1,998 research outputs found
Strong many-particle localization and quantum computing with perpetually coupled qubits
We demonstrate the onset of strong on-site localization in a one-dimensional
many-particle system. The localization is obtained by constructing, in an
explicit form, a bounded sequence of on-site energies that eliminates resonant
hopping between both nearest and remote sites. This sequence leads to
quasi-exponential decay of the single-particle transition amplitude. It also
leads to strong localization of stationary many-particle states in a
finite-length chain. For an {\it infinite} chain, we instead study the time
during which {\it all} many-particle states remain strongly localized. We show
that, for any number of particles, this time exceeds the reciprocal frequency
of nearest-neighbor hopping by a factor already for a moderate
bandwidth of on-site energies. The proposed energy sequence is robust with
respect to small errors. The formulation applies to fermions as well as
perpetually coupled qubits. The results show viability of quantum computing
with time-independent qubit coupling.Comment: 20 pages, 10 figure
Fundamental Limits of Low-Density Spreading NOMA with Fading
Spectral efficiency of low-density spreading non-orthogonal multiple access
channels in the presence of fading is derived for linear detection with
independent decoding as well as optimum decoding. The large system limit, where
both the number of users and number of signal dimensions grow with fixed ratio,
called load, is considered. In the case of optimum decoding, it is found that
low-density spreading underperforms dense spreading for all loads. Conversely,
linear detection is characterized by different behaviors in the underloaded vs.
overloaded regimes. In particular, it is shown that spectral efficiency changes
smoothly as load increases. However, in the overloaded regime, the spectral
efficiency of low- density spreading is higher than that of dense spreading
Random Pilot and Data Access in Massive MIMO for Machine-type Communications
A massive MIMO system, represented by a base station with hundreds of
antennas, is capable of spatially multiplexing many devices and thus naturally
suited to serve dense crowds of wireless devices in emerging applications, such
as machine-type communications. Crowd scenarios pose new challenges in the
pilot-based acquisition of channel state information and call for pilot access
protocols that match the intermittent pattern of device activity. A joint pilot
assignment and data transmission protocol based on random access is proposed in
this paper for the uplink of a massive MIMO system. The protocol relies on the
averaging across multiple transmission slots of the pilot collision events that
result from the random access process. We derive new uplink sum rate
expressions that take pilot collisions, intermittent device activity, and
interference into account. Simplified bounds are obtained and used to optimize
the device activation probability and pilot length. A performance analysis
indicates how performance scales as a function of the number of antennas and
the transmission slot duration
Interference Mitigation in Frequency Hopping Ad Hoc Networks
Radio systems today exhibit a degree of flexibility that was unheard of only a few years ago. Software-defined radio architectures have emerged that are able to service large swathes of spectrum, covering up to several GHz in the UHF bands. This dissertation investigates interference mitigation techniques in frequency hopping ad hoc networks that are capable of exploiting the frequency agility of software-defined radio platforms
Exact solution for the dynamical decoupling of a qubit with telegraph noise
We study the dissipative dynamics of a qubit that is afflicted by classical
random telegraph noise and it is subject to dynamical decoupling. We derive
exact formulas for the qubit dynamics at arbitrary working points in the limit
of infinitely strong control pulses (bang-bang control) and we investigate in
great detail the efficiency of the dynamical decoupling techniques both for
Gaussian and non-Gaussian (slow) noise at qubit pure dephasing and at optimal
point. We demonstrate that control sequences can be successfully implemented as
diagnostic tools to infer spectral proprieties of a few fluctuators interacting
with the qubit. The analysis is extended in order to include the effect of
noise in the pulses and we give upper bounds on the noise levels that can be
tolerated in the pulses while still achieving efficient dynamical decoupling
performance
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