120 research outputs found
Single electron transistors with high quality superconducting niobium islands
Deep submicron Al-AlOx-Nb tunnel junctions and single electron transistors
with niobium islands were fabricated by electron beam gun shadow evaporation.
Using stencil masks consisting of the thermostable polymer polyethersulfone
(PES) and germanium, high quality niobium patterns with good superconducting
properties and a gap energy of up to 2Delta = 2.5 meV for the niobium were
achieved. The I(U) characteristics of the transistors show special features due
to tunneling of single Cooper pairs and significant gate modulation in both the
superconducting and the normal state.Comment: 4 pages, 4 figure
Single-charge devices with ultrasmall Nb/AlOx/Nb trilayer Josephson junctions
Josephson junction transistors and 50-junction arrays with linear junction
dimensions from 200 nm down to 70 nm were fabricated from standard Nb/AlOx/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.Comment: 19 pages incl. 2 tables and 11 figure
Josephson tunnel junctions with nonlinear damping for RSFQ-qubit circuit applications
We demonstrate that shunting of Superconductor-Insulator-Superconductor
Josephson junctions by Superconductor-Insulator-Normal metal (S-I-N) structures
having pronounced non-linear I-V characteristics can remarkably modify the
Josephson dynamics. In the regime of Josephson generation the phase behaves as
an overdamped coordinate, while in the superconducting state the damping and
current noise are strikingly small, that is vitally important for application
of such junctions for readout and control of Josephson qubits. Superconducting
Nb/AlO/Nb junction shunted by Nb/AlO/AuPd junction of S-I-N type
was fabricated and, in agreement with our model, exhibited non-hysteretic I-V
characteristics at temperatures down to at least 1.4 K.Comment: 4 pages incl. 3 figure
Ground-state characterization of Nb charge-phase Josephson qubits
We present investigations of Josephson charge-phase qubits inductively
coupled to a radio-frequency driven tank-circuit enabling the readout of the
states by measuring the Josephson inductance of the qubit. The circuits
including junctions with linear dimensions of 60 nm and 80 nm are fabricated
from Nb trilayer and allowing the determination of relevant sample parameters
at liquid helium temperature. The observed partial suppression of the
circulating supercurrent at 4.2 K is explained in the framework of a quantum
statistical model. We have probed the ground-state properties of qubit
structures with different ratios of the Josephson coupling to Coulomb charging
energy at 20 mK, demonstrating both the magnetic control of phase and the
electrostatic control of charge on the qubit island.Comment: 8 pages, 8 figure
Single flux quantum circuits with damping based on dissipative transmission lines
We propose and demonstrate the functioning of a special Rapid Single Flux
Quantum (RSFQ) circuit with frequency-dependent damping. This damping is
achieved by shunting individual Josephson junctions by pieces of open-ended RC
transmission lines. Our circuit includes a toggle flip-flop cell, Josephson
transmission lines transferring single flux quantum pulses to and from this
cell, as well as DC/SFQ and SFQ/DC converters. Due to the desired
frequency-dispersion in the RC line shunts which ensures sufficiently low noise
at low frequencies, such circuits are well-suited for integrating with the
flux/phase Josephson qubit and enable its efficient control.Comment: 6 pages incl. 6 figure
Measurement of coherent charge transfer in an adiabatic Cooper pair pump
We study adiabatic charge transfer in a superconducting Cooper pair pump,
focusing on the influence of current measurement on coherence. We investigate
the limit where the Josephson coupling energy between the various parts
of the system is small compared to the Coulomb charging energy . In this
case the charge transferred in a pumping cycle , the charge of one
Cooper pair: the main contribution is due to incoherent Cooper pair tunneling.
We are particularly interested in the quantum correction to , which is due
to coherent tunneling of pairs across the pump and which depends on the
superconducting phase difference between the electrodes: . A measurement of tends to destroy the phase
coherence. We first study an arbitrary measuring circuit and then specific
examples and show that coherent Cooper pair transfer can in principle be
detected using an inductively shunted ammeter
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