Project: Long-range quantum bus for electron spin qubits in silicon

Acronym	Si QuBus (Reference Number: QuantERA2017-Si QuBus)
Duration	01/04/2018
Project Topic	Electron spin qubits in silicon form a serious contender for the realization of a large-scale solid-state quantum computer. Solid-state realizations are considered scalable in general, but silicon spin qubits stand out due to their extremely long coherence times, compatibility with reliable and reproducible industrial fabrication techniques and the feasibility to integrate classical electronics. Universal electron spin manipulation with excellent fidelities was demonstrated in this system. However, a key challenge for true scalability of silicon quantum computer architectures remains the realization of a coherent link between separated quantum registers on a chip. Such a link would serve two important functions. First, it enables a modular structure that allows expanding the number of qubits beyond the limits of local registers. Second, coherent links between registers create space for classical control circuits to be integrated with the qubits on-chip. We aim at the demonstration of a fault-tolerant quantum bus (QuBus) that coherently transfers a single electron with an arbitrary spin qubit state between quantum dots separated by 1 to 10 microns. The quantum dots (QDs) will be electrostatically defined in Si/SiGe heterostructures and silicon-on-insulator (SOI) nanostructures – two very promising systems that we will fabricate in cooperation with industry. We shuttle the electron using an array of electrostatic gates following two strategies: (I) An electron spin conveyer i.e. a potential minimum that smoothly moves laterally. (II) An electron spin bucket brigade, i.e. the electron is passed through a series of QDs via adiabatic passage. In both cases, the final goal is a 1 to 10 micron long QuBus, which is controlled by a maximum of four time-dependent voltage inputs. We will compare the different approaches both experimentally and theoretically by modelling the effects of electrical disorder in the heterostructures, including valley splitting variations, g-factor variations and effects of phonons and electrical noise. Experimentally, the problem of qubit shuttling is separated into three intermediate steps: (I) Reliable charge transfer. (II) Reliable transfer of the spin projection (III) High-fidelity transfer of arbitrary spin states. To enable true networks, we will also demonstrate a QuBus that forms branches and investigate the benefits for fault-tolerance. As a benchmark for the coherent and integrated operation of the QuBus, we will use it to distribute an EPR pair and to realize the first probabilistic quantum teleportation performed in silicon. These advances will pave the way to scalable, topologically protected circuits of electron spin qubits in quantum dots.
Project Results (after finalisation)
Website	visit project website
Network	QuantERA
Call	QuantERA Call 2017

Project partner

Number	Name	Role	Country
1	Rheinisch-Westfaelische Technische Hochschule Aachen	Coordinator	Germany
2	Forschungszentrum Jülich	Partner	Germany
3	INSTITUT NEEL - CNRS	Partner	France
4	Commissariat à l'Energie Atomique et aux Energies Alternatives	Partner	France
5	Delft University of Technology	Partner	Netherlands
6	Institute of Physics, Polish Academy of Sciences	Partner	Poland