1,183 research outputs found
Experimental demonstration of quantum teleportation of a squeezed state
We demonstrate an unconditional high-fidelity teleporter capable of preserving the broadband entanglement in an optical squeezed state. In particular, we teleport a squeezed state of light and observe -0.8 ± 0.2dB of squeezing in the teleported (output) state. We show that the squeezing criterion translates directly into a sufficient criterion for entanglement of the upper and lower sidebands of the optical field. Thus, this result demonstrates the first unconditional teleportation of broadband entanglement. Our teleporter achieves sufficiently high fidelity to allow the teleportation to be cascaded, enabling, in principle, the construction of deterministic non-Gaussian operations
Inequivalent Quantizations of the N = 3 Calogero model with Scale and Mirror-S_3 Symmetry
We study the inequivalent quantizations of the N = 3 Calogero model by
separation of variables, in which the model decomposes into the angular and the
radial parts. Our inequivalent quantizations respect the ` mirror-S_3\rq\
invariance (which realizes the symmetry under the cyclic permutations of the
particles) and the scale invariance in the limit of vanishing harmonic
potential. We find a two-parameter family of novel quantizations in the angular
part and classify the eigenstates in terms of the irreducible representations
of the S_3 group. The scale invariance restricts the quantization in the radial
part uniquely, except for the eigenstates coupled to the lowest two angular
levels for which two types of boundary conditions are allowed independently
from all upper levels. It is also found that the eigenvalues corresponding to
the singlet representations of the S_3 are universal (parameter-independent) in
the family, whereas those corresponding to the doublets of the S_3 are
dependent on one of the parameters. These properties are shown to be a
consequence of the spectral preserving SU(2) (or its subrgoup U(1))
transformations allowed in the family of inequivalent quantizations.Comment: 24 pages, LaTe
Sequential Quantum Teleportation of Optical Coherent States
We demonstrate a sequence of two quantum teleportations of optical coherent
states, combining two high-fidelity teleporters for continuous variables. In
our experiment, the individual teleportation fidelities are evaluated as F_1 =
0.70 \pm 0.02 and F_2 = 0.75 \pm 0.02, while the fidelity between the input and
the sequentially teleported states is determined as F^{(2)} = 0.57 \pm 0.02.
This still exceeds the optimal fidelity of one half for classical teleportation
of arbitrary coherent states and almost attains the value of the first
(unsequential) quantum teleportation experiment with optical coherent states.Comment: 5page, 4figure
Theory of tunneling spectroscopy of normal metal/ferromagnet/spin-triplet superconductor junctions
We study the tunneling conductance of a ballistic normal metal / ferromagnet
/ spin-triplet superconductor junction using the extended
Blonder-Tinkham-Klapwijk formalism as a model for a -axis oriented Au /
SrRuO / SrRuO junction. We compare chiral -wave (CPW) and
helical -wave (HPW) pair potentials, combined with ferromagnet magnetization
directions parallel and perpendicular to the interface. For fixed ,
where is a direction of magnetization in the ferromagnet measured
from the -axis, the tunneling conductance of CPW and HPW clearly show
different voltage dependencies. It is found that the cases where the -vector
is perpendicular to the magnetization direction (CPW with
and HPW with ) are identical. The obtained results serve as a
guide to determine the pairing symmetry of the spin-triplet superconductor
SrRuO.Comment: 12 pages, 7 figures. There is also a supplementary (not uploaded
Extremely strong-coupling superconductivity and anomalous lattice properties in the beta-pyrochlore oxide KOs2O6
Superconducting and normal-state properties of the beta-pyrochlore oxide
KOs2O6 are studied by means of thermodynamic and transport measurements. It is
shown that the superconductivity is of conventional s-wave type and lies in the
extremely strong-coupling regime. Specific heat and resistivity measurements
reveal that there are characteristic low-energy phonons that give rise to
unusual scattering of carriers due to strong electron-phonon interactions. The
entity of the low-energy phonons is ascribed to the heavy rattling of the K ion
confined in an oversized cage made of OsO6 octahedra. It is suggested that this
electron-rattler coupling mediates the Cooper pairing, resulting in the
extremely strong-coupling superconductivity.Comment: 17 pages (only 4 pages included here. go to
http://hiroi.issp.u-tokyo.ac.jp/Published%20papers/K-SC6.pdf for full pages),
to be published in PR
All-optical generation of states for "Encoding a qubit in an oscillator"
Both discrete and continuous systems can be used to encode quantum
information. Most quantum computation schemes propose encoding qubits in
two-level systems, such as a two-level atom or an electron spin. Others exploit
the use of an infinite-dimensional system, such as a harmonic oscillator. In
"Encoding a qubit in an oscillator" [Phys. Rev. A 64 012310 (2001)], Gottesman,
Kitaev, and Preskill (GKP) combined these approaches when they proposed a
fault-tolerant quantum computation scheme in which a qubit is encoded in the
continuous position and momentum degrees of freedom of an oscillator. One
advantage of this scheme is that it can be performed by use of relatively
simple linear optical devices, squeezing, and homodyne detection. However, we
lack a practical method to prepare the initial GKP states. Here we propose the
generation of an approximate GKP state by using superpositions of optical
coherent states (sometimes called "Schr\"odinger cat states"), squeezing,
linear optical devices, and homodyne detection.Comment: 4 pages, 3 figures. Submitted to Optics Letter
Optical implementation and entanglement distribution in Gaussian valence bond states
We study Gaussian valence bond states of continuous variable systems,
obtained as the outputs of projection operations from an ancillary space of M
infinitely entangled bonds connecting neighboring sites, applied at each of
sites of an harmonic chain. The entanglement distribution in Gaussian valence
bond states can be controlled by varying the input amount of entanglement
engineered in a (2M+1)-mode Gaussian state known as the building block, which
is isomorphic to the projector applied at a given site. We show how this
mechanism can be interpreted in terms of multiple entanglement swapping from
the chain of ancillary bonds, through the building blocks. We provide optical
schemes to produce bisymmetric three-mode Gaussian building blocks (which
correspond to a single bond, M=1), and study the entanglement structure in the
output Gaussian valence bond states. The usefulness of such states for quantum
communication protocols with continuous variables, like telecloning and
teleportation networks, is finally discussed.Comment: 15 pages, 6 figures. To appear in Optics and Spectroscopy, special
issue for ICQO'2006 (Minsk). This preprint contains extra material with
respect to the journal versio
Experimental demonstration of quantum teleportation of a squeezed state
Quantum teleportation of a squeezed state is demonstrated experimentally. Due
to some inevitable losses in experiments, a squeezed vacuum necessarily becomes
a mixed state which is no longer a minimum uncertainty state. We establish an
operational method of evaluation for quantum teleportation of such a state
using fidelity, and discuss the classical limit for the state. The measured
fidelity for the input state is 0.85 0.05 which is higher than the
classical case of 0.730.04. We also verify that the teleportation process
operates properly for the nonclassical state input and its squeezed variance is
certainly transferred through the process. We observe the smaller variance of
the teleported squeezed state than that for the vacuum state input.Comment: 7 pages, 1 new figure, comments adde
Experimental Demonstration of Macroscopic Quantum Coherence in Gaussian States
We witness experimentally the presence of macroscopic coherence in Gaussian
quantum states using a recently proposed criterion (E.G. Cavalcanti and M.
Reid, Phys. Rev. Lett. 97, 170405 (2006)). The macroscopic coherence stems from
interference between macroscopically distinct states in phase space and we
prove experimentally that even the vacuum state contains these features with a
distance in phase space of shot noise units (SNU). For squeezed
states we found macroscopic superpositions with a distance of up to
SNU. The proof of macroscopic quantum coherence was investigated
with respect to squeezing and purity of the states.Comment: 5 pages, 6 figure
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