47 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.
Generation of Kerr non-Gaussian motional states of trapped ions
Non-Gaussian states represent a powerful resource for quantum information
protocols in the continuous variables regime. Cat states, in particular, have
been produced in the motional degree of freedom of trapped ions by controlled
displacements dependent on the ionic internal state. An alternative method
harnesses the Kerr nonlinearity naturally existent in this kind of system. We
present detailed calculations confirming its feasibility for typical
experimental conditions. Additionally, this method permits the generation of
complex non-Gaussian states with negative Wigner functions. Especially,
superpositions of many coherent states are achieved at a fraction of the time
necessary to produce the cat state.Comment: 6 pages, 5 figure
Witnessing Entanglement with Second-Order Interference
Second-order interference and Hanbury-Brown and Twiss type experiments can
provide an operational framework for the construction of witness operators that
can test classical and nonclassical properties of a Gaussian squeezed state
(GSS), and provide entanglement witness operators to study the separability
properties of correlated Gaussian squeezed sates.Comment: 10 pages, 12 figure
Quantum leap: how to complete a quantum walk in a single step
Quantum walks provide simple models of various fundamental processes. It is pivotal to know when the dynamics underlying a walk lead to quantum advantages just by examining its statistics. A walk with many indistinguishable particles and measurements of non-classical multi-particle correlations is likely to reveal the quantum nature. The number of elements in a setup realizing walks grows with their length or spread . We introduce the concept of a quantum leap, a process which can be achieved with fewer or complementary resources and which in a single step simulates another long process. The process and its leap are described by the same Hamiltonian but, the latter parametrizes the evolution with a tunable parameter of a setup. In the case of walks, a leap immediately gives a probability distribution which results only after many steps. This may be appealing for simulation of processes which are lengthy or require dynamical control. We discuss a leap based on the multi-particle Hong--Ou--Mandel interference, an inherently quantum phenomenon. It reproduces a quantum walk enabling perfect state transfer through spin chains. It requires a beam splitter, two detectors and particles to mimic a walk on a chain of size , for time fixed by beam-splitter's reflectivity. Our results apply to a broad class of systems where the HOM-like effects can be observed, and may constitute a new approach to simulation of complex Hamiltonians with passive interferometers
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
Witnessing Entanglement of EPR States With Second-Order Interference
The separability of the continuous-variable EPR state can be tested with
Hanbury-Brown and Twiss type interference. The second-order visibility of such
interference can provide an experimental test of entanglement. It is shown that
time-resolved interference leads to the Hong, Ou and Mandel deep, that provides
a signature of quantum non-separability for pure and mixed EPR states. A
Hanbury-Brown and Twiss type witness operator can be constructed to test the
quantum nature of the EPR entanglement.Comment: 9 pages, 5 figure
Violation of Bell's inequality using classical measurements and non-linear local operations
We find that Bell's inequality can be significantly violated (up to
Tsirelson's bound) with two-mode entangled coherent states using only homodyne
measurements. This requires Kerr nonlinear interactions for local operations on
the entangled coherent states. Our example is a demonstration of
Bell-inequality violations using classical measurements. We conclude that
entangled coherent states with coherent amplitudes as small as 0.842 are
sufficient to produce such violations.Comment: 6 pages, 5 figures, to be published in Phys. Rev.