirs. Furthermore, these experiments also highlighted the critical role of the external load when optimizing the performance of a QD heat engine in practice. To provide further insight into the operation of these engines we use the Anderson impurity model together with a Master equation approach to perform power and efficiency calculations up to co-tunneling order. This is combined with additional thermoelectric experiments on a QD embedded in a nanowire where the power is measured using two methods. We use the measurements to present an experimental procedure for efficiently finding the external load RP which should be connected to the engine to optimize power output. Our theoretical estimates of RP show good agreement with the experimental results, and we show that second order tunneling processes and nonlinear effects have little impact close to maximum power, allowing us to derive a simple analytic expression for RP. In contrast, we find that the electron contribution to the thermoelectric efficiency is significantly reduced by second order tunneling processes, even for rather weak tunnel couplings.
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| 4. |
- Majidi, Danial, et al.
(author)
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Quantum Confinement Suppressing Electronic Heat Flow below the Wiedemann–Franz Law
- 2022
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In: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 22:2, s. 630-635
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Journal article (peer-reviewed)abstract
- The Wiedemann–Franz law states that the charge conductance and the electronic contribution to the heat conductance are proportional. This sets stringent constraints on efficiency bounds for thermoelectric applications, which seek a large charge conduction in response to a small heat flow. We present experiments based on a quantum dot formed inside a semiconducting InAs nanowire transistor, in which the heat conduction can be tuned significantly below the Wiedemann–Franz prediction. Comparison with scattering theory shows that this is caused by quantum confinement and the resulting energy-selective transport properties of the quantum dot. Our results open up perspectives for tailoring independently the heat and electrical conduction properties in semiconductor nanostructures.
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| 5. |
- Svilans, Artis, et al.
(author)
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Thermoelectric Characterization of the Kondo Resonance in Nanowire Quantum Dots
- 2018
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In: Physical Review Letters. - 0031-9007. ; 121:20
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Journal article (peer-reviewed)abstract
- We experimentally verify hitherto untested theoretical predictions about the thermoelectric properties of Kondo correlated quantum dots (QDs). The specific conditions required for this study are obtained by using QDs epitaxially grown in nanowires, combined with a recently developed method for controlling and measuring temperature differences at the nanoscale. This makes it possible to obtain data of very high quality both below and above the Kondo temperature, and allows a quantitative comparison with theoretical predictions. Specifically, we verify that Kondo correlations can induce a polarity change of the thermoelectric current, which can be reversed either by increasing the temperature or by applying a magnetic field.
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| 6. |
- Aasen, David, et al.
(author)
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Milestones toward Majorana-based quantum computing
- 2016
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In: Physical Review X. - 2160-3308. ; 6:3
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Journal article (peer-reviewed)abstract
- We introduce a scheme for preparation, manipulation, and read out of Majorana zero modes in semiconducting wires with mesoscopic superconducting islands. Our approach synthesizes recent advances in materials growth with tools commonly used in quantum-dot experiments, including gate control of tunnel barriers and Coulomb effects, charge sensing, and charge pumping. We outline a sequence of milestones interpolating between zero-mode detection and quantum computing that includes (1) detection of fusion rules for non-Abelian anyons using either proximal charge sensors or pumped current, (2) validation of a prototype topological qubit, and (3) demonstration of non-Abelian statistics by braiding in a branched geometry. The first two milestones require only a single wire with two islands, and additionally enable sensitive measurements of the system's excitation gap, quasiparticle poisoning rates, residual Majorana zero-mode splittings, and topological-qubit coherence times. These pre-braiding experiments can be adapted to other manipulation and read out schemes as well.
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| 7. |
- Awoga, Oladunjoye A., et al.
(author)
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Controlling Majorana hybridization in magnetic chain-superconductor systems
- 2024
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In: Physical Review Research. - 2643-1564. ; 6:3
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Journal article (peer-reviewed)abstract
- We propose controlling the hybridization between Majorana zero modes at the ends of magnetic adatom chains on superconductors by an additional magnetic adatom deposited close by. By tuning the additional adatom's magnetization, position, and coupling to the superconductor, we can couple and decouple the Majorana modes, as well as control the ground-state parity. The scheme is independent of microscopic details in ferromagnetic and helical magnetic chains on superconductors, with and without spin-orbit coupling, which we show by studying their full microscopic models and their common low-energy description. Our results show that scanning tunneling microscopy and electron-spin-resonance techniques are promising tools for controlling the Majorana hybridization in magnetic adatoms-superconductor setups, providing a basis for Majorana parity measurements, fusion, and braiding techniques.
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| 8. |
- Awoga, Oladunjoye A., et al.
(author)
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Mitigating disorder-induced zero-energy states in weakly coupled superconductor-semiconductor hybrid systems
- 2023
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In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 107:18
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Journal article (peer-reviewed)abstract
- Disorder has appeared as one of the main mechanisms to induce topologically trivial zero-energy states in superconductor-semiconductor systems, thereby challenging the detection of topological superconductivity and Majorana bound states. Here, we demonstrate that, for disorder in any part of the system, the formation of disorder-induced trivial zero-energy states can, to a large extent, be mitigated by keeping the coupling between the semiconductor and superconductor weak. The only exception is strong disorder in the semiconductor, where instead the strong-coupling regime is somewhat more robust against disorder. Furthermore, we find that the topo-logical phase in this weak-coupling regime is robust against disorder, with a large and well-defined topological gap which is highly beneficial for topological protection. Our work shows the advantages and disadvantages of weak and strong couplings under disorder, important for designing superconductor-semiconductor hybrid structures.
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| 9. |
- Benestad, Jacob, et al.
(author)
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Machine-learned tuning of artificial Kitaev chains from tunneling spectroscopy measurements
- 2024
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In: Physical Review B. - 2469-9950. ; 110:7
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Journal article (peer-reviewed)abstract
- We demonstrate reliable machine-learned tuning of quantum-dot-based artificial Kitaev chains to Majorana sweet spots, using the covariance matrix adaptation algorithm. We show that a loss function based on local tunneling spectroscopy features of a chain with two additional sensor dots added at its ends provides a reliable metric to navigate parameter space and find points where crossed Andreev reflection and elastic cotunneling between neighboring sites balance in such a way to yield near-zero-energy modes with very high Majorana quality. We simulate tuning of two- and three-site Kitaev chains, where the loss function is found from calculating the low-energy spectrum of a model Hamiltonian that includes Coulomb interactions and finite Zeeman splitting. In both cases, the algorithm consistently converges towards high-quality sweet spots. Since tunneling spectroscopy provides one global metric for tuning all on-site potentials simultaneously, this presents a promising way towards tuning longer Kitaev chains, which are required for achieving topological protection of the Majorana modes.
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| 10. |
- Debbarma, Rousan, et al.
(author)
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Effects of Parity and Symmetry on the Aharonov-Bohm Phase of a Quantum Ring
- 2021
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In: Nano Letters. - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992.
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Journal article (peer-reviewed)abstract
- We experimentally investigate the properties of one-dimensional quantum rings that form near the surface of nanowire quantum dots. In agreement with theoretical predictions, we observe the appearance of forbidden gaps in the evolution of states in a magnetic field as the symmetry of a quantum ring is reduced. For a twofold symmetry, our experiments confirm that orbital states are grouped pairwise. Here, a π-phase shift can be introduced in the Aharonov-Bohm relation by controlling the relative orbital parity using an electric field. Studying rings with higher symmetry, we note exceptionally large orbital contributions to the effective g-factor (up to 300), which are many times higher than those previously reported. These findings show that the properties of a phase-coherent system can be significantly altered by the nanostructure symmetry and its interplay with wave function parity.
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