Measurement-based interleaved randomised benchmarking using IBM processors

Quantum computers have the potential to outperform classical computers at certain computational tasks, such as prime factorisation and unstructured searching. However, experimental realisations of quantum computers are subject to noise. Quantifying the noise is of fundamental importance, since noise is often the dominant factor preventing the successful realisation of advanced quantum computations. Here we propose an interleaved randomised benchmarking protocol for measurement-based quantum computers, in which any single-qubit measurement-based 2-design can be used to estimate the fidelity of any single-qubit measurement-based gate. We test our protocol by using a weak approximate measurement-based 2-design to estimate the fidelity of the Hadamard gate and the T gate (a universal single-qubit set) on IBM superconducting quantum computers. To this end, single-qubit measurements were performed on entangled linear cluster states of up to 31 qubits. Our estimated gate fidelities show good agreement with gate fidelities calculated from process tomography results. Furthermore, by artificially increasing noise in the measurement-based gates, we were able to show that our protocol is able to detect large noise variations in different measurement-based implementations of a gate.Comment: 13 pages, 6 figures, appendi

Demonstration of Shor's factoring algorithm for N=21 on IBM quantum processors

We report a proof-of-concept demonstration of a quantum order-finding algorithm for factoring the integer 21. Our demonstration involves the use of a compiled version of the quantum phase estimation routine, and builds upon a previous demonstration by Mart\'in-L\'{o}pez et al. in Nature Photonics 6, 773 (2012). We go beyond this work by using a configuration of approximate Toffoli gates with residual phase shifts, which preserves the functional correctness and allows us to achieve a complete factoring of N=21. We implemented the algorithm on IBM quantum processors using only 5 qubits and successfully verified the presence of entanglement between the control and work register qubits, which is a necessary condition for the algorithm's speedup in general. The techniques we employ may be useful in carrying out Shor's algorithm for larger integers, or other algorithms in systems with a limited number of noisy qubits.Comment: 11 pages, 10 figures, appendi

Robust-to-loss entanglement generation using a quantum plasmonic nanoparticle array

We introduce a scheme for generating entanglement between two quantum dots using a plasmonic waveguide made from an array of metal nanoparticles. We show that the scheme is robust to loss, enabling it to work over long distance plasmonic nanoparticle arrays, as well as in the presence of other imperfections such as the detuning of the energy levels of the quantum dots. The scheme represents an alternative strategy to the previously introduced dissipative driven schemes for generating entanglement in plasmonic systems. Here, the entanglement is generated by using dipole-induced interference effects and detection-based postselection. Thus, contrary to the widely held view that loss is major problem for quantum plasmonic systems, we provide a robust-to-loss entanglement generation scheme that could be used as a versatile building block for quantum state engineering and control at the nanoscale.Comment: 32 pages, 11 figure

Fusing multiple W states simultaneously with a Fredkin gate

We propose an optical scheme to prepare large-scale entangled networks of W states. The scheme works by simultaneously fusing three polarization-encoded W states of arbitrary size via accessing only one qubit of each W state. It is composed of a Fredkin gate (controlled-swap gate), two fusion gates [as proposed in S. K. Ozdemir et al., New J. Phys. 13, 103003 (2011)], and an H-polarized ancilla photon. Starting with three n-qubit W states, the scheme prepares a new W state with 3(n - 1) qubits after postselection if both fusion gates operate successfully, i.e., a fourfold coincidence at the detectors. The proposed scheme reduces the cost of creating arbitrarily large W states considerably when compared to previously reported schemes.Publisher's Versio