A research team led by Prof. PAN Jianwei, Prof. WAN Zhensheng and Prof. DENG Youjin from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences observed counterflow superfluidity in two-component Mott insulator for the first time. The study was published in Nature Physics.

In the 1930s, superfluidity was discovered in liquid helium, which spurred the development of quantum many-body physics as well as advancements in low-temperature technologies, providing crucial tools for the development of quantum simulation. 

In the early 2000s, Kuklov and others theoretically proposed the quantum phase of counterflow superfluidity, which involves two counterflowing superfluids composed of particles with different spins. The two types of particles flow relative to each other, exhibiting superfluidity. However, the strict correlation between the two counterflowing spin currents ensures that the total particle flow is zero, therefore forming a Mott insulator. 

Due to the extremely low temperatures required for the preparation of the counterflow superfluid (CSF) state and the need for measurements with single-atom spin resolution, the coexistence of superfluidity and insulation in this novel quantum state is yet to be confirmed.

In this study, researchers prepared a defect-free, low-entropy initial state of a unity-filling spin-Mott state by conveying two spin-1/2 bosonic atoms at every single lattice site. By controlling the interaction between the two atoms, they adiabatically evolved the system to the CSF phase at 1.2 nK. 

Using self-developed spin-resolved quantum-gas microscopy, researchers observed that while the particle number fluctuations of the two spins increased, the total particle number fluctuation remained small, indicating an anti-correlation in the particle number fluctuations of the two spin atoms.

Further time-of-flight (TOF) measurements revealed a non-zero CSF correlation function between the two spins, providing key experimental evidence for the CSF phase in Mott insulator.

This study demonstrates that ultracold atomic quantum simulation offers rich quantum modulation and observational techniques for exploring novel quantum states, which can be extended to study multi-component spin superfluid systems, advancing the experimental investigation of topological quantum states.