277 research outputs found
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Precision LCVD System Design with Real Time Process Control
A Laser Chemical Vapor Deposition (LCVD) system was designed using a fixed 100
Watt C02 laser focused on a moveable substrate. Temperature and height measurement devices
monitor the reaction at the point of deposition to provide feedback for controlling the process.
The LCVD system will use rapid prototyping technology to directly fabricate fully threedimensional ceramic, metallic, and composite parts of arbitrary shape. Potential applications
include high temperature structures, electronic/photonic devices, and orthopaedic implants.Mechanical Engineerin
Parity effect and single-electron injection for Josephson-junction chains deep in the insulating state
We have made a systematic investigation of charge transport in 1D chains of
Josephson junctions where the characteristic Josephson energy is much less than
the single-island Cooper-pair charging energy, . Such
chains are deep in the insulating state, where superconducting phase coherence
across the chain is absent, and a voltage threshold for conduction is observed
at the lowest temperatures. We find that Cooper-pair tunneling in such chains
is completely suppressed. Instead, charge transport is dominated by tunneling
of single electrons, which is very sensitive to the presence of BCS
quasiparticles on the superconducting islands of the chain. Consequently we
observe a strong parity effect, where the threshold voltage vanishes sharply at
a characteristic parity temperature , which is significantly lower than
the the critical temperature, . A measurable and thermally-activated
zero-bias conductance appears above , with an activation energy equal to
the superconducting gap, confirming the role of thermally-excited
quasiparticles. Conduction below and above the voltage threshold occurs
via injection of single electrons/holes into the Cooper-pair insulator, forming
a non-equilibrium steady state with a significantly enhanced effective
temperature. Our results explicitly show that single-electron transport
dominates deep in the insulating state of Josephson-junction arrays. This
conduction process has mostly been ignored in previous studies of both
superconducting junction arrays and granular superconducting films below the
superconductor-insulator quantum phase transition.Comment: 8 pages, 6 figure
Photon generation in an electromagnetic cavity with a time-dependent boundary
We report the observation of photon generation in a microwave cavity with a
time-dependent boundary condition. Our system is a microfabricated quarter-wave
coplanar waveguide cavity. The electrical length of the cavity is varied using
the tunable inductance of a superconducting quantum interference device. It is
measured in the quantum regime, where the temperature is significantly less
than the resonance frequency (~ 5 GHz). When the length is modulated at
approximately twice the static resonance frequency, spontaneous oscillations of
the cavity field are observed. Time-resolved measurements of the dynamical
state of the cavity show multiple stable states. The behavior is well described
by theory. Connections to the dynamical Casimir effect are discussed.Comment: 5 pages, 3 Figure
Tunability of a 2e periodic single Cooper pair box
We have measured the fully 2e periodic Coulomb staircase of a single Cooper pair box (SCB) in superconducting quantum interference design geometry, using a radio-frequency single-electron transistor. We have determined the energies of the SCB with microwave spectroscopy and compared the calculated shape of the Coulomb staircases to the measured staircases. We find excellent agreement as the Josephson coupling energy is tuned by an external magnetic field
Charge noise in single-electron transistors and charge qubits may be caused by metallic grains
We report on measurements of low-frequency noise in a single-electron transistor (SET) from a few hertz up to 10 MHz. Measurements were done for different bias and gate voltages, which allow us to separate noise contributions from different noise sources. We find a 1/f noise spectrum with two Lorentzians superimposed. The cut-off frequency of one of the Lorentzians varies systematically with the potential of the SET island. Our data is consistent with two single-charge fluctuators situated close to the tunnel barrier. We suggest that these are due to random charging of aluminum grains, each acting as a single-electron box with tunnel coupling to one of the leads and capacitively coupled to the SET island. We are able to fit the data to our model and extract parameters for the fluctuators
Fast tuning of superconducting microwave cavities
Photons are fundamental excitations of the electromagnetic field and can be
captured in cavities. For a given cavity with a certain size, the fundamental
mode has a fixed frequency {\it f} which gives the photons a specific "color".
The cavity also has a typical lifetime , which results in a finite
linewidth {\it f}. If the size of the cavity is changed fast compared
to , and so that the frequency change {\it f} {\it
f}, then it is possible to change the "color" of the captured photons. Here we
demonstrate superconducting microwave cavities, with tunable effective lengths.
The tuning is obtained by varying a Josephson inductance at one end of the
cavity. We show data on four different samples and demonstrate tuning by
several hundred linewidths in a time . Working in the few
photon limit, we show that photons stored in the cavity at one frequency will
leak out from the cavity with the new frequency after the detuning. The
characteristics of the measured devices make them suitable for different
applications such as dynamic coupling of qubits and parametric amplification.Comment: 2nd International Workshop on Solid-State Quantum Computing, June
2008, Taipei, Taiwa
Direct Observation of Josephson Capacitance
The effective capacitance has been measured in the split Cooper pair box
(CPB) over its phase-gate bias plane. Our low-frequency reactive measurement
scheme allows to probe purely the capacitive susceptibility due to the CPB band
structure. The data are quantitatively explained using parameters determined
independently by spectroscopic means. In addition, we show in practice that the
method offers an efficient way to do non-demolition readout of the CPB quantum
state.Comment: 4 page
Observation of quantum capacitance in the Cooper-pair transistor
We have fabricated a Cooper-pair transistor (CPT) with parameters such that
for appropriate voltage biases, the sub-gap charge transport takes place via
slow tunneling of quasiparticles that link two Josephson-coupled charge
manifolds. In between the quasiparticle tunneling events, the CPT behaves
essentially like a single Cooper-pair box (SCB). The effective capacitance of a
SCB can be defined as the derivative of the induced charge with respect to gate
voltage. This capacitance has two parts, the geometric capacitance, C_geom, and
the quantum capacitance C_Q. The latter is due to the level anti-crossing
caused by the Josephson coupling. It depends parametrically on the gate voltage
and is dual to the Josephson inductance. Furthermore, it's magnitude may be
substantially larger than C_geom. We have been able to detect C_Q in our CPT,
by measuring the in-phase and quadrature rf-signal reflected from a resonant
circuit in which the CPT is embedded. C_Q can be used as the basis of a charge
qubit readout by placing a Cooper-pair box in such a resonant circuit.Comment: 3 figure
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Fabrication of Advanced Thermionic Emitters Using Laser Chemical Vapor Deposition-Rapid Prototyping 498
Laser Chemical Vapor Deposition-Rapid Prototyping (LCVD-RP) is a relatively new manufacturing process. Its capabilities are ideally suited for the manufacturing of a type of electron emitter called an integrated-grid thermionic emitter. The integrated-grid thermionic emitter is composed of wagon wheel-like structures of alternating layers of boron nitride and molybdenum on tungsten. The goal of this paper is to determine the feasibility of using LCVDRP technology to manufacture advanced thermionic emitters.Mechanical Engineerin
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