  In condensed matter systems, the formation of long range order (LRO) with
broken symmetry is often accompanied by new types of excitations. However, in
many magnetic pyrochlore oxides, geometrical frustration suppresses
conventional LRO while at the same time non-trivial spin correlations are
observed. For such materials, a natural question to ask then is what is the
nature of the excitations in this highly correlated state without broken
symmetry? Frequently the application of a symmetry breaking field can stabilize
excitations whose properties still reflect certain aspects of the anomalous
state without long-range order. Here we report evidence of novel magnetic
excitations in the quantum spin ice material Yb$_2$Ti$_2$O$_7$, obtained from
time-domain terahertz spectroscopy (TDTS). At large fields, both magnon and
two-magnon-like excitations are observed in a $<$001$>$ directed magnetic field
illustrating the stabilization of a field induced LRO state. The unique ability
of TDTS to measure complex response functions allows a direct study of magnetic
responses in different polarization channels, revealing the existence of an
unusual left-hand polarized magnon. The g-factors of these excitations are
dramatically enhanced in the low-field limit, showing a cross-over of these
one- and two-magnon states into features consistent with quantum string-like
excitations proposed to exist in quantum spin ice in a small $<$001$>$ applied
field.
