Quantum networks could provide essentially unhackable communication channels, but first they need a reliable source of single photons. Normally these require cryogenic temperatures, making them impractical and expensive. But now researchers have created a high quality single-photon source (SPS) that operates at room temperature.
The central promise of quantum communication is the ability to share information in a way that is impervious to eavesdroppers. It involves encoding information into the quantum states of photons before transmitting them across the network. The beauty of the approach is that these quantum states collapse as soon as they are measured, which means the receiver will know immediately if someone has read the message before them.
However, this works only if you can reliably produce individual photons. If you produce more than one, a hacker could simply intercept a spare photon, read it, and let the others pass through untampered with, leaving the receiver none the wiser. You also need to produce these single photons fast enough to transmit reasonable amounts of data.
Materials capable of reliably producing large numbers of single photons exist, but they typically have to be cooled to cryogenic temperatures using complex and expensive equipment. Others that operate at room temperature have been discovered, but squeezing enough photons out of them to support quantum communication has proven difficult.
Now though, a team from the University of Technology Sydney has created a reliable SPS that doesn’t require cryogenic cooling, and it can produce more than 10 million single photons per second, enough to carry out useful quantum-communication tasks. The trick was to combine one of the more promising room-temperature single-photon emitters with a commercially available lens that focuses the output, making it possible to collect six times as many photons.
“To be honest, it’s not something genius,” says Igor Aharonovich, a professor in at the university’s school of mathematical and physical sciences, who led the project. “It’s like wearing glasses, right? You need to see better, so you use a lens.”
The device relies on a 2D material called hexagonal boron nitride (hBN), which has emerged as a promising SPS due to the fact that impurities in its structure emit single photons when excited by either a laser or electricity. Crucially, the SPS does this while at room temperature; it is relatively stable and rarely produces more than one photon when excited. But while it is bright (a measure of how many photons it produces in a second) compared to other room-temperature SPS, by itself it is still not efficient enough to support practical applications, says Aharonovich.
So his team placed a special optical component called a solid-immersion lens over the top of the material, which increases the angles from which photons can be collected. When a laser is shone at the hBN it gives off single photons, which are then focused by the lens and collected by a microscope that feeds into an optical fiber.
In a paper in Optics Letters, the researchers demonstrate that this system produces enough single photons to support quantum-key distribution, a form of quantum communication in which an encryption key is shared over a quantum channel to create an ultrasecure data link.
“A room-temperature single-photon source would be a fantastic tool for quantum networks and quantum technologies,” says Alisa Javadi, a postdoctoral Researcher at the University of Basel, in Switzerland. “The authors have taken a step forward in demonstrating single-photon generation with improved efficiency at room temperature.”
However, Javadi says, we’re still a long way from a practical device. While the gains in collection efficiency the researchers have made are significant, he estimates the overall figure is still only about 1 percent. And even after collection, another 56 percent of the photons are lost as they are fed into the optical fiber.
Another issue the device faces is the quality of photons, says Alexander Ling, director of the quantum-engineering program at the National University of Singapore. For most quantum-communication applications, photons need to be indistinguishable apart from their quantum state, but this is typically not the case for photons emitted by hBN. There are currently no good physical models that explain why this happens, he says, which makes it hard to build controllable SPS that would be useful for quantum communication out of these materials.
The device could have applications beyond quantum communication as well, though, according to Aharonovich, thanks to its relatively low cost and simplicity. Previous efforts to boost the efficiency of hBN-based SPS have relied on complex nano-fabrication that is both difficult and expensive, but by using readily available components the group’s device costs roughly US $15,000. It’s also relatively compact, fitting in a 50-by-50-centimeter box and weighing only 10 kilograms.
Given the increasing importance of quantum technology, Aharonovich says the device could be a great way to help teach students about this emerging field. “For us, our big vision is really education,” he says. “Having a device like that in every undergraduate laboratory would be great, because you can measure true quantum properties of light out of the box. Students can see it, they can do all the measurements they like, and it’s scalable, available, and there is no need to invest hundreds of thousands of dollars.”
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Edd Gent is a freelance science and technology writer based in Bangalore, India. His writing focuses on emerging technologies across computing, engineering, energy and bioscience. He's on Twitter at @EddytheGent and email at edd dot gent at outlook dot com. His PGP fingerprint is ABB8 6BB3 3E69 C4A7 EC91 611B 5C12 193D 5DFC C01B. His public key is here. DM for Signal info.