Beating Classical Computing Without a Quantum Computer

Photons performing quantum random walks can be used to calculate matrix properties without actually implementing quantum logic gates. [Also see Reports by Childs et al., Broome et al., and Spring et al.] Quantum computers are expected to be able to solve mathematical problems that are not feasible on a classical computer. Although considerable progress has already been made, building a full-scale quantum computer would require controlled interactions between the quantum bits, or qubits, in order to implement the logic operations required for addition, subtraction, and multiplication. On pages 798 and 794 of this issue, Spring et al. (1) and Broome et al. (2), as well as Tillmann et al. (3), have shown that quantum systems—in this case, photons interacting along waveguides—could outperform a classical computer for certain kinds of matrix calculations without the need for logic operations.