Ultracold atoms shed light on properties of quantum systems

A team of physicists has illuminated certain properties of quantum systems by observing how their fluctuations spread over time. The research offers a new understanding of a complex phenomenon that is foundational to quantum computing — a method that can perform certain calculations more efficiently than conventional computing. Dries Sels, an author of the paper, which appeared in the journal Nature Physics, said the new research reconstructs the full state of a quantum liquid, consistent with the predictions of a quantum field theory — similar to those that describe the fundamental particles in the universe. “In an era of quantum computing it’s vital to generate a precise characterisation of the systems we are building,” Sels said.

Sels said that the breakthrough offers promise for technological advancement. “Quantum computing relies on the ability to generate entanglement between different subsystems, and that’s exactly what we can probe with our method. The ability to do such precise characterisation could also lead to better quantum sensors — another application area of quantum technologies,” Sels said.

The research team, which included scientists from TU Wien (Vienna), ETH Zurich, Free University of Berlin and the Max-Planck Institute of Quantum Optics, calculated quantum information measures of a quantum system using a tomography procedure — the reconstruction of a specific quantum state with the aim of seeking experimental evidence of a theory. The quantum system consisted of ultracold atoms — slow-moving atoms that expose the quantum aspects of matter because of their near-zero temperature — trapped on an atom chip.

The scientists created two ‘copies’ of this quantum system — cigar-shaped clouds of atoms that evolve over time without influencing each other. At different stages of this process, the team performed a series of experiments that revealed the two copies’ correlations. By constructing an entire history of these correlations, the researchers were able to determine the initial quantum state of the system and extract its properties.

“Initially, we have a very strongly coupled quantum liquid, which we split into two so that it evolves as two independent liquids, and then we recombine it to reveal the ripples that are in the liquid. It’s like watching the ripples in a pond after throwing a rock in it and inferring the properties of the rock, such as its size, shape, and weight,” Sels said.

Image caption: An atom chip, in a laboratory at TU Wien (Vienna), created by physicists in order to trap ultracold atoms and study the properties of quantum systems. Image credit: Thomas Schweigler.