14 research outputs found
Photon exchange and entanglement formation during the transmission through a rectangular quantum barrier
When a quantum particle traverses a rectangular potential created by a
quantum field both photon exchange and entanglement between particle and field
take place. We present analytic results for the transition amplitudes of any
possible photon exchange processes for an incoming plane wave and initial Fock,
thermal and coherent field states. We show that for coherent field states the
entanglement correlates the particle's position to the photon number in the
field instead of the particle's energy as usual. Besides entanglement
formation, remarkable differences to the classical field treatment also appear
with respect to the symmetry between photon emission and absorption, resonance
effects and if the field initially occupies the vacuum state.Comment: 6 pages (double column), 6 figure
Spin geometry of entangled qubits under bilocal decoherence modes
The Lindblad generators of the master equation define which kind of
decoherence happens in an open quantum system. We are working with a two qubit
system and choose the generators to be projection operators on the eigenstates
of the system and unitary bilocal rotations of them. The resulting decoherence
modes are studied in detail. Besides the general solutions we investigate the
special case of maximally entangled states - the Bell singlet states. The
results are depicted in the so-called spin geometry picture which allows to
illustrate the evolution of the (nonlocal) correlations stored in a certain
state. The question for which conditions the path traced out in the geometric
picture depends only on the relative angle between the bilocal rotations is
addressed.Comment: 12 pages, 3 figure
Energy entanglement in neutron interferometry
Entanglement between degrees of freedom, namely between the spin, path and
(total) energy degrees of freedom, for single neutrons is exploited. We
implemented a triply entangled Greenberger-Horne-Zeilinger(GHZ)-like state and
coherently manipulated relative phases of two-level quantum subsystems. An
inequality derived by Mermin was applied to analyze the generated GHZ-like
state: we determined the four expectation values and finally obtained M=2.558
+/- 0.004 which is clearly above the threshold of 2. This demonstrates the
violation of a Mermin-like inequality for triply entangled GHZ-like state in a
single-particle system, which, in turn, exhibits a clear inconsistency between
noncontextual assumptions and quantum mechanics and confirms quantum
contextuality.Comment: 4 pages, 3 figure
Engineering of triply entangled states in a single-neutron system
We implemented a triply entangled Greenberger-Horne-Zeilinger(GHZ)-like state
and coherently manipulated the spin, path, and energy degrees of freedom in a
single neutron system. The GHZ-like state was analyzed with an inequality
derived by Mermin: we determined the four expectation values and finally
obtained M = 2.558 +/- 0.004 > 2, which exhibits a clear violation of the
noncontextual assumption and confirms quantum contextuality.Comment: 4 pages, 2figure
Decoherence modes of entangled qubits within neutron interferometry
We study two different decoherence modes for entangled qubits by considering
a Liouville - von Neumann master equation. Mode A is determined by projection
operators onto the eigenstates of the Hamiltonian and mode B by projectors onto
rotated states. We present solutions for general and for Bell diagonal states
and calculate for the later the mixedness and the amount of entanglement given
by the concurrence.
We propose a realization of the decoherence modes within neutron
interferometry by applying fluctuating magnetic fields. An experimental test of
the Kraus operator decomposition describing the evolution of the system for
each mode is presented.Comment: 15 pages, 5 figure
Kochen-Specker theorem studied with neutron interferometer
The Kochen-Specker theorem theoretically shows evidence of the
incompatibility of noncontextual hidden variable theories with quantum
mechanics. Quantum contextuality is a more general concept than quantum
non-locality which is quite well tested in experiments by using Bell
inequalities. Within neutron interferometry we performed an experimental test
of the Kochen-Specker theorem with an inequality, which identifies quantum
contextuality, by using spin-path entanglement in a single neutron system. Here
entanglement is achieved not between different particles, but between degrees
of freedom, i.e., between spin and path degree of freedom. Appropriate
combinations of the spin analysis and the position of the phase shifter allow
an experimental verification of the violation of an inequality of the
Kochen-Specker theorem. The observed value of (2.291 +/- 0.008), which is above
the threshold of 1, clearly shows that quantum mechanical predictions cannot be
reproduced by noncontextual hidden variable theories.Comment: 5 pages, 3 figure
Berry phase in entangled systems: a proposed experiment with single neutrons
The influence of the geometric phase, in particular the Berry phase, on an
entangled spin-1/2 system is studied. We discuss in detail the case, where the
geometric phase is generated only by one part of the Hilbert space. We are able
to cancel the effects of the dynamical phase by using the ``spin-echo'' method.
We analyze how the Berry phase affects the Bell angles and the maximal
violation of a Bell inequality. Furthermore we suggest an experimental
realization of our setup within neutron interferometry.Comment: 10 pages, 6 figures, Introduction extended, References adde
Decoherence of entangled kaons and its connection to entanglement measures
We study the time evolution of the entangled kaon system by considering the
Liouville - von Neumann equation with an additional term which allows for
decoherence. We choose as generators of decoherence the projectors to the
2-particle eigenstates of the Hamiltonian. Then we compare this model with the
data of the CPLEAR experiment and find in this way an upper bound on the
strength of the decoherence. We also relate to an effective
decoherence parameter considered previously in literature. Finally we
discuss our model in the light of different measures of entanglement, i.e. the
von Neumann entropy , the entanglement of formation and the concurrence
, and we relate the decoherence parameter to the loss of
entanglement: .Comment: comments and references added, 18 pages, 1 figur