  Detections of local maxima in the density perturbation spectrum at
characteristic comoving scales L~100-200h^{-1}Mpc have previously been claimed.
Here, this cosmic standard ruler is sought in the ``10K'' release of the 2dF
QSO Redshift Survey (2QZ-10K), by estimating the comoving, spatial correlation
functions \xi(r) of the three-dimensional distribution of the N=2378 quasars in
the most completely observed and ``covered'' sky regions, over the three
redshift ranges 0.6 < z < 1.1, 1.1 < z < 1.6 and 1.6 < z < 2.2. Because of the
selection method of the survey and sparsity of the data, the analysis was done
conservatively to avoid non-cosmological artefacts.
  (i) Avoiding 'a priori' estimates of the length scales of features, local
maxima in \xi(r) are found in all three redshift ranges. The requirement that a
local maximum be present in all three redshift ranges at a fixed comoving
length scale implies strong, purely geometric constraints on the local
cosmological parameters. The length scale of the local maximum common to the
three redshift ranges is 2L= (244\pm17)h^{-1}Mpc.
  (ii) For a standard FLRW model, the matter density \Omm and cosmological
constant \Omega_\Lambda are constrained to \Omm= 0.25\pm0.10,
\Omega_\Lambda=0.65\pm0.25 (68% confidence), \Omm= 0.25\pm0.15,
\Omega_\Lambda=0.60\pm0.35 (95%), respectively, *from the 2QZ-10K alone*.
Independently of the SNe Ia data, the zero cosmological constant model
(\Omega_\Lambda=0) is rejected at the 99.7% confidence level.
  (iii) For an effective quintessence (w_Q) model and zero curvature, w_Q<-0.5
(68%), w_Q<-0.35 (95%) are found, again *from the 2QZ-10K alone*.
