Quantum dots are an excellent platform for hosting and manipulating a single spin, which is the quintessence of a two level system. Furthermore, they have promising prospects for scalable quantum computing due to their compatibility with standard semiconductor technology and their long coherence. The long coherence comes from the weak interaction of the spin with the environment, which at the same time presents a challenge for their mutual coupling. A tunable coupling is actually necessary for two-qubit gate operations allowing universal quantum computing. Several approaches have been suggested in the literature. One of them utilizes the Heisenberg exchange interaction [1,2]. But since the interaction is short-ranged and couples only neighboring spins it is difficult to scale this scheme up. Another approach suggests that a coherent spin-spin coupling could be achieved by means of spin-photon interaction [3]. The spin does not directly couple to the electric field of the resonator, since it does not have an electric dipole moment. However, the spin can be endowed with an effective electric dipole through a spin-orbit interaction. Therefore strong coupling between the spin and the resonator electric field can be achieved and provides a way to couple two distant spins via a photon.

In the Nanoelectronics group we work on long-distance coherent spin-spin coupling mediated by a photon, which could be achieved with hole spins hosted in Ge hut wire quantum dots connected via a high-impedance microwave resonator. Our system could provide several advantages compared to the other competing systems, namely strong spin-orbit interaction [4,5], suppressed hyperfine induced decoherence [6], large lever arm and low parasitic capacitance.

The achievement of such coupling in our system would not only be fundamentally interesting in the field of quantum mechanical open systems and hole physics, but would also pave the way to a scalable quantum processor based on spin qubits.

References:

[1] Baart, T., Fujita, T., Reichl, C. et al. Coherent spin-exchange via a quantum mediator. Nature nanotechnology 12, 26–30 (2017).

[2] Malinowski, F.K., Martins, F., Smith, T.B. et al. Fast spin exchange across a multielectron mediator. Nature communications 10, 1–6 (2019).

[3] Burkard, G., Gullans, M.J., Mi, X. et al. Superconductor–semiconductor hybrid-circuit quantum electrodynamics. Nat Rev Phys 2, 129–140 (2020).

[4] Kloeffel, C., Trif, M., Loss, D. Strong spin-orbit interaction and helical hole states in Ge/Si nanowires. Physical Review B 84, 195314 (2011).

[5] Kloeffel, C., Rancic, M. J., Loss, D. Direct Rashba spin-orbit interaction in Si and Ge nanowires with different growth directions. Physical Review B 97, 35422 (2018).

[6] Fischer, J., Coish, W., Bulaev, D., Loss, D. Spin decoherence of a heavy hole coupled to nuclear spins in a quantum dot. Physical Review B 78, 155329 (2008).