title: High-fidelity spin entanglement using optimal control.
abstract: Precise control of quantum systems is of fundamental importance in quantum information processing, quantum metrology and high-resolution spectroscopy. When scaling up quantum registers, several challenges arise: individual addressing of qubits while suppressing cross-talk, entangling distant nodes and decoupling unwanted interactions. Here we experimentally demonstrate optimal control of a prototype spin qubit system consisting of two proximal nitrogen-vacancy centres in diamond. Using engineered microwave pulses, we demonstrate single electron spin operations with a fidelity F≈0.99. With additional dynamical decoupling techniques, we further realize high-quality, on-demand entangled states between two electron spins with F>0.82, mostly limited by the coherence time and imperfect initialization. Crosstalk in a crowded spectrum and unwanted dipolar couplings are simultaneously eliminated to a high extent. Finally, by high-fidelity entanglement swapping to nuclear spin quantum memory, we demonstrate nuclear spin entanglement over a length scale of 25 nm. This experiment underlines the importance of optimal control for scalable room temperature spin-based quantum information devices.
authors.affiliation: 1] 3rd Institute of Physics, University of Stuttgart, IQST and SCOPE, Pfaffenwaldring 57, 70569 Stuttgart, Germany [2].
authors.forename: Florian
authors.initials: F
authors.lastname: Dolde
authors.affiliation: 1] ISI Foundation, Via Alassio 11/c, Torino 10126, Italy [2].
authors.forename: Ville
authors.initials: V
authors.lastname: Bergholm
authors.affiliation: 1] 3rd Institute of Physics, University of Stuttgart, IQST and SCOPE, Pfaffenwaldring 57, 70569 Stuttgart, Germany [2].
authors.forename: Ya
authors.initials: Y
authors.lastname: Wang
authors.affiliation: 3rd Institute of Physics, University of Stuttgart, IQST and SCOPE, Pfaffenwaldring 57, 70569 Stuttgart, Germany.
authors.forename: Ingmar
authors.initials: I
authors.lastname: Jakobi
authors.affiliation: Institute for Quantum Optics and IQST, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany.
authors.forename: Boris
authors.initials: B
authors.lastname: Naydenov
authors.affiliation: Institute for Experimental Physics II, Linnéstraße 5, University of Leipzig, 04103 Leipzig, Germany.
authors.forename: Sébastien
authors.initials: S
authors.lastname: Pezzagna
authors.affiliation: Institute for Experimental Physics II, Linnéstraße 5, University of Leipzig, 04103 Leipzig, Germany.
authors.forename: Jan
authors.initials: J
authors.lastname: Meijer
authors.affiliation: Institute for Quantum Optics and IQST, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany.
authors.forename: Fedor
authors.initials: F
authors.lastname: Jelezko
authors.affiliation: 3rd Institute of Physics, University of Stuttgart, IQST and SCOPE, Pfaffenwaldring 57, 70569 Stuttgart, Germany.
authors.forename: Philipp
authors.initials: P
authors.lastname: Neumann
authors.affiliation: Department of Chemistry, Technical University Munich, 85747 Garching, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Schulte-Herbrüggen
authors.affiliation: ISI Foundation, Via Alassio 11/c, Torino 10126, Italy.
authors.forename: Jacob
authors.initials: J
authors.lastname: Biamonte
authors.affiliation: 3rd Institute of Physics, University of Stuttgart, IQST and SCOPE, Pfaffenwaldring 57, 70569 Stuttgart, Germany.
authors.forename: Jörg
authors.initials: J
authors.lastname: Wrachtrup