In recent years the interest in the manipulation of quantum systems has furthered new strategies for maintaining
their coherence, continuously threatened by unwanted and uncontrollable interactions with the environment.
Photons interact weakly with the surroundings. Even so decoherence may significantly affect their polarization
state during the propagation within dispersive media because of the unavoidable presence of more than a single
frequency in the envelope of the photon pulse. Here we report on a suppression of the polarization decoherence in
a ring cavity obtained by properly retooling for the photon qubit the "bang-bang" protection technique already
employed for nuclear spins and nuclear-quadrupole qubits. Our results show that bang-bang control can be
profitably extended to all quantum information processes involving flying polarization qubits.
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 propose to use a linear array of singly trapped electrons to implement a spin chain for quantum communication.
The effective spin-spin interaction is realized by means of a magnetic field gradient, which couples the
electron spin to the motional degrees of freedom. Then the Coulomb repulsion between the particles transmits
this coupling throughout the array. The resulting system can be described in terms of a Heisenberg model with
long-range interactions showing a dipolar decay. We estimate the fidelity of the system in reproducing an ideal
spin chain by taking into account the influence of the electron spatial motion.
One-way quantum channels play a fundamental role in the security of the communication between two distant parties, in particular within the frame of "quantum key distribution". Nevertheless quite recently it has been introduced the possibility of using two-way quantum channels for the same purpose. Although the first attempts in this direction did not feature any particular advantage with respect to the one-way counterpart some recent results obtained by our group suggest that this new class of protocols provides higher thresholds of security.
We apply the quantum locking scheme recently proposed by Courty <i>et al.</i> [Phys. Rev. Lett. <b>90</b>, 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.