349 research outputs found
Subventricular zone stem cells are heterogeneous with respect to their embryonic origins and neurogenic fates in the adult olfactory bulb
Wedetermined the embryonic origins of adult forebrain subventricular zone (SVZ) stem cells by Cre-lox fate mapping in transgenic mice. We found that all parts of the telencephalic neuroepithelium, including the medial ganglionic eminence and lateral ganglionic eminence (LGE) and the cerebral cortex, contribute multipotent, self-renewing stem cells to the adult SVZ. Descendants of the embryonic LGE and cortex settle in ventral and dorsal aspects of the dorsolateral SVZ, respectively. Both populations contribute new (5-bromo-2(')-deoxyuridine- labeled) tyrosine hydroxylase- and calretinin-positive interneurons to the adult olfactory bulb. However, calbindin-positive interneurons in the olfactory glomeruli were generated exclusively by LGE- derived stem cells. Thus, different SVZ stem cells have different embryonic origins, colonize different parts of the SVZ, and generate different neuronal progeny, suggesting that some aspects of embryonic patterning are preserved in the adult SVZ. This could have important implications for the design of endogenous stem cell-based therapies in the future
Realistic lower bounds for the factorization time of large numbers on a quantum computer.
Published versio
A quantum-mechanical Maxwell's demon
A Maxwell's demon is a device that gets information and trades it in for
thermodynamic advantage, in apparent (but not actual) contradiction to the
second law of thermodynamics. Quantum-mechanical versions of Maxwell's demon
exhibit features that classical versions do not: in particular, a device that
gets information about a quantum system disturbs it in the process. In
addition, the information produced by quantum measurement acts as an additional
source of thermodynamic inefficiency. This paper investigates the properties of
quantum-mechanical Maxwell's demons, and proposes experimentally realizable
models of such devices.Comment: 13 pages, Te
Multi-valued Logic Gates for Quantum Computation
We develop a multi-valued logic for quantum computing for use in multi-level
quantum systems, and discuss the practical advantages of this approach for
scaling up a quantum computer. Generalizing the methods of binary quantum
logic, we establish that arbitrary unitary operations on any number of d-level
systems (d > 2) can be decomposed into logic gates that operate on only two
systems at a time. We show that such multi-valued logic gates are
experimentally feasible in the context of the linear ion trap scheme for
quantum computing. By using d levels in each ion in this scheme, we reduce the
number of ions needed for a computation by a factor of log d.Comment: Revised version; 8 pages, 3 figures; to appear in Physical Review
Noise spectroscopy of a single spin with spin polarized STM
We show how the noise in a spin polarized STM tunneling current gives
valuable spectroscopic information on the temporal susceptibility of a single
magnetic atom residing on a non-magnetic surface.Comment: 6 pages, 1 figure. To appear in Physical Review
Rotational Cooling of Polar Molecules by Stark-tuned Cavity Resonance
A general scheme for rotational cooling of diatomic heteronuclear molecules
is proposed. It uses a superconducting microwave cavity to enhance the
spontaneous decay via Purcell effect. Rotational cooling can be induced by
sequentially tuning each rotational transition to cavity resonance, starting
from the highest transition level to the lowest using an electric field.
Electrostatic multipoles can be used to provide large confinement volume with
essentially homogeneous background electric field.Comment: 10 pages, 6 figure
Measurement of conditional phase shifts for quantum logic
Measurements of the birefringence of a single atom strongly coupled to a
high-finesse optical resonator are reported, with nonlinear phase shifts
observed for intracavity photon number much less than one. A proposal to
utilize the measured conditional phase shifts for implementing quantum logic
via a quantum-phase gate (QPG) is considered. Within the context of a simple
model for the field transformation, the parameters of the "truth table" for the
QPG are determined.Comment: 4 pages in Postscript format, including 4 figures (attached as
uuencoded version of a gzip-file
Recurrences in Driven Quantum Systems
We consider an initially bound quantum particle subject to an external
time-dependent field. When the external field is large, the particle shows a
tendency to repeatedly return to its initial state, irrespective of whether the
frequency of the field is sufficient for escape from the well. These
recurrences, which are absent in a classical calculation, arise from the system
evolving primarily like a free particle in the external field.Comment: 10 pages in RevTeX format, with three PS files appende
Probabilistic manipulation of entangled photons
We propose probabilistic controlled-NOT and controlled-phase gates for qubits
stored in the polarization of photons. The gates are composed of linear optics
and photon detectors, and consume polarization entangled photon pairs. The
fraction of the successful operation is only limited by the efficiency of the
Bell-state measurement. The gates work correctly under the use of imperfect
detectors and lossy transmission of photons. Combined with single-qubit gates,
they can be used for producing arbitrary polarization states and for designing
various quantum measurements.Comment: 4 pages, 3 figure
Factoring in a Dissipative Quantum Computer
We describe an array of quantum gates implementing Shor's algorithm for prime
factorization in a quantum computer. The array includes a circuit for modular
exponentiation with several subcomponents (such as controlled multipliers,
adders, etc) which are described in terms of elementary Toffoli gates. We
present a simple analysis of the impact of losses and decoherence on the
performance of this quantum factoring circuit. For that purpose, we simulate a
quantum computer which is running the program to factor N = 15 while
interacting with a dissipative environment. As a consequence of this
interaction randomly selected qubits may spontaneously decay. Using the results
of our numerical simulations we analyze the efficiency of some simple error
correction techniques.Comment: plain tex, 18 pages, 8 postscript figure
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