Chinese physicists have made a new experimental observation, confirming the many-particle pairing of fermions, before they reach a critical temperature and exhibit remarkable quantum superfluidity.

In a paper published in Nature [1], researchers at the University of Science and Technology of China (USTC) observed and quantified the pseudogap pairing in a strongly attractively interacting cloud of fermionic lithium atoms, which may help unlock the mysterious microscopic mechanism of high-temperature superconductivity.

Quantum superfluidity/superconductivity is the most intriguing phenomenon of quantum physics. In the superfluid state, quantum particles collectively flow in a frictionless way without losing any kinetic energy. High-temperature superconducting materials therefore hold the prospect of solving the current world’s energy crisis. However, despite substantial efforts spanning the past 38 years, the origin of high-temperature superconductivity, especially the emergence of an energy gap in the normal state preceding superconductivity, continues to elude researchers.

This energy gap without superconducting is the well-known pseudogap, whose origin is controversial and is the subject of a long-lasting debate in the condensed matter community. In one of the two mainstream interpretations, the scenario of preformed pairs, the pseudogap is produced by incoherent fluctuations of the pairing field that describes the entangled fermions due to attraction; however, superfluidity does not arise because large phase fluctuations of the pairing field cannot allow quantum particles to march in the exact same manner above the critical temperature.

The central aim of the USTC researchers’ work was to quantum emulate a simple text-book model to examine the scenario of preformed pairs, by using a table-top system of ultracold atoms.

Quantum simulation using ultracold atoms is a rapidly developing field that was established following the Nobel Prize-winning research on laser cooling of atoms in the late 1980s and on Bose-Einstein condensation in 1995. A cloud of atoms can be cooled down to incredibly low temperatures of a billionth of a degree Kelvin, just above absolute 0. At these temperatures, fermionic atoms in different hyperfine states, as the atomic analog of electrons and protons which are the building blocks of all matter, can bind together due to attraction to form Cooper pairs and consequently condense into a macroscopic quantum state, precisely following the rule of quantum mechanics.

The unique advantage of ultracold atoms is their unprecedented controllability. By utilizing Feshbach resonances and applying a magnetic field at the right strength, interactions between atoms can be controlled with great precision—from arbitrarily weak to arbitrarily strong. This allows scientists to monitor, step by step, the emergence and the evolution of the pairing field in a strongly interacting Fermi cloud of about a million atoms, with decreasing temperature.

At USTC, Professor Xing-Can Yao, Professor Jian-Wei Pan, and their colleagues confined strongly attracting fermionic lithium-6 atoms into a uniform box potential and cooled them down to near absolute 0. By developing a novel technology of momentum-resolved microwave spectroscopy with high energy resolution, the pseudogap pairing above the critical superfluid transition temperature was clearly revealed for the first time.

The investigation of pseudogap pairing with ultracold atoms was attempted in 2010. However, at that time the cloud of fermionic atoms was not homogeneously distributed, leading to an inhomogeneity-related spectral broadening in the spectroscopy. Moreover, atoms that were experimentally probed suffered from some additional, unwanted interatomic collisions during the spectroscopic imaging. The two obstacles made the previous observation of pseudogap inconclusive.

In the USTC experiment, state-of-the-art methods were utilized for preparing the homogeneous Fermi cloud and for removing unwanted interatomic collisions, with ultra-stable magnetic field control at unprecedented levels of a quarter of ppm (i.e., less than one millionth). As a result of these technical advances, the suppression of spectral weight near the Fermi surface in the normal state was unequivocally revealed in microwave spectroscopy. This is the smoking gun of a pseudogap, without the need to invoke any specific microscopic theories to fit the experimental data.

This discovery will undoubtedly have far-reaching implications for the future study of strongly interacting Fermi systems and could lead to potential applications in future quantum technologies.

An immediate application is to probe the long-sought inhomogeneous Fermi superfluidity in a spin-population imbalanced Fermi gas, where the standard pairing field will be modified and become spatially inhomogeneous. This exotic pairing scenario was proposed by Fulde, Ferrell, Larkin, and Ovchinnikov 60 years ago. It may underlie the imbalanced nucleon superfluidity of neutron-proton pairs at the core of neutron stars and may have some relevance in explaining the sudden variations (glitches) in the rotation periods of pulsars.