Session B44: Hole Spins in Semiconductor Quantum Dots

High-fidelity single and two-qubit gates in germanium

The promise of quantum computation with quantum dots inspired over two decades of intensive research on creating and manipulating single spins in semiconductor structures. The main focus was initially on GaAs, because of device maturity, and later on silicon, that can be isotopically purified to support extremely long quantum coherence times. Here, I will present germanium as a material that can combine these assets to form an excellent quantum material [1]. I will present the first planar germanium quantum dots, hosting single holes. I will show excellent quantum dot behavior with great control [2]. Single qubits can be defined on these holes with fidelities over 99% and can be coupled to execute two-qubit logic gates. Interestingly, these systems can also make contact to superconductors. I will present our latest efforst on hybrid structures, including micro meter long supercurrents and supercurrent discretization [2,3]. Germanium bears therefore great promise for fast and coherent quantum hardware.

[1] Sammak et al. arXiv:1809.02365
[2] Hendrickx et al. Nature Communications 9, 2835 (2018)
[3] Hendrickx et al. arXiv:1808.00763   

Hole spins in quantum wires and quantum dots

The spin states of heavy holes in semiconductor nanostructures are attracting significant attention for quantum information applications, with rapid progress being made by a number of groups over the past few years. In this talk I will discuss our recent progress studying spin properties of holes in silicon and gallium arsenide quantum wires and dots. In GaAs hole quantum wires we observe highly anisotropic spin properties, which can be used to directly probe the spin-orbit interaction in holes [1.2], as well as evidence for an emergeny spin gap. In silicon single quantum dots we are able to study the spin shell filling sequence for the first 8 holes, which is consistent with the Fock-Darwin states of a circular 2D quantum dot. However while the spin filling obeys Hund’s first rule, the hole-hole interaction energy is 90% of the orbital energy [3]. In few hole GaAs and Si double quantum dots we observe Pauli spin blockade, and find that the lifting of the spin blockade by an external magnetic field is highly anisotropic. These results highlight the promise, and challenges, of using holes for spin qubits.

[1] D. S. Miserev, A. Srinivasan, O. A. Tkachenko, V. A. Tkachenko, I. Farrer, D. A. Ritchie, A. R. Hamilton, and O. P. Sushkov, “Mechanisms for Strong Anisotropy of In-Plane g -Factors in Hole Based Quantum Point Contacts,” Physical Review Letters 119 (2017).
[2] A. Srinivasan, D. S. Miserev, K. L. Hudson, O. Klochan, K. Muraki, Y. Hirayama, D. Reuter, A. D. Wieck, O. P. Sushkov, et al., “Detection and Control of Spin-Orbit Interactions in a GaAs Hole Quantum Point Contact,” Physical Review Letters 118 (2017).
[3] S. D. Liles, R. Li, C. H. Yang, F. E. Hudson, M. Veldhorst, A. S. Dzurak, and A. R. Hamilton, “Spin and orbital structure of the first six holes in a silicon metal-oxide-semiconductor quantum dot,” Nature Communications 9 (2018).   

A Ge heavy hole spin qubit

The interest in hole spins as potential qubits has strongly increased in the past few years. Due to the intrinsically large spin orbit coupling, hole spin qubits should be electrically tunable and show high Rabi frequencies. Indeed in 2016, the first electrically tunable hole spin qubit with Rabi frequencies as high as 70MHz was demonstrated [1]. In our group we work with Ge. Results on the first Ge hole spin qubit will be presented. The qubit is formed in a Ge hut wire [2] double quantum dot. Rabi-frequencies of 140 MHz were reached [3] and Ramsey experiments revealed dephasing times T2* exceeding 130 ns. Furthermore, by measuring the spin relaxation time of holes in Ge hut wires an upper bound for the coherence time could be extracted [4]. While these experiments underline the potential of hole spins in Ge, randomly grown nanowires are not very appealing for the creation of more complex qubit devices. Latest results on positioned hut wires which emphasize the potential of this material platform for scalable quantum devices will be presented.

[1] R. Maurand et al., Nature Communications 7, 13575 (2016).
[2] J. J. Zhang et al., Phys. Rev. Lett. 109, 085502 (2012).
[3] H. Watzinger, J. Kukucka et al., Nature Communications 9, 3902 (2018).
[4] L. Vukušić et al., arXiv:1803.01775.   

Quantum electronics with holes in Si/Ge

Silicon and germanium form the core materials of the well-established microelectronics industry. Lately, they have enabled remarkable progress also in the raising field of quantum technologies, generating at the same time new fundamental questions and technological challenges. In fact, there is still a lot to know about these well-known semiconducting materials and their potential for quantum electronics. In this talk, I will focus on hole-based systems made from silicon and silicon-germanium nanostructures. I will present recent experiments dealing with spin-related effects in siicon quantum dot devices, and discuss their implications for hole-spin qubits. I will also report the realization of prototypical hybrid superconductor-semiconductor devices exploiting the superconducting proximity effect in a high-mobility two-dimensional hole gas confined to a germanium quantum well.   

Hole quantum dots in planar silicon and in GeSi nanowires

Ge/Si core/shell nanowires are suitable candidates for electrically driven spin qubits, and for the creation of Majorana fermions [1]. In highly tuneable hole quantum dots [2, 3], we observe shell filling of new orbitals and corresponding Pauli spin blockade [4]. In nanowires with superconducting Al leads we create a Josephson junction via proximity-induced superconductivity. A gate-tuneable supercurrent is observed with a maximum of ~60 nA [5]. We identify two different regimes: Cooper pair tunnelling via multiple subbands in the open regime the device [6], while near depletion the supercurrent is carried by single-particle levels of a quantum dot operating in the few-hole regime [5].

Secondly, we create ambipolar quantum dots in silicon nanoMOSFETs. We investigate the conformity of Al, Ti and Pd nanoscale gates by means of transmission electron microscopy [7]. We define low-disorder electron quantum dots with Pd gates [8], and depletion-mode hole quantum dots in undoped silicon [9]. For the latter we use fixed charge in a SiO₂/Al₂O₃ dielectric stack to induce a 2DHG at the Si/SiO₂ interface. The depletion-mode design avoids complex multilayer architectures requiring precision alignment and allows directly adopting best practices already developed for depletion dots in other material systems.

[1] C. Kloeffel et al., Phys. Rev. B 84, 195314 (2011).
[2] M. Brauns et al., Applied Physics Letters 109, p. 143113 (2016).
[3] F. Froning et al., Applied Physics Letters 113, 073102 (2018).
[4] M. Brauns et al., Phys. Rev. B 94, 041441(R) (2016).
[5] J. Ridderbos et al., Advanced Materials, 1802257, (2018).
[6] J. Xiang et al., Nature Nanotechnology 1, 3 (2006).
[7] P. C. Spruijtenburg et al., Nanotechnology, (2018).
[8] M. Brauns et al., Scientific Reports 8, 5690, (2018).
[9] S. V. Amitonov et al., Applied Physics Letters 112, p. 023102 (2018).