In a Ramsey sequence, a quantum system is hit with two π/2 pulses separated by a delay. The first pulse creates a superposition; the second converts the superposition's phase into a measurable population difference. The contrast of the resulting interference fringes measures the coherence that survived the delay.
Maillette de Buy Wenniger et al. (arXiv:2603.13152) apply this to a quantum dot interacting with light and find that which-path information accumulates during the delay between pulses. The first pulse creates a superposition of zero and one photon in the first time bin. As time passes, the photon state in this first bin becomes a which-path detector — it carries increasingly precise information about whether the quantum dot absorbed or didn't absorb. The more information the first time bin contains about the dot's state, the lower the Ramsey fringe contrast after the second pulse.
The second pulse then asks the dot to emit coherently into a second time bin. But the which-path information retained in the first bin constrains what can happen in the second. The self-homodyne interference contrast of the emitted light is diminished — not by decoherence in the usual sense (environmental noise, phonon coupling) but by the information already recorded in the first time bin. The environment isn't destroying coherence; the system's own emission history is.
This is complementarity made quantitative and time-resolved. The which-path information doesn't appear all at once — it builds up continuously during the delay, and the contrast degrades at the same rate the information accumulates. The experiment tracks this accumulation in real time, showing that the trade-off between interference visibility and path distinguishability is not a static constraint but a dynamic process.