QOPT seminar by Claire Le Gall, University of Cambridge
Collective ancilla memory for semiconductor quantum dots
Interfaces between single photons and single spins underpin the promise of flexible quantum architectures and unconditional security for communication. Their versatility arises from our ability to measure the spin information deterministically and use photons to generate entanglement between spins over large distances.
Using leading solid-state systems, semiconductor quantum dots (QDs) and Nitrogen Vacancies in diamond, important milestones have been reached . In the case of QDs, their near-ideal optical properties have allowed to distribute entanglement at an unprecedented rate of 7.3 kHz. Thus far, fewer praises can be sung about their spin coherence. The electron couples to a mesoscopic ensemble of N~100,000 nuclei and gaining control over this many-body system to the point where nuclei are a resource is a frontier challenge in the field.
In this talk, I will present our latest efforts to control collective nuclear states [2, 3]. In our experiments, we operate the electron both as a control and a probe of the total nuclear spin polarisation, Iz. For a thermal nuclear ensemble, fluctuations of this polarisation (~√N) broaden the electron spin linewidth to ~100MHz (pink curve in Fig. 1D). Using an all-optical method to prepare the nuclei, we reduce the uncertainty on nuclear polarisation (purple curve in Fig. 1D) to well below the nuclear Zeeman energy (wz~20MHz), thus allowing to resolve the hyperfine levels of the electron-nuclear system (Fig. 1C) and access magnon modes, appearing as weakly allowed sideband transitions (Fig. 1E and F) or Hartmann-Hahn resonances . The overarching goal is to use these collective nuclear modes for quantum storage , a decisive step towards a scalable distributed network.
 W. B. Gao et al. Nature Photonics 9, 373-373 (2015)
 D. A. Gangloff et al. Science 364, 62-66 (2019)
 J. H. Bodey et al. npj QI 5, 95 (2019)
 E. V. Denning et al. PRL, 123, 140502 (2019)