We report non-invasive single-charge detection of the full probability distribution Pn of the initialization of a quantum dot with n electrons for rapid decoupling from an electron reservoir. We analyze the data in the context of a model for sequential tunneling pinch-off, which has generic solutions corresponding to two opposing mechanisms. One limit considers sequential "freeze out" of an adiabatically evolving grand canonical distribution, the other one is an athermal limit equivalent to the solution of a generalized decay cascade model. We identify the athermal capturing mechanism in our sample, testifying to the high precision of our combined theoretical and experimental methods. The distinction between the capturing mechanisms allows to derive efficient experimental strategies for improving the initialization. PACS numbers:The fast formation of quantum dots (QDs) out of a two-dimensional electron system (2DES) constitutes an open problem within the field of nanoscale electronics [1]. The initialization process in these dynamic QDs is a key ingredient in, e.g., devices for quantum information processing [2,3], single-electron current sources [4-6], or nanoelectronic circuits [7,8]. The outcome of the initialization is characterized by a probability distribution P n for trapping exactly n electrons in the QD. The goal is to attain a predictable low dispersion distribution thus making dynamic QDs reliable and reproducible sources of electrons on demand. Deviations from this ideal case may be caused, for instance, by backtunneling [9][10][11] or non-adiabatic excitations [12][13][14].

The future redefinition of the international system of units in terms of natural constants requires a robust, high-precision quantum standard for the electrical base unit ampere. However, the reliability of any single-electron current sources generating a nominally quantized output current I = ef by delivering single electrons with charge e at a frequency f is eventually limited by the stochastic nature of the underlying quantum mechanical tunnelling process. We experimentally explore a path to overcome this fundamental limitation by serially connecting clocked single-electron emitters with multiple insitu single-electron detectors. Correlation analysis of the detector signatures during current generation reveals erroneous pumping events and enables us to determine the deviation of the output current from the nominal quantized value ef . This demonstrates the concept of a self-referenced single-electron source for electrical quantum metrology. 1 arXiv:1312.5669v1 [cond-mat.mes-hall]

Josephson junction transistors and 50-junction arrays with linear junction dimensions from 200 nm down to 70 nm were fabricated from standard Nb/AlO x /Nb trilayers. The fabrication process includes electron beam lithography, dry etching, anodization, and planarization by chemical-mechanical polishing. The samples were characterized at temperatures down to 25 mK. In general, all junctions are of high quality and their I-U characteristics show low leakage currents and high superconducting energy gap values of ∆ ≈ 1.35 meV. The characteristics of the transistors and arrays exhibit some features in the subgap area, associated with tunneling of Cooper pairs, quasiparticles and their combinations due to the redistribution of the bias voltage between the junctions. Total island capacitances of the transistor samples ranged from 1.5 fF to 4 fF, depending on the junction sizes. Devices made of junctions with linear dimensions below 100 nm by 100 nm demonstrate a remarkable single-electron behavior in both superconducting and normal state. We also investigated the area dependence of the junction capacitances for transistor and array samples.PACS numbers: 73.23. Hk, 74.78.Db, 74.78.Na, 85.35.Gv Superconducting circuits with small tunnel junctions offer the possibility of manipulating single Cooper pairs and may be used to construct e.g. quantum current standards 1 and charge qubits. 2 For many devices, the use of a material with a larger superconducting gap energy ∆ -compared to the commonly used Al-based devices (∆ Al ≈ 0.2 meV) -appears to be attractive. With the use of Nb-based junctions (∆ Nb ≈ 1.5 meV) it should be possible to increase the Josephson coupling energy E J by almost one order of magnitude for the same tunnel resistance (or transparency) of a tunnel barrier. This opens a direct way towards the realization of the required relation between E J and the charging energy E C -namelyT is the thermal energy -in very small Nb circuits. Owing to the fact that both parameters, E J and E C , are large compared with the thermal energy, such circuits are particularly promising for the development of sensitive single Cooper pair electrometers 3 and charge-phase qubits. 4 Traditionally, single Cooper pair and single electron tunneling (SET) devices are fabricated by angle evaporation techniques using suspended masks, defined by high-resolution electron beam lithography (EBL). This technique works successfully for soft materials like aluminum which is the most widely used metal for the preparation of SET devices. Attempts to use this technique for Nb-based devices resulted in a deterioration of the superconducting properties of the Nb; the transition temperature T c , and hence the effective gap energy, is reduced to about 1 meV or smaller 5,6,7,8 , i.e. significantly below the Nb bulk value. Furthermore, appreciable leakage currents were observed in the subgap region. 8 Nb/AlO x /Nb junctions with areas as small as 0.002 µm 2 , good I-U characteristics and a gap energy of about 1.25 meV have been achieved by a sloped-ed...

We investigate a quadratic-quartic anharmonic oscillator formed by a potential well between two potential barriers. We realize this novel potential with a dc SQUID at near-zero current bias and flux bias near half a flux quantum. Escape out of the central well can occur via tunneling through either of the two barriers. We find good agreement with a generalized double-path macroscopic quantum tunneling theory. We also demonstrate an "optimal line" in current and flux bias along which the oscillator, which can be operated as a phase qubit, is insensitive to decoherence due to low-frequency current fluctuations.

We have developed a recently proposed concept of a Josephson traveling-wave parametric amplifier with threewave mixing. The amplifier consists of a microwave transmission line formed by an array of nonhysteretic one-junction SQUIDs. These SQUIDs are flux-biased in a way that the phase drops across the Josephson junctions are equal to 90°. Such a onedimensional metamaterial possesses a large quadratic nonlinearity and zero cubic (Kerr-like) nonlinearity. This property allows phase matching and exponential power gain to take place over a wide frequency range. The proof-of-principle experiment performed at a temperature of T = 4.2 K on Nb trilayer samples has demonstrated that our concept of a practical broadband Josephson parametric amplifier is valid and very promising for achieving quantum-limited performance.