Quadrupolar exchange-only spin qubit*

We propose a quadrupolar exchange-only (QUEX) spin qubit that is highly robust against charge noise and nuclear spin dephasing, the dominant decoherence mechanisms in quantum dots [1]. Building on ideas developed for the exchange-only qubit [2], the hybrid qubit [3], and the exchange-only singlet-only spin qubit [4], the QUEX the qubit consists of four electrons trapped in three quantum dots, and operates in a decoherence-free subspace to mitigate dephasing due to nuclear spins. To reduce sensitivity to charge noise, the qubit can be completely operated at an extended charge noise sweet spot that is first-order insensitive to electrical fluctuations. Due to on site exchange mediated by the Coulomb interaction, the qubit energy splitting is electrically controllable and can amount to several GHz even in the "off" configuration, making it compatible with conventional microwave cavities. A symmetric readout and initialization protocol can be used to perform fast and high fidelity measurements.
[1] M. Russ, J. R. Petta, and G. Burkard, PRL 121, 177701 (2018)
[2] D. P. DiVincenzo et al., Nature 408, 339 (2000)
[3] Z. Shi et al., Phys. Rev. Lett. 108, 140503 (2012)
[4] A. Sala and J. Danon, Phys. Rev. B 95, 241303 (2017)