Researchers from the Technical University of Denmark (DTU) and international collaborators from the US, Canada, and South Korea have shown that entangled light can drastically reduce the number of measurements required to comprehend the behavior of a complex, noisy quantum system. The study was published in Science.
The squeezer, an optical parametric oscillator (OPO) that uses a nonlinear crystal inside an optical cavity to manipulate the quantum fluctuations of light, is responsible for the entanglement. Image Credit: Jonas Schou Neergaard-Nielsen.
Entangled light allows researchers to analyze a system’s noise using significantly fewer measurements, according to a recent experiment.
This is the first proven quantum advantage for a photonic system.
Ulrik Lund Andersen, Study Corresponding Author and Professor, Physics, Technical University of Denmark (DTU)
“Knowing that such an advantage is possible with a straightforward optical setup should help others look for areas where this approach would pay off, such as sensing and machine learning,” said Ulrik Lund Andersen.
Entanglement is Key
At the heart of this research lies a common challenge across science and engineering: understanding or characterizing a physical system, like a device, typically requires repeated measurements. These measurements help reveal key properties, such as the device’s unique “noise fingerprint.”
In quantum devices, however, the process becomes more complex. Quantum noise is intrinsic to the measurements themselves, making it harder to separate the system’s behavior from the noise it generates. The number of experiments required for intricate systems can increase exponentially with the size of the system, making it rapidly impractical or even unfeasible. The researchers aimed to discover an alternative approach utilizing entangled light.
Entanglement represents a fundamental principle in quantum mechanics, wherein two particles or light beams are interconnected to such a degree that measuring one immediately provides information about the other.
“We built a process we could control and asked a simple question: Does entanglement reduce the number of measurements you need to learn such a system? And the answer is yes, by a lot. We learned the behavior of our system in 15 minutes, while a comparable classical approach would take around 20 million years,” said Ulrik Lund Andersen.
Something No Classical System Can Do
Following the establishment of the theoretical foundation in the 2024 publication 'Entanglement-Enabled Advantage for Learning a Bosonic Random Displacement Channel', the researchers were confident that entangled light would probably address the problem.
The experiment was conducted in the basement of DTU Physics and operates at telecom wavelengths, utilizing familiar optical components. It remains effective even in the presence of typical losses within the setup. This is significant, according to the researchers, as it demonstrates that the advantage arises from the measurement approach rather than the quality of the measuring instrument.
The system was composed of an optical channel where multiple light pulses shared the same noise characteristics. Two beams of light were prepared – or more accurately, squeezed – to achieve entanglement. One beam serves to probe the system, while the other acts as a reference. A joint measurement is conducted to compare them simultaneously, and this comparison effectively cancels much of the measurement noise, extracting more information per trial than examining the probe alone.
Jonas Schou Neergaard-Nielsen, an associate professor at DTU Physics and co-author of the study, emphasizes that the researchers have not yet focused on a specific real-world system.
Even though a lot of people are talking about quantum technology and how they outperform classical computers, the fact remains that today, they don't. So, what satisfies us is primarily that we have finally found a quantum mechanical system that does something no classical system will ever be able to do.
Jonas Schou Neergaard-Nielsen, Associate Professor and Study C-Author, Physics, Technical University of Denmark (DTU)
Journal Reference:
Liu, Z., et al. (2025) Quantum learning advantage on a scalable photonic platform. Science. doi.org/10.1126/science.adv2560