We have examined the occurrence of Extremely Red Objects ( EROs ) in the fields of 13 luminous quasars ( 11 radio-loud and two radio-quiet ) at 1.8 < z < 3.0 .
The average surface density of K _ { s } \leq 19 mag EROs is two-three times higher than in large , random-field surveys , and the excess is significant at the \approx 3 \sigma level even after taking into account that the ERO distribution is highly inhomogeneous .
This is the first systematic investigation of the surface density of EROs in the fields of radio-loud quasars above z \approx 2 , and shows that a large number of the fields contain clumps of EROs , similar to what is seen only in the densest areas in random-field surveys .
The high surface densities and angular distribution of EROs suggest that the excess originates in high-redshift galaxy concentrations , possibly young clusters of galaxies .
The fainter EROs at K _ { s } \gtrsim 19 mag show some evidence of being more clustered in the immediate 20 arcsec region around the quasars , suggesting an association with the quasars .
Comparing with predictions from spectral synthesis models , we find that if the K _ { s } \approx 19 mag ERO excess is associated with the quasars at z \approx 2 , their magnitudes are typical of \gtrsim L ^ { * } passively evolving galaxies formed at z \approx 3.5 ( \Omega _ { m } = 0.3 , \Omega _ { \Lambda } = 0.7 , and H _ { 0 } = 70 km s ^ { -1 } Mpc ^ { -1 } ) .
Another interpretation of our results is that the excess originates in concentrations of galaxies at z \approx 1 
lying along the line of sight to the quasars .
If this is the case , the EROs may be tracing massive structures responsible for a magnification bias of the quasars .