26,688 research outputs found
Optimizing inhomogeneous spin ensembles for quantum memory
We propose a general method to maximize the fidelity of writing, storage and
reading of quantum information (QI) in a spectrally inhomogeneous spin ensemble
used as quantum memory. The method is based on preselecting the optimal
spectral portion of the ensemble by a judiciously designed pulse. It allows
drastic improvement of quantum memory realized by spin ensembles that store QI
from a resonator or an optical beam.Comment: Corrected m
Optimal learning rules for discrete synapses
There is evidence that biological synapses have a limited number of discrete weight states. Memory storage with such synapses behaves quite differently from synapses with unbounded, continuous weights, as old memories are automatically overwritten by new memories. Consequently, there has been substantial discussion about how this affects learning and storage capacity. In this paper, we calculate the storage capacity of discrete, bounded synapses in terms of Shannon information. We use this to optimize the learning rules and investigate how the maximum information capacity depends on the number of synapses, the number of synaptic states, and the coding sparseness. Below a certain critical number of synapses per neuron (comparable to numbers found in biology), we find that storage is similar to unbounded, continuous synapses. Hence, discrete synapses do not necessarily have lower storage capacity
Increasing Flash Memory Lifetime by Dynamic Voltage Allocation for Constant Mutual Information
The read channel in Flash memory systems degrades over time because the
Fowler-Nordheim tunneling used to apply charge to the floating gate eventually
compromises the integrity of the cell because of tunnel oxide degradation.
While degradation is commonly measured in the number of program/erase cycles
experienced by a cell, the degradation is proportional to the number of
electrons forced into the floating gate and later released by the erasing
process. By managing the amount of charge written to the floating gate to
maintain a constant read-channel mutual information, Flash lifetime can be
extended. This paper proposes an overall system approach based on information
theory to extend the lifetime of a flash memory device. Using the instantaneous
storage capacity of a noisy flash memory channel, our approach allocates the
read voltage of flash cell dynamically as it wears out gradually over time. A
practical estimation of the instantaneous capacity is also proposed based on
soft information via multiple reads of the memory cells.Comment: 5 pages. 5 figure
Three-dimensional holographic disks
We describe optical disks that store data holographically in three dimensions by using either angle multiplexing or wavelength multiplexing. Data are stored and retrieved in parallel blocks or pages, and each page consists of approximately 106 bits. The storage capacity of such disks is derived as a function of disk thickness, pixel size, page size, and scanning parameters. The optimum storage density is approximately 120 bits/µm^2
Decoherence of a quantum memory coupled to a collective spin bath
We study the quantum dynamics of a single qubit coupled to a bath of
interacting spins as a model for decoherence in solid state quantum memories.
The spin bath is described by the Lipkin-Meshkov-Glick model and the bath spins
are subjected to a transverse magnetic field. We investigate the qubit
interacting via either an Ising- or an XY-type coupling term to subsets of bath
spins of differing size. The large degree of symmetry of the bath allows us to
find parameter regimes where the initial qubit state is revived at well defined
times after the qubit preparation. These times may become independent of the
bath size for large baths and thus enable faithful qubit storage even in the
presence of strong coupling to a bath. We analyze a large range of parameters
and identify those which are best suited for quantum memories. In general we
find that a small number of links between qubit and bath spins leads to less
decoherence and that systems with Ising coupling between qubit and bath spins
are preferable.Comment: 13 pages, 8 figure
Quantum Storage of Photonic Entanglement in a Crystal
Entanglement is the fundamental characteristic of quantum physics. Large
experimental efforts are devoted to harness entanglement between various
physical systems. In particular, entanglement between light and material
systems is interesting due to their prospective roles as "flying" and
stationary qubits in future quantum information technologies, such as quantum
repeaters and quantum networks. Here we report the first demonstration of
entanglement between a photon at telecommunication wavelength and a single
collective atomic excitation stored in a crystal. One photon from an
energy-time entangled pair is mapped onto a crystal and then released into a
well-defined spatial mode after a predetermined storage time. The other photon
is at telecommunication wavelength and is sent directly through a 50 m fiber
link to an analyzer. Successful transfer of entanglement to the crystal and
back is proven by a violation of the Clauser-Horne-Shimony-Holt (CHSH)
inequality by almost three standard deviations (S=2.64+/-0.23). These results
represent an important step towards quantum communication technologies based on
solid-state devices. In particular, our resources pave the way for building
efficient multiplexed quantum repeaters for long-distance quantum networks.Comment: 5 pages, 3 figures + supplementary information; fixed typo in ref.
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