Physicists at the University of Stuttgart, Germany have teleported a quantum state between photons generated by two different semiconductor quantum dot light sources located several metres apart. Though the distance involved in this proof-of-principle “quantum repeater” experiment is small, members of the team describe the feat as a prerequisite for future long-distance quantum communications networks.
“Our result is particularly exciting because such a quantum Internet will encompass these types of distant quantum nodes and will require quantum states that are transmitted among these different nodes,” explains Tim Strobel, a PhD student at Stuttgart’s Institute of Semiconductor Optics and Functional Interfaces (IHFG) and the lead author of a paper describing the research. “It is therefore an important step in showing that remote sources can be effectively interfaced in this way in quantum teleportation experiments.”
In the Stuttgart study, one of the quantum dots generates a single photon while the other produces a pair of photons that are entangled – meaning that the quantum state of one photon is closely linked to the state of the other, no matter how far apart they are. One of the photons in the entangled pair then travels to the other quantum dot and interferes with the photon there. This process produces a superposition that allows the information encapsulated in the single photon to be transferred to the distant “partner” photon from the pair.
Quantum frequency converters
Strobel says the most challenging part of the experiment was making photons from two remote quantum dots interfere with each other. Such interference is only possible if the two particles are indistinguishable, meaning they must be similar in every regard, be it in their temporal shape, spatial shape or wavelength. In contrast, each quantum dot is unique, especially in terms of its spectral properties, and each one emits photons at slightly different wavelengths.
To close the gap, the team used devices called quantum frequency converters to precisely tune the wavelength of the photons and match them spectrally. The researchers also used the converters to shift the original wavelengths of the photons emitted from the quantum dots (around 780 nm) to a wavelength commonly used in telecommunications (1515 nm) without altering the quantum state of the photons. This offers further advantages: “Being at telecommunication wavelengths makes the technology compatible with the existing global optical fibre network, an important step towards real-life applications,” Strobel tells Physics World.
Proof-of-principle experiment
In this work, the quantum dots were separated by an optical fibre just 10 m in length. However, the researchers aim to push this to considerably greater distances in the future. Strobel notes that the Stuttgart study was published in Nature Communications back-to-back with an independent work carried out by researchers led by Rinaldo Trotta of Sapienza University in Rome, Italy. The Rome-based group demonstrated quantum state teleportation across the Sapienza University campus at shorter wavelengths, enabled by the brightness of their quantum-dot source.
“These two papers that we published independently strengthen the measurement outcomes, demonstrating the maturity of quantum dot light sources in this domain,” Strobel says. Semiconducting quantum dots are particularly attractive for this application, he adds, because as well as producing both single and entangled photons on demand, they are also compatible with other semiconductor technologies.
Fundamental research pays off
Simone Luca Portalupi, who leads the quantum optics group at IHFG, notes that “several years of fundamental research and semiconductor technology are converging into these quantum teleportation experiments”. For Peter Michler, who led the study team, the next step is to leverage these advances to bring quantum-dot-based teleportation technology out of a controlled laboratory environment and into the real world.
Strobel points out that there is already some precedent for this, as one of the group’s previous studies showed that they could maintain photon entanglement across a 36-km fibre link deployed across the city of Stuttgart. “The natural next step would be to show that we can teleport the state of a photon across this deployed fibre link,” he says. “Our results will stimulate us to improve each building block of the experiment, from the sample to the setup.”
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