Mach-Zehnder interferometer

In plain terms

Here is the problem in its cleanest hardware. A beam splitter sends light down two separate paths; mirrors bring the paths back together at a second beam splitter; two detectors wait at the outputs. Which detector fires depends on the DIFFERENCE between the two path lengths -- interference. So far, classical optics.

Now dim the source until only one particle is in flight at a time. The interference stays. One photon, and its arrival statistics depend on both paths at once. Block either path and the interference dies instantly, replaced by a fifty-fifty split.

Every hard question about quantum mechanics can be staged in this box: what happened in between? was there a path? when was it decided? That is why the box appears in every foundations argument since 1892.

In CSD

This is the standard arena for the corpus's interference twins -- the bomb tester and the quantum eraser both live inside it. Its value is diagnostic: it forces any account to say something definite about the unmeasured alternative, which is precisely where interpretations stop agreeing.

The eraser variant is the sharpest instrument in the set: which-path information destroys interference, and DISCARDING that information afterwards restores it. Any story in which the particle simply took one definite path has real explaining to do about the restoration. CSD's account runs through the region structure rather than through path-stories, and reproducing both twins is treated as a pass-fail constraint, not decoration.

Mathematically

Two balanced beam splitters, two paths, output intensities set by the relative phase between arms. Unlike the Michelson layout the paths are spatially separated and traversed once, which is what makes arm-specific interventions -- blockers, detectors, erasers -- cleanly statable, and hence what makes it the canonical which-path laboratory.

The name

Ludwig Zehnder (1854-1949), Swiss, doctorate under Rontgen at Wurzburg, published the design in 1891; Ludwig Mach (1868-1951) refined it independently in 1892. Neither had quantum mechanics in mind -- the instrument was built for measuring refractive indices.

The younger Mach was the son of Ernst Mach, whose positivism -- only what is observable is real -- shaped Einstein and the Vienna Circle. There is a neat irony in the son's instrument becoming, a century later, the standard stage for arguments about whether unobserved paths are real. The father would have had opinions.