When matter is cooled close to absolute zero, quantum-mechanical properties become obvious. Using a self-designed turbo-expander, Kapitsa produced large amounts of liquid helium and in 1937 reported the phenomenon of superfluidity. Superfluid helium flows through thin capillaries with almost zero viscosity, enabling low-loss energy transport and ultra-sensitive rotation sensors. He also measured heat conduction and the interface thermal resistance—now called Kapitza resistance—laying the foundation for cooling in MRI scanners and rocket engines. Today, low-temperature physics is essential for quantum computers and superconductivity research.

Kapitsa built a closed-cycle Helium refrigerator based on gas compression and rapid expansion, achieving several litres per hour of liquid helium near 2 K. This allowed him to study Helium II below its 2.17 K transition, confirming zero viscosity and the two-fluid model. Heat-capacity measurements using first- and second-sound propagation provided experimental support for Bose condensation and Fermi-liquid theory. By measuring heat flux across metal–helium interfaces he found κ = A T³ behavior, establishing the concept of Kapitza resistance. The resulting cryogenic techniques feed directly into bolometers, SQUIDs, dilution refrigerators and enable precision studies such as the quantum Hall effect and superconducting qubits.

Kapitsa’s closed-cycle plant employing a high-speed turbine delivered ≈100 W of cooling at 2 K, enabling systematic measurements of quantum turbulence and critical velocities in He II. In interfacial heat transport he introduced the phonon mismatch model and Kapitza length, foreshadowing acoustic-mismatch theory. Combining 12 T Bitter magnets with a rotary cryostat, he visualized vortex-line density via second-sound resonance. He-3 diffusion experiments stimulated p-wave pairing theory and guided the design of dilution refrigerators and adiabatic demagnetization stages. His thermal-property tables became reference values for ITS-90 and serve today to benchmark quantum-fluid simulations.