Researchers at the University of Copenhagen, in colla­boration with Ruhr University Bochum, have solved a problem that has caused quantum researchers headaches for years. The researchers can now control two quantum light sources rather than one. Trivial as it may seem to those unini­tiated in quantum, this results allows researchers to create entanglement better. 

For years, researchers around the world have strived to develop stable quantum light sources and achieve quantum mechanical entangle­ment. Entanglement is the very basis of quantum networks and central to the develop­ment of an efficient quantum computer. Researchers from the Niels Bohr Institute succeeded in doing just that. According to Peter Lodahl, it is a crucial step in the effort to take the development of quantum technology to the next level and to quantize society’s computers, encryption and the internet.

“We can now control two quantum light sources and connect them to each other. It might not sound like much, but it’s a major advance­ment and builds upon the past 20 years of work. By doing so, we’ve revealed the key to scaling up the techno­logy, which is crucial for the most ground-breaking of quantum hardware appli­cations,” says Lodahl. So far, they have only been able to control one light source at a time until now. This is because light sources are extra­ordinarily sensitive to outside noise, making them very difficult to copy. In their new result, the research group succeeded in creating two identical quantum light sources rather than just one.

“Entanglement means that by controlling one light source, you immediately affect the other. This makes it possible to create a whole network of entangled quantum light sources, all of which interact with one another, and which you can get to perform quantum bit operations in the same way as bits in a regular computer, only much more power­fully,” explains postdoc Alexey Tiranov. This is because a quantum bit can be both a 1 and 0 at the same time, which results in processing power that is unattainable using today's computer tech­nology. According to Lodahl, just 100 photons emitted from a single quantum light source will contain more information than the world's largest super­computer can process.

By using 20 to 30 entangled quantum light sources, there is the potential to build an uni­versal error-corrected quantum computer – the ultimate “holy grail” for quantum technology, that large IT companies are now pumping many billions into. According to Lodahl, the biggest challenge has been to go from controlling one to two quantum light sources. Among other things, this has made it necessary for researchers to develop extremely quiet nanochips and have precise control over each light source.

With the new research result, the funda­mental quantum physics research is now in place. Now it is time for other actors to take the researchers´ work and use it in their quests to deploy quantum physics in a range of techno­logies including computers, the internet and encryp­tion. “It is too expensive for an university to build a setup where we control 15 to 20 quantum light sources. So, now that we have contributed to under­standing the fundamental quantum physics and taken the first step along the way, scaling up further is very much a techno­logical task," says Lodahl. (Source: U. Copenhagen)

Reference: A. Tiranov et al.: Collective super- and subradiant dynamics between distant optical quantum emitters, Science 379, 389 (2023); DOI: 10.1126/science.ade9324

Link: Center for Hybrid Quantum Networks (Hy-Q), Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark