  We present a new method for proving lower bounds on quantum query algorithms.
The new method is an extension of adversary method, by analyzing the eigenspace
structure of the problem.
  Using the new method, we prove a strong direct product theorem for quantum
search. This result was previously proven by Klauck, Spalek and de Wolf
(quant-ph/0402123) using polynomials method. No proof using adversary method
was known before.
