Report

A new way to explore the world with quantum light

The project advanced a novel measurement technique based on the counterintuitive laws of quantum physics. The researchers succeeded in generating pairs of quantum-entangled photons at specific wavelengths. These particles of light are linked in a way that allows them to behave as a single quantum system, even when they are spatially separated. The technology extended the spectral range accessible to measurements into the mid-infrared, often known as the "molecular fingerprint region" because almost all molecules absorb light there at characteristic wavelengths.

Two world records in one project

The project successfully pursued two major goals. First, the team became the first in the world to efficiently generate entangled photon pairs at wavelengths between 5 and 10 micrometres using a special opGaP crystal. This spectral range is particularly valuable because many biological and chemical substances-including proteins, plastics and drug molecules-have distinctive "fingerprints" there.

The second key goal was to increase the brightness of the quantum light source. The researchers initially achieved a rate of more than 10 billion photon pairs per second-a world record for this combination of wavelengths. By adding a special optical resonator, they subsequently increased the rate to around 1 trillion photon pairs per second. This makes the source the brightest known source of entangled photons in the mid-infrared.

From fundamental research to practical applications

The techniques developed are relevant not only to fundamental research; they have already found practical applications. Using the quantum light source, the team achieved the first successful detection of microplastic particles smaller than 6 micrometres-an urgent environmental problem that is difficult to investigate using conventional methods. The researchers also demonstrated that the technology, which is relatively inexpensive and can be set up in a compact form, enables very fast spectroscopic measurements at kilohertz rates.

In the longer term, the technology could also be used in medical diagnostics, for example to analyse tissue samples. Quantum sensing could offer significantly greater sensitivity than classical measurement techniques by overcoming limitations inherent to conventional light sources.

Challenges and achievements

The project faced considerable technical challenges. A major setback occurred when the world's only manufacturer of the opGaAs crystals planned for the project discontinued production. The research teams had to redesign their experimental approach and ultimately found a creative solution through collaboration with a research group in Edinburgh. Unexpected problems also arose during crystal optimisation, including thermal effects and absorption in the mid-infrared. A targeted redesign using shorter crystals largely resolved these issues.

Outlook and further development

The project has laid important foundations for future applications. Several scientific papers reporting the results are in preparation. To further develop the work, a collaboration was established with the Fraunhofer Institute for Optronics, System Technologies and Image Exploitation (IOSB) in Ettlingen, where new types of opGaAs crystal can be manufactured.

Although the ultimate goal of reaching the "high-gain regime" has not yet been fully achieved, the key routes towards it have been clearly identified, and initial promising simulations have been carried out. The project demonstrates how fundamental research can go hand in hand with a focus on practical applications-one of the Einstein Foundation Berlin's core aims. The quantum technologies developed have the potential to advance environmental protection, medical diagnostics and chemical analysis.