An on-chip detection scheme for high frequency signals is used to detect noise generated by a quantum dot formed in a single wall carbon nanotube. The noise detection is based on photon assisted tunneling in a superconductor-insulator-superconductor junction. Measurements of shot noise over a full Coulomb diamond are reported with excited states and inelastic cotunneling clearly resolved. Super-Poissonian noise is detected in the case of inelastic cotunneling. DOI: 10.1103/PhysRevLett.96.026803 PACS numbers: 73.63.Kv, 73.23.Hk, 73.63.Fg, 74.40.+k The study of shot noise, i.e., nonequilibrium current fluctuations due to the discreteness of charge carriers, is an important tool for studying correlations induced in mesoscopic transport by different types of interactions [1,2]. Current is characterized by Poissonian shot noise when transport is determined by an uncorrelated stochastic process. Electron-electron interactions, such as Coulomb repulsion or resulting from the Pauli exclusion principle, can correlate electron motion and suppress shot noise. The noise power density is defined as the Fourier transform of the current-current correlator S I ! R 1 ÿ1 d e i! h I t I t i. This definition is valid for both positive and negative frequencies !, corresponding to energy absorption or emission by the device [3][4][5]. When jeVj j@!j; k B T (V is the voltage bias and T the temperature), shot noise dominates over other types of noise and the power density has a white spectrum that can be expressed as S I ÿ! S I ! FeI. Here I is the average current and the Fano factor F indicates the deviation from Poissonian shot noise for which F 1. If the noise detector cannot distinguish between emission and absorption processes, a symmetrized version S symm I ! S I ! S I ÿ! is used. The Schottky formula S symm I 2eI refers to this symmetrized case.For electron transport through a quantum dot (QD), shot noise can be either suppressed or enhanced with respect to the Poissonian value. First, for resonant tunneling, when a QD ground state is aligned between the Fermi levels in the leads, the Fano factor can vary between 1=2 and 1. The exact value is determined by the ratio of tunneling rates between the dot and the two leads [6]. For strongly asymmetric barriers, transport is dominated by the most opaque one and shot noise is Poissonian. If the barriers are symmetric, the resonant charge state is occupied 50% of the time and a F 1=2 shot-noise suppression is predicted. Second, when the QD is in Coulomb blockade, first-order sequential tunneling is energetically forbidden. Transport can still occur via cotunneling processes [7], elastic or inelastic. These are second-order processes, with a virtual intermediate state, allowing electron transfer between the leads. The elastic process leaves the QD in its ground state and transport is Poissonian. Inelastic cotunneling switches the system from a ground to an excited state and can lead to super-Poissonian noise with a Fano factor up to F 3 [8]. Experiments have shown shot-noise suppress...

Aims. We describe the design, construction, and characterization of the Band 9 heterodyne receivers (600-720 GHz) for the Atacama Large Millimeter/submillimeter Array (ALMA). First-light Band 9 data, obtained during ALMA commissioning and science verification phases, are presented as well. Methods. The ALMA Band 9 receiver units (so-called "cartridges"), which are installed in the telescope's front end, have been designed to detect and down-convert two orthogonal linear polarization components of the light collected by the ALMA antennas. The light entering the front end is refocused with a compact arrangement of mirrors, which is fully contained within the cartridge. The arrangement contains a grid to separate the polarizations and two beam splitters to combine each resulting beam with a local oscillator signal. The combined beams are fed into independent double-sideband mixers, each with a corrugated feedhorn coupling the radiation by way of a waveguide with backshort cavity into an impedance-tuned superconductor-insulator-superconductor (SIS) junction that performs the heterodyne down-conversion. Finally, the generated intermediate frequency (IF) signals are amplified by cryogenic and room-temperature HEMT amplifiers and exported to the telescope's IF back end for further processing and, finally, correlation. Results. The receivers have been constructed and tested in the laboratory and they show an excellent performance, complying with ALMA requirements. Performance statistics on all 73 Band 9 receivers are reported. Importantly, two different tunnel-barrier technologies (necessitating different tuning circuits) for the SIS junctions have been used, namely conventional AlO x barriers and the more recent high-current-density AlN barriers. On-sky characterization and tests of the performance of the Band 9 cartridges are presented using commissioning data. Continuum and line images of the low-mass protobinary IRAS 16293-2422 are presented which were obtained as part of the ALMA science verification program. An 8 GHz wide Band 9 spectrum extracted over a 0.3 × 0.3 region near source B, containing more than 100 emission lines, illustrates the quality of the data.

High critical current-density ͑10 to 420 kA/ cm 2 ͒ superconductor-insulator-superconductor tunnel junctions with aluminum nitride barriers have been realized using a remote nitrogen plasma from an inductively coupled plasma source operated in a pressure range of 10 −3 -10 −1 mbar. We find a much better reproducibility and control compared to previous work. From the current-voltage characteristics and cross-sectional transmission electron microscopy images it is inferred that, compared to the commonly used AlO x barriers, the polycrystalline AlN barriers are much more uniform in transmissivity, leading to a better quality at high critical current densities.

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