Before revision: Any superfluid can theoretically act as a quantum solvent, but in practice the only viable superfluid medium that can currently be used is helium-4, and it has been successfully accomplished in controlled conditions.
After revision: Any superfluid can theoretically act as a quantum solvent, but in practice the only viable superfluid medium that can currently be used is helium-4, and it has been successfullyy accomplished in controlled conditions.