3,919 research outputs found
Faithful qubit transmission against collective noise without ancillary qubits
We present a faithful qubit transmission scheme with linear optics against
collective noise, not resorting to ancillary qubits. Its set-up is composed of
three unbalanced polarization interferometers, based on a polarizing beam
splitter, a beam splitter and a half-wave plate, which makes this scheme more
feasible than others with present technology. The fidelity of successful
transmission is 1, independent of the parameters of the collective noise, and
the success probability for obtaining an uncorrupted state can be improved to
100% with some time delayers. Moreover, this scheme has some good applications
in one-way quantum communication for rejecting the errors caused by the
collective noise in quantum channel.Comment: 3 pages, 1 figur
Switchable resonant coupling of flux qubits
We propose a coupling scheme, where two or more flux qubits with different
eigenfrequencies share Josephson junctions with a coupler loop devoid of its
own quantum dynamics. Switchable two-qubit coupling is realized by tuning the
frequency of the AC magnetic flux through the coupler to a combination
frequency of two of the qubits. The coupling allows any or all of the qubits to
be simultaneously at the degeneracy point and can change sign.Comment: REVTeX 4, 4 pages, 2 figures, v2: reference added, v3: final version
published in Phys. Rev.
Study of Phonon Modes in Germanium Nanowires
The observation of pure phonon confinement effect in germanium nanowires is
limited due to the illumination sensitivity of Raman spectra. In this paper we
measured Raman spectra for different size germanium nanowires with different
excitation laser powers and wavelengths. By eliminating the local heating
effect, the phonon confinement effect for small size nanowires was clearly
identified. We have also fitted the Raman feature changes to estimate the size
distribution of nanowires for the first time.Comment: 11 pages,15 figure
Measuring the quality factor of a microwave cavity using superconduting qubit devices
We propose a method to create superpositions of two macroscopic quantum
states of a single-mode microwave cavity field interacting with a
superconducting charge qubit. The decoherence of such superpositions can be
determined by measuring either the Wigner function of the cavity field or the
charge qubit states. Then the quality factor Q of the cavity can be inferred
from the decoherence of the superposed states. The proposed method is
experimentally realizable within current technology even when the value is
relatively low, and the interaction between the qubit and the cavity field is
weak.Comment: 8 page
Efficient quantum key distribution over a collective noise channel
We present two efficient quantum key distribution schemes over two different
collective-noise channels. The accepted hypothesis of collective noise is that
photons travel inside a time window small compared to the variation of noise.
Noiseless subspaces are made up of two Bell states and the spatial degree of
freedom is introduced to form two nonorthogonal bases. Although these protocols
resort to entangled states for encoding the key bit, the receiver is only
required to perform single-particle product measurements and there is no basis
mismatch. Moreover, the detection is passive as the receiver does not switch
his measurements between two conjugate measurement bases to get the key.Comment: 6 pages, 1 figure; the revised version of the paper published in
Phys. Rev. A 78, 022321 (2008). Some negligible errors on the error rates of
eavesdropping check are correcte
Preparation of macroscopic quantum superposition states of a cavity field via coupling to a superconducting charge qubit
We propose how to generate macroscopic quantum superposition states using a
microwave cavity containing a superconducting charge qubit. Based on the
measurement of charge states, we show that the superpositions of two
macroscopically distinguishable coherent states of a single-mode cavity field
can be generated by a controllable interaction between a cavity field and a
charge qubit. After such superpositions of the cavity field are created, the
interaction can be switched off by the classical magnetic field, and there is
no information transfer between the cavity field and the charge qubit. We also
discuss the generation of the superpositions of two squeezed coherent states.Comment: 6 page
Dimensionless ratios: characteristics of quantum liquids and their phase transitions
Dimensionless ratios of physical properties can characterize low-temperature
phases in a wide variety of materials. As such, the Wilson ratio (WR), the
Kadowaki-Woods ratio and the Wiedemann\--Franz law capture essential features
of Fermi liquids in metals, heavy fermions, etc. Here we prove that the phases
of many-body interacting multi-component quantum liquids in one dimension (1D)
can be described by WRs based on the compressibility, susceptibility and
specific heat associated with each component. These WRs arise due to additivity
rules within subsystems reminiscent of the rules for multi-resistor networks in
series and parallel --- a novel and useful characteristic of multi-component
Tomonaga-Luttinger liquids (TLL) independent of microscopic details of the
systems. Using experimentally realised multi-species cold atomic gases as
examples, we prove that the Wilson ratios uniquely identify phases of TLL,
while providing universal scaling relations at the boundaries between phases.
Their values within a phase are solely determined by the stiffnesses and sound
velocities of subsystems and identify the internal degrees of freedom of said
phase such as its spin-degeneracy. This finding can be directly applied to a
wide range of 1D many-body systems and reveals deep physical insights into
recent experimental measurements of the universal thermodynamics in ultracold
atoms and spins.Comment: 12 pages (main paper), (6 figures
Size-dependent decoherence of excitonic states in semiconductor microcrystallites
The size-dependent decoherence of the exciton states resulting from the
spontaneous emission is investigated in a semiconductor spherical
microcrystallite under condition . In general, the
larger size of the microcrystallite corresponds to the shorter coherence time.
If the initial state is a superposition of two different excitonic coherent
states, the coherence time depends on both the overlap of two excitonic
coherent states and the size of the microcrystallite. When the system with
fixed size is initially in the even or odd coherent states, the larger average
number of the excitons corresponds to the faster decoherence. When the average
number of the excitons is given, the bigger size of the microcrystallite
corresponds to the faster decoherence. The decoherence of the exciton states
for the materials GaAs and CdS is numerically studied by our theoretical
analysis.Comment: 4 pages, two figure
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