Measurement of spin coherence using Raman scattering

TL;DR

This paper introduces a novel optical method using Raman scattering to measure spin coherence times in quantum emitters, overcoming limitations of traditional Ramsey interferometry affected by nuclear spin polarization.

Contribution

The authors develop and validate a Raman-based technique for directly measuring spin $T_2^*$ times, providing a more accurate alternative to Ramsey interferometry in quantum dot systems.

Findings

Raman coherence directly reflects spin coherence, enabling precise $T_2^*$ measurement.

The method distinguishes between coherent and incoherent resonance fluorescence contributions.

Results agree with traditional Ramsey measurements, confirming validity.

Abstract

Ramsey interferometry provides a natural way to determine the coherence time of most qubit systems. Recent experiments on quantum dots however, demonstrated that dynamical nuclear spin polarization can strongly influence the measurement process, making it difficult to extract the $T_2^*$ coherence time using optical Ramsey pulses. Here, we demonstrate an alternative method for spin coherence measurement that is based on first-order coherence of photons generated in spin-flip Raman scattering. We show that if a quantum emitter is driven by a weak monochromatic laser, Raman coherence is determined exclusively by spin coherence, allowing for a direct determination of spin $T_2^*$ time. When combined with coherence measurements on Rayleigh scattered photons, our technique enables us to identify coherent and incoherent contributions to resonance fluorescence, and to minimize the latter. We verify the validity of our technique by comparing our results to those determined from Ramsey interferometry for electron and heavy-hole spins.