Session S4: Quantum Dot-Cavity Hybrid Systems

Probing light-matter interactions at the level of single photons and electrons

Pioneering experiments by Fujisawa \textit{et al.} examined the detuning dependence of the current in semiconductor double quantum dots (DQDs) and highlighted the important role of electron-phonon coupling in inelastic transport.$^1$ Later experiments by the same group directly measured orbital relaxations rates, which were consistent with a phonon-mediated relaxation process.$^2$ By placing semiconductor DQDs inside of high quality factor microwave cavities it is now feasible to achieve charge-cavity coupling rates that are comparable to the phonon emission rate. I will describe recent experiments that examine masing in cavity-coupled semiconductor DQDs. The application of a source-drain bias results in single electron tunneling and population inversion. The interdot tunneling process generates photons and leads to gain in the cavity transmission. We measure the detuning dependence of the gain and find that the gain feature is very broad compared to the cavity linewidth. Recent theory accounts for the broad gain feature by considering a second-order process that involves the simultaneous emission of a cavity photon and a phonon.$^3$ With sufficient cavity driving, it is feasible to achieve above-threshold maser action, which is verified by comparing the statistics of the emitted microwave field above and below the maser threshold.$^4$ \newline \newline References \newline 1. T. Fujisawa \textit{et al.}, Science \textbf{282}, 932 (1998). \newline 2. T. Fujisawa \textit{et al.}, Nature \textbf{419}, 278 (2002). \newline 3. M. J. Gullans \textit{et al.}, Phys. Rev. Lett. \textbf{114}, 196802 (2015). \newline 4. Y.-Y. Liu \textit{et al.}, Science \textbf{347}, 285 (2015).   

Theory of strongly driven cavity coupled quantum dots: from micromasers to BECs of light

Embedding a double quantum dot (DQD) in a low loss microwave resonator results in a large electric dipole interaction between the charge states of the DQD and single microwave photons in the resonator. In the regime of a few electrons and photons, this system is reminiscent of well-known models of cavity quantum electrodynamics from atomic physics; however, there are important deviations due to the strong coupling of the DQD to the electronic reservoirs in the leads, as well as phonons in the lattice. In this talk, we explore how external control and driving of this unique hybrid system can be used to induce non-equilibrium states of light in the resonator.   

Cavity quantum electrodynamics with carbon nanotube quantum dots

Cavity quantum electrodynamics techniques have turned out to be instrumental to probe or manipulate the electronic states of nanoscale circuits. Recently, cavity QED architectures have been extended to quantum dot circuits. These circuits are appealing since other degrees of freedom than the traditional ones (e.g. those of superconducting circuits) can be investigated. I will show how one can use carbon nanotube based quantum dots in that context. In particular, I will focus on the coherent coupling of a single spin [1] or non-local Cooper pairs to cavity photons. Quantum dots also exhibit a wide variety of many body phenomena. The cQED architecture could also be instrumental for understanding them. One of the most paradigmatic phenomenon is the Kondo effect which is at the heart of many electron correlation effects. I will show that a cQED architecture has allowed us to observe the decoupling of spin and charge excitations in a Kondo system. [1] J. J. Viennot, M.C. Dartiailh, A. Cottet and T. Kontos Science 349 408 (2015).