Session G42: Spin-valley Qubits in 2D Semiconductor Quantum Dots

Theory of spin and valley lifetimes in bilayer graphene quantum dots

The low nuclear spin density and weak spin-orbit coupling found in graphene allows for long electron spin relaxation and coherence times. The spin and valley degrees of freedom of localized electrons can therefore be seen as potential embodiments of classical or quantum bits for computation [1,2]. However, the formation of localized states in quantum dots requires some form of badgap engineering, and the mechanisms for spin and valley relaxation have so far not been completely understood. Bilayer graphene has an electrically controllable bandgap that allows for the formation of electrostatically defined quantum dots. We present theoretical considerations regarding the formation of quantum dots in graphene and report on recent progress in understanding the relevant physical mechanisms of spin and valley relaxation in electrostatically gated bilayer graphene quantum dots [3]. We then compare theoretically obtained spin and valley relaxation times and their dependence on the applied magnetic field with the latest experimental data [4,5,6].

[1] B. Trauzettel, D.V. Bulaev, D. Loss, and G. Burkard, Nature Phys. 3, 192 (2007).

[2] M. Droth and G. Burkard, Spintronics with graphene quantum dots, Phys. Status Solidi RRL 10, 75 (2016).

[3] L. Wang and G. Burkard, manuscript in preparation (2023).

[4] L. M. Gächter, R. Garreis, et al., PRX Quantum 3, 020343 (2022).

[5] L. Banzerus, K. Hecker, et al., Nature Comm. 13, 3637 (2022).

[6] R. Garreis, C. Tong, et al., arXiv:2304.00980 (2023).   

Symmetry protected spin-valley blockade in graphene quantum dots

Particle-hole symmetry plays an important role for the characterization of topological phases in solid-state systems. It is found, for example, in free-fermion systems at half filling, and it is closely related to the notion of antiparticles in relativistic field theories. In the low energy limit, graphene is a prime example of a gapless particle-hole symmetric system described by an effective Dirac equation, where topological phases can be understood by studying ways to open a gap by preserving (or breaking) symmetries. An important example is the intrinsic Kane-Mele spin-orbit gap of graphene, which leads to a lifting of the spin-valley degeneracy and renders graphene a topological insulator in a quantum spin Hall phase, while preserving particle-hole symmetry. Here, we show that bilayer graphene allows realizing electron-hole double quantum-dots that exhibit nearly perfect particle-hole symmetry, where transport occurs via the creation and annihilation of single electron-hole pairs with opposite quantum numbers [1]. Moreover, we show that the particle-hole symmetric spin and valley textures lead to a protected single-particle spin-valley blockade. The latter will allow robust spin-to-charge conversion and valley-to-charge conversion, which is essential for the operation of spin and valley qubits.

 

[1] L. Banszerus, S. Möller, K. Hecker, E. Icking, K. Watanabe, T. Taniguchi, F. Hassler, C. Volk, and C. Stampfer, Nature 618, 51 (2023)   

Gate-defined accumulation-mode quantum dots in monolayer and bilayer WSe2

This talk discusses the fabrication and measurement of single and double quantum dot devices based on monolayer and bilayer WSe₂ with a target application of spin-valley qubits.  For single dot devices, an intrinsic readout method in bilayer WSe₂ for spin-valley lifetime measurement will be described along with devices designed to implement it.  For double dot devices, I will describe local gate control of monolayer WSe₂ to create single-electron/single-hole p-n junctions for on-demand single photon emission.   

Towards spin-valley qubits based on in-gap quantum dots in the transition metal dichalcogenides

Spins confined to point defects and colour centres have become active components in emerging quantum technologies, with applications in quantum sensing, computation, simulation, and communication. Different from conventional semiconductors, electron spins in atomically-thin (2D) semiconductors with hexagonal lattices – in the presence of inversion asymmetry and strong spin-orbit coupling – are strongly coupled to an additional valley degree of freedom. Such “spin-valley locking” can be inherited by in-gap states due to atomic point defects [1] making them promising candidates for spin-valley quantum bits (qubits), that may be coherently controlled electrically and optically.

Towards this end, we have developed a robust device architecture allowing for transparent electrical contact to atomically-thin layers of MoS₂, down to temperatures as low as T=150mK. The high spectral resolution (50 μeV) enables us to probe the spin-valley eigenspectrum in the ground state transitions of in-gap quantum dots [1]. For the first time, we demonstrate spin-valley locking at the single-electron level, reflected in a pronounced Zeeman anisotropy with an effective out-of-place g-factor as large as g ≈ 8. From a small but finite in-plane g-factor (g ≅ 0.7) we quantify the spin-orbit coupling strength, and hence the degree of spin-valley locking. 

Our results provide new insights into the spin texture of spin-valley locked in-gap states in MoS₂, relevant towards their application as electrically-driven spin-valley qubits.

[1] Krishnan et al. Nano Letters 23, 6171 (2023)   

Theory of coherent control of spin-valley qubits

Certain semiconductors have an electronic conduction or valence band with a multi-valley structure. This feature opens up the possibility of leveraging the combination of the electronic spin and valley degrees of freedom for coherent quantum dynamics, including opportunities for quantum information processing. In this theory talk, I will introduce the concept of a spin-valley qubit via specific example materials, and review recent progress and open challenges toward exploiting these qubits for quantum information.