23 research outputs found
Time-resolved homodyne characterization of individual quadrature-entangled pulses
We describe a simple and efficient setup to generate and characterize
femtosecond quadrature-entangled pulses. Quantum correlations equivalent to
about 2.5 dB squeezing are efficiently and easily reached using the
non-degenerate parametric amplification of femtosecond pulses through a
single-pass in a thin (0.1 mm) potassium niobate crystal. The entangled pulses
are then individually sampled to characterize the non-separability and the
entropy of formation of the states. The complete experiment is analysed in the
time-domain, from the pulsed source of quadrature entanglement to the
time-resolved homodyne detection. This particularity allows for applications in
quantum communication protocols using continuous-variable entanglement.Comment: 7 pages, 5 figure
Robustness of raw quantum tomography
We scrutinize the effects of non-ideal data acquisition on the homodyne
tomograms of photon quantum states. The presence of a weight function,
schematizing the effects of the finite thickness of the probing beam or
equivalently noise, only affects the state reconstruction procedure by a
normalization constant. The results are extended to a discrete mesh and show
that quantum tomography is robust under incomplete and approximate knowledge of
tomograms.Comment: 7 pages, 1 figure, published versio
Entanglement purification of unknown quantum states
A concern has been expressed that ``the Jaynes principle can produce fake
entanglement'' [R. Horodecki et al., Phys. Rev. A {\bf 59}, 1799 (1999)]. In
this paper we discuss the general problem of distilling maximally entangled
states from copies of a bipartite quantum system about which only partial
information is known, for instance in the form of a given expectation value. We
point out that there is indeed a problem with applying the Jaynes principle of
maximum entropy to more than one copy of a system, but the nature of this
problem is classical and was discussed extensively by Jaynes. Under the
additional assumption that the state of the copies of the
quantum system is exchangeable, one can write down a simple general expression
for . We show how to modify two standard entanglement purification
protocols, one-way hashing and recurrence, so that they can be applied to
exchangeable states. We thus give an explicit algorithm for distilling
entanglement from an unknown or partially known quantum state.Comment: 20 pages RevTeX 3.0 + 1 figure (encapsulated Postscript) Submitted to
Physical Review
Image processing as state reconstruction in optics
The image reconstruction of partially coherent light is interpreted as the
quantum state reconstruction. The efficient method based on maximum-likelihood
estimation is proposed to acquire information from registered intensity
measurements affected by noise. The connection with totally incoherent image
restoration is pointed out. The feasibility of the method is demonstrated
numerically. Spatial and correlation details significantly smaller than the
diffraction limit are revealed in the reconstructed pattern.Comment: 10 pages, 5 figure