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Given a function f as an oracle, the collision problem is to find two distinct inputs i and j such that f(i)=f(j), under the promise that such inputs exist. In this paper, we prove that any quantum algorithm for finding a collision in an r-to-one function must evaluate the function /spl Omega/ ((n/r)/sup 1/3/) times, where n is the size of the domain and r|n. This lower bound matches, up to a constant factor, the upper bound of Brassard, Hoyer and Tapp (1997), which uses the quantum algorithm of Grover (1996) in a novel way. The previously best quantum lower bound is /spl Omega/ ((n/r)/sup 1/5/) evaluations, due to Aaronson (2002). Our result implies a quantum lower bound of /spl Omega/ (n/sup 2/3/) queries to the inputs for another well studied problem, the element distinctness problem, which is to determine whether or not the given n real numbers are distinct. The previous best lower bound is /spl Omega/ (/spl radic/n) queries in the black-box model; and /spl Omega/ (/spl radic/n log n) comparisons in the comparisons-only model, due to Hoyer Neerbek, and Shi (2001).

Quantum lower bounds for the collision and the element distinctness problems