Condensate fraction in liquid 4He

The condensate fraction n(0) in the ground state of liquid He-4 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 n0 is maximum, approximate to30%) up to the melting density. The condensate fraction remains finite in the high pressure metastable liquid, decaying exponentially with density.