19 research outputs found
Quantum enhanced positioning and clock synchronization
A wide variety of positioning and ranging procedures are based on repeatedly
sending electromagnetic pulses through space and measuring their time of
arrival. This paper shows that quantum entanglement and squeezing can be
employed to overcome the classical power/bandwidth limits on these procedures,
enhancing their accuracy. Frequency entangled pulses could be used to construct
quantum positioning systems (QPS), to perform clock synchronization, or to do
ranging (quantum radar): all of these techniques exhibit a similar enhancement
compared with analogous protocols that use classical light. Quantum
entanglement and squeezing have been exploited in the context of
interferometry, frequency measurements, lithography, and algorithms. Here, the
problem of positioning a party (say Alice) with respect to a fixed array of
reference points will be analyzed.Comment: 4 pages, 2 figures. Accepted for publication by Natur
Optical codeword demodulation with error rates below standard quantum limit using a conditional nulling receiver
The quantum states of two laser pulses---coherent states---are never mutually
orthogonal, making perfect discrimination impossible. Even so, coherent states
can achieve the ultimate quantum limit for capacity of a classical channel, the
Holevo capacity. Attaining this requires the receiver to make joint-detection
measurements on long codeword blocks, optical implementations of which remain
unknown. We report the first experimental demonstration of a joint-detection
receiver, demodulating quaternary pulse-position-modulation (PPM) codewords at
a word error rate of up to 40% (2.2 dB) below that attained with
direct-detection, the largest error-rate improvement over the standard quantum
limit reported to date. This is accomplished with a conditional nulling
receiver, which uses optimized-amplitude coherent pulse nulling, single photon
detection and quantum feedforward. We further show how this translates into
coding complexity improvements for practical PPM systems, such as in deep-space
communication. We anticipate our experiment to motivate future work towards
building Holevo-capacity-achieving joint-detection receivers.Comment: 6 pages, 4 figure