Abstract
Besides Shor's polynomial-time quantum algorithms for factoring and discrete log, all progress in understanding when quantum algorithms have an exponential advantage over classical algo- rithms has been through oracle problems. Here we give efficient quantum algorithms for two more non-oracle problems. The first is Pell's equation. Given a positive non-square integer d, Pell's equation is x2 − dy2 = 1 and the goal is to find its integer solutions. Factoring integers reduces to finding integer solutions of Pell's equation, but a reduction in the other direction is not known and appears more difficult. The second problem is theprincipal ideal problem in real quadratic number fields. Solving this problem is at least as hard as solving Pell's equation, and is the basis of a cryptosystem which is broken by our algorithm. We also state some related open problems from the area of computational algebraic number theory.

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