We present an experiment using a sample of laser-cooled Rb atoms to show that cross-phase modulation schemes continue to benefit from electromagnetically-induced transparency (EIT) even as the transparency window is made narrower than the signal bandwidth (i.e., for signal pulses much shorter than the response time of the EIT system). Addressing concerns that narrow EIT windows might not prove useful for such applications, we show that while the peak phase shift saturates in this regime, it does not drop, and the time-integrated effect continues to scale inversely with EIT window width. This integrated phase shift is an important figure of merit for tasks such as the detection of single-photon-induced cross phase shifts.The interaction between individual photons is notoriously weak. This is an obstacle, for instance, to optical quantum computing; while the lack of interactions is helpful for coherent communications, it leaves the construction of deterministic logic gates between photonic qubits a difficult challenge.Non-linear optical properties of matter have been used to mediate an effective interaction between two optical fields. For example, an intensity-dependent refractive index provided by an atomic medium can lead to a conditional phase shift written on one optical 'probe' field in the presence of a second 'signal' field. This effective interaction serves as the basis for a two-photon gate proposal, which requires a π rad phase shift to be acquired by the probe field [1]. The difficulty of achieving such a large phase shift has prompted the proposal of a new approach that relies merely on the single-shot detectability of any observable cross-phase shift, not necessarily π rad [2]. However, even the non-linearities required for this 'weak non-linearity' scheme are many orders of magnitude larger than those achievable in typical situations.A number of approaches have been proposed for greatly enhancing the strength of cross-phase modulation (XPM), including cavity QED [3], unconventional media such as photonic crystal fibers [4] and hollow-core fibers filled with alkali gas [5], and finally, novel physical effects such as electromagnetically induced transparency (EIT) [6,7]; this Letter addresses the latter.The original 'N-scheme' proposed by Schmidt anḋ Imamoglu [8] made use of a single EIT window, while later works investigated double-Lambda systems [9,10] to overcome limitations arising from the group-velocity mismatch between signal and probe pulses. Many variations on these multi-level schemes have been introduced [11][12][13][14][15][16] and the ability to store light using EIT has even led to cross-phase modulation between stored pulses of light [17]. In the past 3 years, remarkably high nonlinearities have been observed relying on Rydberg blockade in the presence of EIT [18][19][20][21], presaging potentially huge cross-phase shifts.The enhancement provided by EIT schemes arises from the spectral narrowness of the transparency window, which results in a steep refractive index profile for the probe ...