Recently, we have shown the advantages of two-way quantum communications in continuous variable quantum
cryptography. Thanks to this new approach, two honest users can achieve a non-trivial security enhancement as
long as the Gaussian interactions of an eavesdropper are independent and identical. In this work, we consider asymmetric strategies where the Gaussian interactions can be different and classically correlated. For several attacks of this kind, we prove that the enhancement of security still holds when the two-way protocols are used in direct reconciliation.
We consider an optical cavity made by two moving mirrors and driven by an intense classical laser field. We
determine the steady state of he optomechanical system and show that two vibrational modes of the mirrors,
with effective mass of the order of micrograms, can be entangled thanks to the effect of radiation pressure. The
resulting entanglement is however quite fragile with respect to temperature.
We apply the quantum locking scheme recently proposed by Courty et al. [Phys. Rev. Lett. 90, 083601 (2003)]
for the reduction of back action noise to the realistic case of a gravitational wave interferometer. We show that by applying an active control to each mirror of the interferometer it is possible to improve significatively its sensitivity by reducing the radiation pressure noise.
We study the possibility to reveal a weak coherent force acting on a movable mirror (probe) by coupling it to a radiation field (meter) in a cavityless configuration. We determine the sensitivity of such a model and we show that the use of entangled meter state greatly improves the ultimate detection limit. A comparison of the presented model with that involving optical cavity is also done.
We present a detailed study of how phase-sensitive feedback schemes
can be used to improve the performance of optomechanical devices.
Considering the case of a cavity mode coupled to an oscillating mirror by the radiation pressure, we show how feedback can be used to reduce the position noise spectrum of the mirror, cool it to its quantum ground state, or achieve position squeezing. Then, we show that even though feedback is not able to improve the sensitivity of stationary position spectral measurements, it is possible to design a
nonstationary strategy able to increase this sensitivity.
We predict the appearance of purely quantum effects within a radiation field upon reflection on a movable mirror. The model of an optical cavity having an oscillating end mirror is employed, and the role of thermal noise associated to this mechanical motion is studied.
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