Possible hundredfold enhancement in the direct magnetic coupling of a single-atom electron spin to a circuit resonator
Bahman Sarabi, Peihao Huang, Neil M. Zimmerman
We report on the challenges and limitations of direct coupling of the magnetic eld from a circuit resonator to an electron spin bound to a donor potential. We propose a device consisting of a lumped-element superconducting resonator and a single donor implanted in enriched 28-Si. The resonator, in contrast to coplanar waveguide resonators, includes a nano- scale spiral inductor to spatially focus the magnetic eld from the photons within. The design promises approximately two orders of magnitude increase in the local magnetic eld, and thus the spin to photon coupling rate g, compared to the estimated coupling rate to coplanar transmission-line resonators. We show that by using niobium (aluminum) as the resonator's superconductor and a single phosphorous (bismuth) atom as the donor, a coupling rate of g=240 kHz (390 kHz) can be achieved in the single photon regime. For this truly linear cavity quantum electrodynamic system, such enhancement in g is sucient to enter the strong coupling regime. Finally, this relatively large coupling rate allows us to employ resonators with quality factors limited by deposited (lossy) dielectrics for the construction of qubits and quantum memory bits, and for the coupling of distant qubits.