Download QIM 2013 Verifying Entanglement poster final

Measuring Entanglement by Attempting to
Undo it with Ultra-Broadband Bi-Photons
Yaakov Shaked, Roey Pomeranz and Avi Pe’er
Department of Physics and BINA Center for Nano-technology, Bar-Ilan University, Ramat-Gan 52900, Israel
[email protected]
Abstract:
We demonstrate a unique source of ultra broadband bi-photons with a correlation time of 10-14sec, emitting an ultra-high flux of
>1012 time-energy entangled photon-pairs per second. Using a pairwise “Mach-Zehnder” interferometer we fully characterize
the quantum state of the bi-photons (amplitude and phase), and show that the interference visibility is a direct measure for the
bi-photons purity. We support our results with a theoretical model, which fully reconstructs the observed square root
dependence of the fringe visibility on internal loss.
Pairwise interference of bi-photons (CCD image)
Interpretation
Concept
(a)
st
1 Crystal
Spectral nd
2 Crystal
Phase
Pump
DC
(b)
Pump
Pump
BS1
0
BS2
DC
f(w)
(a) Bi-photons produced in a first non-linear (NL)
crystal via spontaneous parametric down conversion
(SPDC) enter an identical second NL crystal together
with the original pump, and are either enhanced by
further down conversion (DC), or up-converted back
to the pump, depending on the relative phase between
the pump and the bi-photons. Since the two
possibilities for bi-photon generation, in the first or
second NL crystal, are indistinguishable, their
probability
amplitudes
interfere
quantum
mechanically. Thus, the setup is analogous to a MachZehnder interferometer (b) for bi-photons, where the
crystals represent two-photon beam splitters that
couple the pump and DC fields.
Results
Two calibrated fringe
spectra with a p phase
shift between them
(blue
and
dotted
green lines), and the
corresponding
calculated
spectral
phase j(w)/2 of the biphotons (red line).
ArXiv:1209.4194 [physics.optics]
Conceptually, the second nonlinear
medium serves as a physical
detector of entanglement, where
the existence of an entangled pair
is detected by attempting to
annihilate it. Since this operation
affects bi-photons, only, the fringe
contrast is a direct measure of the
bi-photon
purity,
thereby
providing a method to measure
entanglement by attempting to
undo it.
By attenuating the bi-photon (and
pump) field between the two
crystals we reduce the single
photon flux linearly, but the
”surviving”
bi-photon
flux
quadratically, thereby reducing the
quantum bi-photon state purity
and
reducing
the
pairwise
interference visibility. We compare
this scenario with attenuation of
the two fields before the first
crystal reducing the bi-photon flux
but not the quantum bi-photon
state purity, and indeed the
interference visibility remains
constant.
Experimental layout
Bi-photons are generated in the 1st crystal (12mm
long PPKTP), pumped by a single-frequency diode
laser at 880 nm with up to 1 W power. Reflection of
the bi-photons from mirrors M2 and M3 is
accompanied by a spectral phase shift. Both biphotons and pump enter the 2nd identical crystal.
The resulting bi-photons spectrum is measured by a
home-built spectrometer composed of a prism (SF11)
and a CCD IR camera. The last mirror (M4) separates
the pump from DC light, allowing a pump power
measurement. Attenuation is achieved either before
the 1st crystal by a half-wave plate and polarizer
beam splitter, or between the two crystals with a
polarizer P1, and a polarizer P2 in front of the camera.
The inset shows a measured intensity spectrum of the
ultra-broadband bi-photons after the first crystal.
Interference
visibility Vs. loss
Measured interference
visibility as a function
of attenuation with
corresponding
theoretical fits before
the
first
crystal
(squares + dotted fit)
and between crystals
(circles +
solid fit).