Highly tunable exchange-only singlet-only qubit in a GaAs triple quantum dot

We propose an implementation of a singlet-only spin qubit in a GaAs-based triple quantum dot with a (1, 4, 1) charge occupation. In the central multielectron dot, the interplay between Coulomb interaction and an out-of-plane magnetic field creates an energy spectrum with a tunable singlet-triplet splitting, which can be exploited to create a six-particle singlet-only qubit with a qubit splitting that can straightforwardly be tuned over tens of $\mu \mathrm{eV}$ by adjusting the external magnetic field. We confirm the full exchange-based electric control of the qubit and demonstrate its superior coherence properties due to its singlet-only nature.

Figure 1

(a) Field-dependent low-energy part of the spectrum of a four-electron quantum dot with $\kappa =0.5$ and $g=-0.4$. Dots present numerical results and solid lines show the perturbative results of (5). (b) The numerically evaluated energy of the state $|S_{\alpha }\rangle$ (green lines) relative to $|T_{\beta }^0\rangle$ for two values of $\kappa$.

Figure 2

(a) Sketch of the linear triple-dot setup in a (1, 4, 1) charge configuration with a perpendicular magnetic field applied. (b) Six-electron charge stability diagram around the (1, 4, 1) ground state, as a function of $V_m$ and $V_d$.

Figure 3

Low-energy part of the spectrum of the Hamiltonian (6) (a) as a function of $V_d$ at $V_m/{\omega }_0=0.27$ and (b) as a function of $\delta t$ at the SS, $V_d=V_m=0$. The green and blue lines show the spin-singlet qubit states $|0\rangle$ and $|1\rangle$ respectively; the gray lines show the spin triplet and quintuplet states. (c) The qubit splitting as a function of the magnetic field, where ${\omega }_c/{\omega }_0=0.1$ corresponds to $B\approx 75\mathrm{mT}$. (d) The derivative $d{J}_x/dq$ for $q\in \{\delta t,V_d\}$ as a function of $V_m$ and at $V_d=0$.