Coherent storage of microwave photons over 100ms in an ensemble of electron spins.

Electron and nuclear spins in crystals are attractive for storing quantum information due to their long
coherence times in pure matrices. Bismuth donors in silicon have the particular appeal to possess
special biasing points (the so-called “clock transitions”) at which the electron spin becomes
insensitive to first-order to magnetic field noise, yielding long coherence times [1]. Here we report
the coupling of a small ensemble of \approx 10^4 Bismuth donors in silicon to a superconducting
micro-resonator at a clock-transition, at millikelvin temperatures. At this point, we measure a Hahn-
echo coherence time of up to T_2=300ms. This makes it possible to store coherently a train of weak
microwave pulses of \approx 10 photons over 100ms. [2]
[1] G. Wolfowicz et al., Nature Nano (2013)
[2] V. Ranjan et al., in preparation (2019)