133 research outputs found
Method for direct observation of coherent quantum oscillations in a superconducting phase qubit
Time-domain observations of coherent oscillations between quantum states in
mesoscopic superconducting systems were so far restricted to restoring the
time-dependent probability distribution from the readout statistics. We propose
a new method for direct observation of Rabi oscillations in a phase qubit. The
external source, typically in GHz range, induces transitions between the qubit
levels. The resulting Rabi oscillations of supercurrent in the qubit loop are
detected by a high quality resonant tank circuit, inductively coupled to the
phase qubit. Detailed calculation for zero and non-zero temperature are made
for the case of persistent current qubit. According to the estimates for
dephasing and relaxation times, the effect can be detected using conventional
rf circuitry, with Rabi frequency in MHz range.Comment: 5 pages, 1 figure, to appear in Phys.Rev.
Scanning Quantum Decoherence Microscopy
The use of qubits as sensitive magnetometers has been studied theoretically
and recent demonstrated experimentally. In this paper we propose a
generalisation of this concept, where a scanning two-state quantum system is
used to probe the subtle effects of decoherence (as well as its surrounding
electromagnetic environment). Mapping both the Hamiltonian and decoherence
properties of a qubit simultaneously, provides a unique image of the magnetic
(or electric) field properties at the nanoscale. The resulting images are
sensitive to the temporal as well as spatial variation in the fields created by
the sample. As an example we theoretically study two applications of this
technology; one from condensed matter physics, the other biophysics. The
individual components required to realise the simplest version of this device
(characterisation and measurement of qubits, nanoscale positioning) have
already been demonstrated experimentally.Comment: 11 pages, 5 low quality (but arXiv friendly) image
Josephson-phase qubit without tunneling
We show that a complete set of one-bit gates can be realized by coupling the
two logical states of a phase qubit to a third level (at higher energy) using
microwave pulses. Thus, one can achieve coherent control without invoking any
tunneling between the qubit levels. We propose two implementations, using
rf-SQUIDs and d-wave Josephson junctions.Comment: REVTeX4, 4pp., 6 EPS figure files; N.B.: "Alec" is my first, and
"Maassen van den Brink" my family name. v2: gate universality fleshed out,
small fix in d-wave decoherence para, discussion expanded, two Refs. added.
v3: some more Refs., a molecular example, and a few minor fixes; final, to
appear in PRB Rapid
Zero-energy Andreev surface bound states in the lattice model
The conditions for zero-energy Andreev surface bound states to exist are
found for the lattice model of d-wave superconductor with arbitrary surface
orientation. Both nearest neighbors and next nearest neighbors models are
considered. It is shown that the results are very sensitive to the surface
orientation. In particular, for half-filled -surface zero-energy Andreev
surface states only appear under the condition that and are odd
simultaneouslyComment: 9 pages, 1 figur
Strong interconversion of non-polar phonons and Josephson plasma oscillations induced by equilibrium Josephson currents in high T_c superconductors
We analyze consequences of dynamical modulations of Josephson current by
non-polar lattice mode in the Josephson junction barrier. In the high
junctions, the effect of such modulations can be anomalously strong due to the
proximity of the insulating barrier to the superconducting state. Accordingly,
the interconversion of sound (as well as other non-polar phonons) and the
Josephson plasma oscillations mediated by stationary Josephson currents, which
may be present in the junction due to various reasons, becomes possible. We
suggest that this effect can be employed for imaging of the stationary
Josephson currents. Estimates of the effect are given.Comment: 11 RevTeX pages, no figure
Induced paramagnetic states by localized -loops in grain boundaries
Recent experiments on high-temperature superconductors show paramagnetic
behavior localized at grain boundaries (GB). This paramagnetism can be
attributed to the presence unconventional d-wave induced -junctions. By
modeling the GB as an array of and conventional Josephson junction we
determine the conditions of the occurrence of the paramagnetic behavior.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Voting 'against all' in postcommunist Russia
Since the early 1990s voters in Russia (and most of the other post-Soviet republics) have been offered
the opportunity to vote ‘against all’ parties and candidates. Increasing numbers have done so. The
evidence of two post-election surveys indicates that ‘against all’ voters are younger than other voters,
more urban and more highly educated. They do not reject liberal democracy, but are critical of the
contemporary practice of Russian politics and find no parties that adequately reflect their views. With
the ending of the ‘against all’ facility in 2006 and other changes in the Russian electoral system under
the Putin presidency, levels of turnout are likely to fall further and the protest vote will seek other
outlets within or outside the parliamentary system
Quantum state engineering with Josephson-junction devices
We review recent theoretical and experimental progress in quantum state
engineering with Josephson junction devices. The concepts of quantum computing
have stimulated an increased activity in the field. Either charges or phases
(fluxes) of the Josephson systems can be used as quantum degrees of freedom,
and their quantum state can be manipulated coherently by voltage and current
pulses. They thus can serve as qubits, and quantum logic gates can be
performed. Their phase coherence time, which is limited, e.g., by the
electromagnetic fluctuations in the control circuit, is long enough to allow a
series of these manipulations. The quantum measurement process performed by a
single-electron transistor, a SQUID, or further nanoelectronic devices is
analyzed in detail.Comment: An article prepared for Reviews of Modern Physics, 46 pages, 23
figure
Zero-bias conductance peak splitting due to multiband effect in tunneling spectroscopy
We study how the multiplicity of the Fermi surface affects the zero-bias peak
in conductance spectra of tunneling spectroscopy. As case studies, we consider
models for organic superconductors -(BEDT-TTF)Cu(NCS) and
(TMTSF)ClO. We find that multiplicity of the Fermi surfaces can lead to
a splitting of the zero-bias conductance peak (ZBCP). We propose that the
presence/absence of the ZBCP splitting is used as a probe to distinguish the
pairing symmetry in -(BEDT-TTF)Cu(NCS).Comment: 7 pages, 7 figure
Mobile kinks and half-integer zero-field-like steps in highly discrete alternating Josephson junction arrays
The dynamics of a one-dimensional, highly discrete, linear array of
alternating and Josephson junctions is studied numerically, under
constant bias current at zero magnetic field. The calculated current - voltage
characteristics exhibit half-integer and integer zero-field-like steps for even
and odd total number of junctions, respectively. Inspection of the
instantaneous phases reveals that, in the former case, single kink
excitations (discrete semi-fluxons) are supported, whose propagation in the
array gives rise to the step, while in the latter case, a pair of
kink -- antikink appears, whose propagation gives rise to the
step. When additional kinks are inserted in the array, they are
subjected to fractionalization, transforming themselves into two closely spaced
kinks. As they propagate in the array along with the single kink or
the kink - antikink pair, they give rise to higher half-integer or
integer zero-field-like steps, respectively.Comment: 7 pages, 8 figures, submitted to Supercond. Sci. Techno
- …