An Experiment to Probe the Condensate in Superfluid ^4He

J. Mayers

Richard N. Silver, Paul E. Sokol

Paul M. Walmsley, А. А. Левченко, Andrei I. Golov

Valentin Anatol'evich Zagrebnov

This short review is devoted to the celebration of two major events in quantum physics. The first one is the birth of the concept of Bose–Einstein condensation (1925) and the second is the experimental proof that it does exist and appears in liquid 4He simultaneously with superfluidity below the λ-point (1975).Both of these events are tightly related to the Bogoliubov theory of superfluidity (1947). The existence of condensate in the system of interacting bosons is the key ansatz of this theory. Therefore, the experiments started at JINR-Dubna in 1975 confirmed this prediction of the Bogoliubov theory that superfluidity of liquid 4He (He II) should emerge at the same time as the Bose–Einstein condensation. Corrected: 13 November 2025 (the captions to Figures 1 and 2 were changed)26 November 2025 (changes were made in formulas (53) and (55))

Nicolas Laflorencie, Frédéric Mila

Since the discovery of superfluidity in 4He and Landau's phenomenological theory, the relationship between Bose condensation and superfluidity has been intensely debated. 4He is known by now to be both superfluid and condensed at low temperature, and more generally, in dimension D≥2, all superfluid bosonic models realized in experiments are condensed in their ground state, the most recent example being provided by ultracold bosonic atoms trapped in an optical lattice. In this Letter, it is shown that a 2D gas of bosons which is not condensed at T=0 can be achieved by populating a layer through a frustrated proximity effect from a superfluid reservoir. This condensate-free bosonic fluid is further shown to be a superfluid with incommensurate correlations.