Quantum uncertainty captured in real time using femtosecond light pulses

Researchers from the University of Arizona, working with an international team, have captured and controlled quantum uncertainty in real time using ultrafast pulses of light. Their discovery, published in the journal Light: Science & Applications, could lead to more secure communication and the development of ultrafast quantum optics.

At the heart of the breakthrough is “squeezed light,” said Mohammed Hassan, the paper’s corresponding author and associate professor of physics and optical sciences.

In quantum physics, light is identified by two linked properties that roughly correspond to a particle’s position and intensity—but can never be known with perfect precision, a concept known as uncertainty. The product of these two measurements cannot fall below a certain threshold, much like the fixed amount of air in a balloon, with each measurement representing one side of the balloon.

“Ordinary light is like a round balloon, with uncertainty spread evenly between its measurements,” Hassan said. “Squeezed light—also known as quantum light—is stretched into an oval, where one property becomes quieter and more precise, while the other grows noisier.”

This squeeze has real-world applications: gravitational-wave detectors already use squeezed light to cut through background noise and detect faint ripples in spacetime caused by distant celestial bodies.

Previous applications of squeezed light relied on laser pulses lasting milliseconds. Hassan wanted to explore whether it was possible to generate squeezed light with ultrafast pulses measured in femtoseconds, or one quadrillionth of a second.

“Creating quantum light with ultrafast laser pulses would be a revolutionary step, and the first real implementation combining quantum optics and ultrafast science,” he said. “The main technical challenge was phase-matching between lasers of different colors, which usually requires complex setups. I realized our technology could overcome this problem.”

Hassan and his colleagues developed a new method for producing extremely short bursts of light using an existing process called four-wave mixing, in which different light sources interact and combine with one another. Building on Hassan’s previous work with ultrafast pulses, the team split a laser into three identical beams and focused them into fused silica, producing ultrafast squeezed light.

Earlier approaches to ultrafast squeezed light reduce uncertainty in a photon’s phase, or its position within a waveform relative to its wavelength. Hassan’s team instead squeezed a photon’s intensity and demonstrated the ability to fluctuate between intensity and phase-squeezing by adjusting the position of the silica relative to the split beam.

If the silica is perpendicular, the photons all arrive together. Adjust the incident angle slightly, and one photon arrives later than the other. That small change is what controls the squeeze.

“This is the first-ever demonstration of ultrafast squeezed light, and the first real-time measurement and control of quantum uncertainty,” Hassan said. “By combining ultrafast lasers with quantum optics, we are opening the door to a new field: ultrafast quantum optics.”

The team has already applied their technique to the field of secure communications. While ultrafast and squeezed light pulses have previously been separately used to transmit binary data, combining them enhances both speed and security.

“If someone intercepts data sent with quantum light, the network will immediately detect the intrusion—but the intruder could still acquire some information with a decoding key,” Hassan said. “Using our method, an eavesdropper not only disturbs the quantum state but also must know both the key and the exact pulse amplitude. Their interference affects the amplitude squeezing, meaning they cannot determine the correct uncertainty, and any decoded data is inaccurate.”

Beyond secure communications, Hassan hopes ultrafast quantum light will advance quantum sensing, chemistry and biology, leading to more precise diagnostics, new drug discovery methods and ultrasensitive detectors for environmental monitoring.

Hassan worked alongside Mohamed Sennary, a graduate student studying optics and physics and the paper’s first author; Mohammed ElKabbash, assistant professor of optical science; and collaborators from the Barcelona Institute of Science and Technology, Ludwig Maximilian University of Munich and the Catalan Institution for Research and Advanced Studies.