In chapter 9 we concluded that in quantal interference experiments a single atom passes through both branches of an interferometer. In chapter 10.1 we firmed up that everyday-language expression to the technical phrase “there is an amplitude for the atom to go through either branch”. Exactly what do these strange statements mean? How can our minds grow familiar with a real quantal atom, which behaves so unlike a small, hard marble? To prepare for these questions, this chapter examines two variations of the quantal interference experiment. This chapter is not absolutely essential for the logical development of the book, but it dramatically underscores that quantal interference demands a total rethinking of our picture of the atom — no simple trick will suffice.

The Aharonov–Bohm effect

It's possible to build a box called a “corkscrew” from a uniform magnetic field twisted into one turn of a spiral (see figure below). At the left edge of the box the magnetic field points straight out of the page (that is, in the +x direction). Moving towards the right the magnetic field slowly dips until it points straight down, then continues to twist until it points into the page, then straight up, until finally — at the right edge of the box — the magnetic field again points straight out of the page.