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Digital-Analog Quantum Computation (DAQC) has recently been proposed as an alternative to the standard paradigm of digital quantum computation. DAQC creates entanglement through a continuous or analog evolution of the whole device, rather than by applying two-qubit gates. This manuscript describes an in-depth analysis of DAQC by extending its implementation to arbitrary connectivities and by performing the first systematic study of its scaling properties. We specify the analysis for three examples of quantum algorithms, showing that except for a few specific cases, DAQC is in fact disadvantageous with respect to the digital case., Comment: 16+5 pages, 11 figures

Quantum computers have the potential to efficiently simulate the dynamics of nanoscale NMR systems. In this work, we demonstrate that a noisy intermediate-scale quantum computer can be used to simulate and predict nanoscale NMR resonances. In order to minimize the required gate fidelities, we propose a superconducting application-specific Co-Design quantum processor that reduces the number of SWAP gates by over 90% for chips with more than 20 qubits. The processor consists of transmon qubits capacitively coupled via tunable couplers to a central co-planar waveguide resonator with a quantum circuit refrigerator (QCR) for fast resonator reset. The QCR implements the nonunitary quantum operations required to simulate nuclear hyperpolarization scenarios. The authors would like to thank Caspar Ockeloen-Korppi, Alessandro Landra, and Johannes Heinsoo for their help in de- veloping the idea of the star-architecture chip, Jani Tuorila for his support in developing the gate theory, Amin Hosseinkhani and Tianhan Liu for reviewing the manuscript, and Hen- rikki Mäkynen and Hoang-Mai Nguyen for graphic design. J.C. additionally acknowledges the Ramón y Cajal program (RYC2018-025197-I). We further acknowledge support from Atos with the Quantum Learning Machine (QLM). Finally, the authors acknowledge financial support to BMBF through the Q-Exa Project No. FZK: 13N16062.

Tunable coupling of superconducting qubits has been widely studied due to its importance for isolated gate operations in scalable quantum processor architectures. Here, we demonstrate a tunable qubit-qubit coupler based on a floating transmon device which allows us to place qubits at least 2 mm apart from each other while maintaining over 50 MHz coupling between the coupler and the qubits. In the introduced tunable-coupler design, both the qubit-qubit and the qubit-coupler couplings are mediated by two waveguides instead of relying on direct capacitive couplings between the components, reducing the impact of the qubit-qubit distance on the couplings. This leaves space for each qubit to have an individual readout resonator and a Purcell filter needed for fast high-fidelity readout. In addition, the large qubit-qubit distance reduces unwanted non-nearest neighbor coupling and allows multiple control lines to cross over the structure with minimal crosstalk. Using the proposed flexible and scalable architecture, we demonstrate a controlled-$Z$ gate with $(99.81 \pm 0.02)\%$ fidelity., Comment: 24 pages, 12 figures, Figure 1 updated, Appendix A extended

We describe a path-integral molecular dynamics implementation of our recently developed golden-rule quantum transition-state theory(GR-QTST). The method is applied to compute the reaction rate in various models of electron transfer and benchmarked against the exactresults. We demonstrate that for systems exhibiting two or more transition states, rates computed using Wolynes theory [P. G. Wolynes, J. Chem. Phys. 87, 6559 (1987)] can be overestimated by orders of magnitude, whereas the GR-QTST predictions are numerically accu-rate. This is the case both at low temperature, where nuclear tunneling makes a considerable contribution, and also in the classical limit,where only GR-QTST rigorously tends to the correct result. Analysis shows that the saddle-point approximation employed by Wolynestheory is not valid in this case, which results in the predictions of unphysical reaction pathways, while the energy constraint employedby GR-QTST resolves this problem. The GR-QTST method is also seen to give accurate results for a strongly anharmonic system bysampling configurations around the instanton pathway without making the semiclassical approximation. These promising results indicatethat the GR-QTST method could be an efficient and accurate approach for simulating electron-transfer reactions in complex molecularsystems., The Journal of Chemical Physics, 151 (21), ISSN:0021-9606, ISSN:1089-7690