Researchers develop new quantum light source | MIT News

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Using new materials that have been extensively studied as potential new solar photovoltaics, MIT researchers have shown that nanoparticles of these materials can emit a stream of identical single photons.

While the work is currently a landmark discovery of the capabilities of these materials, it could ultimately pave the way for new optical-based quantum computers, as well as possible quantum teleportation devices for communication, say the researchers. The findings appear today in the journal Nature Photonics, in a paper by graduate student Alexander Kaplan, Moungi Bawendi professor of chemistry, and six others at MIT.

Most concepts for quantum computing use ultracold atoms or the spins of individual electrons to act as the quantum bits, or qubits, that form the basis of such devices. But about two decades ago some researchers proposed the idea of ​​using light instead of physical objects as the basic unit of qubits. Among other benefits, this would eliminate the need for complex and expensive equipment to control qubits and insert and extract data from them. Instead, ordinary mirrors and optical detectors would be all that was needed.

With these qubit-like photons, Kaplan explains, with just domestic linear optics, you can build a quantum computer, provided you have properly prepared photons.

The preparation of those photons is the key thing. Each photon must exactly match the quantum characteristics of the previous one, and so on. Once that perfect match is achieved, the really big paradigm shift is changing from needing very fancy optics, very fancy equipment, to needing just simple equipment. The thing that has to be special is the light itself.

Then, explains Bawendi, they take these identical and indistinguishable single photons and interact them with each other. That indistinguishability is crucial: if you have two photons, and everything is the same about them, and you can’t tell number one and number two, you can’t keep track of them that way. This is what allows them to interact in certain non-classical ways.

Kaplan says that if we want the photon to have this very specific property, of being very well-defined in terms of energy, polarization, spatial mode, time, all of which we can quantum encode, we need the source to be very well-defined quantum as well. mechanically.

The source they ended up using is a nanoparticle form of lead-halite perovskite. Lead-halide perovskite thin films are being widely pursued as a potential next-generation photovoltaics, among other things, because they could be much lighter and easier to work with than today’s standard silicon-based photovoltaics. In nanoparticle form, lead halide perovskites are notable for their incredibly fast cryogenic radiative rate, which distinguishes them from other colloidal semiconducting nanoparticles. The faster the light is emitted, the more likely it is that the output has a well-defined wave function. The fast radiative rates then uniquely position the lead halide perovskite nanoparticles to emit quantum light.

To verify that the photons they generate do indeed have this indistinguishable property, a standard test is to detect a specific type of interference between two photons, known as Hong-Ou-Mandel interference. This phenomenon is central to many quantum technologies, says Kaplan, and so demonstrating its presence was a hallmark to confirm that a photon source can be used for these purposes.

Very few materials can emit light that meets this test, he says. They can practically be listed on one side. While their new source is still not perfect, producing HOM interference only about half the time, their other sources have significant problems with achieving scalability. The reason the other sources are consistent is that they are made of the purest materials, and are made individually one by one, atom by atom. So, there’s very poor scalability and very poor reproducibility, says Kaplan.

In contrast, perovskite nanoparticles are made in a solution and simply deposited onto a substrate material. We were basically just rotating them on a surface, in this case just a regular glass surface, says Kaplan. And we were seeing them undergo this behavior that was previously only seen under the most rigorous conditions of preparation.

So while these materials may not be perfect yet, they are very scalable, we can make a lot of them. and currently they are very poorly optimized. We can build them into devices and we can make them even better, says Kaplan.

At this stage, he says, this work is a very interesting fundamental discovery, showing the capabilities of these materials. The significance of the work is that it will hopefully encourage people to look into how to further improve them across various device architectures.

And, Bawendi adds, by integrating these emitters into reflective systems called optical cavities, as has already been done with other sources, we have full confidence that integrating them into an optical cavity will bring their properties to the level of the competition.

The research team included Chantalle Krajewska, Andrew Proppe, Weiwei Sun, Tara Sverko, David Berkinsky and Hendrik Utzat. The work was supported by the US Department of Energy and the Natural Sciences and Engineering Research Council of Canada.

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