A Pauli spin blockade in quantum dot systems occurs when the charge transport is allowed only for some spin states, and it has been an efficient tool in spin-based qubit devices in semiconductors. We theoretically investigate a Pauli spin blockade in a triple quantum dot molecule consisting of three identical quantum dots in a semiconductor in the presence of an external magnetic field through the molecule. When the three-electron state is on resonance with two- or four-electron states, the Aharonov–Bohm oscillation and the Zeeman splitting lead to a periodic spin blockade effect. We focus on the spin blockade at a two- and three-electron resonance and show that we can tune the magnetic field to selectively allow only either a spin-singlet or spin-triplet state to add an additional electron from tunnel-coupled leads. This spin blockade maintains the three quantum dots at the optimal sweet spot against the charge noise, demonstrating its potential as an efficient readout scheme for the qubits in quantum dot systems.
Energy spectra and spin configurations of a system of
N=4 electrons in lateral double quantum dots (quantum dot helium molecules) are investigated using exact diagonalization (EXD), as a function of interdot separation, applied magnetic field
(B), and strength of interelectron repulsion. As a function of the magnetic field, the energy spectra exhibit a low-energy band consisting of a group of six states, with the number six being a consequence of the conservation of the total spin and the ensuing spin degeneracies for four electrons. The energies of the six states appear to cross at a single value of the magnetic field, and with increasing Coulomb repulsion they tend to become degenerate, with a well-defined energy gap separating them from the higher-in-energy excited states. The appearance of the low-energy band is a consequence of the formation of a Wigner supermolecule, with the four electrons (two in each dot) being localized at the vertices of a rectangle. Using spin-resolved pair-correlation distributions, a method for mapping the complicated EXD many-body wave functions onto simpler spin functions associated with a system of four localized spins is introduced. Detailed interpretation of the EXD spin functions and EXD spectra associated with the low-energy band via a four-site Heisenberg cluster (with
B-dependent exchange integrals) is demonstrated. Aspects of spin entanglement, referring to the well-known
N-qubit Dicke states, are also discussed.
We present a microscopic theory of a lateral quantum dot molecule in a radial magnetic field with a Greenberger-Horne-Zeilinger (GHZ) maximally entangled three particle ground state. The quantum dot molecule consists of three quantum dots with one electron spin each forming a central equilateral triangle. The antiferromagnetic spin-spin interaction is changed to the ferromagnetic interaction by additional doubly occupied quantum dots, one dot near each side of a triangle. The magnetic field is provided by micromagnets. The interaction among the electrons is described within an extended Hubbard Hamiltonian which is solved by using exact diagonalization techniques. The set of parameters is established for which the ground state of the molecule in a radial magnetic field is well approximated by a GHZ state.