Quantum Lower Bound for the Collision Problem

Scott Aaronson
(Professor Umesh Vazirani)
DARPA grant and NSF Graduate Fellowship

The collision problem is to decide whether a function X:{1,..,n}->{1,..,n} is one-to-one or two-to-one, given that one of these is the case. We show a lower bound of Ω(n1/5) on the number of queries needed by a quantum computer to solve this problem with bounded error probability. The best known upper bound is O(n1/3), but obtaining any lower bound better than Ω(1) was an open problem since 1997. Our proof uses the polynomial method augmented by some new ideas. We also give a lower bound of Ω(n1/7) for the problem of deciding whether two sets are equal or disjoint on a constant fraction of elements. Finally we give implications of these results for quantum complexity theory.

[1]
S. Aaronson, "Quantum Lower Bound for the Collision Problem," Proc. ACM Proc. ACM Symp. Theory of Computing, Montreal, Canada, May 2002.

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