Physicists Smash Record For Wave-Particle Duality

Researchers have observed quantum superposition in molecules containing around 5000 protons, 5000 neutrons and 5000 electrons

One of the central concepts of quantum mechanics is wave-particle duality—that all objects can be thought of as either a wave or as a particle. That’s hard, if not impossible, to imagine. But it’s trivial to demonstrate in a school laboratory.

In the famous double slit experiment, a beam of particles such as electrons, travels through a double slit and then hits a screen behind where the arrival of each electron is recorded at a specific point on the screen.

But while each electron arrives like a particle, many electrons together create an interference pattern that can only be explained if the electrons also behave like waves. These waves become superposed and so interfere.

Stranger still is the prediction that the same effect ought to be measurable for much larger objects too. According to quantum mechanics, wave-particle duality and quantum superpositions must also occur for macroscopic objects such as viruses, cells and even baseballs larger objects.

Of course, nobody has seen the quantum superposition of a baseball or anything anywhere near that size. The experiment would be impossibly difficult. But physicists have seen this wave-particle duality for protons, atoms and increasingly large molecules such as buckyballs.

And that raises an interesting question: how big an object can physicists observe behaving like a wave? Today, Sandra Eibenberger at the University of Vienna in Austria and a few pals say they’ve smashed the record for a quantum superposition by observing wavelike behaviour in giant molecules containing over 800 atoms.

These kinds of experiments are hard to do. In principle, they are like the double slit experiments that reveal the wave-like behaviour of electrons. So they require a beam of particles, a set of slits for the beam to pass through and a detector that records the position of each particle on the other side.

But while creating a coherent beam of electrons or even atoms is relatively straightforward, that’s not the case with molecules.

Eibenberger and co do it by heating a sample of molecules so that it vapourises and the gas passes through a narrow slit to form a beam that curves due to the force of gravity.

Another horizontal slit then filters out only those molecules with a specific velocity, since those that are faster or slower travel in a parabola that passes above or below the slit. The result is a beam of molecules that have a specific kinetic energy.

The problem is that few large molecules can survive this process. These molecules must be volatile so that they can form a gas but also strong enough not to decompose when heated. For technical reasons to do with the experimental set up, these molecules also need to be polarisable and transparent to light at certain specific frequencies.

That’s quite an ask. Eibenberger and co solve it by creating tree-like molecules that have a porphyrin core with perfluoroalkyl chains added on. This molecule has the nominal formula: C284.H190.F320.N4.S12. It has relatively low intermolecular binding forces and so is volatile and yet is relatively stable when heated.

Having created a beam of these molecules, Eibenberger and co pass them through a series of slits that reveal any wavelike characteristics. Sure enough, the molecules form an interference pattern at the detector which implies that they must have been superposed while passing through the slits.

The team’s measurements imply that this molecule has a wavelength of about 500 femtometres, which is about four orders of magnitude smaller than the diameter of a molecule by itself. “Our data confirm the fully coherent quantum delocalization of single compounds composed of about 5000 protons, 5000 neutrons and 5000 electrons,” they say.

That’s an impressive result and a significant step forward for the detection of wave-particle duality and quantum superposition in macroscopic objects.

However, it still leaves open the question of how big an object can be and still be observed forming a quantum superposition. These molecules are of course tiny but they are within an order of magnitude or so of the smallest viruses.

So it’s not beyond the realms of possibility that physicists will be able to see quantum-like behaviour at this level. And that opens the door to other quantum phenomenon such as teleportation.

Of viruses? On this evidence, don’t bet against it.

Ref: arxiv.org/abs/1310.8343: Matter-wave interference with particles selected from a molecular library with masses exceeding 10 000 amu (Phys. Chem. Chem. Phys., 2013,15, 14696-14700)