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The difference between the phases of superconducting order parameter plays in superconducting circuits the role similar to that played by the electrostatic potential difference required to drive a current in conventional circuits. This fundamental property can be altered by inserting in a superconducting circuit a particular type of weak link, the so-called Josephson π-junction having inverted current-phase relation and enabling a shift of the phase by π. We demonstrate the operation of three superconducting circuits -two of them are classical and one quantum -which all utilize such π-phase shifters realized using superconductor-ferromagnet-superconductor sandwich technology. The classical circuits are based on single-flux-quantum cells, which are shown to be scalable and compatible with conventional niobium-based superconducting electronics. The quantum circuit is a π-phase biased qubit, for which we observe coherent Rabi oscillations and compare the measured coherence time with that of conventional superconducting phase qubits. 1 arXiv:1005.1581v1 [cond-mat.supr-con]

We develop a concept of the traveling-wave Josephson parametric amplifier exploiting quadratic nonlinearity of a serial array of one-junction SQUIDs embedded in a superconducting transmission line. The external magnetic flux applied to the SQUIDs makes it possible to efficiently control the shape of their current-phase relation and, hence, the balance between quadratic and cubic (Kerrlike) nonlinearities. This property allows us to operate in the favorable three-wave-mixing mode with minimal phase mismatch, an exponential dependence of the power gain on number of sections N , a large bandwidth, a high dynamic range, and substantially separated signal (ωs) and pump (ωp) frequencies obeying relation ωs + ωi = ωp, where ωi is the idler frequency. An estimation of the amplifier characteristics with typical experimental parameters, a pump frequency of 12 GHz, and N = 300 yields a flat gain of 20 dB in the bandwidth of 5.6 GHz.

We propose a transistorlike circuit including two serially connected segments of a narrow superconducting nanowire joint by a wider segment with a capacitively coupled gate in between. This circuit is made of amorphous NbSi film and embedded in a network of on-chip Cr microresistors ensuring a sufficiently high external electromagnetic impedance. Assuming a virtual regime of quantum phase slips (QPS) in two narrow segments of the wire, leading to quantum interference of voltages on these segments, this circuit is dual to the dc SQUID. Our samples demonstrated appreciable Coulomb blockade voltage (analog of critical current of the SQUIDs) and periodic modulation of this blockade by an electrostatic gate (analog of flux modulation in the SQUIDs). The model of this QPS transistor is discussed.

We have analyzed the possibility of using the resistive coaxial cable, Thermocoax(R) Philips as a microwave cryofilter for single electron experiments performed in a top-loading system dilution refrigerator. The biasing and signal lines made of this cable are assumed to link a single electron device kept at the lowest temperature Td to the rf filter having a temperature Tf of several kelvin. The attenuation in a wide frequency range has been calculated and it turned out that a 40-cm-long piece of this cable with an outer diameter of 0.5 mm could drastically reduce the effect of noise from the 50 Ω source anchored at Tf (below 25 K), making its effect comparable with the effect of the thermal fluctuations in the device for Td≊40 mK. The calculated attenuation is in reasonable agreement with the data measured at frequencies up to 18 GHz.

Using an optimally coupled nanometer-scale SQUID, we measure the magnetic flux originating from an individual ferromagnetic Ni nanotube attached to a Si cantilever. At the same time, we detect the nanotube's volume magnetization using torque magnetometry. We observe both the predicted reversible and irreversible reversal processes. A detailed comparison with micromagnetic simulations suggests that vortexlike states are formed in different segments of the individual nanotube. Such stray-field free states are interesting for memory applications and noninvasive sensing.

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