MIT researchers harness nanoparticles for new source of quantum light

What is the Hong-Ou-Mandel effect?

The key to optical quantum computers is creating photons with identical properties. These indistinguishable and identical photons must then show the Hong–Ou–Mandel effect.

The Hong-Ou-Mandel effect is a two-photon interference central to quantum-based systems. This effect is observed when two identical photons enter a beam-splitter and come out of it together rather than splitting apart. 

Very few quantum light sources exhibit this phenomenon. The team's goal was to test if the photons produced by CsPbBr3 nanocrystals showed the Hong-Ou-Mandel effect, which is crucial for their use in quantum technologies. 

In a press release, Bawendi explained, "If you have two photons, and everything is the same about them, and you can't say number one and number two, you can't keep track of them that way. That's what allows them to interact in certain ways that are non-classical." 

The team used thin films of lead-halide perovskite materials as their quantum light source. These materials are being studied for their photovoltaic properties, lightweight, and easier production process compared to silicon-based photovoltaics. 

Lead-halide perovskites can be identified in nanoparticle form by their blindingly rapid cryogenic radiative rate, which is the rate at which photons are created. The faster the photons are emitted, the better chance there is to have a well-defined wave function. 

Having a well-defined wave function means that their quantum properties, such as polarization, energy spatial mode, and time are consistent and precisely determined. This allows for precise control and manipulation of these properties, enabling accurate encoding, processing, and utilization of quantum information in various quantum technologies and applications.

The team observed that the perovskite nanoparticles only produced Hong-Ou-Mandel interference only about half the time and are, therefore, not perfect. However, unlike other quantum sources made with pure materials, perovskite nanoparticles can be produced in large quantities using a solution-based method.

"The reason other sources are coherent is they're made with the purest materials, and they're made individually one by one atom by atom. So, there's very poor scalability and very poor reproducibility," explained Kaplan in a press release.

"We're basically just spinning them onto a surface, in this case, just a regular glass surface. And we're seeing them undergo this behavior that previously was seen only under the most stringent of preparation conditions," Kaplan added. 

This work is a significant fundamental discovery and highlights the potential capabilities of perovskite nanoparticle materials. By integrating the emitters into optical cavities, similar to what has been done with other sources, the researchers are confident that the properties of the perovskite nanoparticles can reach a competitive level.

The findings of the study are published in the journal Nature Photonics on June 22 and can be found here.

Study abstract:

In the search for novel, robust quantum emitters, inorganic lead halide perovskite nanocrystals have emerged as potential colloidal sources of coherent single photons. While colloidal perovskite nanocrystals offer a great source of synthetically scalable, tunable photon sources, observation of two-photon quantum interference from the emission of any colloidal nanoparticle has not been previously reported. In this work, we prepare large CsPbBr₃ nanocrystals and observe direct evidence of interference between indistinguishable single photons sequentially emitted from a single nanocrystal. We measure Hong–Ou–Mandel interference from photons in CsPbBr₃ nanocrystals, showing corrected visibilities of up to 0.56 ± 0.12 in the absence of any radiative enhancement or photonic architecture. These results demonstrate the unique potential of perovskite nanocrystals to serve as scalable, colloidal sources of indistinguishable single photons.