  Key performance characteristics are demonstrated for the microwave SQUID
multiplexer ($\mu$MUX) coupled to transition edge sensor (TES) bolometers that
have been optimized for cosmic microwave background (CMB) observations. In a
64-channel demonstration, we show that the $\mu$MUX produces a white, input
referred current noise level of 29~pA$/\sqrt{\mathrm{Hz}}$ at -77~dB microwave
probe tone power, which is well below expected fundamental detector and photon
noise sources for a ground-based CMB-optimized bolometer. Operated with
negligible photon loading, we measure 98~pA$/\sqrt{\mathrm{Hz}}$ in the
TES-coupled channels biased at 65% of the sensor normal resistance. This noise
level is consistent with that predicted from bolometer thermal fluctuation
(i.e., phonon) noise. Furthermore, the power spectral density exhibits a white
spectrum at low frequencies ($\sim$~100~mHz), which enables CMB mapping on
large angular scales that constrain the physics of inflation. Additionally, we
report cross-talk measurements that indicate a level below 0.3%, which is less
than the level of cross-talk from multiplexed readout systems in deployed CMB
imagers. These measurements demonstrate the $\mu$MUX as a viable readout
technique for future CMB imaging instruments.
