5,318 research outputs found
Quantum vortex tunneling in thin films
Cuprate films offer a unique opportunity to observe vortex tunneling effects,
due to their unusually low superfluid density and short coherence length. Here,
we measure the magnetoresistance (\textit{MR}) due to vortex motion of a long
meander line of a superconducting film made of underdoped
. At low temperatures (\textit{T}), the \textit{MR}
shows a significant deviation from Arrhenius activation. The data is consistent
with two dimensional Variable Range Hopping (VRH) of single vortices, i.e.
. The VRH temperature scale depends on the
vortex tunneling rates between pinning sites. We discuss its magnitude with
respect to estimated parameters of the meander thin film.Comment: 5 figure
Variable sequence of events during the past seven terminations in two deep-sea cores from the Southern Ocean
The relationships among internally consistent records of summer sea-surface temperature (SSST), winter sea
ice (WSI), and diatomaceous stable isotopes were studied across seven terminations over the last 660 ka in
sedimentary cores from ODP sites 1093 and 1094. The sequence of events at both sites indicates that SSST
and WSI changes led the carbon and nitrogen isotopic changes in three Terminations (TI, TII and TVI) and followed
them in the other four Terminations (TIII, TIV, TV and TVII). In both TIII and TIV, the leads and lags between
the proxies were related to weak glacial mode, while in TV and TVII they were due to the influence of
the mid-Pleistocene transition. We show that the sequence of events is not unique and does not follow the
same pattern across terminations, implying that the processes that initiated climate change in the Southern
Ocean has varied through time
Unextendible Product Bases, Uncompletable Product Bases and Bound Entanglement
We report new results and generalizations of our work on unextendible product
bases (UPB), uncompletable product bases and bound entanglement. We present a
new construction for bound entangled states based on product bases which are
only completable in a locally extended Hilbert space. We introduce a very
useful representation of a product basis, an orthogonality graph. Using this
representation we give a complete characterization of unextendible product
bases for two qutrits. We present several generalizations of UPBs to arbitrary
high dimensions and multipartite systems. We present a sufficient condition for
sets of orthogonal product states to be distinguishable by separable
superoperators. We prove that bound entangled states cannot help increase the
distillable entanglement of a state beyond its regularized entanglement of
formation assisted by bound entanglement.Comment: 24 pages RevTex, 15 figures; appendix removed, several small
corrections, to appear in Comm. Math. Phy
Quantum Key Distribution with Classical Bob
Secure key distribution among two remote parties is impossible when both are
classical, unless some unproven (and arguably unrealistic)
computation-complexity assumptions are made, such as the difficulty of
factorizing large numbers. On the other hand, a secure key distribution is
possible when both parties are quantum.
What is possible when only one party (Alice) is quantum, yet the other (Bob)
has only classical capabilities? We present a protocol with this constraint,
and prove its robustness against attacks: we prove that any attempt of an
adversary to obtain information (and even a tiny amount of information)
necessarily induces some errors that the legitimate users could notice.Comment: 4 and a bit pages, 1 figure, RevTe
Quantum disentanglers
It is not possible to disentangle a qubit in an unknown state from a
set of (N-1) ancilla qubits prepared in a specific reference state . That
is, it is not possible to {\em perfectly} perform the transformation
. The question is then how well we can do? We consider a number of
different methods of extracting an unknown state from an entangled state formed
from that qubit and a set of ancilla qubits in an known state. Measuring the
whole system is, as expected, the least effective method. We present various
quantum ``devices'' which disentangle the unknown qubit from the set of ancilla
qubits. In particular, we present the optimal universal disentangler which
disentangles the unknown qubit with the fidelity which does not depend on the
state of the qubit, and a probabilistic disentangler which performs the perfect
disentangling transformation, but with a probability less than one.Comment: 8 pages, 1 eps figur
Optimal Universal Disentangling Machine for Two Qubit Quantum States
We derive the optimal curve satisfied by the reduction factors, in the case
of universal disentangling machine which uses only local operations.
Impossibility of constructing a better disentangling machine, by using
non-local operations, is discussed.Comment: 15 pages, 2 eps figures, 1 section added, 1 eps figure added, minor
corrections, 2 reference numbers correcte
Perfect state transfers by selective quantum interferences within complex spin networks
We present a method that implement directional, perfect state transfers
within a branched spin network by exploiting quantum interferences in the
time-domain. That provides a tool to isolate subsystems from a large and
complex one. Directionality is achieved by interrupting the spin-spin coupled
evolution with periods of free Zeeman evolutions, whose timing is tuned to be
commensurate with the relative phases accrued by specific spin pairs. This
leads to a resonant transfer between the chosen qubits, and to a detuning of
all remaining pathways in the network, using only global manipulations. As the
transfer is perfect when the selected pathway is mediated by 2 or 3 spins,
distant state transfers over complex networks can be achieved by successive
recouplings among specific pairs/triads of spins. These effects are illustrated
with a quantum simulator involving 13C NMR on Leucine's backbone; a six-spin
network.Comment: 5 pages, 3 figure
Finite lifetime eigenfunctions of coupled systems of harmonic oscillators
We find a Hermite-type basis for which the eigenvalue problem associated to
the operator acting on becomes a three-terms recurrence. Here and are two constant
positive definite matrices with no other restriction. Our main result provides
an explicit characterization of the eigenvectors of that lie in the
span of the first four elements of this basis when .Comment: 11 pages, 1 figure. Some typos where corrected in this new versio
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
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