Detection of Ramsey oscillations in germanium doped with shallow donors upon the excitation of the $1s\to2p_0$ transition

Ramsey oscillations have been observed in germanium doped with shallow impurities exposed to terahertz pulses from the NovoFEL facility involving free-electron lasers in experiments performed using the standard method with the action of a sequence of two optical pulses at the frequency close to the frequency of the $1s(A_1)\to2p_0$ impurity transition. The coherent state of the ensemble of donors has been detected by measuring the photocurrent caused by the thermal ejection of electrons from the $2p_0$ state to the conduction band. The revealed effect is quite stable under experimental conditions, in particular, to the temperature regime, which allows the further improvement in artificial systems based on shallow donors in germanium.