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Ultra-Slow Orbital and Spin Dynamics in an Electrically Tunable Quantum Dot Molecule

Christopher Thalacker, Michelle Lienhart, Markus Stöcker, N. Akhlaq, Irina Ivanova, Nikolai Bart, Arne Ludwig, Johannes Schall, Stephan Reitzenstein, D. Reuter, Steffen Wilksen, Christopher Gies, Krzysztof Gawarecki, Paweł Machnikowski, Kai Müller, Jonathan James Finley

Published Aug 10, 2026• 0 citations
Publisher

Abstract

Tunnel-coupled optically active quantum dot molecules (QDMs) have the potential to operate as spin-photon interfaces with coupled spins that interact with two different photon frequencies at the same time. A prerequisite is to deterministically prepare two (electron or hole) spins in the QDM and be able to electrically tune the orbital state couplings. Here, we demonstrate the sequential optical charging of a single QDM with two-electron spins while simultaneously maintaining the ability to widely tune orbital couplings using static electric fields and optically drive the system for quantum light generation. We optically prepare one- and two-spin states, initialize via optical pumping and explore orbital and spin relaxation dynamics for one- and two-spin states as a function of the energy detuning and hybridization of orbital states. For two-spin states, remarkably long singlet-triplet relaxation times are observed, extending beyond ∼ 100 μ s with strong dependence on the relative energy of ground and excited two-spin states. Qualitative agreement is observed with k · p calculations of phonon-mediated spin relaxation. Our results provide new quantitative understanding of the dynamics of one- and two-spin states and address a number of key prerequisites for using QDMs to create multidimensional photonic cluster states.

DOI
10.1103/qgr2-968q

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Tunnel-coupled optically active quantum dot molecules (QDMs) have the potential to operate as spin-photon interfaces with coupled spins that interact with two different photon frequencies at the same time. A prerequisite is to deterministically prepare two (electron or hole) spins in the QDM and be able to electrically tune the orbital state couplings. Here, we demonstrate the sequential optical charging of a single QDM with two-electron spins while simultaneously maintaining the ability to widely tune orbital couplings using static electric fields and optically drive the system for quantum light generation. We optically prepare one- and two-spin states, initialize via optical pumping and explore orbital and spin relaxation dynamics for one- and two-spin states as a function of the energy detuning and hybridization of orbital states. For two-spin states, remarkably long singlet-triplet relaxation times are observed, extending beyond ∼ 100 μ s with strong dependence on the relative energy of ground and excited two-spin states. Qualitative agreement is observed with k · p calculations of phonon-mediated spin relaxation. Our results provide new quantitative understanding of the dynamics of one- and two-spin states and address a number of key prerequisites for using QDMs to create multidimensional photonic cluster states.
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A pair of tunnel-coupled quantum dots has advantages over single dots in tunability and spin coherence, but far less work has been done to measure and understand the quantum optics of this system. In particular, the two-electron singlet-triplet system with one electron in each dot generates an eight-level system in which two-level and four-level double-\ensuremath{\Lambda} systems are coupled only through the hyperfine interaction and other spin relaxation processes. We first measure the emission spectrum under resonant driving with a continuous-wave laser and then perform time-correlated Hanbury Brown-Twiss (HBT) and Hong-Ou-Mandel (HOM) interferometric measurements to examine nonclassical photon properties of spin-flip Raman emission and resonance fluorescence. The shapes of second-order correlation functions $[{g}^{(2)}(\ensuremath{\tau})]$ from HBT are strikingly different between resonance fluorescence and spin-flip Raman emission, reflecting the spin dynamics of the singlet-triplet system. In addition, our two-photon HOM measurements demonstrate a high raw visibility of 0.96, with a coherence time exceeding the radiative lifetime.
We show that spin-orbit coupling in a quantum dot molecule allows for coherent manipulation of two-electron spin states using Raman transitions. Such two-electron spin states defined by the singlet and triplet states of two exchange-coupled quantum dots can have favorable coherence properties. In addition, two of the four metastable ground states in this system can be used as auxiliary states that could facilitate implementation of tasks such as mapping of spin states to that of a single propagating photon. We find that even weak spin-orbit effects---manifesting themselves as slightly different ggg factors for the electron and the hole---would allow for the coherent Raman coupling of the singlet-triplet states. We also discuss the possibilities for implementing quantum optical techniques for spin preparation and manipulation.
Recent studies in self-assembled InAs quantum dots (QDs) for applications in quantum information processing have demonstrated initialization, readout and long decoherence time of an electron spin confined in a single QD. These arguably fulfill three out of the five DiVincenzo criteria for the physical implementation of quantum computation. Based on recent developments self-assembled InAs quantum dot molecules (QDMs), several advancements in the optical manipulation of two-electron spin states have been made. As a continuation of these studies towards a full two-qubit system, this thesis addresses one of the remaining criteria concerning a universal set of quantum gates. The physical platform for two-qubit gates is provided by two electrons confined in the QDM where the Coulomb exchange interaction gives rise to the singlet and triplet manifolds. In a transverse magnetic field, an eight-level system consisting of four singlet-triplet spin states, four optical excited states and twelve dipole allowed transitions arises. Spin initialization via multi-laser optical pumping is demonstrated with near unity fidelity for three of the spin states, while the remaining one can, in principle, be achieved by coherent optical pumping using four CW lasers. The effects of dynamic nuclear spin polarization, arising from the coupling between the electrons and the surrounding nuclei, is evident in the frequency pulling and pushing lineshapes in absorption profiles. This thesis shows that the optical nuclear spin locking that was demonstrated in a single QD earlier is effective in QDMs, yielding a long spin decoherence time of about 1 microsecond. Spectroscopic evidence suggests that this is accompanied by the first evidence of a narrowing in the Overhauser field distribution. The results reveal that stabilization of nuclear spin polarization in both QDs is achieved by optical manipulations in the top QD, demonstrating the effect of non-local nuclear spin locking. Finally, it is shown theoretically that pulsed excitation results in single spin rotations and in conjunction with the Coulomb exchange interaction, provides the ingredients for a universal two-qubit gate. A feasible experimental demonstration of the two-qubit gate is proposed, along with the methodology for the population readout of individual spin states.
We study the loading of electrons into a quantum dot with dynamically controlled tunnel barriers. We introduce a method to measure tunneling rates for individual discrete states and to identify their relaxation paths. Exponential selectivity of the tunnel coupling enables loading into specific quantum dot states by tuning independently energy and rates. While for the single-electron case orbital relaxation leads to fast transition into the ground state, for electron pairs triplet-to-singlet relaxation is suppressed by long spin-flip times. This enables the fast gate-controlled initialization of either a singlet or a triplet electron pair state in a quantum dot with broad potential applications in quantum technologies.
Connecting quantum processors over long distances using photon qubits would enable more complex quantum computing architectures and quantum networks. Recently a protocol to transfer a quantum state from a photonic qubit to a singlet-triplet spin qubit in GaAs/AlGaAs heterostructure has been theoretically demonstrated. In practice, the coherent transfer is facilitated by a hybrid device where a GaAs/AlGaAs gate-defined quantum dot (GDQD) is tunnel coupled to a self-assembled quantum dot (SAQD), in which a conversion between a photon state and a spin state takes place. This work addressed the three requirements of the optical-interface devices based on tunnel coupling between the GDQD and the SAQD: (i) a properly designed GaAs/AlGaAs heterostructure where the two-dimensional electron gas (2DEG) can be electrically tuned into resonance with the InAs SAQD; (ii) high-mobility 2DEG to form and operate a GDQD; (iii) precise alignment between the SAQD and the GDQD. It is proven that the InAs SAQD can be tuned to be charged, discharged, and resonant with the 2DEG by applying a local electric field.