4 min readNew DelhiFeb 5, 2023 08:11 AM IST
First published on: Feb 2, 2023 at 05:37 PM IST
In a major breakthrough, researchers at the University of Copenhagen and Ruhr University have figured out how to control two quantum light sources rather than one. While a small step up from one to two might sound too anticlimatic to be termed a breakthrough, this new technology could potentially be developed to create a universal error-corrected quantum computer— often referred to as the holy grail of quantum computing.
According to the University of Copenhagen, researchers around the world have been striving for years to develop stable quantum light sources to achieve something known as quantum mechanical entanglement. In the context of photonic quantum computing, entanglement refers to when two light sources can affect each other instantly, potentially even across large geographic distances. Entanglement is a concept that is central to the development of an efficient quantum computer.
When two light sources are entangled, it means that if you control one of the light sources, the other is also affected almost immediately. This technology can then be expanded to create a whole network of entangled quantum light sources, which can be used to perform “quantum bit operations” just like you do with regular bits in a computer.
Researchers were unable to make two entangled light sources owing to their extreme sensitivity to noise. “The problem is very much ‘charge noise’ of carriers in the vicinity of the quantum emitter that gives spectral jitter. We overcome this by using ultra-clean materials and by applying a low-noise bias voltage across the quantum dot emitter,” Peter Lodahl, co-author of the research paper published in the journal Science, informed indianexpress.com over email.
To achieve the feat, researchers used a nanochip as large as the diameter of human hair. Over the past five years, the team developed this nanochip, eventually improving its performance.
“We start with ultra-clean materials grown in a UHV molecular-beam epitaxy chamber by our colleagues in Bochum, Germany. After that, we fabricate small chip devices using our dedicated and very well-tested etching processes. Finally, we fabricate electrical contact on the samples and shield the experiment from picking up excess electrical noise,” explained Lodahl.
According to the researchers, this technology can be adapted to use 20-30 entangled quantum light sources that can potentially be used to build a “universal error-corrected quantum computer,” an effort that technology companies are pouring billions of dollars into.
Part of the research team is pictured here. From left to right, Peter Lodahl, Anders Sørensen, Vasiliki Angelopoulou, Ying Wang, Alexey Tiranov, Cornelis van Diepen. Photo: Ola J. Joensen. (Image credit: Niels Bohr Institute)
The primary distinction between a classical computer and a quantum computer is their differing rule sets, according to the EU Research and Innovation Magazine. Unlike classical computers, quantum computers don’t use zeros and ones or “bits.” Instead, they work with “qubits.”
Bits can be thought of as a light switch—it is either on or off, one or zero. Qubits have a special property that allows them to exist in a state where they are both zero and one. This superposition will theoretically allow quantum computers to do things that are beyond classical computers.
“I think quantum computers would primarily be used for solving hard quantum problems. For example, in the context of understanding complex chemical reactions like in a drug discovery pipeline or for engineering new materials. Quantum computers are not yet very mature, and around the world, different qubit platforms are being researched, each having their pros and cons,” Lodahl said, referring to the various different quantum technologies.
“Photonics is an increasingly serious contender, mainly because it seems easier to scale up to large processors compared to some of the competing approaches. Our work is a key stepping stone to use deterministic single-photon sources for photonic quantum computing,” added Lodahl.
According to the researchers, it is too expensive for a university to build a setup where it can control ten, fifteen or more light sources. So now, it is up to other actors, like private companies and laboratories, to take the research work further and find applications for the technology.
An artist's illustration of two entangled light source. (Image credit: Niels Bohr Institute) 
Part of the research team is pictured here. From left to right, Peter Lodahl, Anders Sørensen, Vasiliki Angelopoulou, Ying Wang, Alexey Tiranov, Cornelis van Diepen. Photo: Ola J. Joensen. (Image credit: Niels Bohr Institute)
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