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Abstract:
The thermal conductance of a single channel is limited by its unique
quantum value G(Q), as was shown theoretically(1) in 1983. This result
closely resembles the well-known quantization of electrical conductance
in ballistic one-imensional conductors(2,3). Interestingly, all
particles - irrespective of whether they are bosons or fermions - have
the same quantized thermal conductance(4,5) when they are confined
within dimensions that are small compared to their characteristic
wavelength. The single-mode heat conductance is particularly relevant
in nanostructures. Quantized heat transport through submicrometre
dielectric wires by phonons has been observed(6), and it has been
predicted to influence cooling of electrons in metals at very low
temperatures due to electromagnetic radiation(7). Here we report
experimental results showing that at low temperatures heat is
transferred by photon radiation, when electron - phonon(8) as well as
normal electronic heat conduction is frozen out. We study heat exchange
between two small pieces of normal metal, connected to each other only
via superconducting leads, which are ideal insulators against
conventional thermal conduction. Each superconducting lead is
interrupted by a switch of electromagnetic ( photon) radiation in the
form of a DC-SQUID ( a superconducting loop with two Josephson tunnel
junctions). We find that the thermal conductance between the two metal
islands mediated by photons indeed approaches the expected quantum
limit of GQ at low temperatures. Our observation has practical
implications - for example, for the performance and design of
ultra-sensitive bolometers ( detectors of far-infrared light) and
electronic micro-refrigerators(9), whose operation is largely dependent
on weak thermal coupling between the device and its environment.
