In 1998, D. Loss and D. DiVicenzo made a seminal proposal to use spin qubits in semiconductor quantum dots as a resource for manipulating quantum information [1]. They should offer long coherence times in a scalable architecture. It was however quickly found that the presence of nuclear spins in the host material is a major source of decoherence. A lot of effort has thus been put on fighting against this noise, either through complex control protocols or by developing new technologies in materials with fewer nuclear spins.
Recently, it was shown that by exploiting the slowness of the nuclear spins fluctuations, the dephasing time of electron spin qubits in a 100% nuclear spins material (GaAs) could be enhanced from 10 ns to microseconds. Working in this regime however implies to operate the electron spin faster than the decorrelation time of its environmental noise. We experimentally investigate this regime in detail and find non trivial signatures of the noise short time dynamics in the dephasing of the electron spin.
The other approach to improve the performance of spin qubits is to remove the nuclear spins of the environment. Fault tolerant operations have been demonstrated in expensive purified 28Si devices. We have developed an architecture based on standard industry silicon, which contains 4.8% of nuclear spins, where we demonstrated the operation of two spin qubits at the fault tolerance threshold. Finally, scalability of these architectures will also be discussed through the extension of the number of operated qubits.
[1] D. Loss and D. P. DiVincenzo, Phys. Rev. A 57, 120 (1998).
Quantum Functional System Research Group, Center for Emergent Matter Science, Riken, Hirosawa, Wako-shi, Saitama, Japan