Our paper entitled Decoupling a hole spin qubit from the nuclear spins was published in Nature Materials. An electron spin in a quantum dot interacts with the nuclear spins (via the hyperfine interaction) and this limits the spin dephasing times to quite modest values. What about a hole spin? Theory predicts that a heavy-hole spin decouples from the nuclear spins in an in-plane magnetic field. This is an idealized limit. How does it work out in practice? We have attempted to answer this question with an experiment: we apply an in-plane magnetic field, we measure the hole Zeeman frequency spectroscopically with an ultrahigh resolution technique (“coherent population trapping”, equivalently “dark-state spectroscopy”) and then look for changes in the hole Zeeman frequency as we polarize the nuclear spins. We tried to carry out this experiment several years ago but charge noise defeated us. Now though we have super-quiet hole devices thanks to the efforts of our partners Arne Ludwig and Andreas Wieck in Bochum and the dark-state spectroscopy works really nicely. The result? The hole spin decouples from the nuclear spins, even when we look with 10 neV resolution. The paper was featured in UniNews.
Our paper entitled Role of the electron spin in determining the coherence of the nuclear spins in a quantum dot was published in Nature Nanotechnology. The work is a close collaboration between the Poggio and Loss groups in Basel and our partners Arne Ludwig and Andreas Wieck in Bochum. Our motivation was to determine the limits set by the hyperfine interaction on the decoherence rate of an electron spin inside a quantum dot. To do this, we decided to measure the coherence of the nuclear spins as the nuclear spins have long-lived coherence in the absence of an electron. We found that the nuclear spin coherence time (as measured with a Hahn echo protocol) falls from several milli-seconds to around 20 μs on adding a single electron to the quantum dot. The nuclear spin recovers to its original value on adding two electrons pointing to a spin-based and not a charge-based decoherence mechanism (in the case of two electrons, a spin singlet is formed). We propose that a single electron mediates a nuclear spin-nuclear spin coupling, an RKKY-like effect. This hypothesis is backed up with a quantitative calculation. The paper was featured in UniNews.
Immo Söllner’s paper entitled Deterministic Single-Phonon Source Triggered by a Single Photon was published in Physical Review Letters. In the manuscript, a collaboration with Leonardo Midolo and Peter Lodahl from the Niels Bohr Institute, we propose an optomechanical crystal that allows for the simultaneous control of a quantum dot’s photonic and phononic environments. We go on to show how this platform enables the generation of single phonons at gigahertz frequencies triggered by single photons in the near infrared. Future work will focus on the experimental realization of these ideas, in collaboration with groups at the University of Bochum and at the Niels Bohr Institute.