Hiroshima University researchers develop a new experimental method to demonstrate that interference physically delocalizes each photon.
The nature of quantum particles has long puzzled scientists. While single-particle interference suggests that a photon can behave like a spread-out wave, we only ever detect a whole photon in one specific place. Traditional interpretations of quantum mechanics often address this by suggesting the particle is in a superposition of being here and there at the same time. However, this tells us only where the particle is when it is measured, not where the particle physically is when no detector is present.
A research team led by Hiroshima University, led by Holger F. Hofmann , professor at the Graduate School of Advanced Science and Engineering , has now developed a method to measure this delocalization without disturbing the photon's wave-like path.
In a study published in the New Journal of Physics on March 23, 2026, the researchers applied a modification of the well-established method of "weak measurements" to a two-path interferometer. As the photon traveled, they applied a tiny rotation by a positive angle in one path and a negative angle in the other. If the two paths interfere in the output, the average rotation angle is always zero. However, this is only a statistical average. To identify the variation of angles for individual photons, the researchers looked at the rate of quantum jumps to orthogonal polarizations. The rate of these jumps depends on the square of the rotation angle, and this squared value can tell us whether the photons were in only one path or not.
Under the rules of classical physics, a single particle is a solid object that can only be in one place at a time. Therefore, it should have been in either path A (+1) or path B (-1). In both scenarios, squaring that number results in a value of 1. However, the experimental data revealed values that did not fit this either-or model. The results showed that the photon's physical presence was distributed across both paths simultaneously, demonstrating that the particle is truly delocalized until a detector forces it into a single location.
The findings have significant implications for high-tech sensors. In the quantum world, the uncertainty of a quantity in a superposition is actually an asset: It allows researchers to achieve phase sensitivity sharper than previously thought possible for a single particle in ultra-precise measurements, such as those used in GPS, atomic clocks, or deep-space communication.
They also have deep philosophical implications for how we understand the universe. In our daily lives, we assume that the Moon is there even if we aren't looking at it. This is because the Moon is "easy to see." It is constantly interacting with its environment (sunlight, gravity, etc.), which measures its position for us.
In the microscopic world of photons, however, particles have very few interactions with their environment. Hofmann's team is suggesting that reality is not a fixed thing that exists independently of us.
"Whenever we imagine an object, we assume that we can see it without effort," explained Hofmann. "At the microscopic level, objects have almost no recordable interactions with their environment. Our results show that the macroscopic world is easy to see, but in the microscopic world, we must learn to distinguish between what is 'there' and how our measurements define reality."
Ryuya Fukuda, Masataka Iinuma and Yuto Matsumoto at the Graduate School of Advanced Science and Engineering, Hiroshima University, co-authored the paper.
This research was supported by the Japan Science and Technology Agency (JST) ERATO (JPMJER2402) and JST SPRING (JPMJSP2132).
