A note on the runtime of a faulty Hamiltonian oracle

In these notes we show that it is impossible to obtain a quantum speedup for a faulty Hamiltonian oracle. The effect of dephasing noise to this continuous time oracle model has first been investigated in [1]. The authors consider a faulty oracle described by a continuous time master equation that acts as dephasing noise in the basis determined by the marked item. The analysis focuses on the implementation with a particular driving Hamiltonian. A universal lower bound for this oracle model, which rules out a better performance with a different driving Hamiltonian has so far been lacking. In this note, we derive an adversary type lower bound which shows that the evolution time T has to be at least in the order of N, i.e. the size of the search space, when the error rate of the oracle is constant. For the standard quantum oracle model this result was first proven in [2]. This note can be seen as an extension of their result to the continuous time setting.

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