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Quantum theory provides the foundation for much of the technology of modern society. And yet, physicists are still not sure how to think about some of its underlying concepts. One of these is the history of a quantum particle: when we measure an electron or photon in a detector, we can’t be sure of what trajectory it took to get there from the source. Whereas a classical particle traveling between two points always has a single trajectory as its history (think of a canon ball), the history of a quantum particle arriving at a detector is made up of every path it could have taken from where it started. Writing in Physical Review Letters, Ariel Danan and colleagues at Tel-Aviv University, Israel, present a modern version of the famous double-slit experiment with light that underscores how difficult it is to know a quantum particle’s past [1]. They show that even when their experiment blocks light from taking certain paths to a detector, these forbidden paths still appear to be part of the photons’ history.

In the double-slit interference experiment (Fig. 1, top), a beam of photons impinges on two closely spaced slits. A light-sensitive screen placed on the opposite side of the slits will reveal a pattern due to the wavelike interference between the two possible paths—top slit or bottom slit—the photon can take to any point on the screen. But which slit did the photon travel through on its way to the screen? Two common answers are “both slits” and “one cannot say,” but either way, the photon’s trajectory (its history) is ambiguous.

One can try to find which path a photon takes by inserting a lens after each slit—the idea being that each lens will only collimate those photons arriving from its respective slit. In this “which-way” measurement [2], the interference pattern on the screen should disappear and be replaced by two distinct spots, one for each slit (Fig. 1, middle). It would then seem safe to assume that a photon detected in, say, the upper spot, must have taken the route through the upper slit. That is, it appears that there is no confusion in this case: the photon had a single trajectory as its history.

The Tel-Aviv experiment calls even this seemingly safe assertion into question. Danan et al. use a Mach-Zehnder interferometer (MZI), in which the two output ports of a beam splitter—the reflection path and the transmission path—take the place of the two slits (Fig. 1, bottom). These two outputs lead to an upper and lower arm, which recombine the photons at a second beam splitter. They set up the path lengths of the two arms so that their outputs interfere constructively, ensuring that all photons exit the second beam splitter in the same direction. Instead of asking “which slit?” the photon travels through, the authors ask “which arm?”

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