50 research outputs found
Superconducting Spin Qubits
We propose and theoretically investigate spin superconducting qubits. Spin
superconducting qubit consists of a single spin confined in a Josephson
junction. We show that owing to spin-orbit interaction, superconducting
difference across the junction can polarize this spin. We demonstrate that this
enables single qubit operations and more complicated quantum gates, where spins
of different qubits interact via a mutual inductance of superconducting loop
where the junctions are embedded. Recent experimental realizations of Josephson
junctions made of semiconductor quantum dots in contact with superconducting
leads have shown that the number of electrons in the quantum dot can be tuned
by a gate voltage. Spin superconducting qubit is realized when the number of
electrons is odd. We discuss the qubit properties at phenomenological level. We
present a microscopic theory that enables us to make accurate estimations of
the qubit parameters by evaluating the spin-dependent Josephson energy in the
framework of fourth-order perturbation theory.Comment: 11 pages, 8 figure
The Effect of Mechanical Resonance on Josephson Dynamics
We study theoretically dynamics in a Josephson junction coupled to a
mechanical resonator looking at the signatures of the resonance in d.c.
electrical response of the junction. Such a system can be realized
experimentally as a suspended ultra-clean carbon nanotube brought in contact
with two superconducting leads. A nearby gate electrode can be used to tune the
junction parameters and to excite mechanical motion. We augment theoretical
estimations with the values of setup parameters measured in the samples
fabricated.
We show that charging effects in the junction give rise to a mechanical force
that depends on the superconducting phase difference. The force can excite the
resonant mode provided the superconducting current in the junction has
oscillating components with a frequency matching the resonant frequency of the
mechanical resonator. We develop a model that encompasses the coupling of
electrical and mechanical dynamics. We compute the mechanical response (the
effect of mechanical motion) in the regime of phase bias and d.c. voltage bias.
We thoroughly investigate the regime of combined a.c. and d.c. bias where
Shapiro steps are developed and reveal several distinct regimes characteristic
for this effect. Our results can be immediately applied in the context of
experimental detection of the mechanical motion in realistic superconducting
nano-mechanical devices.Comment: 18 pages, 11 figure
Quartet currents in a biased three-terminal diffusive Josephson junction
Biasing a three-terminal Josephson junction (TTJ) with symmetrical voltages
leads to new kinds of DC currents, namely quartet Josephson currents
and phase-dependent multiple Andreev reflection (MAR) currents. We study these
currents in a system where a normal diffusive metallic node is connected to
three terminals by barriers of arbitrary transparency. We use the
quantum circuit theory to calculate the current in each terminal, including
decoherence. In addition to the stationary combination
of the terminal phases ,
the bias voltage appears as a new and unusual control variable for a DC
Josephson current. A general feature is the sign changes of the current-phase
characteristics, manifesting in minima of the quartet ``critical current".
Those sign changes can be triggered by the voltage, by the junction
transparency or by decoherence. We study the possible separation of quartet
currents from MAR currents in different regimes of parameters, including an
"funnel" regime with very asymmetric couplings to . In the regime of
low transparency and asymetric couplings, we provide an analytic perturbative
expression for the currents which shows an excellent agreement with the full
numerical results
Spin-dependent tunneling between individual superconducting bound states
Magnetic impurities on superconductors induce discrete bound levels inside
the superconducting gap, known as Yu-Shiba-Rusinov (YSR) states. YSR levels are
fully spin-polarized such that the tunneling between YSR states depends on
their relative spin orientation. Here, we use scanning tunneling spectroscopy
to resolve the spin dynamics in the tunneling process between two YSR states by
experimentally extracting the angle between the spins. To this end, we exploit
the ratio of thermally activated and direct spectral features in the
measurement to directly extract the relative spin orientation between the two
YSR states. We find freely rotating spins down to 7mK, indicating a purely
paramagnetic nature of the impurities. Such a non-collinear spin alignment is
essential not only for producing Majorana bound states but also as an outlook
manipulating and moving the Majorana state onto the tip.Comment: 22 pages, 7 figures, including supplementary materia
Impedance spectroscopy method for investigation of the polycrystalline inhomogeneous ceramics
Spatial navigation deficits — overlooked cognitive marker for preclinical Alzheimer disease?
Detection of incipient Alzheimer disease (AD) pathophysiology is critical to identify preclinical individuals and target potentially disease-modifying therapies towards them. Current neuroimaging and biomarker research is strongly focused in this direction, with the aim of establishing AD fingerprints to identify individuals at high risk of developing this disease. By contrast, cognitive fingerprints for incipient AD are virtually non-existent as diagnostics and outcomes measures are still focused on episodic memory deficits as the gold standard for AD, despite their low sensitivity and specificity for identifying at-risk individuals. This Review highlights a novel feature of cognitive evaluation for incipient AD by focusing on spatial navigation and orientation deficits, which are increasingly shown to be present in at-risk individuals. Importantly, the navigation system in the brain overlaps substantially with the regions affected by AD in both animal models and humans. Notably, spatial navigation has fewer verbal, cultural and educational biases than current cognitive tests and could enable a more uniform, global approach towards cognitive fingerprints of AD and better cognitive treatment outcome measures in future multicentre trials. The current Review appraises the available evidence for spatial navigation and/or orientation deficits in preclinical, prodromal and confirmed AD and identifies research gaps and future research priorities
Impedance spectroscopy method for investigation of the polycrystalline inhomogeneous ceramics
The present paper introduces the principles of the Impedance Spectroscopy technique, as an important method in the characterization of the discrete materials working in real conditions in circuits. The method was firstly verified by using an RC series-parallel circuit and then was exemplified in studying two types of (Ni, Zn) ferrite ceramics