  Universal control of multiple qubits -- the ability to entangle qubits and to
perform arbitrary individual qubit operations -- is a fundamental resource for
quantum computation, simulation, and networking. Here, we implement a new
laser-free scheme for universal control of trapped ion qubits based on
microwave magnetic fields and radiofrequency magnetic field gradients. We
demonstrate high-fidelity entanglement and individual control by creating
symmetric and antisymmetric two-qubit maximally entangled states with
fidelities in the intervals [0.9983, 1] and [0.9964, 0.9988], respectively, at
68% confidence, corrected for state initialization error. This technique is
robust against multiple sources of decoherence, usable with essentially any
trapped ion species, and has the potential to perform simultaneous entangling
operations on many pairs of ions without increasing control signal power or
complexity.
