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Coulomb interactions strongly influence the spectrum and the wave functions of few electrons or holes confined in a quantum dot. In particular, when the confinement potential is not too strong, the Coulomb repulsion triggers the formation of a correlated state, the Wigner molecule, where the particles tend to split apart. We show that the anisotropy of the confinement potential strongly enhances the molecularization process and affects the performances of quantum-dot systems used as spin qubits. Relying on analytical and numerical solutions of the two-particle problem -- both in a simplified single-band approximation and in realistic setups -- we highlight the exponential suppression of the singlet-triplet gap with increasing anisotropy. We compare the molecularization effects in different semiconductor materials and discuss how they specifically hamper Pauli spin blockade readout and reduce the exchange interactions in two-qubit gates., 12 pages and 8 figures in the main text + 5 pages of appendices

Alloyed ${\mathrm{InAs}}_{0.2}{\mathrm{P}}_{0.8}/\mathrm{InP}$ nanowire quantum dot molecules reveal nontrivial electric-field evolution of the bright-exciton spectra; this was studied here using the atomistic theory. For a quantum dot molecule composed of two nanowire quantum dots of dissimilar sizes, the overall field dependence resembles the typical self-assembled quantum dot molecule spectra with an avoided crossing of direct and indirect excitons. However, for coupled nanowire quantum dots of identical dimensions and chemical compositions---where the bright-exciton splitting is triggered by alloy randomness---the notion of direct/indirect excitons is mostly lost, with the bright-exciton splitting field evolution varying strongly between various random realizations of nominally identical systems. Nonetheless, for several random samples, lower-higher excitonic branch mixing leads to the reduction of bright-exciton splitting below the $1\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{eV}$ threshold but with the restoration of pronounced optical activity away from the crossing. Thus, a simultaneous reduction of the bright-exciton splitting, without the detrimental reduction in the lower excitonic branch optical activity, makes alloyed nanowire quantum dot molecules a possible platform for applications in quantum optics and information.

We report measurements of the radiative lifetimes and coherence times of the dark and bright excitons in an asymmetric GaAs/AlGaAs quantum dot. The dots, fabricated by partial infilling of asymmetric in situ etched nanoholes, have low symmetry, which leads to significant dark-bright mixing as demonstrated by dark-bright anticrossing in magnetophotoluminescence spectra. Using an orthogonal excitation-detection waveguiding geometry and quasiresonant excitation, we compare the coherence properties, measured by Michelson interferometry, of the dark and bright exciton from the same dot in the absence of an external magnetic field.

When a detuned and strong laser pulse acts on an optical transition, a Stark shift of the corresponding energies occurs. We analyze how this optical Stark effect can be used to prepare and control the dark exciton occupation in a semiconductor quantum dot. The coupling between the bright and dark exciton states is facilitated by an external magnetic field. Using sequences of laser pulses, we show how the dark exciton and different superposition states can be prepared. We give simple analytic formulas, which yield a good estimate for optimal preparation parameters. The preparation scheme is quite robust against the influence of acoustic phonons. We further discuss the experimental feasibility of the used Stark pulses. Giving a clear physical picture our results will stimulate the usage of dark excitons in schemes to generate photons from quantum dots., 10 pages, 6 figures

Proposals for quantum information applications are frequently based on the coherent manipulation of spins confined to quantum dots. For these applications, $p$-type III-V material systems promise a reduction of the hyperfine interaction while maintaining large $g$ factors and strong spin-orbit interaction. In this Letter, we study bottom-gated device architectures to realize single and serial multiquantum dot systems in Schottky-contacted $p$-type GaSb nanowires. We find that the effect of potentials applied to gate electrodes on the nanowire is highly localized to the immediate vicinity of the gate electrode only, which prevents the formation of double quantum dots with commonly used device architectures. We further study the transport properties of a single quantum dot induced by bottom gating and find large gate-voltage dependent variations of the ${g}^{*}$ factors up to $8.1\ifmmode\pm\else\textpm\fi{}0.2$ as well as spin-orbit energies between 110 and 230 $\ensuremath{\mu}\mathrm{eV}$.

Recent work has demonstrated a new route to discrete time crystal physics in quantum spin chains by periodically driving nearest-neighbor exchange interactions in gate-defined quantum dot arrays [arXiv:2006.10913]. Here, we present a detailed analysis of exchange-driven Floquet physics in small arrays of GaAs quantum dots, including phase diagrams and additional diagnostics. We also show that emergent time-crystalline behavior can benefit the protection and manipulation of multi-spin states. For typical levels of nuclear spin noise in GaAs, the combination of driving and interactions protects spin-singlet states beyond what is possible in the absence of exchange interactions. We further show how to construct a time-crystal-inspired CZ gate between singlet-triplet qubits with high fidelity. These results show that periodically driving exchange couplings can enhance the performance of quantum dot spin systems for quantum information applications., Comment: 12 pages, 16 figures