12,094 research outputs found
Controlling qubit transitions during non-adiabatic rapid passage through quantum interference
In adiabatic rapid passage, the Bloch vector of a qubit is inverted by slowly
inverting an external field to which it is coupled, and along which it is
initially aligned. In non-adiabatic twisted rapid passage, the external field
is allowed to twist around its initial direction with azimuthal angle \phi(t)
at the same time that it is non-adiabatically inverted. For polynomial twist,
\phi(t) \sim Bt^{n}. We show that for n \ge 3, multiple qubit resonances can
occur during a single inversion of the external field, producing strong
interference effects in the qubit transition probability. The character of the
interference is controllable through variation of the twist strength B.
Constructive and destructive interference are possible, greatly enhancing or
suppressing qubit transitions. Experimental confirmation of these controllable
interference effects has already occurred. Application of this interference
mechanism to the construction of fast fault-tolerant quantum CNOT and NOT gates
is discussed.Comment: 8 pages, 7 figures, 2 tables; submitted to J. Mod. Op
Fermi Surface Nesting and Nanoscale Fluctuating Charge/Orbital Ordering in Colossal Magnetoresistive Oxides
We used high resolution angle-resolved photoemission spectroscopy to reveal
the Fermi surface and key transport parameters of the metallic state of the
layered Colossal Magnetoresistive (CMR) oxide La1.2Sr1.8Mn2O7. With these
parameters the calculated in-plane conductivity is nearly one order of
magnitude larger than the measured DC conductivity. This discrepancy can be
accounted for by including the pseudogap which removes at least 90% of the
spectral weight at the Fermi energy. Key to the pseudogap and many other
properties are the parallel straight Fermi surface sections which are highly
susceptible to nesting instabilities. These nesting instabilities produce
nanoscale fluctuating charge/orbital modulations which cooperate with
Jahn-Teller distortions and compete with the electron itinerancy favored by
double exchange
Holographic data storage in a DX-center material
We report on the optical storage of digital data in a semiconductor sample containing DX centers. The diffraction efficiency and the bit-error-rate performance of multiplexed data images are shown to agree well with a simple model of the material. Uniform storage without an exposure schedule is demonstrated. The volume sensitivity is found to be ~10^3 times that of LiNBO3:Fe. The importance of coherent addition of scattered light with diffracted light in holographic data storage is discussed
Realization of logically labeled effective pure states for bulk quantum computation
We report the first use of "logical labeling" to perform a quantum
computation with a room-temperature bulk system. This method entails the
selection of a subsystem which behaves as if it were at zero temperature -
except for a decrease in signal strength - conditioned upon the state of the
remaining system. No averaging over differently prepared molecules is required.
In order to test this concept, we execute a quantum search algorithm in a
subspace of two nuclear spins, labeled by a third spin, using solution nuclear
magnetic resonance (NMR), and employing a novel choice of reference frame to
uncouple nuclei.Comment: PRL 83, 3085 (1999). Small changes made to improve readability and
remove ambiguitie
Deutsch-Jozsa algorithm as a test of quantum computation
A redundancy in the existing Deutsch-Jozsa quantum algorithm is removed and a
refined algorithm, which reduces the size of the register and simplifies the
function evaluation, is proposed. The refined version allows a simpler analysis
of the use of entanglement between the qubits in the algorithm and provides
criteria for deciding when the Deutsch-Jozsa algorithm constitutes a meaningful
test of quantum computation.Comment: 10 pages, 2 figures, RevTex, Approved for publication in Phys Rev
Electronic bandstructure and optical gain of lattice matched III-V dilute nitride bismide quantum wells for 1.55 m optical communication systems
Dilute nitride bismide GaNBiAs is a potential semiconductor alloy for near-
and mid-infrared applications, particularly in 1.55 m optical
communication systems. Incorporating dilute amounts of Bismuth (Bi) into GaAs
reduces the effective bandgap rapidly, while significantly increasing the
spin-orbit-splitting energy. Additional incorporation of dilute amounts of
Nitrogen (N) helps to attain lattice matching with GaAs, while providing a
route for flexible bandgap tuning. Here we present a study of the electronic
bandstructure and optical gain of the lattice matched
GaNBiAs/GaAs quaternary alloy quantum well (QW) based on the
16-band kp model. We have taken into consideration the interactions
between the N and Bi impurity states with the host material based on the band
anticrossing (BAC) and valence band anticrossing (VBAC) model. The optical gain
calculation is based on the density matrix theory. We have considered different
lattice matched GaNBiAs QW cases and studied their energy dispersion curves,
optical gain spectrum, maximum optical gain and differential gain; and compared
their performances based on these factors. The thickness and composition of
these QWs were varied in order to keep the emission peak fixed at 1.55 m.
The well thickness has an effect on the spectral width of the gain curves. On
the other hand, a variation in the injection carrier density has different
effects on the maximum gain and differential gain of QWs of varying
thicknesses. Among the cases studied, we found that the 6.3 nm thick
GaNBiAs lattice matched QW was most suited for 1.55
m (0.8 eV) GaAs-based photonic applications.Comment: Accepted in AIP Journal of Applied Physic
Investing in China\u27s Telecommunications Market: Reflections on the Rule of Law and Foreign Investment in China
The lack of clarity in China\u27s investment laws has translated into an investment environment that is often uncertain, risky, and mired in red tape. In fact, there have been cases where foreign corporations have invested in joint ventures following what they thought to be all the requisite guidelines, only to find out after the money had exchanged hands that something was terribly wrong with the entire agreement. A perfect example of this is the birth, development, and eventual demise of the ill-fated China-China-Foreign (hereinafter CCF ) investment vehicles used for investment in China\u27s telecommunications industry in the past few years. This comment will use CCF investment in China\u27s telecommunications industry as a case study to show how the vague legal framework for foreign investment in China can make investment in China an unpredictable venture. It will also discuss how a weak rule of law has contributed to developing this vague legal framework through promoting the existence of multiple interpretations of each law. Furthermore, this comment will reflect on the impact of the U.S.-China Bilateral WTO Agreement (hereinafter U.S.-China WTO Agreement ), signed in November 1999, and China\u27s impending accession into the World Trade Organization (hereinafter WTO ) on the case study. It will also touch upon the possible ramifluations that entry into the WTO will have on the rule of law in China
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