Abstract
We investigate the spin of a multielectron GaAs quantum dot in a sequence of nine charge occupancies, by exchange coupling the multielectron dot to a neighboring two-electron double quantum dot. For all nine occupancies, we make use of a leakage spectroscopy technique to reconstruct the spectrum of spin states in the vicinity of the interdot charge transition between a single- and a multielectron quantum dot. In the same regime we also perform time-resolved measurements of coherent exchange oscillations between the single- and multielectron quantum dot. With these measurements, we identify distinct characteristics of the multielectron spin state, depending on whether the dot's occupancy is even or odd. For three out of four even occupancies we do not observe any exchange interaction with the single quantum dot, indicating a spin-0 ground state. For the one remaining even occupancy, we observe an exchange interaction that we associate with a spin-1 multielectron quantum dot ground state. For all five of the odd occupancies, we observe an exchange interaction associated with a spin-1/2 ground state. For three of these odd occupancies, we clearly demonstrate that the exchange interaction changes sign in the vicinity of the charge transition. For one of these, the exchange interaction is negative (i.e. triplet-preferring) beyond the interdot charge transition, consistent with the observed spin-1 for the next (even) occupancy. Our experimental results are interpreted through the use of a Hubbard model involving two orbitals of the multielectron quantum dot. Allowing for the spin correlation energy (i.e. including a term favoring Hund's rules) and different tunnel coupling to different orbitals, we qualitatively reproduce the measured exchange profiles for all occupancies.
Highlights
Spins in semiconducting nanostructures offer a wide variety of approaches to quantum computing
We apply two methods developed for spin qubits to study the spin properties of a mesoscopic multielectron quantum dot, namely, spin leakage spectroscopy and measurement of coherent exchange oscillations
By studying in detail the interaction of the multielectron dot with a neighboring electron, we discover a counterintuitive exchange profile between the neighboring electron and the odd-occupied multielectron quantum dot
Summary
Spins in semiconducting nanostructures offer a wide variety of approaches to quantum computing. An attractive alternative that has recently been proposed [52,53] and demonstrated [54,55] is to base two-qubit coupling on exchange interactions, using an intermediate quantum system as a mediator This approach makes use of the high speed associated with exchange processes, without the need to arrange quantum dots in direct contact with each other, and is attractive for current fabrication techniques. (3) The level spacing of the multielectron quantum dot and the relevant tunnel couplings must be larger than both the energy of the thermal fluctuations (kBT ≈ 10 μeV for T 1⁄4 100 mK) and the excitation spectrum of the control voltage pulses (≈20 μeV for 5 GHz bandwidth) This condition is necessary to guarantee that the mediator will be prepared in the ground state and to avoid its accidental excitation.
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