Robust valley quantum coherence in synthetic 3R-phase MoS₂: A material for future valley based devices

Two-dimensional layered transition metal dichalcogenides (TMDCs) offer the ideal platform to optically initialize the qubits utilizing the superposition of valley polarized photons. However, in the commonly available 2H phase TMDCs, thickness sensitive crystal inversion symmetry demands the precise control over the number of layers, which makes the study of valley physics challenging. On the other hand, 3R-polymorphic phase TMDCs are non-centrosymmetric irrespective of the number of layers and therefore retain valley degree of freedom for all the thicknesses. Here, using polarization resolved photoluminescence measurements under linearly polarized excitations, we demonstrate the coherent superposition of the valley photons (known as valley coherence) in synthetic 3R-phase MoS₂ with different numbers of layers ranging from 3 layers to 11 layers. With near-resonant excitation (633 nm for AX⁰), the degree of linear polarization (a measure of the valley coherence) is observed up to 70% at 80 K measurement temperatures and is found to be layer thickness independent. Robust valley coherence is attributed to the high optical quality and the 3R crystal symmetry of the MoS₂. Using angle resolved polarization dependent measurements, it is established that the net dipolar emission aligns with the polarization orientations of the incident laser, which signifies the valley coherence to be independent of the crystal orientation. This study paves the way toward using 3R-phase MoS₂ as a key material for the development of future quantum technologies utilizing valley polarized photons.