Abstract
Microwave cavities have been widely used to investigate the behavior of closed few-level systems. Here, we show that they also represent a powerful probe for the dynamics of charge transfer between a discrete electronic level and fermionic continua. We have combined experiment and theory for a carbon nanotube quantum dot coupled to normal metal and superconducting contacts. In equilibrium conditions, where our device behaves as an effective quantum dot-normal metal junction, we approach a universal photon dissipation regime governed by a quantum charge relaxation effect. We observe how photon dissipation is modified when the dot admittance turns from capacitive to inductive. When the fermionic reservoirs are voltage biased, the dot can even cause photon emission due to inelastic tunneling to/from a Bardeen-Cooper-Schrieffer peak in the density of states of the superconducting contact. We can model these numerous effects quantitatively in terms of the charge susceptibility of the quantum dot circuit. This validates an approach that could be used to study a wide class of mesoscopic QED devices.
Highlights
We have combined experiment and theory for a carbon nanotube quantum dot coupled to normal metal and superconducting contacts
In equilibrium conditions, where our device behaves as an effective quantum dot-normal metal junction, we approach a universal photon dissipation regime governed by a quantum charge relaxation effect
Circuit QED techniques [1] have recently been put forward to investigate the electronic dynamics in quantum dot circuits [2,3,4] or, more generally, mesoscopic circuits [5]
Summary
Circuit QED techniques [1] have recently been put forward to investigate the electronic dynamics in quantum dot circuits [2,3,4] or, more generally, mesoscopic circuits [5]. Quantum dot circuits with superconducting reservoirs have been coupled neither to microwave cavities nor to a direct ac excitation, so far Despite this lack of experiments, photon-assisted tunneling between a dot and a superconductor has created theoretical interest for more than 15 years [50,51,52,53,54,55,56,57,58,59]. Our work gives wide experimental perspectives since microwave cavities appear as a powerful probe for quantum charge relaxation, photon-assisted tunneling, and all other effects involving tunneling between a discrete level and fermionic reservoirs.
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