MIT researchers harness nanoparticles for new source of quantum light

These findings could significantly boost optical quantum computers by making them scalable and affordable without the need for complex equipment.
Tejasri Gururaj
Lead halide perovskite nanocrystals were used as the quantum light source.
Lead halide perovskite nanocrystals were used as the quantum light source.

Kaplan et al.  

Optical quantum computers are of interest to researchers for a variety of reasons. In general, quantum computers use spins of single electrons or ultra-cold atoms as quantum bits or qubits, analogous to classical bits. 

However, around two decades ago, scientists proposed using photons, which are particles of light, as qubits. Using photons eliminates the need for expensive and complex equipment and would instead require optical mirrors and detectors only. 

Now, a group of researchers from the Massachusetts Institute of Technology (MIT) have developed a new source of quantum light using nanoparticles of novel materials. 

The team of scientists was led by Prof. Moungi Bawendi and graduate student Alexander Kaplan, both from MIT. They used CsPbBr3 nanocrystals, which are inorganic lead halide perovskite materials made of cesium (Cs), lead (Pb), and bromine (Br).

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. 

MIT researchers harness nanoparticles for new source of quantum light
Lead halide perovskite nanocrystals were used as the quantum light source.

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.

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