Phonon heat transfer across a vacuum through quantum fluctuations*

In quantum mechanics, quantum fields are never at rest but constantly fluctuate even at zero temperature. These fluctuations are attributed as the cause of various extraordinary physical phenomena, such as spontaneous emission, Hawking radiation, and the Casimir force. Recent theories predict that quantum fluctuations of electromagnetic fields can assist phonon coupling across a vacuum gap and result in a new form of heat transfer. In this talk, we present the first experimental observation of such a bizarre phenomenon. We use nanomechanical systems to realize strong Casimir phonon coupling, and observe thermal energy exchange between individual phonon modes by monitoring their thermal Brownian motions. Control experiments were performed to eliminate the consequences of other effects such as thermal radiation and electrostatic interaction. Our experiment reveals a new mechanism of heat transfer in addition to the conventional conduction, convection and thermal radiation. It opens up new opportunities to study nanoscale energy transport and quantum thermodynamics.