Individuals often adopt the behaviors of those they live in close proximity. Can such a change in behavior due to proximity (let’s call it “proximity effect”) also occur in the quantum mechanical world? Yes, it can. In our recent Nature Materials paper, we show that an artificial quantum mechanical two-level system (also known as a quantum emitter) placed in close proximity to a local magnetic moment can emit a stream of circularly polarized single photons, a behavior that usually manifests under very high magnetic field generated by bulky superconducting magnets.¹ Such an ability holds the key to realizing quantum information networks. As an alternative to applying high magnetic fields, in the past, the coupling of quantum emitters to highly complex nanoscale photonics/plasmonic structures² or the injection of spin-polarized carriers/excitons into quantum emitters^3,4 has been utilized to achieve this goal. The former required costly nanofabrication and integration processes, while the latter’s effectiveness in some materials is limited due to the rapid decay of spin polarization in the material. Our work shows that the proximity effects approach can be exploited to achieve the highly desired circularly polarized single photon stream without suffering these drawbacks.

Because the proximity effect arises from the spread of the electronic wavefunction of one material into a few atomic layers of another crystal in close contact, the effect exists only at the interface of two bulk crystals and is usually negligible. However, for two-dimensional materials consisting of only a single atomic layer, the proximity effect necessarily dominates their behaviors, dramatically changing their electrical and optical properties.^5,6 Recently, researchers have demonstrated that by simply placing a semiconducting monolayer of transition-metal dichalcogenide (such as WSe₂, WS₂) onto ferromagnetic materials (such as EuS, CdGeTe, FeGeTe),^7-10 the effect of an external magnetic field on the energy splitting between the two electronic states coupled to left and right circularly polarized light (known as Zeeman splitting) can be enhanced by more than a factor of ten. These experiments suggest that, for a monolayer semiconductor, emitting circularly polarized light might be possible without using an external magnetic field. 

Aiming to realize this possibility, we investigated the proximity effect on a tungsten di-selenide (WSe₂) monolayer placed on top of nickel-phosphorus tri-sulfide (NiPS₃) crystals. At this point, scientists familiar with materials like NiPS₃ might question our choice of material for achieving our goal. In previous attempts to achieve emission of circularly polarized light from WSe₂, researchers coupled it with a ferromagnetic material exhibiting a net out-of-plane magnetization. However, NiPS₃ is not a ferromagnetic material; rather, it is an antiferromagnetic semiconductor in which the spins of consecutive Ni atoms align in an antiparallel manner with those of the adjacent atomic row (Top Figure).^11-13 As the result, the magnetic moments of the Ni atoms naturally cancel each other out, leading to no out-of-plane magnetization. Indeed, our initial experiments yielded disappointing results. Coupling a WSe₂ monolayer to the NiPS₃ substrate resulted only in partial quenching of the WSe₂ photoluminescence (PL) emission, and the PL exhibited no circular polarization. 

Despite this, Xiangzhi Li, the postdoctoral researcher spearheading the experiment, made one more attempt with a slight twist in this material combination. He utilized atomic force microscopy (AFM) to create a series of nanometer-scale indentations in the WSe₂-NiPS₃ stack, employing nanoscale strain engineering. Surprisingly, this time, we not only observed bright, spectrally sharp emission from the nanoscale indents, the emission also exhibited strong circular polarization at zero magnetic field under linearly polarized laser excitation. Next, we conducted Hunbury Brown-Twiss quantum optical experiments to determine the probability of detecting two photons simultaneously in one optical excitation-emission cycle of such emission. Our experiment revealed that a nano-indent emits only one photon in 90% of such cycles. These two experiments together provide clear evidence that we have realized a highly desired, quantum light source capable of producing a consistent stream of circularly polarized single photons.

We hypothesize that our nano-scale strain engineering with the AFM tip has induced two important changes in both the WSe₂ layer and the NiPS₃ crystal, leading to the formation of circularly polarized (chiral) quantum light emitters. Firstly, the strain engineering from the indentation creates a potential well that quantum mechanically confines electron-hole pairs (excitons) within the WSe_{2 ­}monolayer to discrete quantum states capable of emitting a stream of single photons. At the same location, the nano-indent disrupts the antiferromagnetic spin alignment of the NiPS₃ substrate, generating a local magnetic moment that points out of the substrate plane (Bottom Figure). As a result, a quantum state emitting single photons and a local magnetic moment required for circularly polarizing the photons are created in close proximity to each other. This synergy results in the emission of what we term “chiral quantum light.” 

To gather further evidence in support of our hypothesis, we conducted two experiments: First, in collaboration with Patrick Maletinsky’s group at the University of Basel, Switzerland, we performed scanning diamond nitrogen-vacancy center microscopy on an indented NiPS₃ crystal. The experiment revealed a small out-of-plane magnetization along the edge of the indentation, confirming that the disturbance of the antiferromagnetic order indeed leads to some localized out-of-plane magnetic moments. Next, to demonstrate the direct association of chiral light emission with the antiferromagnetic state of NiPS₃, we performed field-cooling magneto photoluminescence experiments, following the suggestion of Scott Crooker from the National High Magnetic Field Laboratory at Los Alamos National Laboratory. By applying a high magnetic field at elevated temperature and maintaining it while cooling the sample to cryogenic temperature, we prevented the spins of Ni atoms from re-establishing their anti-ferromagnetic order. We observed that the quantum emitter can no longer emit circularly polarized light under this condition, demonstrating that chiral quantum light emission is directly linked to a defect in the antiferromagnetic order of the NiPS₃ crystal. 

In summary, our observations highlight that local strain engineering can be utilized not only to create quantum emitters but also to localize ferromagnetic proximity effects, thus facilitating the creation of chiral single photon emitters in WSe₂/NiPS₃ heterostructures. Interestingly, a similar chiral localized excitonic emission have also been observed in our more recent experiments performed on WSe₂/MnPS₃ and WSe₂/FePS₃ heterostructures with nanoindentations. These discoveries establish the transition metal dichalcogenides /TMPX₃ semiconductor heterostructures as an exciting material platform for further exploration of novel emergent phenomena and the development of the solid-state quantum transduction and sensing technologies.

Figure. Top: Atomic structure of NiPS₃ crystal. The electronic spins of Ni atoms are aligned parallel (blue arrows) and antiparallel (red arrows) to the crystallographic a-axis, forming a zig-zag antiferromagnetic order. Bottom: Schematic illustration of a nano-indent in a monolayer of WSe₂(orange balls) stacked on top of a thin NiPS₃ crystal (green and red balls). The strain from the nano-indent causes the formation of potential well, trapping the electron-hole pairs (excitons) in WSe₂, along with a defect possessing an out-of-plane magnetic moment in the antiferromagnetic NiPS₃ crystal at the same location. The proximity interaction between the localized electron-hole pair and the magnetic moment gives rise to the emission of chiral quantum light. 

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