S. Ashhab, P. C. de Groot, Franco Nori
We use analytical and numerical calculations in order to obtain speed limits for various unitary quantum operations in multi-qubit systems. The operations that we consider include single-, two- and three-qubit gates, as well as quantum state transfer in a chain of qubits. We find in particular that simple methods for implementing two-qubit gates generally provide the fastest possible implementations of these gates. We also find that the three-qubit Toffoli gate time varies greatly depending on the system's geometry, taking only slightly longer than a two-qubit CNOT gate for a triangle geometry. The speed limit for quantum state transfer across a qubit chain is set by the maximum spin wave speed in the chain.
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http://arxiv.org/abs/1202.5872
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