Limits to improvements?

Alexei S. Trifonov

Liqiang Liu, Haidong Yuan

In this article we study the effect of feedback control on the improvement of the precision limit in quantum parameter estimation. In particular we investigate the case with a qubit in a magnetic field at the presence of spontaneous emission, and by detecting the emitted photons we design a feedback control to improve the precision limit of estimating the magnitude of the magnetic field. We show that the feedback control can improve the precision limit.

Yakov Borisovich Zeldovich

Duger Ulam-Orgikh, Masahiro Kitagawa

It is known that a partially entangled state gives improved sensitivity in generalized Ramsey spectroscopy in the presence of decoherence, whereas a maximally entangled state does not. However, it has been an open question whether the known decoherence limit in the improvement is achievable. We show that every spin-squeezed state possesses pairwise entanglement, thereby improving the spectroscopic sensitivity, and that even suboptimal entanglement in its easiest implementation suffices to asymptotically reach the decoherence limit without any measurement optimization.

Richard E. Slusher, Bernard Yurke

Coherent lightwave communications systems are approaching a limit where the error rates and channel capacities are limited by the quantum properties of light. This is often referred to as the shot-noise limit. If ideal laser light is used in the system, there is no way to avoid this limit. However, new states of the light field called squeezed states have recently been developed that allow an improvement in error rates below the shot-noise limit. Squeezed light concepts and recent experiments are reviewed with emphasis on aspects important to coherent communications. It is shown that channel capacity can be improved using squeezed light by only a factor of two. Larger improvements are in principle possible for error rates, e.g. a factor of three reduction in the number of required photons per bit for a 10/sup -9/ bit error rate. An example of a recent high-performance system is described where optical losses and electronic noise reduce the improvement expected using squeezed light to the 10-20% level. It is concluded that squeezed light only offers significant improvement in bit error rates for very-high-efficiency systems.>

R. Shankar