When liquid helium-3 is cooled to a millikelvin temperature, it converts to a superfluid substance. Outwardly, superfluid helium-3 looks like a simple, transparent object. However, its internal structure is vibrant and is often described as “the Universe in drops” [1]. In helium-3, some composite atoms play the role of quantum vacuum — the largest quantum region with the lowest possible potential — and others act as matter, interacting with analogs of electromagnetic and gravitational forces. To date, researchers have learned how to “smell” the structure by examining the weak magnetic fields around the helium-3 nuclei, “touching” it by pushing foreign objects through the liquid and measuring reaction forces, and by “hearing” the speaker and microphone inside the liquid. Now, Theo Noble and his colleagues at the University of Lancaster in the UK have introduced a fourth concept with a camera that can “see” the internal details of the helium-3 universe [2]. This ability could allow researchers to understand the chaos in quantum liquids better and discover the topological properties that are predicted to hide in this highly liquid world.
In superfluid helium-3, atomic pairs called Cooper pairs to form a vacuum, which exhibits unusual dynamics, such as flow without collision. In contrast, single atoms form a compound called fermionic quasiparticles. The strength of these quasiparticles cannot be limited, which may vary from place to place in the inhomogeneous system. As a result, the scattering quasiparticle may not be able to penetrate other regions of such a system and will instead be expressed through a process called Andreev reflection [3]. The incoming quasiparticle triggers a partner to form a Cooper pair, which becomes part of the vacuum in this process. The partner leaves behind a pleasure known as a hole that differs differently from the incoming quasiparticle. The inhomogeneity that causes this demonstration can be produced, as in the case of Josephson superconducting — devices used in qubits in quantum computers. But it can also be internal, like the one exploited by Noble and his colleagues.
The superfluid that flows rapidly past the barrier cannot meet the obstacles of proper fluid flow and is damaged in the environment. Quantum mechanics and topology incorporate this flexibility into a linear structure called the quantized vortex, which is found in helium-3 above water as a fiber with a thickness of less than 100 nm. The Superfluid flow around this vortex carries a single quantum cycle. Flow changes the amount of energy-restricted to quasiparticles by a value depending on the distance to the vortex. This intrinsic inhomogeneity leads to Andreev’s reflection of quasiparticles moving near the vortex.
In the presence of dynamic variables of many vortices, the superfluid can mimic a variety of complex flow patterns, the maximum of which is not limited to the erratic movement of the relevant liquid. A functional area of study called quantum turbulence explores how the interaction of simple, uniform vortices on a small scale leads to complex dynamics on a large scale [4]. Noble and colleagues are focused on this area to showcase their camera skills for the first time.
Their equipment consists of three parts, wholly immersed in the bath of superfluid helium-3. The first part is the source of the quasiparticle: a closed box in which a moving machine breaks the Cooper pear into a leaky particle in an anchor. Outside the box, the temperature is much lower than the superfluid transition rate of helium-3, and temperature changes break a few Cooper pairs, so quasiparticles fly in a straight line like light rays. The second part is the vortex source: an oscillating semicircular loop that produces vortices when the flow velocity around the loop area reaches a critical value. Finally, the third part is a camera: a five-five-row series of quartz repair forks. The blurring of the oscillation of each division corresponds to the number of adjacent quasiparticles, which are translated into the corresponding pixel light in the image captured by the camera.
Before turning on the vortex generator, Noble and colleagues demonstrated that the image produced by quasiparticles is clearly defined by the laws of light created by light. After turning on the generator, they found that the vortices prevented some of the quasiparticles from reaching the camera due to the scattering of the Andreev reflection (Fig. 1), creating the shadow of the vortex tangle in the image captured by the camera. Unexpectedly, the authors found that the outer edge of the wire loop produces more vortices than the inner edge. However, the flow velocity should be approximately the same on both ends. This effect has not yet been described, but it does show how the camera provides new insights into the superfluid helium-3.
Visualizing the flow is essential in understanding the ancient chaos, and there is an ongoing effort to achieve a higher level of detail in quantum turbulence testing. A common way to visualize is to follow the movements of small tracking particles added to the liquid. Still, suitable tracers are hard to find and photograph in near-zero temperatures. The Noble strategy and its partners work without tracers and do not attack miraculously because Andreev’s demonstration does not use almost the force of the object. Due to nanofabrication technology, future cameras can contain many pixels and may work faster, making it easier to switch from still images to video recording.
