How MPL Scientists Measured Entanglement Through 99% Light Loss

Scientists at the Max Planck Institute for the Science of Light (MPL) have demonstrated a method for measuring entanglement in light even after 99.7% of the signal is lost, a significant step toward practical quantum technologies. The team overcame this extreme signal degradation by first amplifying squeezed light, a resource crucial for quantum computing and precision measurements like those used at the LIGO gravitational wave observatory, before separating and analyzing its quantum channels. Squeezed light is particularly vulnerable to loss, presenting a significant hurdle for maintaining its delicate quantum properties; however, the researchers successfully measured multiple modes of this light simultaneously. “Amplifying a quantum state before detection is like properly packaging fragile glass before shipping it,” says Mahmoud Kalash, a PhD student at Friedrich-Alexander-Universität Erlangen-Nürnberg and first author of the paper published in Nature Communications 17, ( ). /s41467- -0, detailing a technique that could unlock more scalable quantum systems.

Multimode Squeezed Light Generation via Parametric Down-Conversion

Researchers have, for the first time, successfully measured entanglement in multimode squeezed light despite experiencing a 99.7% loss of signal, a feat previously considered insurmountable and a major step toward practical quantum technologies. This achievement centers on squeezed light, a non-classical state of light crucial for enhancing the sensitivity of instruments like LIGO, which detects gravitational waves, and increasingly important for quantum computing. The challenge lies in the inherent noise present in all light sources; even “perfect” laser light exhibits random fluctuations known as shot noise, obscuring faint quantum signals. Squeezed light offers a solution by reducing this noise below the standard quantum limit, but it is particularly vulnerable to loss, making detection exceptionally difficult. To address this, the team, led by Prof. Maria Chekhova of MPL, employed a multimode optical parametric amplifier (MOPA) to boost the signal before measurement.

By first amplifying the light and then separating it into individual modes using a spatial light modulator, the researchers were able to measure squeezing of up to 7.9 decibels. The team also monitored eight other modes simultaneously. All modes showed significant squeezing and high purity, and groups of modes showed quantum entanglement. These results show that multimode quantum light can be measured even under extreme losses. Marcello Passos, research group leader at the Centro Brasileiro de Pesquisas Físicas, notes that “The method presented in this work opens up new possibilities for high-dimensional quantum information processing, particularly in quantum computing with complex networks, where many modes can process information simultaneously.”

Multimode Optical Parametric Amplification Overcomes Detection Losses

Researchers are now successfully measuring entangled states of light even with signal degradation previously considered insurmountable; conventional quantum measurements are exceptionally sensitive, with even minor losses obscuring delicate quantum properties before they can be detected. This noise, known as shot noise, represents random fluctuations in the electromagnetic field, even in what appears to be perfect laser light. Despite experiencing losses exceeding 99.7%, the team was able to measure squeezing of up to 7.9 decibels. The team also monitored eight other modes simultaneously. All modes showed significant squeezing and high purity, and groups of modes showed quantum entanglement. These results show that multimode quantum light can be measured even under extreme losses.

Demonstrated 7.9dB Squeezing and Entanglement Across Eight Modes

The team, collaborating with scientists from institutions in Germany, Brazil, and France, recently demonstrated the ability to measure squeezing and entanglement across eight modes of light even after experiencing a 99.7% loss of signal. This achievement, published in Nature Communications (Kalash, M., Sudharsanam, A., M. Real-time monitoring of multimode squeezing. Nat Commun 17, ( ). /s41467- -0), hinges on a novel approach to amplification before detection. Instead of attempting to directly measure the fragile squeezed light, the scientists first boosted its intensity using a multimode optical parametric amplifier (MOPA). The team successfully measured squeezing levels up to 7.9 decibels, corresponding to a noise level one-sixth that of a perfect laser. Crucially, the researchers didn’t just demonstrate squeezing in individual modes; they also confirmed entanglement between groups of these modes.