Joseph F. Fitzsimons, Elham Kashefi
Blind Quantum Computing (BQC) allows a client to have a server carry out a quantum computation for them such that the client's input, output and computation remain private. Recently the authors together with Broadbent proposed a universal unconditionally secure BQC scheme where the client only needs to be able to prepare single qubits in separable states randomly chosen from a finite set and send them to the server, who has the balance of the required quantum computational resources. A desirable property for any BQC protocol is verification, whereby the client can verify with high probability whether the server has followed the instructions of the protocol, or if there has been some deviation resulting in a corrupted output state. A verifiable BQC protocol can be viewed as an interactive proof system leading to consequences for complexity theory. In this paper we extend the BQC protocol presented in [Broadbent, Fitzsimons and Kashefi, FOCS 2009 p517] with new functionality allowing blind computational basis measurements, which we use to construct a new verifiable universal BQC protocol based on a new class of resource states. We rigorously prove that the probability of detecting an incorrect output is exponentially small in a security parameter, while resource overhead remains polynomial in this parameter. The new resource state allows entangling gates to be performed between arbitrary pairs of logical qubits with only constant overhead. This is a significant improvement on the original scheme, which required that all computations to be performed must first be put into a nearest neighbour form, incurring linear overhead in the number of qubits. Such an improvement has important consequences for efficiency and fault-tolerance thresholds.
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http://arxiv.org/abs/1203.5217
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