All over the world, physicists are putting huge amounts of effort into designing devices that harness the power of quantum mechanics. “A promise of quantum communication is to be able to exchange information with absolute security by making sure that any eavesdropper is detected in advance”, describes Gregor Weihs, head of the Department of Experimental Physics at the University of Innsbruck, Austria. Quantum communication relies on information transfer with small light particles called “photons”. To generate single photons, the researchers used quantum dots, which are “tiny semiconductor crystals, that can be easily incorporated into chip-scale devices”, according to Armando Rastelli, who leads the Semiconductor Physics Division at Johannes Kepler University of Linz and has fabricated these quantum dots.
When laser light excites a quantum dot, high-quality single photons are produced. But this can be tricky if the wavelength (colour) of the laser light is identical to that of the generated photon, and therefore sophisticated filtering schemes are required, which results in half of the photons being lost. To overcome this issue, a new method called the Swing UP of quantum emittER population (SUPER) scheme was proposed last year. Doris Reiter from the Technical University of Dortmund, leader of the theory team, explains: “The SUPER scheme uses two red-detuned laser pulses below the energy of the quantum dot to generate single photons.” Because this technique does not require any advanced filtering, the generated photon rate can in principle be doubled.
The research published in Nano Letters was a collaboration between researchers from Austria and Germany. “The intense exchange between the theory and experiment teams allowed a successful implementation of the proposal”, emphasizes Thomas Bracht from University of Münster, who performed theoretical calculations. To implement the experiment, the researchers needed to use two different lasers, but, “in our work we shaped two colours out of one laser source, using a special device called spatial light modulator”, says Yusuf Karli, who together with Florian Kappe and Vikas Remesh conducted the experiments at the University of Innsbruck on quantum dots from the University of Linz and showed that the experimental results are in excellent agreement with the theoretical prediction.
This proof-of-principle experiment, published in the latest issue of Nano Letters, achieves a remarkable step in bringing quantum communication devices from the laboratory to the real world.