We present a low-temperature experimental test of the fluctuation theorem for electron transport through a double quantum dot. The rare entropy-consuming system trajectories are detected in the form of single charges flowing against the source-drain bias by using time-resolved charge detection with a quantum point contact. We find that these trajectories appear with a frequency that agrees with the theoretical predictions even under strong nonequilibrium conditions, when the finite bandwidth of the charge detection is taken into account

Hybrid excitations, called polaritons, emerge in systems with strong light-matter coupling. Usually, they dominate the linear and nonlinear optical properties with applications in quantum optics. Here, we show the crucial role of the electronic component of polaritons in the magnetotransport of a cavity-embedded 2D electron gas in the ultrastrong coupling regime. We show that the linear dc resistivity is significantly modified by the coupling to the cavity even without external irradiation. Our observations confirm recent predictions of vacuum-induced modification of the resistivity. Furthermore, photo-assisted transport in presence of a weak irradiation field at sub-THz frequencies highlights the different roles of localized and delocalized states.The strong light-matter coupling regime [1, 2] is realized when the coupling Ω between photons and a material's excitation of frequency ω exceeds the losses γ tot of both components. An especially interesting situation is attained when quantum fluctuations of the electromagnetic field ground state give rise to the so-called vacuum Rabi splitting of the cavity polaritons. Solid-state systems [3][4][5] have recently proven to be instrumental in achieving the ultimate limit of this kind of coupling. The ultrastrong coupling regime [6-20], realized in the limit of Ω/ω 0.1, exploits the collective nature of the matter excitations [6,21,22] to achieve a peculiar situation where the ground state of the system is constituted by non-trivial quantum vacua [6].The (ultra-)strong coupling regime has so far mostly been investigated by interrogating the photonic component of the polariton quasi-particle weakly probing the coupled system with low photon fluxes [1-5, 7, 9, 10, 13-17, 19, 23-25]. Notable exceptions have been the measurements of the matter part of an exciton polariton condensate with an excitonic 1s-2p transition[26] and a transport experiment in molecules coupled to a plasmonic resonance [18].Recently we pioneered a new experimental platform, the Landau polaritons, to study ultrastrong light matter interactions [15,27] allowing to reach record-high nor-malized light-matter coupling ratios Ω/ω cav > 1 [28]. The inter-Landau level (cyclotron) transition ω c = eB m * (m * : effective electron mass) of a two-dimensional electron gas (2DEG) under strong magnetic field is coupled to a complementary electronic LC resonator [29] at frequencies of 100's of GHz, which plays effectively the role of the optical cavity. This system is especially well suited to study the matter part of ultrastrongly coupled polaritons using low temperature magneto-transport.It was recently proposed theoretically [30] that such transport is actually driven by the bright polariton operator, i.e. the same operator driving the optical response.Here we find experimental evidence consistent with this picture, in which most tellingly the longitudinal resistivity ρ xx bears the signatures of the polariton branches.Further confirmation for polaritonic effects acting on magneto-transport is obtained by observ...

We measure weak quasiparticle tunneling across a constriction in the second Landau level. At ν = 7/3, 8/3 and 5/2, comparison of temperature and DC bias dependence to weak tunneling theory allows extracting parameters that describe the edges' quasiparticle excitations. At ν = 8/3, our results are well described by a particle-hole conjugate Laughlin state, but not compatible with proposed non-Abelian quasiparticle excitations. For ν = 5/2, our measurements are in good agreement with previous experiments and favor the Abelian (3,3,1) or (1,1,3)-states. At these filling factors, we further investigate the influence of the backscattering strength on the extracted scaling parameters. For ν = 7/3, the backscattering strength strongly affects the scaling parameters, whereas quasiparticle tunneling at ν = 8/3 and 5/2 appears more robust. Our results provide important additional insight about the physics in the second Landau level and contribute to the understanding of the physics underlying the fractional quantum Hall states at ν = 7/3, 8/3 and 5/2.

Scanning gate microscopy is used to locally investigate electron transport in a high-mobility two-dimensional electron gas formed in a GaAs/AlGaAs heterostructure. Using quantum point contacts, we observe branches caused by electron backscattering decorated with interference fringes similar to previous observations by Topinka et al (2000 Science 289 2323). We investigate the branches at different points of a conductance plateau as well as between plateaus, and demonstrate that the most dramatic changes in branch pattern occur at the low-energy side of the conductance plateaus. The branches disappear at magnetic fields as low as 50 mT, demonstrating the importance of backscattering for the observation of the branching effect. The spacing between the interference fringes varies by more than 50% for different branches across scales of microns. Several scenarios are discussed to explain this observation.

Quantum point contacts are fundamental building blocks for mesoscopic transport experiments and play an important role in recent interference-and fractional quantum Hall experiments. However, it is not clear how electron-electron interactions and the random disorder potential influence the confinement potential and give rise to phenomena like the mysterious 0.7 anomaly. Novel growth techniques of Al X Ga 1−X As heterostructures for high-mobility two-dimensional electron gases enable us to investigate quantum point contacts with a strongly suppressed disorder potential. These clean quantum point contacts indeed show transport features that are obscured by disorder in standard samples. From this transport data, we are able to extract the parameters of the confinement potential which describe its shape in longitudinal and transverse direction. Knowing the shape (and hence the slope) of the confinement arXiv:1106.2982v2 [cond-mat.mes-hall]