The coupling of two quantum dots was successful for the first time

Arne Ludwig was responsible for the design and manufacture of the semiconductor structures for the experiment.
Photo Credit: RUB, Kramer

This means a big step towards the technical applicability of quantum technology, for example for arithmetic operations.

A tiny change means a big breakthrough in quantum physics: an international research team from Bochum and Copenhagen has managed to couple two quantum dots in one nanochip. After exciting a quantum point using a laser, a signal is sent out, the origin of which can no longer be related to one of the quantum points, as if both had each sent half of the signal in the form of a single photon. "At first that sounds like a little success, but this signal entanglement, which sits on a single photon, is more than the sum of its parts," says Dr. Arne Ludwig from the Chair of Solid-State Physics at the Ruhr University Bochum. “It represents a big step towards the usability of quantum technology for computer operations. "Together with researchers from the Niels Bohr Institute at the University of Copenhagen, the Bochum team published the results in the journal Science from 27. Published January 2023.

Millions of quantum dots on a host crystal

The Bochum part of the elaborate work included the design and manufacture of the semiconductor structures for the experiment. "With belt structure engineering, we develop structures in which artificial atoms, so-called quantum dots, control themselves in a targeted manner and let them shield from the ambient fluctuations," explains Arne Ludwig. This structure must then be produced in a high-purity ultra-high vacuum environment, taking surface physical processes into account on a host crystal. Dr. Sven Scholz, then doctoral student with Arne Ludwig and chair holder Prof. Dr. Andreas Wieck, taken over by Arne Ludwig under the guidance. The properties of the structures are then measured optically and electronically, the results are analyzed and parameters for optimized structures are worked out. "There are many billion quantum dots in the semiconductor structure, each of which has a diameter of only around ten nanometers," says Arne Ludwig. “If we could link all these quantum dots together and control them for quantum computing operations, we would have an unimaginably powerful computer. However, this is currently still completely utopian."

Illustration of a chip with two entangled quantum light sources
Illustration Credit: Niels Bohr Institute of the University of Copenhagen

The way to the nanochip

In coordination with the researchers in Copenhagen, the structures were then further optimized until electric fields, quantum mechanical energy levels, optical reflection properties and the coupling between photons and the quantum dots are correct. In Copenhagen, the structure was further worked on and refined into a nanochip. Individual quantum dots can then be excited in this component using a laser. The coupling results in the emission of individual photons from two of these quantum points.

"The peculiarity of this optical communication is that we can transport information in the light signal absolutely tap-proof," explains Arne Ludwig. The coupling of two quantum points was successful for the first time and represents a major step towards the applicability of quantum technology for technical purposes. Modern computer chips consist of several billion transistors, each of which can be switched as one or zero, i.e. binary. 100 photons from a single quantum dot, on the other hand, have a complexity that far exceeds that of modern large computing systems. "The step from one quantum point to two seems to be a small contribution, but it is a fundamentally important hurdle that we have now overcome with our colleagues from Copenhagen," said Arne Ludwig. Peter Lodahl, senior scientist at the team at the Niels-Bohr-Institut, continues: “If it were possible to pair 20 to 30 quantum points, this opened up the possibility of building a universal, error-corrected quantum computer - the ultimate sacred Grail of quantum technology. Our contribution shows an important step in how this can be achieved. The structures that Arne Ludwig developed in Bochum are unique in the world. Their quality is second to none and allows this great progress.”

Funding:

The work was funded by Danmarks Grundforskningsfond (DNRF 139, Hy-Q Center for Hybrid Quantum Networks), the German Research Foundation (funding code 449674892), the European Union as part of the Marie Skłodowska Curie project no. 801199, the German-French University (CDFA-05-06) and the Federal Ministry of Education and Research (QR.X Project 16 KISQ 009).

Published in journal: Science

Source/Credit: Ruhr University Bochum

Reference Number: qs012723_01

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