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Here’s what you’ll learn when you read this story:
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Researchers have figured out how to create a gas that, when cooled to freezing temperatures, will not experience resistance or diffusion.
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The atoms in this gas do not abide by the usual ballistic or diffusive modes of transport. They collide in a straight line and do not allow for friction or loss.
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This experiment proved that it is possible for atoms to travel in a straight line—and collide in a straight line—without slowing down.
Flow is always happening in some way or another, whether it is water gushing through a drainpipe or electricity coursing through computer wires, but there is usually something holding it back. Resistance created by friction or collisions between particles can slow down or break the flow. This used to be inevitable—until it wasn’t.
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Transport is the phenomenon of movement through a medium. It can describe the passage of particles, energy, momentum, or mass. Metals, insulators, and superconductors are all defined by their transport properties. Led by physicist Frederik Møller of the Atominstitut at Vienna University of Technology, a team of researchers have figured out how to create a consistent flow of rubidium atoms in an ultracold quantum gas that flowed without resistance, though it was plagued with atomic collisions. This is the most efficient form of transport ever seen.
The two existing types of transport are stark opposites. In diffusive transport, particles move freely through a medium, though constant collisions occur randomly. Heat conduction occurs when warmer particles smash into cooler ones, and heat in warmer regions will flow to where there is less heat. It is through particle interactions that energy and momentum are transferred until the temperature in the system breaks even. Osmosis is another type of diffusive transport. Water molecules diffuse through cell membranes, sometimes passing through alone, and sometimes with an assist from transport proteins if larger molecules such as sugars or amino acids also need to cross a membrane.
Ballistic transport is the free movement of particles, usually charged particles such as electrons, that zoom through space without collisions and cover much more distance than diffusive transport would in the same time. The movement of electrons in metals is ironically weaker via ballistic transport than diffusive transport.
What Møller and his team achieved was closest to (though not exactly like) ballistic transport. The rubidium atoms used in the experiment were bosonic atoms, which behave like the subatomic particles known as bosons, characterized by their full integer spins. Atoms were turned into a freezing gas with both evaporative cooling, which uses the energy water needs to evaporate in order to cool air, and laser cooling, which takes advantage of an atom’s change in momentum when it absorbs a photon from the laser. The atom will then re-emit the photon and decrease its momentum and therefore its temperature. Above the gas formed by these atoms, scientists placed a chip that created a magnetic trap to restrict their movement.
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“If currents are dissipationless, the corresponding charge carriers propagate ballistically,” the researchers said in a study recently published in Science Advances.
Even though atoms in the gas traveled in a straight line without diffusing, devoid of friction or loss, the transport mode in the experiment is not defined as either diffusive or ballistic. While conditions made it virtually impossible for Møller’s rubidium atoms to diffuse, and collisions still occurred within the atomic current, mass and energy could still keep flowing through the system without scattering. The collisions were also not random. They behaved more like a Newton’s cradle, that desktop fidget contraption of suspended metal spheres that depends on one of those spheres being released so its momentum could be transferred through the entire row of them but only swing the one at the far end outward.
This flawless efficiency has broken the rules of resistance as we know them. Could the same go for shock waves? While the research team explored this theoretically, it was beyond the scope of their study, but stay tuned.
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