Extension of electron spin coherence in a quantum dot via CPT-locking of the Overhauser field

Summary form only given. Semiconductor quantum dots (QD) offer the current state-of-the-art in intensity and coherence for single photon generation, as well as serving as a testbed for various novel approaches to quantum networking. Their high brightness provides a means to produce high-bandwidth spin-photon entanglement [1]. However, the purity and usefulness of the generated entangled state is severely limited by the coherence time of the QD charge carrier spin, typically on the order of 2ns for electrons in a self-assembled InGaAs QD. This limit is imposed by low-frequency fluctuations of the Overhauser field generated by host nuclear spins, affecting the electron spin resonance frequency.Here, we measure an extension of T2 by over an order of magnitude, enabled by prior preparation of the surrounding nuclear spin bath. The interplay of dynamic nuclear polarization and coherent population trapping (CPT) can lead to a controllable mechanism that locks the nuclear spins, and thereby reduces the Overhauser field fluctuations [2,3]. We use the lambda-scheme formed by an optical transition between bound electron and negatively-charged exciton in Voigt geometry at a field of 5T. There, we use the CPT dark-state resonance as a means to lock the polarization state of the nuclear spins surrounding the electron spin. Using ultrafast optical spin rotations, we perform a Ramsey sequence following a CPT preparation of the nuclear spins, to measure the Free Induction Decay of the electron spin coherence [4]. With a pulse sequence composed of 70μs of CPT preparation, followed by 70μs of repeated Ramsey measurements, as shown in Fig. 1a, we observe coherent electron spin oscillations at a Ramsey delay of 29ns, as shown in Fig. 1b. The oscillations are free of the deleterious effects of nuclear polarization typical in such measurements [5], indicating the stabilization of the mean Overhauser field to a fixed value. The measured T2 of 39ns is over an order of magnitude longer than the Ti of 3ns measured on the system with unprepared nuclear spins, as shown in Fig. 1c.