Final Report Summary - S^3NANO (Few Spin Solid-State Nano-systems) The efforts of the S3NANO consortium (www.s3nano.eu) were aimed at the exploration of physics of few spin solid-state nano-systems, to be used in the long-term in the new generation of quantum devices. The applications of such quantum devices will span from fast quantum computing and secure quantum communication, to ultra-sensitive nano-magnetometry. Several types of nano-structured materials were chosen for their unique potential for manipulation of small spin ensembles and individual spins on the nano-scale, and for controlled generation of specially prepared photon states useful for quantum communication and computing. The consortium achieved particularly remarkable progress in these directions using III-V semiconductor quantum dots (e.g. InGaAs/GaAs) and colour centres in diamond (Nitrogen-Vacancy, NV, and Silicon-Vacancy, SiV), and has demonstrated important realisations in graphene nano-structures. In addition to new research delivered by the academic groups, the industrial members of the consortium have also delivered on development of new materials and hardware for advanced experiments on few spin nano-systems. In the 4 years of the project the consortium has delivered 488 person-months of training for junior researchers in the domain of solid state physics and experimental technology. 5 early stage researchers (ESRs) have completed their PhD studies and 4 of them had thesis examination. 6 ESRs are in the final stages of their PhD. The consortium also employed 5 experienced researchers (at postdoctoral level) for 94 months in total. Two of them continue to work in academia (TU Delft and ETH), and two have now moved in a new industrial employment. In total ESRs and ERs have completed 7.5 months of secondments in the groups within and outside the network.The consortium organised two large conferences: ‘Few spin solid-state nano-systems’ workshop in Windsor, 3-6 February 2013, 67 attendees, http://www.s3nano.eu/resources/winter-school-3-6-feb-2013-windsor; ‘Spin-based quantum information processing’ in Konstanz, 170 attendees, 18-21 August 2014, http://spinqubits.uni-konstanz.de. We also contributed to organisation of one of the largest forums for semiconductor physics, International Conference on Physics of Semiconductors, Zurich 29 July-3 August 2012, >1200 attendees. The final network meeting took place in Sheffield on the 11th June 2015, where in addition to network members, members of the Sheffield group, and external researchers, industrial representatives outside the network have attended and gave talks. These included T-Optica, Andor and Janssen Precision Engineering. In addition 3 invited speakers from outside the network also attended. An additional technical ‘wrap-up’ meeting of the network took place on the 29th of January 2016 in TU Munich.The network effort in Science and Technology spanned experimental and theoretical investigations as well as new instrument development and pure material (III-V and diamond) growth. Below we present some highlights of this work, which all together resulted in 54 publications directly acknowledging support by S3NANO funding, and including 24 journal and 4 arxiv.org papers co-authored by the ESRs and ERs of the network. The 54 papers include 13 papers in Nature family journals, 1 in Science, 7 in Physical Review Letters, 2 in Nano Letters. Below we present highlights in the four main S&T objectives.1. Realization and optical control of coherent single spins in nanostructures: Cambridge in collaboration with Sheffield have observed long-lived coherence of the electronic spin ground state of at least 45 ns in Silicon-Vacancy (SV) centre in diamond [Pingault et al, PRL 113, 263601 (2014)], work led by the Cambridge ESR B Pingault. Basel have made significant progress towards understanding of the hole spin coherence and methods for manipulation of the hole spin in III-V quantum dots (QDs) using electric field factor [Houel et al, PRL 112, 107401 (2014), Prechtel et al, PRB 91, 165304 (2015)]. Sheffield has significantly progressed in the understanding of how hole spin interacts with nuclear spins in III-V QDs [Chekhovich et al, Nature Physics 9, 74 (2013)].2. Spin-orbit interaction and spin-orbit qubits in nanostructures: Konstanz and Basel have significantly progressed in understanding the interplay of the electron hyperfine and spin-orbit interactions in theoretical studies of III-V double quantum dots [Rancic et al, PRB 90, 245305 (2014)] and nanowires [Zyuzin et al, PRB 90, 195125 (2014)], work where ESRs M Rancic and V Kornich contributed. Zurich have realised graphene double-dot structures enabling further charge and possibly spin control studies in graphene nanostructures [Bischoff et al, New Journal of Physics 15, 083029 (2013)]. Significant progress has been made by this group with a leading role of their ESR A Varlet in fabrication of graphene/BN hetero-structures necessary to improve transport properties in graphene nanostructures [Varlet et al, Phys. Rev. Lett. 113, 116601 (2014) and 113, 116602 (2014)].3. Advanced techniques for manipulation of nuclear spins on the nanoscale: Major progress has been achieved in nuclear magnetic resonance (NMR) experiments in self-assembled III-V QDs by Sheffield and Basel. In total three new techniques have been developed: (i) pulsed optically detected NMR (Hahn echo etc) [Chekhovich et al, Nature Comms. 6, 6348 (2015)]; (ii) nuclear spin manipulation using frequency-swept radio frequency pulses [Munsch et al, Nature Nano. 9, 671 (2014)]; (iii) most recently work by the Sheffield ESR A Waeber and co-workers - frequency comb NMR [Waeber et al, Nature Physics doi:10.1038/nphys3686 (2016)]. These techniques enable new insight in the nuclear spin coherence and spin-spin interactions on the nanoscale.4. Generation of long-distance entanglement between single spins: Zurich demonstrated heralded quantum entanglement of two quantum-dot hole spins separated by 5 m using single-photon interference [Delteil et al, Nature Physics 12, 218 (2015)], work which A Delteil started when he was employed as the ER in the network. Despite suboptimal photon extraction from NV centres, Delft have also demonstrated a major milestone achievement [Pfaff et al, Science 345, 532 (2014)]: unconditional teleportation of arbitrary quantum states between diamond spin qubits separated by 3 meters. Currently both technologies remain prime candidates for the realization of quantum networks for quantum communication and network-based quantum computing. Several groups (Zurich, Delft and Sheffield) have also started work on tunable microcavities, which in a long term will be used to improve photon collection from colour centres and also study novel spin effects in new light-matter interaction regimes [see work by the Sheffield ESR S Schwarz, Schwarz et al, Nano Lett. 14, 7003 (2014); Dufferwiel, Schwarz et al, Nature Comms. 6, 8579 (2015)].Deliverables: most of the S&T deliverables have been accomplished. However, some difficulties have been encountered in demonstration of D1.2 ‘Release of commercial thermometry system for micro-Kelvin range’ and D2.2 ‘Demonstration of driven hole spin oscillations in a single QD’. For D1.2 Leiden Cryogenics demonstrated temperature measurements as low as 0.9 mK using a Lanthanum doped CMN crystal. However, this method of thermometry was not compatible with measurements in high magnetic fields. The alternative then developed was a SQUID noise thermometer. Using this development Leiden Cryogenics were able to show very good agreement in the temperature range of 10mK - 4K spanning 3 orders of magnitude. This technology will be tested together with Delft to lower temperatures. This work will continue outside S3NANO. For D2.2 many initial steps have been accomplished including work by Basel on coherent manipulation of the hole spin, control of the g-factor, and understanding of the spin noise in QDs. Sheffield’s work has also contributed to understanding of the hole spin interaction with the nuclear spins. All ingredients exist now to carry out electrically driven hole oscillations, the direction that groups in Basel and Sheffield are currently working on.
