12,650 research outputs found
Quantum dense coding over Bloch channels
Dynamics of coded information over Bloch channels is investigated for
different values of the channel's parameters. We show that, the suppressing of
the travelling coded information over Bloch channel can be increased by
decreasing the equilibrium absolute value of information carrier and
consequently decreasing the distilled information by eavesdropper. The amount
of decoded information can be improved by increasing the equilibrium values of
the two qubits and decreasing the ratio between longitudinal and transverse
relaxation times. The robustness of coded information in maximum and partial
entangled states is discussed. It is shown that the maximum entangled states
are more robust than the partial entangled state over this type of channels
Experimental Demonstration of a Synthetic Lorentz Force by Using Radiation Pressure
Synthetic magnetism in cold atomic gases opened the doors to many exciting
novel physical systems and phenomena. Ubiquitous are the methods used for the
creation of synthetic magnetic fields. They include rapidly rotating
Bose-Einstein condensates employing the analogy between the Coriolis and the
Lorentz force, and laser-atom interactions employing the analogy between the
Berry phase and the Aharonov-Bohm phase. Interestingly, radiation pressure -
being one of the most common forces induced by light - has not yet been used
for synthetic magnetism. We experimentally demonstrate a synthetic Lorentz
force, based on the radiation pressure and the Doppler effect, by observing the
centre-of-mass motion of a cold atomic cloud. The force is perpendicular to the
velocity of the cold atomic cloud, and zero for the cloud at rest. Our novel
concept is straightforward to implement in a large volume, for a broad range of
velocities, and can be extended to different geometries.Comment: are welcom
Synthetic Lorentz force in classical atomic gases via Doppler effect and radiation pressure
We theoretically predict a novel type of synthetic Lorentz force for
classical (cold) atomic gases, which is based on the Doppler effect and
radiation pressure. A fairly uniform and strong force can be constructed for
gases in macroscopic volumes of several cubic millimeters and more. This opens
the possibility to mimic classical charged gases in magnetic fields, such as
those in a tokamak, in cold atom experiments.Comment: are welcom
Magnetic structures of Mn3-xFexSn2: an experimental and theoretical study
We investigate the magnetic structure of Mn3-xFexSn2 using neutron powder
diffraction experiments and electronic structure calculations. These alloys
crystallize in the orthorhombic Ni3Sn2 type of structure (Pnma) and comprise
two inequivalent sites for the transition metal atoms (4c and 8d) and two Sn
sites (4c and 4c). The neutron data show that the substituting Fe atoms
predominantly occupy the 4c transition metal site and carry a lower magnetic
moment than Mn atoms. Four kinds of magnetic structures are encountered as a
function of temperature and composition: two simple ferromagnetic structures
(with the magnetic moments pointing along the b or c axis) and two canted
ferromagnetic arrangements (with the ferromagnetic component pointing along the
b or c axis). Electronic structure calculations results agree well with the
low-temperature experimental magnetic moments and canting angles throughout the
series. Comparisons between collinear and non-collinear computations show that
the canted state is stabilized by a band mechanism through the opening of a
hybridization gap. Synchrotron powder diffraction experiments on Mn3Sn2 reveal
a weak monoclinic distortion at low temperature (90.08 deg at 175 K). This
lowering of symmetry could explain the stabilization of the c-axis canted
ferromagnetic structure, which mixes two orthorhombic magnetic space groups, a
circumstance that would otherwise require unusually large high-order terms in
the spin Hamiltonian.Comment: 11 pages, 13 figure
Optimization of Short Coherent Control Pulses
The coherent control of small quantum system is considered. For a two-level
system coupled to an arbitrary bath we consider a pulse of finite duration. We
derive the leading and the next-leading order corrections to the evolution
operator due to the non-commutation of the pulse and the bath Hamiltonian. The
conditions are computed that make the leading corrections vanish. The pulse
shapes optimized in this way are given for and pulses.Comment: 9 pages, 6 figures; published versio
Correlation Functions and Spin
The k-electron correlation function of a free chaotic electron beam is
derived with the spin degree of freedom taken into account. It is shown that it
can be expressed with the help of correlation functions for a polarized
electron beam of all orders up to k and the degree of spin polarization. The
form of the correlation function suggests that if the electron beam is not
highly polarized, observing multi-particle correlations should be difficult.
The result can be applied also to chaotic photon beams, the degree of spin
polarization being replaced by the degree of polarization.Comment: 6 pages, 1 eps figure, accepted to Phys. Rev.
- …