TEP Seminar - Michelle Lienhart

Quantum Dot Molecules as Spin-Photon Interfaces
 
Spin-photon interfaces based on III-V semiconductor quantum dots (QDs) are a promising platform for quantum technologies, combining strong light-matter interactions, robust spin-photon selection rules, and ease of integration into optoelectronic devices. It has been demonstrated that a QD hosting a single spin can be used to generate a string of entangled photons, a linear cluster state [1]. Tunnel-coupled pairs of InGaAs QDs, known as QD molecules (QDMs), can host two electron spins, which may be used to generate multi-dimensional photonic cluster states, an important resource for quantum information applications [2, 3].
In our experiments, we study a single QDM embedded in a p-i-n diode structure. We directly measure the orbital phonon relaxation rate between the energy states of the neutral exciton in the QDM and benchmark our findings against microscopic k·p theory [4]. Using an optical charging protocol, we can deterministically prepare our QDM with one or two spins and subsequently tune their orbital configuration into resonance with both QDs [5]. Under optical excitation, the two-electron state exhibits intriguing features, such as spin pumping from singlet (S) to triplet (T) states and long spin relaxation times. Moreover, we demonstrate coherent spin control in the QDM using ultrashort optical pulses to drive spin rotations and characterize the spin dynamics across different orbital configurations.
 
References
[1]   D. Cogan, et al., Nat. Photon., 17, 324 (2023).
[2]   M. Gimeno-Segovia, et al., Phys. Rev. Lett., 123, 070501 (2019).
[3]   S. E. Economou, et al., Phys. Rev. Lett., 105, 093601 (2010).
[4]   M. Lienhart, et al., Phys. Rev. B 112, 235305 (2025).
[5]   C. Thalacker, et al., PRX Quantum 7, 033056 (2026).