Anil Ananthaswamy - Through two doors at once: the elegant experiment that captures the enigma of our quantum reality

The office is simply the most uncluttered of any physicists office I have ever seen. Theres a chair alongside a small table, with nothing on it. No books, no papers, no lamp, no computer, nothing. A sofa graces the office. Large windows overlook a small lake, the trees around which are bare, except for a few stragglers that are holding on to their fall foliage, defying the approaching winter in this part of Ontario, Canada. Lucien Hardy puts his laptop on the tablepointing out that he does most of his work in cafs and figures that all he needs in his office is a caf-like small table to set down his laptop.

There is the obligatory blackboard, taking up most of one wall of his office. It doesnt take long for Hardy to spring up and start chalking it up with diagrams and equationssomething that most of the quantum physicists I meet seem inclined to do.

We start talking about some esoteric aspect of quantum physics, when he stops and says, I started off the wrong way. To reset our discussion, he says, Imagine you have a factory and they make bombs. He has my attention.

He writes two names on the blackboard: Elitzur and Vaidman. He is talking about something called the Elitzur-Vaidman bomb puzzle. Named after two Israeli physicists, the puzzle exemplifies the counterintuitive nature of the quantum world in ways that non-physicists can appreciate. It confounds physicists too in no small measure.

The problem goes something like this. Theres a factory that makes bombs equipped with triggers. The triggers are so sensitive that a single particle, any particle, even a particle of light, can set them off. Theres a big dilemma, however. The factorys assembly line is faulty. Its churning out both good bombs with triggers and bad bombs without triggers. Hardy writes them as good and bad and quips about the quotation marks: Obviously, you may have a different moral perspective on it.

The task is to identify the good bombs. This means having to check whether the bombs have triggers. But examining each bomb isnt the correct strategy, because in order to do so, youd need to shine light on it, however faint, and that would cause a good bomb to explode. The only ones left unexploded would be the duds without triggers.

So, how does one solve this problem? If it helps, we are allowed one concession: we can detonate some bombs, as long as we are left with some good, undetonated bombs.

From our everyday experience of how the world works, this is an impossible problem to solve. But the quantum worldthe world of very small things like molecules and atoms and electrons and protons and photonsbehaves in bizarre ways. The physics that governs the behavior of this microscopic world is called quantum physics or quantum mechanics. And we can use quantum physics to find good bombs without setting them off. Even with a simple setup, its possible to salvage about half the good bombs. It involves using a modern variation of a 200-year-old experiment.

Called the double-slit experiment, it was first done in the early 1800s to challenge Isaac Newtons ideas about the nature of light. The experiment took center stage again in the early twentieth century, when two of the founders of quantum physics, Albert Einstein and Niels Bohr, grappled with its revelations about the nature of reality. In the 1960s, Richard Feynman extolled its virtues, saying that the double-slit experiment contained all of the mysteries of the quantum world. A simpler and more elegant experiment would be hard to find, the workings of which a high school student can grasp, yet profound enough in its implications to bewilder brains like Einsteins and Bohrs, a confusion that continues to this day.

This is the story of quantum mechanics from the perspective of one classic experiment and its subtle, sophisticated variations (including one that, as well see, solves the Elitzur-Vaidman bomb puzzle), whether these variations are carried out as thought experiments by luminous minds or painstakingly performed in the basement labs of physics departments. Its the story of nature taunting us: catch me if you can.