Fun with an Argon Atom

Photon-recoil bilocation experiment at Heidelberg

A recent experiment on Argon atoms by Jeri Tomkovic and five collaborators at the University of Heidelberg has demonstrated once again the subtle and astonishing reality of the quantum world.

Erwin Schrödinger, who devised the Schrödinger equation that governs quantum behavior, also demonstrated the preposterousness of his own equation by showing that under certain special conditions quantum theory seemed to allow a cat (Schrödinger's Cat) to be alive and dead at the same time. Humans can't yet do this to cats, but clever physicists are discovering how to put larger and larger systems into a "quantum superposition" in which a single entity can comfortably dwell in two distinct (and seemingly contradictory) states of existence.

The Heidelberg experiment with Argon atoms (explained popularly here, in the physics arXiv here and published in Nature here) dramatically demonstrates two important features of quantum reality: 1) if it is experimentally impossible to tell whether a process went one way or the other, then, in reality, IT WENT BOTH WAYS AT ONCE (like a Schrödinger Cat); 2) quantum systems behave like waves when not looked at--and like particles when you look.

The Heidelberg physicists looked at laser-excited Argon atoms which shed their excitation by emitting a single photon of light. The photon goes off in a random direction and the Argon atom recoils in the opposite direction. Ordinary physics so far.

But Tomkovic and pals modified this experiment by placing a gold mirror behind the excited Argon atom. Now (if the mirror is close enough to the atom) it is impossible for anyone to tell whether the emitted photon was emitted directly or bounced off the mirror. According to the rules of quantum mechanics then, the Argon atom must be imagined to recoil IN BOTH DIRECTIONS AT ONCE--both towards and away from the mirror.

But this paradoxical situation is present only if we don't look. Like Schrödinger's Cat, who will be either alive or dead (if we look) but not both, the bilocal Argon atom (if we look) will always be found to be recoiling in only one direction--towards the mirror (M) or away from the mirror (A) but never both at the same time.

To prove that the Argon atom was really in the bilocal superposition state we have to devise an experiment that involves both motions (M and A) at once. (Same to verify the Cat--we need to devise a measurement that looks at both LIVE and DEAD cat at the same time.)

To measure both recoil states at once, the Heidelberg guys set up a laser standing wave by shining a laser directly into a mirror and scattered the bilocal Argon atom off the peaks and troughs of this optical standing wave. Just as a wave of light is diffracted off the regular peaks and troughs of a matter-made CD disk, so a wave of matter (Argon atoms) can be diffracted from a regular pattern of light (a laser shining into a mirror).

When an Argon atom encounters the regular lattice of laser light, it is split (because of its wave nature) into a transmitted (T) and a diffracted (D) wave. The intensity of the laser is adjusted so that the relative proportion of these two waves is approximately 50/50.

In its encounter with the laser lattice, each state (M and A) of the bilocated Argon atom is split into two parts (T and D), so now THE SAME ARGON ATOM is traveling in four directions at once (MT, MD, AT, AD).

Furthermore (as long as we don't look) these four distinct parts act like waves. This means they can constructively and destructively interfere depending on their "phase difference". The two waves MT and AD are mixed and the result sent to particle detector #1. The two waves AT and MD are mixed and sent to particle detector #2. For each atom only one count is recorded--one particle in, one particle out. But the PATTERN OF PARTICLES in each detector will depend on the details of the four-fold experience each wavelet has encountered on its way to a particle detector. This hidden wave-like experience is altered by moving the laser mirror L which shifts the position of the peaks of the optical diffraction grating.

In quantum theory, the amplitude of a matter wave is related to the probability that it will trigger a count in a particle detector. Even though the unlooked-at Argon atom is split into four partial waves, the looked-at Argon particle can only trigger one detector.

The outcome of the Heidelberg experiment consists of counting the number of atoms detected in counters #1 and #2 as a function of the laser mirror position L.

The results of this experiment show that, while it was unobserved, a single Argon atom was 1) in two places at once because of the mirror's ambiguisation of photon recoil, then 2) four places at once after encountering the laser diffraction grating, 3) then at last, only one place at a time when it is finally observed by either atom counter #1 or atom counter #2.

The term "Schrödinger Cat state" has come to mean ANY MACROSCOPIC SYSTEM that can be placed in a quantum superposition. Does an Argon atom qualify as a Schrödinger Cat? Argon is made up of 40 nucleons, each consisting of 3 quarks. Furthermore each Argon atom is surrounded by 18 electrons for a total of 138 elementary particles--each "doing its own thing" while the atom as a whole exists in four separate places at the same time. Now a cat surely has more parts than a single Argon atom, but the Heidelberg experiment demonstrates that, with a little ingenuity, a quite complicated system can be coaxed into quantum superposition.

Today's physics students are lucky. When I was learning quantum physics in the 60s, much of the quantum weirdness existed only as mere theoretical formalism. Now in 2011, many of these theoretical possibilities have become solid experimental fact. This marvelous Heidelberg quadralocated Argon atom joins the growing list of barely believable experimental hints from Nature Herself about how She routinely cooks up the bizarre quantum realities that underlie the commonplace facts of ordinary life.