![]() ![]() We then use this method to implement Hahn echo decoupling 1 without perturbing the nuclear bath, and find the coherence to be governed by the quadrupolar coupling of the nuclear bath to the inhomogeneous strain fields in the quantum dot. Here, we present an all-optical solution to access the quantum dot spin state dynamics free from polarizing the nuclear environment. This nonlinear measurement-induced process, often observed as the dragging of resonance frequencies by detuning resonant optical probes 13, affects both the evolution of electron spin states and the detuning of resonant spin readout to the extent that it can limit state retrieval and obscure the mechanisms influencing the electron-spin state 14. A key challenge to understand electron-nuclear dynamics at non-zero external magnetic fields in these structures is presented by dynamic nuclear spin polarization. Recent studies of electron-spin dynamics have provided a comprehensive understanding at zero and very low (mT) magnetic field 11, 12, but dephasing mechanisms and the limits of spin coherence in self-assembled quantum dots remain unclear at larger fields (few T) where quantum-dot spins are promising quantum bit candidates. While Hahn echo decoupling has been shown to enable the recovery of spin coherence for up to 3 μs 8, identifying the irreversible mechanism preventing coherence from reaching closer to the millisecond spin relaxation time 9, 10 is an unresolved issue. ![]() The effect of the hyperfine interaction is a loss of electron spin coherence within a few nanoseconds in time-averaged measurements 6, 7. The nuclear spins in the quantum dot couple to these gradients via quadrupolar moments, resulting in spatially-dependent shifts of their energy levels, which has recently been shown to provide a natural isolation from dipolar interactions 5. Local field gradients are present in addition as a consequence of the random alloying of atomic species in these systems. This nuclear spin bath is subject to inhomogeneous electric field gradients within the crystal lattice, which arise from the strain-driven self-assembly during epitaxial growth. A particularly rich environment is experienced by electrons confined to self-assembled indium–gallium–arsenide (InGaAs) quantum dots: the electron spin couples via the contact hyperfine interaction to the dense nuclear spin bath of the ∼10 4−10 5 atoms which constitute the quantum dot. They are essential to realising quantum information, computation and simulation protocols by extending the usable coherence time of quantum states, but also provide a powerful spectroscopic tool to understand environmental dynamics 3, 4. These results reveal spin coherence can be improved by applying large magnetic fields and reducing strain inhomogeneity.ĭecoupling techniques, such as Hahn echo, protect quantum states by filtering the effects of correlated environment noise 1, 2. ![]() The high-frequency nuclear dynamics are directly imprinted on the electron spin coherence, resulting in a dramatic jump of coherence times from few tens of nanoseconds to the microsecond regime between 2 and 3 T magnetic field and an exponential decay of coherence at high fields. Through all-optical Hahn echo decoupling we now recover the intrinsic coherence time set by the interaction with the inhomogeneously strained nuclear bath. Reaching the full potential of spin coherence has been hindered by the lack of knowledge of the key irreversible environment dynamics. Coherent qubit control combines with an ultrafast spin–photon interface to make these confined spins attractive candidates for quantum optical networks. The interaction between a confined electron and the nuclei of an optically active quantum dot provides a uniquely rich manifestation of the central spin problem.
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