1,469 research outputs found
Finite Controllability of Infinite-Dimensional Quantum Systems
Quantum phenomena of interest in connection with applications to computation
and communication almost always involve generating specific transfers between
eigenstates, and their linear superpositions. For some quantum systems, such as
spin systems, the quantum evolution equation (the Schr\"{o}dinger equation) is
finite-dimensional and old results on controllability of systems defined on on
Lie groups and quotient spaces provide most of what is needed insofar as
controllability of non-dissipative systems is concerned. However, in an
infinite-dimensional setting, controlling the evolution of quantum systems
often presents difficulties, both conceptual and technical. In this paper we
present a systematic approach to a class of such problems for which it is
possible to avoid some of the technical issues. In particular, we analyze
controllability for infinite-dimensional bilinear systems under assumptions
that make controllability possible using trajectories lying in a nested family
of pre-defined subspaces. This result, which we call the Finite Controllability
Theorem, provides a set of sufficient conditions for controllability in an
infinite-dimensional setting. We consider specific physical systems that are of
interest for quantum computing, and provide insights into the types of quantum
operations (gates) that may be developed.Comment: This is a much improved version of the paper first submitted to the
arxiv in 2006 that has been under review since 2005. A shortened version of
this paper has been conditionally accepted for publication in IEEE
Transactions in Automatic Control (2009
5G Millimeter Wave Cellular System Capacity with Fully Digital Beamforming
Due to heavy reliance of millimeter-wave (mmWave) wireless systems on
directional links, Beamforming (BF) with high-dimensional arrays is essential
for cellular systems in these frequencies. How to perform the array processing
in a power efficient manner is a fundamental challenge. Analog and hybrid BF
require fewer analog-to-digital converters (ADCs), but can only communicate in
a small number of directions at a time,limiting directional search, spatial
multiplexing and control signaling. Digital BF enables flexible spatial
processing, but must be operated at a low quantization resolution to stay
within reasonable power levels. This paper presents a simple additive white
Gaussian noise (AWGN) model to assess the effect of low resolution quantization
of cellular system capacity. Simulations with this model reveal that at
moderate resolutions (3-4 bits per ADC), there is negligible loss in downlink
cellular capacity from quantization. In essence, the low-resolution ADCs limit
the high SNR, where cellular systems typically do not operate. The findings
suggest that low-resolution fully digital BF architectures can be power
efficient, offer greatly enhanced control plane functionality and comparable
data plane performance to analog BF.Comment: To appear in the Proceedings of the 51st Asilomar Conference on
Signals, Systems, and Computers, 201
Information hiding and retrieval in Rydberg wave packets using half-cycle pulses
We demonstrate an information hiding and retrieval scheme with the relative
phases between states in a Rydberg wave packet acting as the bits of a data
register. We use a terahertz half-cycle pulse (HCP) to transfer phase-encoded
information from an optically accessible angular momentum manifold to another
manifold which is not directly accessed by our laser pulses, effectively hiding
the information from our optical interferometric measurement techniques. A
subsequent HCP acting on these wave packets reintroduces the information back
into the optically accessible data register manifold which can then be `read'
out.Comment: 4 pages, 4 figure
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