Coherent population trapping of an electron spin in a single negatively charged quantum dot

Coherent population trapping (CPT) refers to the steady-state trapping of population in a coherent superposition of two ground states that are coupled by coherent optical fields to an intermediate state in a three-level atomic system(1). Recently, CPT has been observed in an ensemble of donor-bound spins in GaAs (ref.2) and in single nitrogen-vacancy centres in diamond(3) by using a fluorescence technique. Here, we report the demonstration of CPT of an electron spin in a single quantum dot. The observation demonstrates both the CPT of an electron spin and the successful generation of Raman coherence between the two spin ground states of the electron(4-6). This technique can be used to initialize, at about a gigahertz rate, an electron spin state in an arbitrary superposition by varying the ratio of the Rabi frequencies between the driving and probe fields. The results show the potential importance of charged quantum dots for a solid-state approach to the implementation of electromagnetically induced transparency(7,8), slow light(9), quantum information storage(10) and quantum repeaters(11,12).