304 research outputs found
Effects of anisotropy on optimal dense coding
We study optimal dense coding with thermal entangled states of a two-qubit
anisotropic \emph{XXZ} model and a Heisenberg model with Dzyaloshinski-Moriya
(DM) interactions. The DM interaction is another kind of anisotropic
antisymmetric exchange interaction. The effects of these two kinds of
anisotropies on dense coding are studied in detail for both the
antiferromagnetic and ferromagnetic cases. For the two models, we give the
conditions that the parameters of the models have to satisfy for a valid dense
coding. We also found that even though there is entanglement, it is unavailable
for our optimal dense coding, which is the same as entanglement teleportation.Comment: Accepted by physica script
Reiterated Commemoration: Hiroshima as National Trauma *
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75748/1/j.1467-9558.2006.00295.x.pd
Detecting the inseparability and distillability of continuous variable states in Fock space
The partial transposition(PT) operation is an effecient tool in detecting the
inseparability of a mixed state. We give an explicit formula for the PT
operation for the continuous variable states in Fock space. We then give the
necessary and sufficient condition for the positivity of Gaussian operators.
Based on this, a number of creterions on the inseparability and distillability
for the multimode Gaussian states are naturally drawn. We finally give an
explicit formula for the state in a subspace of a global Gaussian state. This
formula, together with the known results for Gaussian states, gives the
criterions for the inseparability and distillability in a subspace of the
global Gaussian state.Comment: 8 pages, no figure, some typing errors correcte
Renormalized Bosonic Interaction of Excitons
An effective bosonic Hamiltonian of excitons with ``spin'' degrees of
freedom in two dimension is obtained through a projection procedure, starting
from a conventional electron-hole Hamiltonian . We first
demonstrate that a straightforward transformation of into a
Hamiltonian of bosonic excitons does not give the two-body interaction between
an ``up-spin'' exciton and a ``down-spin'' exciton, which are created by the
left- and right-circularly polarized light beams, respectively. We then show
that this interaction is generated through a projection procedure onto the
subspace spanned by excitons, as a renormalization effect coming from
higher exciton states. The projection also renormalizes the interaction between
excitons with the same spins by a large amount. These renormalization
effects are crucial for the polarization dependence of the optical responses
from semiconductors. The present theory gives the microscopic foundation of the
phenomenology that was successfully applied to the analysis of four-wave mixing
experiments in GaAs quantum wells strongly coupled to the radiation field in a
high-Q micro cavity.Comment: 2 figure
Gaussian quantum marginal problem
The quantum marginal problem asks what local spectra are consistent with a
given spectrum of a joint state of a composite quantum system. This setting,
also referred to as the question of the compatibility of local spectra, has
several applications in quantum information theory. Here, we introduce the
analogue of this statement for Gaussian states for any number of modes, and
solve it in generality, for pure and mixed states, both concerning necessary
and sufficient conditions. Formally, our result can be viewed as an analogue of
the Sing-Thompson Theorem (respectively Horn's Lemma), characterizing the
relationship between main diagonal elements and singular values of a complex
matrix: We find necessary and sufficient conditions for vectors (d1, ..., dn)
and (c1, ..., cn) to be the symplectic eigenvalues and symplectic main diagonal
elements of a strictly positive real matrix, respectively. More physically
speaking, this result determines what local temperatures or entropies are
consistent with a pure or mixed Gaussian state of several modes. We find that
this result implies a solution to the problem of sharing of entanglement in
pure Gaussian states and allows for estimating the global entropy of
non-Gaussian states based on local measurements. Implications to the actual
preparation of multi-mode continuous-variable entangled states are discussed.
We compare the findings with the marginal problem for qubits, the solution of
which for pure states has a strikingly similar and in fact simple form.Comment: 18 pages, 1 figure, material added, references updated, except from
figure identical with version to appear in Commun. Math. Phy
Security Analysis of an Untrusted Source for Quantum Key Distribution: Passive Approach
We present a passive approach to the security analysis of quantum key
distribution (QKD) with an untrusted source. A complete proof of its
unconditional security is also presented. This scheme has significant
advantages in real-life implementations as it does not require fast optical
switching or a quantum random number generator. The essential idea is to use a
beam splitter to split each input pulse. We show that we can characterize the
source using a cross-estimate technique without active routing of each pulse.
We have derived analytical expressions for the passive estimation scheme.
Moreover, using simulations, we have considered four real-life imperfections:
Additional loss introduced by the "plug & play" structure, inefficiency of the
intensity monitor, noise of the intensity monitor, and statistical fluctuation
introduced by finite data size. Our simulation results show that the passive
estimate of an untrusted source remains useful in practice, despite these four
imperfections. Also, we have performed preliminary experiments, confirming the
utility of our proposal in real-life applications. Our proposal makes it
possible to implement the "plug & play" QKD with the security guaranteed, while
keeping the implementation practical.Comment: 35 pages, 19 figures. Published Versio
Alpha-fetoprotein-producing primary lung carcinoma: A case report
Alpha-fetoprotein (AFP)-producing lung adenocarcinoma is a rare type of lung cancer, with its characteristics not yet fully clarified. We recently encountered a case of this type of lung cancer. The patient was a 69-year-old man who consulted an internist with the chief complaint of epigastric pain. Chest X-ray and CT revealed a lobulated mass measuring 70 mm in diameter in the right lower lung field and a metastasis in the right hilar lymph nodes. Of the tumor markers, the serum AFP was elevated (4620 ng/ml), and the serum carcinoembryonic antigen and carbohydrate antigen 19-9 were also slightly elevated. Transbronchial lung biopsy revealed the diagnosis of lung cancer. Under thoracoscopic assistance, right lower lobectomy + mediastinal lymph node dissection was carried out. Immunostaining showed the tumor cells to be AFP-positive. The tumor was thus diagnosed as an AFP-producing lung adenocarcinoma. The patient followed an uneventful clinical course after the surgery, with serum AFP decreasing to the normal range by about 2 weeks after the surgery. As of this writing, no sign of tumor recurrence has been noted. This case is presented here with a review of the literature
Semiconductor-cavity QED in high-Q regimes: Detuning effect
The non-resonant interaction between the high-density excitons in a quantum
well and a single mode cavity field is investigated. An analytical expression
for the physical spectrum of the excitons is obtained. The spectral properties
of the excitons, which are initially prepared in the number states or the
superposed states of the two different number states by the resonant
femtosecond pulse pumping experiment, are studied. Numerical study of the
physical spectrum is carried out and a discussion of the detuning effect is
presented.Comment: 7 pages, 8 figure
- …