“For two years, I was looking at flat lines,” physicist Sebastian Pedalino recalled of a stubborn detector that refused to show anything but noise. The evening when the signal at last expanded out into an interference figure of stripes, the silent frustration of the lab was inverted to a sort of quantifiable strangeness: heaps of some 7,000 sodium atoms had now become a unitary wave, not a small shower of classical particles.
That finding is important since quantum mechanics had never claimed to remain small. Daily life tells us otherwise: stones, dust, and specks of metal are placed on a single spot and on a single course. The separation between those worlds is typically attributed to decoherence an unremitting influence of stray gas molecules, light or fields that disrupt fragile superpositions. The quantum engineering problem then is not so much how to “making things quantum” but to create a space in which more giant things can be left to find their own way.
The experiment of Vienna team did not involve a single atom, it was made of sodium in the form of a nanoparticle 8 nanometres across, which was then formed into clusters of 5,000-10,000 atoms. The arrangement was driving these metal clumps through a near field interferometer consisting of three laser-made gratings, the design of which was due to the extremely short de Broglie wavelengths of massive particles and their inability to be diffracted in the far field. The beam was ready in cold and isolated state via ultra-high vacuum and cryogenicity in order to minimize the environmental effects that would obliterate the effect. The gratings were not just sifts: the standing ultraviolet frequency light waves served as filters and phase shapers with a clearly spaced set of filters that can be used with all sorts of materials, not just atom specific transitions. Only when the device was set up suitably and the remaining perturbations could be minimized did a characteristic interference fringe develop as the matter wave of the cluster could remain coherent along several paths.
This fundamental finding was easy to explain and hard to win: rather than a straight line being drawn on the detector, the sodium clusters showed a periodic structure, i.e. each cluster followed a superposition of spatially distinct pathways and then self-fertilized. One of the reported configurations had the delocalization inserting the cluster in different positions that were a distance of approximately 133 nanometres-a distance that was much larger than that of the particle itself. The punchline can be condensed into one short paragraph: these were “metal clumps,” and they continued to interfere.
The comparison of “bigness” between the widely different demonstrated quantum examples is commonly done by macroscopicity, a measure that collapsed mass, the separation between the superposed states and the duration of the state. In this case, the team recorded μ = 15.5, which is a figure that is characterized as an order-of-magnitude leap over previous matter-wave measurements of this type of spatial superposition. It is not just a trophy figure; it leaves limited space in which the proposed changes to quantum mechanics can be made to predict an inevitable degeneration to classical behaviour at some scale, even in a vacuum.
What the next steps would be is an additional change in the experiment. The clusters are already in a size-mass neighborhood of proteins and small viruses and the same interferometric reasoning can, in theory, be applied to biological samples- provided delicate structures can endure the flight and preparation without disintegrating. Outside this more specific area, other proposals have proposed placing microscopic things into quantum states by coupling them to cryogenic mechanical vibrations, in which case “quantum biology” is typically understood as simply the center-of-mass motion of a prepared object, rather than biochemical activity.
To the engineers of quantum technologies, the implicit implication is more practical in that scaling is not exclusively a computer-science problem. It is a stability issue, which has been solved in vacuum systems, cooling schemes, vibration control, and optical precision, until a system consisting of thousands of atoms can, temporarily, act as though it is not at a single location.
