Wiebke Guichard

Single-mode heat conduction by photons.

The thermal conductance of a single channel is limited by its unique quantum value GQ, as was shown theoretically in 1983. This result closely resembles the well-known quantization of electrical conductance in ballistic one-dimensional conductors. Interestingly, all particles—irrespective of whether they are bosons or fermions—have the same quantized thermal conductance 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, and it has been predicted to influence cooling of electrons in metals at very low temperatures due to electromagnetic radiation. Here we report experimental results showing that at low temperatures heat is transferred by photon radiation, when electron–phonon 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, whose operation is largely dependent on weak thermal coupling between the device and its environment.

Epitaxial rhenium microwave resonators

We have fabricated rhenium microwave resonators from epitaxial films. We have used thin films of different structural qualities depending on their growth conditions. The resonators were coupled to a microwave transmission line that allows the measurement of their resonance frequencies and internal quality factors. From the resonance frequency at a low temperature, the effective penetration depth and the London penetration depth of the rhenium film are extracted.

Unexpectedly Allowed Transition in Two Inductively Coupled Transmons

We present experimental results in which the unexpected zero-two transition of a circuit composed of two inductively coupled transmons is observed. This transition shows an unusual magnetic flux dependence with a clear disappearance at zero magnetic flux. In a transmon qubit, the symmetry of the wave functions prevents this transition to occur due to selection rule. In our circuit, the Josephson effect introduces strong couplings between the two normal modes of the artificial atom. This leads to a coherent superposition of states from the two modes, enabling such transitions to occur.