56 research outputs found
Tunable refraction in a two dimensional quantum metamaterial
In this paper we consider a two-dimensional metamaterial comprising an array
of qubits (two level quantum objects). Here we show that a two-dimensional
quantum metamaterial may be controlled, e.g. via the application of a magnetic
flux, so as to provide controllable refraction of an input signal. Our results
are consistent with a material that could be quantum birefringent (beam
splitter) or not dependent on the application of this control parameter. We
note that quantum metamaterials as proposed here may be fabricated from a
variety of current candidate technologies from superconducting qubits to
quantum dots. Thus the ideas proposed in this work would be readily testable in
existing state of the art laboratories.Comment: 4 pages, 2 figure
Nonlinear Transport in a Quantum Point Contact due to Soft Disorder Induced Coherent Mode Mixing
We show that the coherent mixing of different transverse modes, due to
forward scattering of carriers by soft impurity- or boundary potentials leads
to a nonlinear, asymmetric current response of quantum point contacts (QPC).
The oscillating contribution to the current is sensitive both to driving
voltage and to gate voltage in direct analogy to the electrostatic
Aharonov-Bohm effect.
Our calculations are in a good agreement with recent experimental data
showing small-scale conductivity nonlinearities and asymmetry in QPC.Comment: 4 pages, 2 figures (availiable upon request), REVTEX, Applied Physics
Report 93-4
Spectroscopy of the Potential Profile in a Ballistic Quantum Constriction
We present a theory for the nonlinear current-voltage characteristics of a
ballistic quantum constriction. Nonlinear features first develop because of
above-barrier reflection from the potential profile, created by impurities in
the vicinity of the constriction. The nonlinearity appears on a small voltage
scale and makes it possible to determine distances between impurities as well
as the magnitude of the impurity potentials.Comment: 3 pages, 4 figures (availiable upon request), REVTEX, Applied Physics
Report 93-5
Electromagnetically induced transparency on a single artificial atom
We present experimental observation of electromagnetically induced
transparency (EIT) on a single macroscopic artificial "atom" (superconducting
quantum system) coupled to open 1D space of a transmission line. Unlike in a
optical media with many atoms, the single atom EIT in 1D space is revealed in
suppression of reflection of electromagnetic waves, rather than absorption. The
observed almost 100 % modulation of the reflection and transmission of
propagating microwaves demonstrates full controllability of individual
artificial atoms and a possibility to manipulate the atomic states. The system
can be used as a switchable mirror of microwaves and opens a good perspective
for its applications in photonic quantum information processing and other
fields
Dynamical effects of an unconventional current-phase relation in YBCO dc-SQUIDs
The predominant d-wave pairing symmetry in high temperature superconductors
allows for a variety of current-phase relations in Josephson junctions, which
is to a certain degree fabrication controlled. In this letter we report on
direct experimental observations of the effects of a non-sinusoidal
current-phase dependence in YBCO dc-SQUIDs, which agree with the theoretical
description of the system.Comment: 4 pages, 4 ps figures, to apprear in Phys. Rev. Let
Ultimate on-chip quantum amplifier
We report amplification of electromagnetic waves by a single artificial atom
in open 1D space. Our three-level artificial atom -- a superconducting quantum
circuit -- coupled to a transmission line presents an analog of a natural atom
in open space. The system is the most fundamental quantum amplifier whose gain
is limited by a spontaneous emission mechanism. The noise performance is
determined by the quantum noise revealed in the spectrum of spontaneous
emission, also characterized in our experiments.Comment: 4 pages, 4 figures + supplemenntary materials accepted for
publication in Phys. Rev. Lett
Mechanisms of Spontaneous Current Generation in an Inhomogeneous d-Wave Superconductor
A boundary between two d-wave superconductors or an s-wave and a d-wave
superconductor generally breaks time-reversal symmetry and can generate
spontaneous currents due to proximity effect. On the other hand, surfaces and
interfaces in d-wave superconductors can produce localized current-carrying
states by supporting the T-breaking combination of dominant and subdominant
order parameters. We investigate spontaneous currents in the presence of both
mechanisms and show that at low temperature, counter-intuitively, the
subdominant coupling decreases the amplitude of the spontaneous current due to
proximity effect. Superscreening of spontaneous currents is demonstrated to be
present in any d-d (but not s-d) junction and surface with d+id' order
parameter symmetry. We show that this supercreening is the result of
contributions from the local magnetic moment of the condensate to the
spontaneous current.Comment: 4 pages, 5 figures, RevTe
Operation of universal gates in a DXD superconducting solid state quantum computer
We demonstrate that complete set of gates can be realized in a DXD
superconducting solid state quantum computer (quamputer), thereby proving its
universality.Comment: 4 pages, 2 figure
Andreev bound states in high- superconducting junctions
The formation of bound states at surfaces of materials with an energy gap in
the bulk electron spectrum is a well known physical phenomenon. At
superconductor surfaces, quasiparticles with energies inside the
superconducting gap may be trapped in bound states in quantum wells,
formed by total reflection against the vacuum and total Andreev reflection
against the superconductor. Since an electron reflects as a hole and sends a
Cooper pair into the superconductor, the surface states give rise to resonant
transport of quasiparticle and Cooper pair currents, and may be observed in
tunneling spectra. In superconducting junctions, these surface states may
hybridize and form bound Andreev states, trapped between the superconducting
electrodes. In d-wave superconductors, the order parameter changes sign under
rotation and, as a consequence, Andreev reflection may lead to the
formation of zero energy quasiparticle bound states, midgap states (MGS). The
formation of MGS is a robust feature of d-wave superconductivity and provides a
unified framework for many important effects which will be reviewed: large
Josephson current, low-temperature anomaly of the critical Josephson current,
-junction behavior, junction crossover with temperature,
zero-bias conductance peaks, paramagnetic currents, time reversal symmetry
breaking, spontaneous interface currents, and resonance features in subgap
currents. Taken together these effects, when observed in experiments, provide
proof for d-wave superconductivity in the cuprates.Comment: 52 pages, 20 figures. Review article under consideration for
publication in Superconductor Science and Technolog
Resonance Fluorescence of a Single Artificial Atom
An atom in open space can be detected by means of resonant absorption and
reemission of electromagnetic waves, known as resonance fluorescence, which is
a fundamental phenomenon of quantum optics. We report on the observation of
scattering of propagating waves by a single artificial atom. The behavior of
the artificial atom, a superconducting macroscopic two-level system, is in a
quantitative agreement with the predictions of quantum optics for a pointlike
scatterer interacting with the electromagnetic field in one-dimensional open
space. The strong atom-field interaction as revealed in a high degree of
extinction of propagating waves will allow applications of controllable
artificial atoms in quantum optics and photonics.Comment: 5 pages, 4 figure
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