6,505 research outputs found
Quantum Speed Limit for Perfect State Transfer in One Dimension
The basic idea of spin chain engineering for perfect quantum state transfer
(QST) is to find a set of coupling constants in the Hamiltonian, such that a
particular state initially encoded on one site will evolve freely to the
opposite site without any dynamical controls. The minimal possible evolution
time represents a speed limit for QST. We prove that the optimal solution is
the one simulating the precession of a spin in a static magnetic field. We also
argue that, at least for solid-state systems where interactions are local, it
is more realistic to characterize the computation power by the couplings than
the initial energy.Comment: 5 pages, no figure; improved versio
Approaching Capacity at High-Rates with Iterative Hard-Decision Decoding
A variety of low-density parity-check (LDPC) ensembles have now been observed
to approach capacity with message-passing decoding. However, all of them use
soft (i.e., non-binary) messages and a posteriori probability (APP) decoding of
their component codes. In this paper, we show that one can approach capacity at
high rates using iterative hard-decision decoding (HDD) of generalized product
codes. Specifically, a class of spatially-coupled GLDPC codes with BCH
component codes is considered, and it is observed that, in the high-rate
regime, they can approach capacity under the proposed iterative HDD. These
codes can be seen as generalized product codes and are closely related to
braided block codes. An iterative HDD algorithm is proposed that enables one to
analyze the performance of these codes via density evolution (DE).Comment: 22 pages, this version accepted to the IEEE Transactions on
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Role of interference in quantum state transfer through spin chains
We examine the role that interference plays in quantum state transfer through
several types of finite spin chains, including chains with isotropic Heisenberg
interaction between nearest neighbors, chains with reduced coupling constants
to the spins at the end of the chain, and chains with anisotropic coupling
constants. We evaluate quantitatively both the interference corresponding to
the propagation of the entire chain, and the interference in the effective
propagation of the first and last spins only, treating the rest of the chain as
black box. We show that perfect quantum state transfer is possible without
quantum interference, and provide evidence that the spin chains examined
realize interference-free quantum state transfer to a good approximation.Comment: 10 figure
Spin Star as Switch for Quantum Networks
Quantum state transfer is an important task in quantum information
processing. It is known that one can engineer the couplings of a
one-dimensional spin chain to achieve the goal of perfect state transfer. To
leverage the value of these spin chains, a spin star is potentially useful for
connecting different parts of a quantum network. In this work, we extend the
spin-chain engineering problem to the problems with a topology of a star
network. We show that a permanently coupled spin star can function as a network
switch for transferring quantum states selectively from one node to another by
varying the local potentials only. Together with one-dimensional chains, this
result allows applications of quantum state transfer be applied to more general
quantum networks.Comment: 10 pages, 2 figur
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