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Register a new userSpin-dependent coherent transport of two-dimensional excitons
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We propose a theory of interference contributions to the two-dimensional exciton diffusion coefficient. The theory takes into account four spin states of the heavy-hole exciton. An interplay of the single particle, electron and hole, spin splittings with the electron-hole exchange interaction gives rise to either localization or antilocalization behavior of excitons depending on the system parameters. Possible experimental manifestations of exciton interference are discussed.
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Spin transport of indirect excitons in GaAs/AlGaAs coupled quantum wells was observed by measuring the spatially resolved circular polarization of exciton emission. Exciton spin transport over several microns originates from a long spin relaxation time and long lifetime of indirect excitons.
The dephasing mechanisms of p-shell and s-shell excitons in an InAs self-assembled quantum dot ensemble are examined using two-dimensional coherent spectroscopy (2DCS). 2DCS provides a comprehensive picture of how the energy level structure of dots affects the exciton dephasing rates and recombination lifetimes. We find that at low temperatures, dephasing of s-shell excitons is lifetime limited, whereas p-shell excitons exhibit significant pure dephasing due to scattering between degenerate spin states. At elevated temperatures, quadratic exciton-phonon coupling plays an important role in both s-shell and p-shell exciton dephasing. We show that multiple p-shell states are also responsible for stronger phonon dephasing for these transitions
We investigate the spin dynamics of high-mobility two-dimensional electrons in GaAs/AlGaAs quantum wells grown along the $[001]$ and $[110]$ directions by time-resolved Faraday rotation at low temperatures. In measurements on the $(001)$-grown structures without external magnetic fields, we observe coherent oscillations of the electron spin polarization about the effective spin-orbit field. In non-quantizing magnetic fields applied normal to the sample plane, the cyclotron motion of the electrons rotates the effective spin-orbit field. This rotation leads to fast oscillations in the spin polarization about a non-zero value and a strong increase in the spin dephasing time in our experiments. These two effects are absent in the $(110)$-grown structure due to the different symmetry of its effective spin-orbit field. The measurements are in excellent agreement with our theoretical model.
We propose a robust and efficient way of controlling the optical spectra of two-dimensional materials and van der Waals heterostructures by quantum cavity embedding. The cavity light-matter coupling leads to the formation of exciton-polaritons, a superposition of photons and excitons. Our first principles study demonstrates a reordering and mixing of bright and dark excitons spectral features and in the case of a type II van-der-Waals heterostructure an inversion of intra and interlayer excitonic resonances. We further show that the cavity light-matter coupling strongly depends on the dielectric environment and can be controlled by encapsulating the active 2D crystal in another dielectric material. Our theoretical calculations are based on a newly developed non-perturbative many-body framework to solve the coupled electron-photon Schrodinger equation in a quantum-electrodynamical extension of the Bethe-Salpeter approach. This approach enables the ab-initio simulations of exciton-polariton states and their dispersion from weak to strong cavity light-matter coupling regimes. Our method is then extended to treat van der Waals heterostructures and encapsulated 2D materials using a simplified Mott-Wannier description of the excitons that can be applied to very large systems beyond reach for fully ab-initio approaches.
A fault-tolerant quantum processor may be configured using stationary qubits interacting only with their nearest neighbours, but at the cost of significant overheads in physical qubits per logical qubit. Such overheads could be reduced by coherently transporting qubits across the chip, allowing connectivity beyond immediate neighbours. Here we demonstrate high-fidelity coherent transport of an electron spin qubit between quantum dots in isotopically-enriched silicon. We observe qubit precession in the inter-site tunnelling regime and assess the impact of qubit transport using Ramsey interferometry and quantum state tomography techniques. We report a polarization transfer fidelity of 99.97% and an average coherent transfer fidelity of 99.4%. Our results provide key elements for high-fidelity, on-chip quantum information distribution, as long envisaged, reinforcing the scaling prospects of silicon-based spin qubits.
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