Condensate Fraction in Liquid 4He

The condensate fraction n ₀ in the ground state of liquid ⁴He is computed by means of an unbiased Quantum Monte Carlo technique. Unlike previous calculations of the same type, the method adopted here is in principle exact, errors being only statistical. At the equilibrium density, our estimate for n ₀ is 0.069 ± 0.005; this is consistent with the most recent experimental measurements, but significantly lower than most existing theoretical estimates. Results are provided in the density range from the spinodal (where n ₀ is maximum, ≈30%) up to the melting density. The condensate fraction remains finite in the high pressure metastable liquid, decaying exponentially with density.