  We propose an optical scheme for generating entanglement between co-trapped
identical or dissimilar alkaline earth atomic ions ($^{40}\text{Ca}^+$,
$^{88}\text{Sr}^+$, $^{138}\text{Ba}^+$, $^{226}\text{Ra}^+$) which exhibits
fundamental error rates below $10^{-4}$ and can be implemented with a broad
range of laser wavelengths spanning from ultraviolet to infrared. We also
discuss straightforward extensions of this technique to include the two
lightest Group-2 ions ($\text{Be}^+$, $\text{Mg}^+$) for multispecies
entanglement. The key elements of this wavelength-insensitive geometric phase
gate are the use of a ground ($S_{1/2}$) and a metastable ($D_{5/2}$)
electronic state as the qubit levels within a $\sigma^z \sigma^z$ light-shift
entangling gate. We present a detailed analysis of the principles and
fundamental error sources for this gate scheme which includes photon scattering
and spontaneous emission decoherence, calculating two-qubit-gate error rates
and durations at fixed laser beam intensity over a large portion of the optical
spectrum (300 nm to 2 $\mu \text{m}$) for an assortment of ion pairs. We
contrast the advantages and disadvantages of this technique against previous
trapped-ion entangling gates and discuss its applications to quantum
information processing and simulation with like and multispecies ion crystals.
