32 research outputs found
Towards loophole-free Bell inequality test with preselected unsymmetrical singlet states of light
Can a Bell test with no detection loophole be demonstrated for multi-photon
entangled states of light within the current technology? We examine the
possibility of a postselection-free CHSH-Bell inequality test wih an
unsymmetrical polarization singlet. To that end we employ a preselection
procedure which is performed prior to the test. It allows using imperfect
(coarse-grained) binary photodetection in the test. We show an example of
preselection scheme which improves violation of the CHSH inequality with the
micro-macro polarization singlet produced by the optimal quantum cloning. The
preselection is realized by a quantum filter which is believed to be not useful
for this purpose.Comment: 12 pages, 8 figures, accepted to Phys. Rev.
Filtering of the absolute value of photon-number difference for two-mode macroscopic quantum superpositions
We discuss a device capable of filtering out two-mode states of light with
mode populations differing by more than a certain threshold, while not
revealing which mode is more populated. It would allow engineering of
macroscopic quantum states of light in a way which is preserving specific
superpositions. As a result, it would enhance optical phase estimation with
these states as well as distinguishability of "macroscopic" qubits. We propose
an optical scheme, which is a relatively simple, albeit non-ideal, operational
implementation of such a filter. It uses tapping of the original polarization
two-mode field, with a polarization neutral beam splitter of low reflectivity.
Next, the reflected beams are suitably interfered on a polarizing beam
splitter. It is oriented such that it selects unbiased polarization modes with
respect to the original ones. The more an incoming two-mode Fock state is
unequally populated, the more the polarizing beam splitter output modes are
equally populated. This effect is especially pronounced for highly populated
states. Additionally, for such states we expect strong population correlations
between the original fields and the tapped one. Thus, after a photon-number
measurement of the polarizing beam splitter outputs, a feed-forward loop can be
used to let through a shutter the field, which was transmitted by the tapping
beam splitter. This happens only if the counts at the outputs are roughly
equal. In such a case, the transmitted field differs strongly in occupation
number of the two modes, while information on which mode is more populated is
non-existent (a necessary condition for preserving superpositions).Comment: 11 pages, 12 figure
Homodyne detection for atmosphere channels
We give a systematic theoretical description of homodyne detection in the
case where both the signal and the local oscillator pass through the turbulent
atmosphere. Imperfect knowledge of the local-oscillator amplitude is
effectively included in a noisy density operator, leading to postprocessing
noise. Alternatively, we propose a technique with monitored transmission
coefficient of the atmosphere, which is free of postprocessing noise.Comment: 9 pages, 5 figure