  The paper reports on the optical detection and spectroscopy of ultracold
atoms near a gold surface. A probe light field is used to excite surface
plasmon polaritons. The refractive index of the atomic gas shifts the plasmon
resonance and changes the reflected light power. Thus, the sensitivity of the
detection is plasmonically enhanced. Absorption of photons from the evanescent
wave is avoided by detuning the laser from atomic resonance which makes the
detection scheme potentially nondestructive. The spectrum of the signal is
determined by a Fano resonance. We show that atoms can be detected
nondestructively with single atom resolution for typical parameters in cold
atom experiments. Thus, the method is suitable for quantum nondemolition
measurements of matter wave amplitudes. Experimentally, we measure a
technically-limited sensitivity of 30 atoms and extend the detection scheme to
dispersively image the atom cloud near the surface.
