120 research outputs found
Linear-Optical Hyperentanglement-Assisted Quantum Error-Correcting Code
We propose a linear-optical implementation of a hyperentanglement-assisted
quantum error-correcting code. The code is hyperentanglement-assisted because
the shared entanglement resource is a photonic state hyperentangled in
polarization and orbital angular momentum. It is possible to encode, decode,
and diagnose channel errors using linear-optical techniques. The code corrects
for polarization "flip" errors and is thus suitable only for a
proof-of-principle experiment. The encoding and decoding circuits use a
Knill-Laflamme-Milburn-like scheme for transforming polarization and orbital
angular momentum photonic qubits. A numerical optimization algorithm finds a
unit-fidelity encoding circuit that requires only three ancilla modes and has
success probability equal to 0.0097.Comment: 6 pages, 2 figures, 1 table, Accepted for publication in Physical
Review
Four-level and two-qubit systems, sub-algebras, and unitary integration
Four-level systems in quantum optics, and for representing two qubits in
quantum computing, are difficult to solve for general time-dependent
Hamiltonians. A systematic procedure is presented which combines analytical
handling of the algebraic operator aspects with simple solutions of classical,
first-order differential equations. In particular, by exploiting and sub-algebras of the full SU(4)
dynamical group of the system, the non-trivial part of the final calculation is
reduced to a single Riccati (first order, quadratically nonlinear) equation,
itself simply solved. Examples are provided of two-qubit problems from the
recent literature, including implementation of two-qubit gates with Josephson
junctions.Comment: 1 gzip file with 1 tex and 9 eps figure files. Unpack with command:
gunzip RSU05.tar.g
Dephasing times in quantum dots due to elastic LO phonon-carrier collisions
Interpretation of experiments on quantum dot (QD) lasers presents a
challenge: the phonon bottleneck, which should strongly suppress relaxation and
dephasing of the discrete energy states, often seems to be inoperative. We
suggest and develop a theory for an intrinsic mechanism for dephasing in QD's:
second-order elastic interaction between quantum dot charge carriers and
LO-phonons. The calculated dephasing times are of the order of 200 fs at room
temperature, consistent with experiments. The phonon bottleneck thus does not
prevent significant room temperature dephasing.Comment: 4 pages, 1 figure, accepted for Phys. Rev. Let
Carrier capture dynamics of InAs/GaAs quantum dots
Carrier dynamics of a 1.3 mu m InAs/GaAs quantum dot amplifier is studied using heterodyne pump-probe spectroscopy. Measurements of the recovery times versus injection current reveal a power law behavior predicted by a quantum dot rate equation model. These results indicate that Auger processes dominate the carrier dynamics. (c) 2007 American Institute of Physics. (DOI:10.1063/1.2715115
Optical absorption spectra and monomer interaction in polymers. Investigation of exciton coupling in DNA hairpins
We investigate the effect of exciton coupling on the optical absorption
spectrum of polymer molecules under conditions of strong inhomogeneous
broadening. We demonstrate that the dependence of the maximum in the rescaled
absorption spectrum on the number of monomers is determined by the average
monomer excitation energies and their resonant coupling and insensitive to the
inhomogeneous broadening. Thus the absorption spectrum can be used to determine
optical interactions between monomers. The results are applied to the
absorption spectra of poly-A poly-T DNA hairpins and used to interpret the
dependence of the absorption spectrum on the number of monomers. We also
discuss exciton localization in these hairpins.Comment: Submitted to Journal of Chemical Physic
Quantum dot dephasing by edge states
We calculate the dephasing rate of an electron state in a pinched quantum
dot, due to Coulomb interactions between the electron in the dot and electrons
in a nearby voltage biased ballistic nanostructure. The dephasing is caused by
nonequilibrium time fluctuations of the electron density in the nanostructure,
which create random electric fields in the dot. As a result, the electron level
in the dot fluctuates in time, and the coherent part of the resonant
transmission through the dot is suppressed
Temperature dependence of polarization relaxation in semiconductor quantum dots
The decay time of the linear polarization degree of the luminescence in
strongly confined semiconductor quantum dots with asymmetrical shape is
calculated in the frame of second-order quasielastic interaction between
quantum dot charge carriers and LO phonons. The phonon bottleneck does not
prevent significantly the relaxation processes and the calculated decay times
can be of the order of a few tens picoseconds at temperature K,
consistent with recent experiments by Paillard et al. [Phys. Rev. Lett.
{\bf86}, 1634 (2001)].Comment: 4 pages, 4 figure
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