  The Quantum Scientific Computing Open User Testbed (QSCOUT) at Sandia
National Laboratories is a trapped-ion qubit system designed to evaluate the
potential of near-term quantum hardware in scientific computing applications
for the US Department of Energy (DOE) and its Advanced Scientific Computing
Research (ASCR) program. Similar to commercially available platforms, most of
which are based on superconducting qubits, it offers quantum hardware that
researchers can use to perform quantum algorithms, investigate noise properties
unique to quantum systems, and test novel ideas that will be useful for larger
and more powerful systems in the future. However, unlike most other quantum
computing testbeds, QSCOUT uses trapped $^{171}$Yb$^{+}$ ions as the qubits,
provides full connectivity between qubits, and allows both quantum circuit and
low-level pulse control access to study new modes of programming and
optimization. The purpose of this manuscript is to provide users and the
general community with details of the QSCOUT hardware and its interface,
enabling them to take maximum advantage of its capabilities.
