Quantum and Classical Communication-Space Tradeoffs from Rectangle Bounds

We derive lower bounds for tradeoffs between the communication C and space S for communicating circuits. The first such bound applies to quantum circuits. If for any function f with image Z the multicolor discrepancy of the communication matrix of f is 1/2^d, then any bounded error quantum protocol with space S, in which Alice receives some l inputs, Bob r inputs, and they compute f(x_i,y_j) for the lr pairs of inputs (x_i,y_j) needs communication C=Ω(lrd \log |Z|/S). In particular, n\times n-matrix multiplication over a finite field F requires C=Θ(n^3\log^2 |F|/S). We then turn to randomized bounded error protocols, and derive the bound C=Ω(n^3/S^2) for Boolean matrix multiplication, utilizing a new direct product result for the one-sided rectangle lower bound on randomized communication complexity. This implies a separation between quantum and randomized protocols.

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