  Digital quantum computing paradigm offers highly-desirable features such as
universality, scalability, and quantum error correction. However, physical
resource requirements to implement useful error-corrected quantum algorithms
are prohibitive in the current era of NISQ devices. As an alternative path to
performing universal quantum computation, within the NISQ era limitations, we
propose to merge digital single-qubit operations with analog multi-qubit
entangling blocks in an approach we call digital-analog quantum computing
(DAQC). Along these lines, although the techniques may be extended to any
resource, we propose to use unitaries generated by the ubiquitous Ising
Hamiltonian for the analog entangling block and we prove its universal
character. We construct explicit DAQC protocols for efficient simulations of
arbitrary inhomogeneous Ising, two-body, and $M$-body spin Hamiltonian dynamics
by means of single-qubit gates and a fixed homogeneous Ising Hamiltonian.
Additionally, we compare a sequential approach where the interactions are
switched on and off (stepwise~DAQC) with an always-on multi-qubit interaction
interspersed by fast single-qubit pulses (banged DAQC). Finally, we perform
numerical tests comparing purely digital schemes with DAQC protocols, showing a
remarkably better performance of the latter. The proposed DAQC approach
combines the robustness of analog quantum computing with the flexibility of
digital methods.
