19,073 research outputs found
Measurement in control and discrimination of entangled pairs under self-distortion
Quantum correlations and entanglement are fundamental resources for quantum
information and quantum communication processes. Developments in these fields
normally assume these resources stable and not susceptible of distortion. That
is not always the case, Heisenberg interactions between qubits can produce
distortion on entangled pairs generated for engineering purposes (e. g. for
quantum computation or quantum cryptography). Experimental work shows how to
produce entangled spin qubits in quantum dots and electron gases, so its
identification and control are crucial for later applications. The presence of
parasite magnetic fields modifies the expected properties and behavior for
which the pair was intended. Quantum measurement and control help to
discriminate the original state in order to correct it or, just to try of
reconstruct it using some procedures which do not alter their quantum nature.
Two different kinds of quantum entangled pairs driven by a Heisenberg
Hamiltonian with an additional inhomogeneous magnetic field which becoming
self-distorted, can be reconstructed without previous discrimination by adding
an external magnetic field, with fidelity close to 1 (with respect to the
original state, but without discrimination). After, each state can be more
efficiently discriminated. The aim of this work is to show how combining both
processes, first reconstruction without discrimination and after discrimination
with adequate non-local measurements, it's possible a) improve the
discrimination, and b) reprepare faithfully the original states. The complete
process gives fidelities better than 0.9. In the meanwhile, some results about
a class of equivalence for the required measurements were found. This property
lets us select the adequate measurement in order to ease the repreparation
after of discrimination, without loss of entanglement.Comment: 6 figure
Cotunneling theory of inelastic STM spin spectroscopy
We propose cotunneling as the microscopic mechanism that makes possible
inelastic electron spectroscopy of magnetic atoms in surfaces for a wide range
of systems, including single magnetic adatoms, molecules and molecular stacks.
We describe electronic transport between the scanning tip and the conducting
surface through the magnetic system (MS) with a generalized Anderson model,
without making use of effective spin models. Transport and spin dynamics are
described with an effective cotunneling Hamiltonian in which the correlations
in the magnetic system are calculated exactly and the coupling to the
electrodes is included up to second order in the tip-MS and MS-substrate. In
the adequate limit our approach is equivalent to the phenomenological Kondo
exchange model that successfully describe the experiments . We apply our method
to study in detail inelastic transport in two systems, stacks of Cobalt
Phthalocyanines and a single Mn atom on CuN. Our method accounts both, for
the large contribution of the inelastic spin exchange events to the conductance
and the observed conductance asymmetry.Comment: 12 pages, 6 figure
Spin-transfer torque on a single magnetic adatom
We theoretically show how the spin orientation of a single magnetic adatom
can be controlled by spin polarized electrons in a scanning tunneling
microscope configuration. The underlying physical mechanism is spin assisted
inelastic tunneling. By changing the direction of the applied current, the
orientation of the magnetic adatom can be completely reversed on a time scale
that ranges from a few nanoseconds to microseconds, depending on bias and
temperature. The changes in the adatom magnetization direction are, in turn,
reflected in the tunneling conductance.Comment: 5 pages, 3 figure
Spin dynamics of current driven single magnetic adatoms and molecules
A scanning tunneling microscope can probe the inelastic spin excitations of a
single magnetic atom in a surface via spin-flip assisted tunneling in which
transport electrons exchange spin and energy with the atomic spin. If the
inelastic transport time, defined as the average time elapsed between two
inelastic spin flip events, is shorter than the atom spin relaxation time, the
STM current can drive the spin out of equilibrium. Here we model this process
using rate equations and a model Hamiltonian that describes successfully spin
flip assisted tunneling experiments, including a single Mn atom, a Mn dimer and
Fe Phthalocyanine molecules. When the STM current is not spin polarized, the
non-equilibrium spin dynamics of the magnetic atom results in non-monotonic
curves. In the case of spin polarized STM current, the spin orientation
of the magnetic atom can be controlled parallel or anti-parallel to the
magnetic moment of the tip. Thus, spin polarized STM tips can be used both to
probe and to control the magnetic moment of a single atom.Comment: 15 pages, 12 figure
The emergence of classical behavior in magnetic adatoms
A wide class of nanomagnets shows striking quantum behavior, known as quantum
spin tunneling (QST): instead of two degenerate ground states with opposite
magnetizations, a bonding-antibonding pair forms, resulting in a splitting of
the ground state doublet with wave functions linear combination of two
classically opposite magnetic states, leading to the quenching of their
magnetic moment. Here we study how QST is destroyed and classical behavior
emerges in the case of magnetic adatoms, as the strength of their coupling,
either to the substrate or to each other, is increased. Both spin-substrate and
spin-spin coupling renormalize the QST splitting to zero allowing the
environmental decoherence to eliminate superpositions between classical states,
leading to the emergence of spontaneous magnetization.Comment: 5 pages, 4 figure
Diverging Entanglement Length in Gapped Quantum Spin Systems
We prove the existence of gapped quantum Hamiltonians whose ground states
exhibit an infinite entanglement length, as opposed to their finite correlation
length. Using the concept of entanglement swapping, the localizable
entanglement is calculated exactly for valence bond and finitely correlated
states, and the existence of the so--called string-order parameter is
discussed. We also report on evidence that the ground state of an
antiferromagnetic chain can be used as a perfect quantum channel if local
measurements on the individual spins can be implemented.Comment: 4 page
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