89 research outputs found

### Kinetics of non-equilibrium quasiparticle tunneling in superconducting charge qubits

We directly observe low-temperature non-equilibrium quasiparticle tunneling
in a pair of charge qubits based on the single Cooper-pair box. We measure
even- and odd-state dwell time distributions as a function of temperature, and
interpret these results using a kinetic theory. While the even-state lifetime
is exponentially distributed, the odd-state distribution is more heavily
weighted to short times, implying that odd-to-even tunnel events are not
described by a homogenous Poisson process. The mean odd-state dwell time
increases sharply at low temperature, which is consistent with quasiparticles
tunneling out of the island before reaching thermal equilibrium.Comment: Replaced Figure 1 with color version, corrected more typos. Version
submitted to PR

### Programmable networks for quantum algorithms

The implementation of a quantum computer requires the realization of a large
number of N-qubit unitary operations which represent the possible oracles or
which are part of the quantum algorithm. Until now there are no standard ways
to uniformly generate whole classes of N-qubit gates. We have developed a
method to generate arbitrary controlled phase shift operations with a single
network of one-qubit and two-qubit operations. This kind of network can be
adapted to various physical implementations of quantum computing and is
suitable to realize the Deutsch-Jozsa algorithm as well as Grover's search
algorithm.Comment: 4 pages. Accepted version; Journal-ref. adde

### Characterization of a Differential Radio-Frequency Single-Electron Transistor

We have fabricated and characterized a new type of electrometer that couples
two parallel single-electron transistors (SETs) to a radio-frequency tank
circuit for use as a differential RF-SET. We demonstrate operation of this
device in summing, differential, and single-SET operation modes, and use it to
measure a Coulomb staircase from a differential single Cooper-pair box. In
differential mode, the device is sensitive to uncorrelated input signals while
screening out correlated ones.Comment: 3 pages, 3 figures, submitted to Applied Physics Letter

### Non-ohmicity and energy relaxation in diffusive 2D metals

We analyze current-voltage characteristics taken on Au-doped indium-oxide
films. By fitting a scaling function to the data, we extract the
electron-phonon scattering rate as function of temperature, which yields a
quadratic dependence of the electron-phonon scattering rate on temperature from
1K down to 0.28K. The origin of this enhanced electron-phonon scattering rate
is ascribed to the mechanism proposed by Sergeev and Mitin.Comment: 7 pages, 6 figure

### The Quantum Capacitance Detector: A concept for a pair-breaking radiation detector based on the single Cooper-pair box

We present a proposed design for a pair-breaking photodetector for
far-infrared and sub-millimeter radiation. Antenna-coupled radiation generates
quasiparticles in a superconducting absorber, the density of which are measured
using a single Cooper-pair box. Readout is performed using an electromagnetic
oscillator or a microwave resonator, which is well suited for frequency
multiplexing in large arrays. Theoretical limits to detector sensitivity are
discussed and modeled, with predicted sensitivities rivaling transition-edge
sensors and microwave kinetic inductance detectors. We anticipate that this
detector can be used to address key scientific goals in far-infrared and
sub-millimeter astronomy.Comment: 11 pages, 11 figures. Corrected expression for Fano noise NEP,
improved Fig. 9, removed section on energy resolutio

### Universal saturation of electron dephasing in three-dimensional disordered metals

We have systematically investigated the low-temperature electron dephasing
times $\tau_\phi$ in more than 40 three-dimensional polycrystalline impure
metals with distinct material characteristics. In all cases, a saturation of
the dephasing time is observed below about a (few) degree(s) Kelvin, depending
on samples. The value of the saturated dephasing time $\tau_0$ [$\equiv
\tau_\phi (T \to 0 {\rm K})$] falls basically in the range 0.005 to 0.5 ns for
all samples. Particularly, we find that $\tau_0$ scales with the electron
diffusion constant $D$ as $\tau_0 \sim D^{- \alpha}$, with $\alpha$ close to or
slightly larger than 1, for over two decades of $D$ from about 0.1 to 10
cm$^2$/s. Our observation suggests that the saturation behavior of $\tau_\phi$
is universal and intrinsic in three-dimensional polycrystalline impure metals.
A complete theoretical explanation is not yet available.Comment: 4 pages, 3 eps figure

### Entanglement and decoherence of a micromechanical resonator via coupling to a Cooper box

We analyse the quantum dynamics of a micromechanical resonator capacitively
coupled to a Cooper box. With appropriate quantum state control of the Cooper
box, the resonator can be driven into a superposition of spatially separated
states. The Cooper box can also be used to probe the environmentally-induced
decoherence of the resonator superposition state.Comment: 4 pages, 3 figure

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