Chiral Gravitons in Fractional Quantum Hall Liquids

We elucidate the nature of neutral collective excitations of fractional quantum Hall liquids in the long-wavelength limit. We demonstrate that they are chiral gravitons carrying angular momentum -2, which are quanta of quantum motion of an internal metric, and show up as resonance peaks in the system's response to what is the fractional Hall analog of gravitational waves. The relation with existing and possible future experimental work that can detect these fractional quantum Hall gravitons and reveal their chirality is discussed.