Gate-controlled quantum dots and superconductivity in planar germanium

Hendrickx, Nico W.; Franke, David; Sammak, Amir; Kouwenhoven, Marc; Sabbagh, Diego; Yeoh, L. A.; Li, R.; Tagliaferri, Marco; Virgilio, Michele; Capellini, Giovanni; Scappucci, Giordano; Veldhorst, Menno

doi:10.1038/s41467-018-05299-x

Cited by 141 publications

(126 citation statements)

References 42 publications

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“…Josephson junctions are defined as a weak link between two superconducting reservoirs, which allows a supercurrent to be transported through intrinsically non-superconducting materials, as long as the junction is shorter than the coherence length [1,2]. While early Josephson junctions realized a weak link by using thin layers of oxide, micro-constrictions, point contacts or grain boundaries [3][4][5][6][7], access to complex mesoscopic semiconducting materials have led to Josephson junctions in which control over the charge carrier density enables in situ tuning of the junction transparency and critical current [8][9][10][11][12][13].…”

Section: Introductionmentioning

confidence: 99%

Multiple Andreev reflections and Shapiro steps in a Ge-Si nanowire Josephson junction

Ridderbos

Brauns

et al. 2019

Phys. Rev. Materials

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We present a Josephson junction based on a Ge-Si core-shell nanowire with transparent superconducting Al contacts, a building block which could be of considerable interest for investigating Majorana bound states, superconducting qubits and Andreev (spin) qubits. We demonstrate the dc Josephson effect in the form of a finite supercurrent through the junction, and establish the ac Josephson effect by showing up to 23 Shapiro steps. We observe multiple Andreev reflections up to the sixth order, indicating that charges can scatter elastically many times inside our junction, and that our interfaces between superconductor and semiconductor are transparent and have low disorder. arXiv:1908.07579v1 [cond-mat.mes-hall]

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Section: Introductionmentioning

confidence: 99%

Multiple Andreev reflections and Shapiro steps in a Ge-Si nanowire Josephson junction

Ridderbos

Brauns

et al. 2019

Phys. Rev. Materials

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“…Research on germanium mostly focused on self-assembled nanowires [18][19][20] and demonstrated single-shot spin readout [21] and coherent spin control [15]. However, strained germanium quantum wells were recently shown to support the formation of gate-controlled planar hole quantum dots [22,23]. Now, the crucial challenge is the demonstration of coherent control in this platform and the implementation of qubit-qubit gates for quantum information with holes.Here, we make this step and demonstrate single and two qubit logic with holes in planar germanium.…”

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confidence: 99%

“…Fabrication is based on silicon substrates and standard manufacturing materials. We grow strained germanium quantum wells, measured to have high hole mobilities µ > 500.000 cm 2 /Vs and a low effective hole mass m h = 0.09 m e [22,24], and predicted to reach m h = 0.05 m e at zero density [25,26]. This allows us to define quantum dots of comparatively large size and we find excellent control over the exchange interaction between the two dots.…”

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confidence: 99%

Fast two-qubit logic with holes in germanium

Hendrickx

Franke

Sammak

et al. 2020

Nature

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The promise of quantum computation with quantum dots has stimulated widespread research. Still, a platform that can combine excellent control with fast and high-fidelity operation is absent. Here, we show single and two-qubit operations based on holes in germanium. A high degree of control over the tunnel coupling and detuning is obtained by exploiting quantum wells with very low disorder and by working in a virtual gate space. Spin-orbit coupling obviates the need for microscopic elements and enables rapid qubit control with Rabi frequencies exceeding 100 MHz and a singlequbit fidelity of 99.3 %. We demonstrate fast two-qubit CX gates executed within 75 ns and minimize decoherence by operating at the charge symmetry point. Planar germanium thus matured within one year from a material that can host quantum dots to a platform enabling two-qubit logic, positioning itself as a unique material to scale up spin qubits for quantum information.Gate-defined quantum dots were recognized early on as a promising platform for quantum information [1] and a plethora of materials stacks has been investigated as host material. Initial research mainly focused on the low disorder semiconductor gallium arsenide [2,3]. Steady progress in the control and understanding of this system culminated in the initial demonstration and optimization of spin qubit operations [4,5] and the realization of rudimentary analog quantum simulations [6]. However, the omnipresent hyperfine interactions in group III-V materials seriously deteriorate the spin coherence, despite attempts to mitigate this by nuclear polarization [7]. Drastic improvements to the coherence times could be achieved by switching to the group IV semiconductor silicon, in particular when defining spin qubits in isotopically purified host crystal with vanishing concentrations of nonzero nuclear spin [8]. This enabled single qubit rotations with fidelities beyond 99.9% [9] and the execution of two-qubit logic gates with fidelities up to 98% [10-13], underlining the potential of spin qubits for quantum computation. Nevertheless, quantum dots in silicon are often formed at unintended locations and control over the tunnel coupling determining the strength of two-qubit interactions is limited. Moreover, the absence of a sizable spin-orbit coupling for electrons requires the inclusion of microscopic components such as onchip striplines or nanomagnets close to each qubit, which seriously complicates the design of large and dense 2D-structures. This, combined with the limited control over the location and coupling of the dots, remains an outstanding challenge for the scalability of these systems and a platform that can overcome these limitations would be highly desirable. * These two authors contributed equally to this work Hole states in semiconductors typically exhibit strong spinorbit coupling, which has enabled the demonstration of fast single qubit rotations [14,15] and additionally, unlike electrons, holes do not suffer from nearby valley states. In silicon, unfavorable band alignment...

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“…Therefore, undoped Ge/SiGe quantum wells are preferable for quantum dot fabrication. [11] The transport properties of undoped Ge/SiGe quantum wells are relatively unexplored and effective mass measurements have shown so far conflicting results. In Ref.…”

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confidence: 99%

Light effective hole mass in undoped Ge/SiGe quantum wells

et al. 2019

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We report density-dependent effective hole mass measurements in undoped germanium quantum wells. We are able to span a large range of densities (2.0 − 11 × 10 11 cm −2 ) in top-gated field effect transistors by positioning the strained buried Ge channel at different depths of 12 and 44 nm from the surface. From the thermal damping of the amplitude of Shubnikov-de Haas oscillations, we measure a light mass of 0.061me at a density of 2.2 × 10 11 cm −2 . We confirm the theoretically predicted dependence of increasing mass with density and by extrapolation we find an effective mass of ∼ 0.05me at zero density, the lightest effective mass for a planar platform that demonstrated spin qubits in quantum dots.Holes are rapidly emerging as a promising candidate for semiconductor quantum computing.[1-3] In particular, holes in germanium (Ge) bear favorable properties for quantum operation, such as strong spin-orbit coupling enabling electric driving without the need of microscopic objects,[1-3] large excited state splitting energies to isolate the qubit states, [4] and ohmic contacts to virtually all metals for hybrid superconducting-semiconducting research [5][6][7][8][9]. Furthermore, undoped planar Ge quantum wells with hole mobilities µ > 5 × 10 5 cm 2 /Vs were recently developed[10] and shown to support quantum dots [11,12] and single and two qubit logic,[3] providing scope to scale up the number of qubits.

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Gate-controlled quantum dots and superconductivity in planar germanium

Cited by 141 publications

References 42 publications

Multiple Andreev reflections and Shapiro steps in a Ge-Si nanowire Josephson junction

Multiple Andreev reflections and Shapiro steps in a Ge-Si nanowire Josephson junction

Fast two-qubit logic with holes in germanium

Light effective hole mass in undoped Ge/SiGe quantum wells

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