386 research outputs found
Direct detection of quantum entanglement
Quantum entanglement, after playing a significant role in the development of
the foundations of quantum mechanics, has been recently rediscovered as a new
physical resource with potential commercial applications such as, for example,
quantum cryptography, better frequency standards or quantum-enhanced
positioning and clock synchronization. On the mathematical side the studies of
entanglement have revealed very interesting connections with the theory of
positive maps. The capacity to generate entangled states is one of the basic
requirements for building quantum computers. Hence, efficient experimental
methods for detection, verification and estimation of quantum entanglement are
of great practical importance. Here, we propose an experimentally viable,
\emph{direct} detection of quantum entanglement which is efficient and does not
require any \emph{a priori} knowledge about the quantum state. In a particular
case of two entangled qubits it provides an estimation of the amount of
entanglement. We view this method as a new form of quantum computation, namely,
as a decision problem with quantum data structure.Comment: 4 pages, 1 eps figure, RevTe
Perfect quantum error correction coding in 24 laser pulses
An efficient coding circuit is given for the perfect quantum error correction
of a single qubit against arbitrary 1-qubit errors within a 5 qubit code. The
circuit presented employs a double `classical' code, i.e., one for bit flips
and one for phase shifts. An implementation of this coding circuit on an
ion-trap quantum computer is described that requires 26 laser pulses. A further
circuit is presented requiring only 24 laser pulses, making it an efficient
protection scheme against arbitrary 1-qubit errors. In addition, the
performance of two error correction schemes, one based on the quantum Zeno
effect and the other using standard methods, is compared. The quantum Zeno
error correction scheme is found to fail completely for a model of noise based
on phase-diffusion.Comment: Replacement paper: Lost two laser pulses gained one author; added
appendix with circuits easily implementable on an ion-trap compute
Supersymmetric NambuJona-Lasinio Model on four-dimensional Non(anti)commutative Superspace
We construct the Lagrangian of the four-dimensional generalized
supersymmetric NambuJona-Lasinio (SNJL) model, which has
supersymmetry (SUSY) on non(anti)commutative superspace. A special attention is
paid to the examination on the nonperturbative quantum dynamics: The phenomenon
of dynamical-symmetry-breaking/mass-generation on the deformed superspace is
investigated. The model Lagrangian and the method of SUSY auxiliary fields of
composites are examined in terms of component fields. We derive the effective
action, examine it, and solve the gap equation for self-consistent mass
parameters.Comment: 16 pages, TeX mistakes corrected, accepted for publication in JHEP,
25 Jan. 200
Long-time electron spin storage via dynamical suppression of hyperfine-induced decoherence in a quantum dot
The coherence time of an electron spin decohered by the nuclear spin
environment in a quantum dot can be substantially increased by subjecting the
electron to suitable dynamical decoupling sequences. We analyze the performance
of high-level decoupling protocols by using a combination of analytical and
exact numerical methods, and by paying special attention to the regimes of
large inter-pulse delays and long-time dynamics, which are outside the reach of
standard average Hamiltonian theory descriptions. We demonstrate that dynamical
decoupling can remain efficient far beyond its formal domain of applicability,
and find that a protocol exploiting concatenated design provides best
performance for this system in the relevant parameter range. In situations
where the initial electron state is known, protocols able to completely freeze
decoherence at long times are constructed and characterized. The impact of
system and control non-idealities is also assessed, including the effect of
intra-bath dipolar interaction, magnetic field bias and bath polarization, as
well as systematic pulse imperfections. While small bias field and small bath
polarization degrade the decoupling fidelity, enhanced performance and temporal
modulation result from strong applied fields and high polarizations. Overall,
we find that if the relative errors of the control parameters do not exceed 5%,
decoupling protocols can still prolong the coherence time by up to two orders
of magnitude.Comment: 16 pages, 10 figures, submitted to Phys. Rev.
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