88,869 research outputs found
Media-Based MIMO: A New Frontier in Wireless Communications
The idea of Media-based Modulation (MBM), is based on embedding information
in the variations of the transmission media (channel state). This is in
contrast to legacy wireless systems where data is embedded in a Radio Frequency
(RF) source prior to the transmit antenna. MBM offers several advantages vs.
legacy systems, including "additivity of information over multiple receive
antennas", and "inherent diversity over a static fading channel". MBM is
particularly suitable for transmitting high data rates using a single transmit
and multiple receive antennas (Single Input-Multiple Output Media-Based
Modulation, or SIMO-MBM). However, complexity issues limit the amount of data
that can be embedded in the channel state using a single transmit unit. To
address this shortcoming, the current article introduces the idea of Layered
Multiple Input-Multiple Output Media-Based Modulation (LMIMO-MBM). Relying on a
layered structure, LMIMO-MBM can significantly reduce both hardware and
algorithmic complexities, as well as the training overhead, vs. SIMO-MBM.
Simulation results show excellent performance in terms of Symbol Error Rate
(SER) vs. Signal-to-Noise Ratio (SNR). For example, a LMIMO-MBM is
capable of transmitting bits of information per (complex) channel-use,
with SER at dB (or SER
at dB). This performance is achieved using a single transmission
and without adding any redundancy for Forward-Error-Correction (FEC). This
means, in addition to its excellent SER vs. energy/rate performance, MBM
relaxes the need for complex FEC structures, and thereby minimizes the
transmission delay. Overall, LMIMO-MBM provides a promising alternative to MIMO
and Massive MIMO for the realization of 5G wireless networks.Comment: 26 pages, 11 figures, additional examples are given to further
explain the idea of Media-Based Modulation. Capacity figure adde
Entanglement Stabilization using Parity Detection and Real-Time Feedback in Superconducting Circuits
Fault tolerant quantum computing relies on the ability to detect and correct
errors, which in quantum error correction codes is typically achieved by
projectively measuring multi-qubit parity operators and by conditioning
operations on the observed error syndromes. Here, we experimentally demonstrate
the use of an ancillary qubit to repeatedly measure the and parity
operators of two data qubits and to thereby project their joint state into the
respective parity subspaces. By applying feedback operations conditioned on the
outcomes of individual parity measurements, we demonstrate the real-time
stabilization of a Bell state with a fidelity of in up to 12
cycles of the feedback loop. We also perform the protocol using Pauli frame
updating and, in contrast to the case of real-time stabilization, observe a
steady decrease in fidelity from cycle to cycle. The ability to stabilize
parity over multiple feedback rounds with no reduction in fidelity provides
strong evidence for the feasibility of executing stabilizer codes on timescales
much longer than the intrinsic coherence times of the constituent qubits.Comment: 12 pages, 10 figures. Update: Fig. 5 correcte
Quantum Chaos & Quantum Computers
The standard generic quantum computer model is studied analytically and
numerically and the border for emergence of quantum chaos, induced by
imperfections and residual inter-qubit couplings, is determined. This
phenomenon appears in an isolated quantum computer without any external
decoherence. The onset of quantum chaos leads to quantum computer hardware
melting, strong quantum entropy growth and destruction of computer operability.
The time scales for development of quantum chaos and ergodicity are determined.
In spite the fact that this phenomenon is rather dangerous for quantum
computing it is shown that the quantum chaos border for inter-qubit coupling is
exponentially larger than the energy level spacing between quantum computer
eigenstates and drops only linearly with the number of qubits n. As a result
the ideal multi-qubit structure of the computer remains rather robust against
imperfections. This opens a broad parameter region for a possible realization
of quantum computer. The obtained results are related to the recent studies of
quantum chaos in such many-body systems as nuclei, complex atoms and molecules,
finite Fermi systems and quantum spin glass shards which are also reviewed in
the paper.Comment: Lecture at Nobel symposium on "Quantum chaos", June 2000, Sweden;
revtex, 10 pages, 9 figure
Quantum cryptography: key distribution and beyond
Uniquely among the sciences, quantum cryptography has driven both
foundational research as well as practical real-life applications. We review
the progress of quantum cryptography in the last decade, covering quantum key
distribution and other applications.Comment: It's a review on quantum cryptography and it is not restricted to QK
Multiparticle entanglement purification for graph states
We introduce a class of multiparticle entanglement purification protocols
that allow us to distill a large class of entangled states. These include
cluster states, GHZ states and various error correction codes all of which
belong to the class of two-colorable graph states. We analyze these schemes
under realistic conditions and observe that they are scalable, i.e. the
threshold value for imperfect local operations does not depend on the number of
parties for many of these states. When compared to schemes based on bipartite
entanglement purification, the protocol is more efficient and the achievable
quality of the purified states is larger. As an application we discuss an
experimental realization of the protocol in optical lattices which allows one
to purify cluster states.Comment: 4 pages, 2 figures; V2: some typos corrected; V3: published versio
Tight bounds for LDPC and LDGM codes under MAP decoding
A new method for analyzing low density parity check (LDPC) codes and low
density generator matrix (LDGM) codes under bit maximum a posteriori
probability (MAP) decoding is introduced. The method is based on a rigorous
approach to spin glasses developed by Francesco Guerra. It allows to construct
lower bounds on the entropy of the transmitted message conditional to the
received one. Based on heuristic statistical mechanics calculations, we
conjecture such bounds to be tight. The result holds for standard irregular
ensembles when used over binary input output symmetric channels. The method is
first developed for Tanner graph ensembles with Poisson left degree
distribution. It is then generalized to `multi-Poisson' graphs, and, by a
completion procedure, to arbitrary degree distribution.Comment: 28 pages, 9 eps figures; Second version contains a generalization of
the previous resul
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