An artist’s illustration of the superposition of the massive molecules used in the latest experiment. [Image: Yaakov Fein, University of Vienna]
Researchers in Austria and Switzerland have pushed the boundaries of the quantum world by demonstrating wave-like properties of molecules containing a record-breaking 2,000 atoms (Nat. Phys., doi: 10.1038/s41567-019-0663-9.)
Obtained using a specially built matter-wave interferometer, the new results go some way to ruling out alternatives to the Copenhagen interpretation of quantum mechanics that regard wavefunctions as real and their collapse as an objective process.
Building the interferometer
Physicists have in recent years been using an increasingly wide range of systems to establish just how macroscopic they can make quantum superpositions before they break down into well-defined classical objects. Such systems include superconductors, Bose-Einstein condensates, mechanical oscillators and matter-wave interferometers.
This last category is similar to the classic double-slit experiment with single photons, but using massive objects such as electrons, atoms or even molecules. Sent through a grating one at a time, those objects collectively trace out a set of interference fringes on a screen—thanks to the fact that each of their wavefunctions interferes with itself after passing through the grating.
To carry out the latest research, Markus Arndt, Yaakov Fein and colleagues at the University of Vienna, Austria, built the longest molecular interferometer to date. The device employs three gratings with a total baseline of 2 meters, allowing interference between molecules with de Broglie wavelengths 10 times shorter than previously possible.
The first grating, made from silicon nitride, forces molecules through very narrow slits, which gives them well-defined positions. The consequent extra uncertainty in their transverse velocity causes each molecule’s wavefunction to spread out further down the baseline, where it sweeps across the second grating. Made using a green laser, this grating consists of a standing light wave of multiple antinodes that generate a phase shift within the wavefunction. Matter-wave interference then creates a variation in the density of molecules at the position of the third (silicon nitride) grating, with a plot of that density variation revealing the interference fringes.
The molecules used in the experiment, known as functionalized oligoporphyrins, were prepared by chemists Marcel Mayor and Patrick Zwick at the University of Basel, Switzerland. These molecules consist of up to 2,000 atoms and weigh in at over 25,000 atomic mass units (amu). They are well isolated from each other, thanks to the Teflon-like qualities of the fluorine atoms in the molecules’ outer shell—which bind electrons very tightly, making the molecules harder to polarize.
Arndt and colleagues used these molecules to break the mass record for matter-wave interference—10,000 amu—which they set themselves in 2013. They found that they could generate interference fringes with 90% of the expected visibility and hold the molecules in a superposition for more than 7 milliseconds.
They did so, points out Arndt, despite the fact that the molecules had about 6,000 vibrational degrees of freedom and internal temperatures of a few hundred kelvin. Some researchers, he says, find it hard to believe that they could have created superpositions of such massive molecules for so long, given that infrared photons might be emitted in those conditions. But Arndt maintains that a fine tuning of experimental parameters “luckily, in this case, works out in our favor.”
Ruling out the alternatives
In achieving this superposition, the researchers have also set tighter limits on certain alternatives to standard quantum theory. Such models posit that a particle’s wavefunction spontaneously collapses at a certain rate to a certain minimal radius. The rate would grow with the square of a particle’s mass, which can be represented mathematically by adding a nonlinear term to the Schrödinger equation. One set of parameters, says Arndt, put forward by Stephen Adler of the Institute for Advanced Study in Princeton, N.J., USA, now looks “not entirely ruled out, but becoming very unlikely to be realized by nature.”
Assuming that nature doesn’t work in a similar way, Arndt reckons that most such models could be ruled out once superposition has been demonstrated using molecules of around a billion atomic mass units. He says that improving on his group’s current record by an order of magnitude—to a few times 105 mass units—should be “relatively straightforward” using metal clusters and the replacement of mechanical gratings with ultraviolet ones. Going up by another factor of 10 to 100, he says, might be possible “in a few years,” with the use of cooled dielectric nanoparticles.