25 research outputs found
Using entanglement against noise in quantum metrology
We analyze the role of entanglement among probes and with external ancillas
in quantum metrology. In the absence of noise, it is known that unentangled
sequential strategies can achieve the same Heisenberg scaling of entangled
strategies and that external ancillas are useless. This changes in the presence
of noise: here we prove that entangled strategies can have higher precision
than unentangled ones and that the addition of passive external ancillas can
also increase the precision. We analyze some specific noise models and use the
results to conjecture a general hierarchy for quantum metrology strategies in
the presence of noise.Comment: 7 pages, 4 figures, published versio
Fundamental quantum interferometry bound for the squeezed-light-enhanced gravitational-wave detector GEO600
The fundamental quantum interferometry bound limits the sensitivity of an
interferometer for a given total rate of photons and for a given decoherence
rate inside the measurement device.We theoretically show that the recently
reported quantum-noise limited sensitivity of the squeezed-light-enhanced
gravitational-wave detector GEO600 is exceedingly close to this bound, given
the present amount of optical loss. Furthermore, our result proves that the
employed combination of a bright coherent state and a squeezed vacuum state is
generally the optimum practical approach for phase estimation with high
precision on absolute scales. Based on our analysis we conclude that neither
the application of Fock states nor N00N states or any other sophisticated
nonclassical quantum states would have yielded an appreciably higher
quantum-noise limited sensitivity.Comment: 5 pages, 4 figure
Bayesian quantum frequency estimation in presence of collective dephasing
We advocate a Bayesian approach to optimal quantum frequency estimation - an
important issue for future quantum enhanced atomic clock operation. The
approach provides a clear insight into the interplay between decoherence and
the extent of the prior knowledge in determining the optimal interrogation
times and optimal estimation strategies. We propose a general framework capable
of describing local oscillator noise as well as additional collective atomic
dephasing effects. For a Gaussian noise the average Bayesian cost can be
expressed using the quantum Fisher information and thus we establish a direct
link between the two, often competing, approaches to quantum estimation theoryComment: 15 pages, 3 figure