Spins in semiconductors are extremely well isolated from their environment and can therefore achieve very long coherence times, so that quantum operations on a nuclear spin-based processor could potentially have extremely low error rates. Yet, the major challenge for a quantum processor architecture based on spin quantum bits (qubits) is the realization of gates between distant individual spins.
A promising research direction is to combine spin qubits with superconducting circuits and construct a hybrid quantum system that would inherit the advantages of each one of the different components. Indeed, spins in semiconductors with their long coherence time are a perfect system to reliably store the quantum information while superconducting qubits with their strong coupling with external fields are perfect systems to easily process fast quantum gates.
Efficient transfer of quantum information between these systems requires reaching the so-called “strong coupling regime” where the coupling between the different systems is much larger than their decoherence rates. The coupling constant g of a single electron spin with a superconducting resonator being many orders of magnitude lower than the best resonator energy decay rates, a first possible solution is to couple the resonator not to a single spin but to a large number N~10^12. By that way, the collective magnetization of the ensemble is coupled with an enhanced constant g√N and one can reach the strong coupling regime.
The objective of this project is to go far beyond the limits of current hybrid technologies and to develop a new experimental platform, which we call a “quantum writing machine”. This platform, which builds on our recent developments on flux qubits in circuit QED [1,2] will enable us to store the state of a superconducting qubit into a single spin. Using quantum writing machines, we will be able to initialize the spin, to manipulate it, to read it out and eventually to couple it to other distant spins. This achievements will be a first step towards a spin-based quantum processor.
[1] M. Stern et al., Phys. Rev. Lett. 113, 123601 (2014).
[2] T. Douce et al., Phys. Rev. A, 92, 052335 (2015).