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Scientific Report 2007-2009
Condensed matter physics and biophysics
C45. Experiments on Foundations of Quantum Mechanics
Einstein, Podolsky, and Rosen (EPR) showed that
quantum mechanics cannot be simultaneously local, real,
and complete, and were convinced that quantum theory
should satisfy the reasonable assumption of locality and
reality. Therefore, they concluded that quantum mechanics
is incomplete. In 1964, John Bell discovered his
famous inequality ruling out the possibility to introduce
Local Hidden Variables (LHV). In his proof, he demonstrated
that any LHV model cannot explain the statistical
correlations present in two-qubit (i.e., a quantum
two-level system) entangled states. The huge amount of
experimental data obtained so far by Bells nonlocality
tests, in particular with photons, confirms the quantum
mechanical predictions and leads to the common belief
that quantum mechanics cannot be simultaneously local
and real. Indeed Bells inequality is nowadays exploited
as a tool to detect entanglement in quantum cryptography
and in quantum computation.
Figure 1: Generation of the six-qubit linear cluster state.
a) Scheme of the entangled two-photon six-qubit parametric
source: a UV laser beam (wavelength λ p) impinges on
the Type I BBO crystal after reflection on a small mirror. b)
Spatial superposition between the left (l) and right (r) modes
on the common 50/50 beam splitter BS 1 . ) Spatial superposition
between the internal I (a 2 , a 3 , b 2 , b 3 ) and external
E (a 1 , a 4 , b 1 , b 4 ) modes is performed on BS 2A and BS 2B for
the A and B photon, respectively.
Not so long ago, it was thought that the magnitude
of the violation of local realism, defined as the ratio between
the quantum prediction and the classical bound,
decreases as the size (i.e., number n of particles and/or
the number N of internal degrees of freedom) grows. Now
it is clear that the ratio between the quantum prediction
and the classical bound can grow as 2 (n−1)/2 in the
case of n-qubit systems. Mermins observation that the
magnitude of the violation of local realism, defined as
the ratio between the quantum prediction and the classical
bound, can grow exponentially with the size of the
system has been demonstrated using two-photon hyperentangled
states entangled in polarization and path degrees
of freedom, and local measurements of polarization
and path simultaneously. Latter we reported on
the experimental realization of a four-qubit linear cluster
state via two photons entangled both in polarization
and linear momentum. By use of this state we carried
out a novel nonlocality proof, the so-called stronger
two observer all-versus-nothing test of quantum nonlocality.
Then we created a six-qubit linear cluster state
by transforming a two-photon hyperentangled state in
which three qubits are encoded in each particle, one in
the polarization and two in the linear momentum degrees
of freedom. For this state, we demonstrate genuine sixqubit
entanglement, persistency of entanglement against
the loss of qubits, and higher violation than in previous
experiments on Bell inequalities of the Mermin type [2].
Figure 2: Generation of the microscopic-macroscopic state
for non-locality tests in the multi-photon domain.
In 1981 N. Herbert proposed a gedanken experiment
in order to achieve by the First Laser-Amplified Superluminal
Hookup (FLASH) a faster-than-light (FTL)
communication by quantum nonlocality. In Ref.[3]
we reported the first experimental realization of that
proposal by the optical parametric amplification of a
single photon belonging to an entangled EPR pair into
an output field involving a large number of photons.
A theoretical and experimental analysis explains in
general and conclusive terms the precise reasons for
the failure of the FLASH program as well as of any
similar FTL proposals. As following step a macrostate
consisting of N = 10 3 photons in a quantum superposition
and entangled with a far apart single-photon
state (microstate) has been generated. Precisely, an
entangled photon pair is created by a nonlinear optical
process; then one photon of the pair is injected into
an optical parametric amplifier operating for any input
polarization state, i.e., into a phase-covariant cloning
machine. Such transformation establishes a connection
between the single photon and the multiparticle
fields. We demonstrated the nonseparability of the bipartite
system by adopting a local filtering technique [4].
References
1. G. Vallone et al., Phys. Rev. Lett. 98, 180502 (2007).
2. R. Ceccarelli et al. , Phys. Rev. Lett. 103, 160401 (2009)
3. T. De Angelis et al., Phys. Rev. Lett. 99, 193601 (2007).
4. F. De Martini et al., Phys. Rev. Lett. 100, 253601 (2008).
Authors
P. Mataloni, F. Sciarrino, F. De Martini, G. Vallone 4 , N.
Spagnolo, C. Vitelli, S. Giacomini, G. Milani
http://quantumoptics.phys.uniroma1.it/
Sapienza Università di Roma 98 Dipartimento di Fisica