3,409 research outputs found
Quantum repeaters with imperfect memories: cost and scalability
Memory dephasing and its impact on the rate of entanglement generation in
quantum repeaters is addressed. For systems that rely on probabilistic schemes
for entanglement distribution and connection, we estimate the maximum
achievable rate per employed memory for our optimized partial nesting protocol.
We show that, for any given distance , the polynomial scaling of rate with
distance can only be achieved if quantum memories with coherence times on the
order of or longer, with being the speed of light in the channel, are
available. The above rate degrades as a power of
with distance when the coherence time .Comment: Extended version with 5 figure
Fuzzy Logic Control of Adaptive ARQ for Video Distribution over a Bluetooth Wireless Link
Bluetooth's default automatic repeat request (ARQ) scheme is not suited to video distribution resulting in missed display and decoded deadlines. Adaptive ARQ with active discard of expired packets from the send buffer is an alternative approach. However, even with the addition of cross-layer adaptation to picture-type packet importance, ARQ is not ideal in conditions of a deteriorating RF channel. The paper presents fuzzy logic control of ARQ, based on send buffer fullness and the head-of-line packet's deadline. The advantage of the fuzzy logic approach, which also scales its output according to picture type importance, is that the impact of delay can be directly introduced to the model, causing retransmissions to be reduced compared to all other schemes. The scheme considers both the delay constraints of the video stream and at the same time avoids send buffer overflow. Tests explore a variety of Bluetooth send buffer sizes and channel conditions. For adverse channel conditions and buffer size, the tests show an improvement of at least 4 dB in video quality compared to nonfuzzy schemes. The scheme can be applied to any codec with I-, P-, and (possibly) B-slices by inspection of packet headers without the need for encoder intervention.</jats:p
Quantum Key Distribution over Probabilistic Quantum Repeaters
A feasible route towards implementing long-distance quantum key distribution
(QKD) systems relies on probabilistic schemes for entanglement distribution and
swapping as proposed in the work of Duan, Lukin, Cirac, and Zoller (DLCZ)
[Nature 414, 413 (2001)]. Here, we calculate the conditional throughput and
fidelity of entanglement for DLCZ quantum repeaters, by accounting for the DLCZ
self-purification property, in the presence of multiple excitations in the
ensemble memories as well as loss and other sources of inefficiency in the
channel and measurement modules. We then use our results to find the generation
rate of secure key bits for QKD systems that rely on DLCZ quantum repeaters. We
compare the key generation rate per logical memory employed in the two cases of
with and without a repeater node. We find the cross-over distance beyond which
the repeater system outperforms the non-repeater one. That provides us with the
optimum inter-node distancing in quantum repeater systems. We also find the
optimal excitation probability at which the QKD rate peaks. Such an optimum
probability, in most regimes of interest, is insensitive to the total distance.Comment: 12 pages, 6 figures; Fig. 5(a) is replace
Power-Constrained Fuzzy Logic Control of Video Streaming over a Wireless Interconnect
Wireless communication of video, with Bluetooth as an example, represents a compromise between channel conditions, display and decode deadlines, and energy constraints. This paper proposes fuzzy logic control (FLC) of automatic repeat request (ARQ) as a way of reconciling these factors, with a 40% saving in power in the worst channel conditions from economizing on transmissions when channel errors occur. Whatever the channel conditions are, FLC is shown to outperform the default Bluetooth scheme and an alternative Bluetooth-adaptive ARQ scheme in terms of reduced packet loss and delay, as well as improved video quality
Restricted Discrete Invariance and Self-Synchronization For Stable Walking of Bipedal Robots
Models of bipedal locomotion are hybrid, with a continuous component often
generated by a Lagrangian plus actuators, and a discrete component where leg
transfer takes place. The discrete component typically consists of a locally
embedded co-dimension one submanifold in the continuous state space of the
robot, called the switching surface, and a reset map that provides a new
initial condition when a solution of the continuous component intersects the
switching surface. The aim of this paper is to identify a low-dimensional
submanifold of the switching surface, which, when it can be rendered invariant
by the closed-loop dynamics, leads to asymptotically stable periodic gaits. The
paper begins this process by studying the well-known 3D Linear Inverted
Pendulum (LIP) model, where analytical results are much easier to obtain. A key
contribution here is the notion of \textit{self-synchronization}, which refers
to the periods of the pendular motions in the sagittal and frontal planes
tending to a common period. The notion of invariance resulting from the study
of the 3D LIP model is then extended to a 9-DOF 3D biped. A numerical study is
performed to illustrate that asymptotically stable walking may be obtained.Comment: Conferenc
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