We undertake experimental detection of the entanglement present in arbitrary three-qubit pure quantum states on an NMR quantum information processor. Measurements of only four observables suffice to experimentally differentiate between the six classes of states which are inequivalent under stochastic local operation and classical communication. The experimental realization is achieved by mapping the desired observables onto Pauli
z operators of a single qubit, which is directly amenable to measurement. The detection scheme is applied to known entangled states as well as to states randomly generated using a generic scheme that can construct all possible three-qubit states. The results are substantiated via direct full quantum state tomography as well as via negativity calculations and the comparison suggests that the protocol is indeed successful in detecting tripartite entanglement without requiring any a priori information about the states.
Random local measurements have recently been proposed to construct entanglement witnesses and thereby detect the presence of bipartite entanglement. We experimentally demonstrate the efficacy of one such scheme on a two-qubit NMR quantum-information processor. We show that a set of three random local measurements suffices to detect the entanglement of a general two-qubit state. We experimentally generate states with different amounts of entanglement and show that the scheme is able to clearly witness entanglement. We perform complete quantum state tomography for each state and compute state fidelity to validate our results. Further, we extend previous results and perform a simulation using random local measurements to optimally detect bipartite entanglement in a hybrid system of $2\ensuremath{\bigotimes}3$ dimensionality.
We advocate an alternative paradigm to operate quantum computers that utilizes multipartite entanglement generated in dedicated auxiliary systems during idle times. This stored entanglement enhances the future capabilities of the quantum processor, as it can be flexibly used to assist and speed-up computations when needed. We identify classes of multipartite entangled resource states whose computational power are related to their entanglement features, and in turn to the complexity to generate them. During idle times, one can thus continuously work towards generating more and more powerful auxiliary multipartite entangled states. Idle times include both times prior to the start of a computation, but also any step during the execution of an algorithm where parts of the processor are not actively involved. To illustrate our approach, we consider architectures with limited connectivity, e.g. corresponding to a 1D geometry. We show that
d-dimensional cluster states allow one to flexibly perform multiple long-distance two-qubit gates in parallel, where both the complexity to generate them, as well as the number of achievable gates increases with
d.