386 research outputs found
Semi-optimal Practicable Algorithmic Cooling
Algorithmic Cooling (AC) of spins applies entropy manipulation algorithms in
open spin-systems in order to cool spins far beyond Shannon's entropy bound. AC
of nuclear spins was demonstrated experimentally, and may contribute to nuclear
magnetic resonance (NMR) spectroscopy. Several cooling algorithms were
suggested in recent years, including practicable algorithmic cooling (PAC) and
exhaustive AC. Practicable algorithms have simple implementations, yet their
level of cooling is far from optimal; Exhaustive algorithms, on the other hand,
cool much better, and some even reach (asymptotically) an optimal level of
cooling, but they are not practicable. We introduce here semi-optimal
practicable AC (SOPAC), wherein few cycles (typically 2-6) are performed at
each recursive level. Two classes of SOPAC algorithms are proposed and
analyzed. Both attain cooling levels significantly better than PAC, and are
much more efficient than the exhaustive algorithms. The new algorithms are
shown to bridge the gap between PAC and exhaustive AC. In addition, we
calculated the number of spins required by SOPAC in order to purify qubits for
quantum computation. As few as 12 and 7 spins are required (in an ideal
scenario) to yield a mildly pure spin (60% polarized) from initial
polarizations of 1% and 10%, respectively. In the latter case, about five more
spins are sufficient to produce a highly pure spin (99.99% polarized), which
could be relevant for fault-tolerant quantum computing.Comment: 13 pages, 5 figure
Prospects and Limitations of Algorithmic Cooling
Heat-bath algorithmic cooling (AC) of spins is a theoretically powerful
effective cooling approach, that (ideally) cools spins with low polarization
exponentially better than cooling by reversible entropy manipulations alone.
Here, we investigate the limitations and prospects of AC. For non-ideal and
semioptimal AC, we study the impact of finite relaxation times of reset and
computation spins on the achievable effective cooling. We derive, via
simulations, the attainable cooling levels for given ratios of relaxation times
using two semioptimal practicable algorithms. We expect this analysis to be
valuable for the planning of future experiments. For ideal and optimal AC, we
make use of lower bounds on the number of required reset steps, based on
entropy considerations, to present important consequences of using AC as a tool
for improving signal-to-noise ratio in liquid-state magnetic resonance
spectroscopy. We discuss the potential use of AC for noninvasive clinical
diagnosis and drug monitoring, where it may have significantly lower specific
absorption rate (SAR) with respect to currently used methods.Comment: 12 pages, 5 figure
Experimental Heat-Bath Cooling of Spins
Algorithmic cooling (AC) is a method to purify quantum systems, such as
ensembles of nuclear spins, or cold atoms in an optical lattice. When applied
to spins, AC produces ensembles of highly polarized spins, which enhance the
signal strength in nuclear magnetic resonance (NMR). According to this cooling
approach, spin-half nuclei in a constant magnetic field are considered as bits,
or more precisely, quantum bits, in a known probability distribution.
Algorithmic steps on these bits are then translated into specially designed NMR
pulse sequences using common NMR quantum computation tools. The
cooling of spins is achieved by alternately combining reversible,
entropy-preserving manipulations (borrowed from data compression algorithms)
with , the transfer of entropy from selected spins to the
environment. In theory, applying algorithmic cooling to sufficiently large spin
systems may produce polarizations far beyond the limits due to conservation of
Shannon entropy.
Here, only selective reset steps are performed, hence we prefer to call this
process "heat-bath" cooling, rather than algorithmic cooling. We experimentally
implement here two consecutive steps of selective reset that transfer entropy
from two selected spins to the environment. We performed such cooling
experiments with commercially-available labeled molecules, on standard
liquid-state NMR spectrometers. Our experiments yielded polarizations that
- , so that the entire
spin-system was cooled. This paper was initially submitted in 2005, first to
Science and then to PNAS, and includes additional results from subsequent years
(e.g. for resubmission in 2007). The Postscriptum includes more details.Comment: 20 pages, 8 figures, replaces quant-ph/051115
Doped MXenes—A new paradigm in 2D systems: Synthesis, properties and applications
Since 2011, 2D transition metal carbides, carbonitrides and nitrides known as MXenes have gained huge attention due to their attractive chemical
and electronic properties. The diverse functionalities of MXenes make them a promising candidate for multitude of applications. Recently, doping
MXene with metallic and non-metallic elements has emerged as an exciting new approach to endow new properties to this 2D systems, opening a
new paradigm of theoretical and experimental studies. In this review, we present a comprehensive overview on the recent progress in this emerging
field of doped MXenes. We compare the different doping strategies; techniques used for their characterization and discuss the enhanced properties.
The distinct advantages of doping in applications such as electrocatalysis, energy storage, photovoltaics, electronics, photonics, environmental
remediation, sensors, and biomedical applications is elaborated. Additionally, theoretical developments in the field of electrocatalysis, energy
storage, photovoltaics, and electronics are explored to provide key specific advantages of doping along with the underlying mechanisms. Lastly, we
present the advantages and challenges of doped MXenes to take this thriving field forward
Feminist everyday political economy : space, time and violence
It goes without saying that feminist International Political Economy (IPE) is concerned in one way or another with the everyday – conceptualized as both a site of political struggle and a site within which social relations are (re)produced and governed. Given the long-standing grounding of feminist research in everyday gendered experiences, many would ask: Why do we need an explicit feminist theorization of the everyday? After all, notions of everyday life and everyday political struggle infuse feminist analysis. This paper seeks to interrogate the concept of the everyday – questioning prevalent understandings of the everyday and asking whether there is analytical and conceptual utility to be gained in articulating a specifically feminist understanding of it. We argue that a feminist political economy of the everyday can be developed in ways that push theorizations of social reproduction in new directions. We suggest that one way to do this is through the recognition that social reproduction is the everyday alongside a three-part theorization of space, time and violence (STV). It is an approach that we feel can play an important role in keeping IPE honest – that is, one that recognizes how important gendered structures of everyday power and agency are to the conduct of everyday life within global capitalism
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Doped MXenes—A new paradigm in 2D systems: Synthesis, properties and applications
Since 2011, 2D transition metal carbides, carbonitrides and nitrides known as MXenes have gained huge attention due to their attractive chemical and electronic properties. The diverse functionalities of MXenes make them a promising candidate for multitude of applications. Recently, doping MXene with metallic and non-metallic elements has emerged as an exciting new approach to endow new properties to this 2D systems, opening a new paradigm of theoretical and experimental studies. In this review, we present a comprehensive overview on the recent progress in this emerging field of doped MXenes. We compare the different doping strategies; techniques used for their characterization and discuss the enhanced properties. The distinct advantages of doping in applications such as electrocatalysis, energy storage, photovoltaics, electronics, photonics, environmental remediation, sensors, and biomedical applications is elaborated. Additionally, theoretical developments in the field of electrocatalysis, energy storage, photovoltaics, and electronics are explored to provide key specific advantages of doping along with the underlying mechanisms. Lastly, we present the advantages and challenges of doped MXenes to take this thriving field forward
Nation-Work: A Praxeology of Making and Maintaining Nations
This article bridges the literatures on nationalist projects and everyday nationhood by elucidating a repertoire of actions shared by both. Analysis of such “nation-work” contributes to the cognitive turn in ethnicity and nationalism research by showing how ethnonational categorization operates. The author distinguishes three types of categorization processes at play: (1) we-they distinctions are made across ethnonational groups, (2) these ethnonational distinctions are further specified by linking them with non-ethnonational categories such as gender and class, and (3) differentiations are made within the same ethnonational category by distinguishing exemplary from less exemplary members of the category. Through historical and ethnographic analyses of the tea ceremony in Japan, the author shows how distinctions drawn across national boundaries help select the characteristics of national membership. Yet while nationalism may project an image of a homogeneous “we,” internal heterogeneity is crucial for refining the experience and performance of membership in the nation
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