QUROPE Quantum Information Processing and Communication in Europe

We investigate the dynamic quantum tunneling between two attractors of a mesoscopic driven Duffing oscillator. We find that, in addition to inducing a remarkable quantum shift of the bifurcation point, the mesoscopic nature also results in a perfect linear scaling behavior for the tunneling rate with the driving distance to the shifted bifurcation point. 

We study a superconducting single-electron transistor (SSET) which is coupled to a LC oscillator via the phase difference across one of the Josephson junctions. This leads to a strongly anharmonic coupling between the SSET and the oscillator. The coupling can oscillate with the number of photons, which makes this system very similar to the single-atom injection maser. However, the advantage of a design based on superconducting circuits is the strong coupling and existence of standard methods to measure the radiation field in the oscillator.

We study the photon generation in a transmission line oscillator coupled to a driven qubit in the presence of a dissipative electromagnetic environment. It has been demonstrated previously that a population inversion in the qubit can lead to a lasing state of the oscillator. Here we show that the circuit can also exhibit the effect of “lasing without inversion.” It arises since the coupling to the dissipative environment enhances photon emission as compared to absorption, similar to the recoil effect predicted for atomic systems.

Nowadays there is clear and convincing proof that artificially fabricated macroscopic solid state systems can behave according to the laws of quantum mechanics. Recent experiments have demonstrated that superconducting Josephson circuits can be in superposition of macroscopically distinct quantum states. This has important implications, for instance, for the quantum measurement theory, for the description of the interaction between light and matter as well as for implementation of new generation of quantum limited detectors and quantum information processing devices.

 Frequency-selective readout for superconducting qubits opens up the way towards scaling qubit circuits without increasing the number of measurement lines. Here we demonstrate the readout of an array of 7 flux qubits located on the same chip using a single measurement line. Each qubit is placed near an individual λ/4 resonator which, in turn, is coupled to a common microwave transmission line. We performed spectroscopy and coherent manipulation of all qubits and determined their parameters in a single measurement run.

We present a detailed study of the microstructure of submicron Al/Al−O/Al Josephson junctions fabricated by the conventional shadow evaporation technique. The morphology of the dielectric Al−O layer, which plays the key role for junction transport properties, has been investigated by making use of high resolution electron microscopy. We demonstrate, that the flatness and thickness of the aluminum oxide layer strongly depends on its grain structure.

An ultra-low-noise one-stage SiGe heterojunction bipolar transistor amplifier was designed for cryogenic temperatures and a frequency range of 10 kHz–100 MHz. A noise temperature TN ≈ 1.4 K was measured at an ambient temperature of 4.2 K at frequencies between 100 kHz and 100 MHz for a source resistance of ∼50 Ω. The voltage gain of the amplifier was 25 dB at a power consumption of 720 μW. The input voltage noise spectral density of the amplifier is about 35 pV/Hz^(1/2).