| 1. |
- Kringhøj, A., et al.
(författare)
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Anharmonicity of a superconducting qubit with a few-mode Josephson junction
- 2018
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Ingår i: Physical Review B. - 2469-9950. ; 97:6
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Tidskriftsartikel (refereegranskat)abstract
- Coherent operation of gate-voltage-controlled hybrid transmon qubits (gatemons) based on semiconductor nanowires was recently demonstrated. Here we experimentally investigate the anharmonicity in epitaxial InAs-Al Josephson junctions, a key parameter for their use as a qubit. Anharmonicity is found to be reduced by roughly a factor of 2 compared to conventional metallic junctions and dependent on gate voltage. Experimental results are consistent with a theoretical model, indicating that Josephson coupling is mediated by a small number of highly transmitting modes in the semiconductor junction.
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| 2. |
- Deng, M. T., et al.
(författare)
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Majorana bound state in a coupled quantum-dot hybrid-nanowire system
- 2016
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 354:6319, s. 1557-1562
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Tidskriftsartikel (refereegranskat)abstract
- Hybrid nanowires combining semiconductor and superconductor materials appear well suited for the creation, detection, and control of Majorana bound states (MBSs). We demonstrate the emergence of MBSs from coalescing Andreev bound states (ABSs) in a hybrid InAs nanowire with epitaxial Al, using a quantum dot at the end of the nanowire as a spectrometer. Electrostatic gating tuned the nanowire density to a regime of one or a few ABSs. In an applied axial magnetic field, a topological phase emerges in which ABSs move to zero energy and remain there, forming MBSs. We observed hybridization of the MBS with the end-dot bound state, which is in agreement with a numerical model. The ABS/MBS spectra provide parameters that are useful for understanding topological superconductivity in this system.
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| 3. |
- Krøjer, S., et al.
(författare)
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Demonstrating Majorana non-Abelian properties using fast adiabatic charge transfer
- 2022
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Ingår i: Physical Review B. - 2469-9950. ; 105
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Tidskriftsartikel (refereegranskat)abstract
- Demonstration of Majorana non-Abelian properties is a major challenge in the field of topological superconductivity. In this work, we propose a minimal device and protocol for testing non-Abelian properties using charge-transfer operations between a quantum dot and two Majorana bound states combined with reading the parity state using a second dot. We use an adiabatic perturbation theory to find fast adiabatic paths to perform operations and to account for nonadiabatic errors. We find the ideal parameter sweep and a region in parameter space that reduces the charge-transfer operation time 1–2 orders of magnitude with respect to constant velocity driving. Using realistic parameters, we estimate that the lower bound for the timescale can be reduced to ∼10 ns. Deviations from the ideal parameters lead to the accumulation of an undesired dynamical phase, affecting the outcome of the proposed protocol. We furthermore suggest to reduce the influence from the dynamical phase using a flux echo. The echo protocol is based on the 4π periodicity of the topological state, absent for trivial bound states.
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| 4. |
- Maiani, A., et al.
(författare)
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Topological superconductivity in semiconductor-superconductor-magnetic-insulator heterostructures
- 2021
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Ingår i: Physical Review B. - 2469-9950. ; 103:10
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Tidskriftsartikel (refereegranskat)abstract
- Hybrid superconductor-semiconductor heterostructures are promising platforms for realizing topological superconductors and exploring Majorana bound state physics. Motivated by recent experimental progress, we theoretically study how magnetic insulators offer an alternative to the use of external magnetic fields for reaching the topological regime. We consider different setups, where (1) the magnetic insulator induces an exchange field in the superconductor, which leads to a splitting in the semiconductor by proximity effect, and (2) the magnetic insulator acts as a spin-filter tunnel barrier between the superconductor and the semiconductor. We show that the spin splitting in the superconductor alone cannot induce a topological transition in the semiconductor. To overcome this limitation, we propose to use a spin-filter barrier that enhances the magnetic exchange and provides a mechanism for a topological phase transition. Moreover, the spin-dependent tunneling introduces a strong dependence on the band alignment, which can be crucial in quantum-confined systems. This mechanism opens up a route towards networks of topological wires with fewer constraints on device geometry compared to previous devices that require external magnetic fields.
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| 5. |
- MOLENKAMP, LW, et al.
(författare)
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SCALING OF THE COULOMB ENERGY DUE TO QUANTUM FLUCTUATIONS IN THE CHARGE ON A QUANTUM-DOT
- 1995
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 75:23, s. 4282-4285
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Tidskriftsartikel (refereegranskat)abstract
- The charging energy of a quantum dot is measured through the effect of its potential on the conductance of a second dot. This technique allows a measurement of the scaling of the dots charging energy with the conductance of the tunnel barriers leading to the dot. We find that the charging energy scales quadratically with the reflection probability of the barriers. The observed power law agrees with a recent theory.
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| 6. |
- O'Farrell, E. C.T., et al.
(författare)
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Hybridization of Subgap States in One-Dimensional Superconductor-Semiconductor Coulomb Islands
- 2018
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Ingår i: Physical Review Letters. - 0031-9007. ; 121:25
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Tidskriftsartikel (refereegranskat)abstract
- We present measurements of one-dimensional superconductor-semiconductor Coulomb islands, fabricated by gate confinement of a two-dimensional InAs heterostructure with an epitaxial Al layer. When tuned via electrostatic side gates to regimes without subgap states, Coulomb blockade reveals Cooper-pair mediated transport. When subgap states are present, Coulomb peak positions and heights oscillate in a correlated way with magnetic field and gate voltage, as predicted theoretically, with (anti)crossings in (parallel) transverse magnetic field indicating Rashba-type spin-orbit coupling. Overall results are consistent with a picture of overlapping Majorana zero modes in finite wires.
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| 7. |
- Vaitiekenas, S., et al.
(författare)
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Evidence for spin-polarized bound states in semiconductor–superconductor–ferromagnetic-insulator islands
- 2022
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Ingår i: Physical Review B. - 2469-9950. ; 105
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Tidskriftsartikel (refereegranskat)abstract
- We report Coulomb blockade transport studies of semiconducting InAs nanowires grown with epitaxial superconducting Al and ferromagnetic insulator EuS on overlapping facets. Comparing experiment to a theoretical model, we associate cotunneling features in even-odd bias spectra with spin-polarized Andreev levels. Results are consistent with zero-field spin splitting exceeding the induced superconducting gap. Energies of subgap states are tunable on either side of zero via electrostatic gates.
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