Effective mitigation of nuclear decoherence for electron spin qubits
Electron spins in semiconductors are a promising candidate for solid state quantum
computing. However, decoherence due to the hyperfine interaction with nuclear spins is a
major challenge in many materials. I will present experiments on electron spin qubits in
GaAs double quantum dots which demonstrate effective methods to overcome nuclear
decoherence. At the same time, they explore the intricate quantum dynamics of a
mesoscopic ensemble of nuclear spins coupled to two electron spins.
By designing a feedback mechanism into a dynamic nuclear polarization cycle, we
suppressed slow fluctuations of the hyperfine field and achieved an enhancement of T2* by
nearly an order of magnitude. At the same time, fully electrical, universal qubit control
with few nanosecond gate times is enabled by the nuclear polarization. An independent set
of spin-echo measurements confirms recent predictions regarding T2. At low magnetic
fields, we find collapses and revivals of the echo signal associated with entanglement and
disentanglement of the electron spins with the nuclei. At higher fields, the echo signal
decays as exp(-(/30 s)4), as expected due to dipolar coupling between nuclei. Using
CPMG decoupling pulses, the dephasing time T2 can be extended to at least 200 s, which
represents an improvement by about two orders of magnitude compared to previous
experiments.
These dramatically improved coherence times and the fast control show that nuclear
decoherence is not an insurmountable obstacle for reaching the quantum error correction
threshold.