The sound, which until now was only known to have been heard in neutron stars, makes this breakthrough a mammoth of an achievement. The physicists, a team of six, a part of the MIT-Harvard Center for Ultracold Atoms published their findings in the journal 'Science'. A perfect fluid is characterised by a perfect flow, which in simple terms, refers to a liquid that offers the least friction or viscosity to the flow of soundwaves. It offers the least heat or resistance to whatever pipe that it flows thorugh. Speaking to MIT's news website, Martin Zwerlein expressed his elation and also the implications of his team's achievement. "It’s quite difficult to listen to a neutron star,” says Martin Zwierlein, the Thomas A. Frank Professor of Physics at MIT. “But now you could mimic it in a lab using atoms, shake that atomic soup and listen to it, and know how a neutron star would sound,” he said. To do this, they first generated a gas of strongly interacting fermions. A fermion, like electrons and protons, is a type of elementary particle. They are, however, different from electrons and protons in that they have a half-integer spin, meaning that they assume a different state after a full spin, and that no two neighbouring fermions have the same spin, ensuring that they don't collide....

Scientists have captured the sound of a “perfect fluid,” which flows with the smallest amount of friction allowed by the laws of quantum mechanics. Credit: Christine Daniloff, MIT...

For some, the sound of a “perfect flow” might be the gentle lapping of a forest brook or perhaps the tinkling of water poured from a pitcher. For physicists, a perfect flow is more specific, referring to a fluid that flows with the smallest amount of friction, or viscosity, allowed by the laws of quantum mechanics. Such perfectly fluid behavior is rare in nature, but it is thought to occur in the cores of neutron stars and in the soupy plasma of the early universe. This recording is a product of a glissando of sound waves that the team sent through a carefully controlled gas of elementary particles known as fermions. The pitches that can be heard are the particular frequencies at which the gas resonates like a plucked string. The researchers analyzed thousands of sound waves traveling through this gas, to measure its “sound diffusion,” or how quickly sound dissipates in the gas, which is related directly to a material’s viscosity, or internal friction. Surprisingly, they found that the fluid’s sound diffusion was so low as to be described by a “quantum” amount of friction, given by a constant of nature known as Planck’s constant, and the mass of the individual fermions in the fluid....

Atomic clocks are the most precise timekeepers in the world. These exquisite instruments use lasers to measure the vibrations of atoms, which oscillate at a constant frequency, like many microscopic pendulums swinging in sync. The best atomic clocks in the world keep time with such precision that, if they had been running since the beginning of the universe, they would only be off by about half a second today. Still, they could be even more precise. If atomic clocks could more accurately measure atomic vibrations, they would be sensitive enough to detect phenomena such as dark matter and gravitational waves. With better atomic clocks, scientists could also start to answer some mind-bending questions, such as what effect gravity might have on the passage of time and whether time itself changes as the universe ages. The researchers report today in the journal Nature that they have built an atomic clock that measures not a cloud of randomly oscillating atoms, as state-of-the-art designs measure now, but instead atoms that have been quantumly entangled. The atoms are correlated in a way that is impossible according to the laws of classical physics, and that allows the scientists to measure the atoms’ vibrations more accurately....