The efficient transfer of energy across different quantum systems - an essential ingredient to quantum-information-processing protocols - requires that the coupling between these systems is stronger than the decay rate of either one. Here we report experiments in which we have reached this so-called strong-coupling regime in a hybrid system consisting of a gate-defined double quantum dot and a superconducting resonator. These results underline the potential of circuit-QED experiments with semiconductor nano-structures both for quantum information processing and for fundamental studies of light–matter interactions.

The coupling of double quantum dots and superconducting resonators has been shown before, but the strength of the coupling has been limited to around 25 MHz, an order of magnitude below the observed decoherence rates. Recently, three independent efforts to reach strong coupling in these systems have come to fruition. In an experiment in Princeton, remarkably low decoherence rates were reached in a Si double quantum dot [Mi et al., Science 355, 156 (2017)], resulting in strong coupling to the microwave cavity, while in a parallel effort at ENS Paris (Bruhat, arXiv:1612.05214) a carbon nanotube-based double quantum dot was strongly coupled to a coplanar waveguide resonator. In our approach, the key to achieving strong coupling is an enhancement of the vacuum fluctuations of the resonator electric field due to an increased characteristic impedance. This enabled coupling strengths of up to 155 MHz and together with lowered decoherence rates of 40 MHz placed the hybrid circuit firmly in the strong-coupling regime.

Our approach yields the highest coupling strength reported to date for semiconductor–superconductor hybrid systems. The coupling strength can be engineered in a controlled manner and the method can be applied to a broad variety of charge-based quantum systems. We expect that the technique should be useful in particular for reading out qubit states and for realizing two-qubit gates for electronic systems.

Figure: (a) False-color optical micrograph of a representative device indicating the substrate (dark gray), the superconducting structures (light gray), the gold top gates (yellow) forming the DQD and its source and drain leads and contacts (blue). (b) Optical micrograph displaying a SQUID array resonator (light gray) and its coupling gate to the DQD and the DQD biasing structures (yellow).

Full article: https://journals.aps.org/prx/abstract/10.1103/PhysRevX.7.011030, also in arXiv:1701.03433