  Decoherence in quantum logic gates (qubits) due to interaction with the
surrounding environment is a major obstacle to the practical realization of
quantum information technologies. For solid state electron-spin qubits the
interaction with nuclear spins is the main problem. One particular, neradicable
source of electron decoherence arises from decoherence of the nuclear spin
bath, driven by nuclear-nuclear dipolar interactions. Due to its many-body
nature nuclear decoherence is difficult to predict, especially for an important
class of strained nanostructures where nuclear quadrupolar effects have a
significant but largely unknown impact. Here we report direct measurement of
nuclear spin bath coherence in individual strained InGaAs/GaAs quantum dots:
nuclear spin-echo coherence times in the range T2~1.2 - 4.5 ms are found. Based
on these T2 values we demonstrate that quadrupolar interactions make nuclear
fluctuations in strained quantum dots much slower compared to lattice matched
GaAs/AlGaAs structures. Such fluctuation suppression is particularly strong for
arsenic nuclei due to the effect of atomic disorder of gallium and indium
alloying. Our findings demonstrate that quadrupolar effects can help to solve
the long-standing challenge of designing a scalable hardware for quantum
computation: III-V semiconductor spin-qubits can be engineered to have a
noise-free nuclear spin bath (previously achievable only in nuclear spin-0
semiconductors, where qubit network interconnection and scaling is
challenging).
