520 research outputs found
Virtual noiseless amplification and Gaussian post-selection in continuous-variable quantum key distribution
The noiseless amplification or attenuation are two heralded filtering
operations that enable respectively to increase or decrease the mean field of
any quantum state of light with no added noise, at the cost of a small success
probability. We show that inserting such noiseless operations in a transmission
line improves the performance of continuous-variable quantum key distribution
over this line. Remarkably, these noiseless operations do not need to be
physically implemented but can simply be simulated in the data post-processing
stage. Hence, virtual noiseless amplification or attenuation amounts to perform
a Gaussian post-selection, which enhances the secure range or tolerable excess
noise while keeping the benefits of Gaussian security proofs.Comment: 8 pages, 5 figure
Quantum Communication with an Accelerated Partner
An unsolved problem in relativistic quantum information research is how to
model efficient, directional quantum communication between localised parties in
a fully quantum field theoretical framework. We propose a tractable approach to
this problem based on solving the Heisenberg evolution of localized field
observables. We illustrate our approach by analysing, and obtaining approximate
analytical solutions to, the problem of communicating coherent states between
an inertial sender, Alice and an accelerated receiver, Rob. We use these
results to determine the efficiency with which continuous variable quantum key
distribution could be carried out over such a communication channel.Comment: Additional explanatory text and typo in Eq.17 correcte
Gaussian Post-selection for Continuous Variable Quantum Cryptography
We extend the security proof for continuous variable quantum key distribution
protocols using post selection to account for arbitrary eavesdropping attacks
by employing the concept of an equivalent protocol where the post-selection is
implemented as a series of quantum operations including a virtual distillation.
We introduce a particular `Gaussian' post selection and demonstrate that the
security can be calculated using only experimentally accessible quantities.
Finally we explicitly evaluate the performance for the case of a noisy Gaussian
channel in the limit of unbounded key length and find improvements over all
pre-existing continuous variable protocols in realistic regimes.Comment: 4+4 pages. arXiv admin note: substantial text overlap with
arXiv:1106.082
Secure Coherent-state Quantum Key Distribution Protocols with Efficient Reconciliation
We study the equivalence between a realistic quantum key distribution
protocol using coherent states and homodyne detection and a formal entanglement
purification protocol. Maximally-entangled qubit pairs that one can extract in
the formal protocol correspond to secret key bits in the realistic protocol.
More specifically, we define a qubit encoding scheme that allows the formal
protocol to produce more than one entangled qubit pair per coherent state, or
equivalently for the realistic protocol, more than one secret key bit. The
entanglement parameters are estimated using quantum tomography. We analyze the
properties of the encoding scheme and investigate its application to the
important case of the attenuation channel.Comment: REVTeX, 11 pages, 2 figure
Continuous variable quantum cryptography using coherent states
We propose several methods for quantum key distribution (QKD) based upon the
generation and transmission of random distributions of coherent or squeezed
states, and we show that they are are secure against individual eavesdropping
attacks. These protocols require that the transmission of the optical line
between Alice and Bob is larger than 50 %, but they do not rely on
"non-classical" features such as squeezing. Their security is a direct
consequence of the no-cloning theorem, that limits the signal to noise ratio of
possible quantum measurements on the transmission line. Our approach can also
be used for evaluating various QKD protocols using light with gaussian
statistics.Comment: 5 pages, 1 figure. In v2 minor rewriting for clarity, references
adde
Quantum cryptography using balanced homodyne detection
We report an experimental quantum key distribution that utilizes balanced
homodyne detection, instead of photon counting, to detect weak pulses of
coherent light. Although our scheme inherently has a finite error rate, it
allows high-efficiency detection and quantum state measurement of the
transmitted light using only conventional devices at room temperature. When the
average photon number was 0.1, an error rate of 0.08 and "effective" quantum
efficiency of 0.76 were obtained.Comment: Errors in the sentence citing ref.[20] are correcte
Enhancing single-molecule photostability by optical feedback from quantum-jump detection
We report an optical technique that yields an enhancement of single-molecule
photostability, by greatly suppressing photobleaching pathways which involve
photoexcitation from the triplet state. This is accomplished by dynamically
switching off the excitation laser when a quantum-jump of the molecule to the
triplet state is optically detected. This procedure leads to a lengthened
single-molecule observation time and an increased total number of detected
photons. The resulting improvement in photostability unambiguously confirms the
importance of photoexcitation from the triplet state in photobleaching
dynamics, and may allow the investigation of new phenomena at the
single-molecule level
Surface-induced charge state conversion of nitrogen-vacancy defects in nanodiamonds
We present a study of the charge state conversion of single nitrogen-vacancy
(NV) defects hosted in nanodiamonds (NDs). We first show that the proportion of
negatively-charged NV defects, with respect to its neutral counterpart
NV, decreases with the size of the ND. We then propose a simple model
based on a layer of electron traps located at the ND surface which is in good
agreement with the recorded statistics. By using thermal oxidation to remove
the shell of amorphous carbon around the NDs, we demonstrate a significant
increase of the proportion of NV defects in 10-nm NDs. These results are
invaluable for further understanding, control and use of the unique properties
of negatively-charged NV defects in diamondComment: 6 pages, 4 figure
Experimental investigation of continuous variable quantum teleportation
We report the experimental demonstration of quantum teleportation of the
quadrature amplitudes of a light field. Our experiment was stably locked for
long periods, and was analyzed in terms of fidelity, F; and with signal
transfer, T_{q}=T^{+}+T^{-}, and noise correlation, V_{q}=V_{in|out}^{+}
V_{in|out}^{-}. We observed an optimum fidelity of 0.64 +/- 0.02, T_{q}= 1.06
+/- 0.02 and V_{q} =0.96 +/- 0.10. We discuss the significance of both T_{q}>1
and V_{q}<1 and their relation to the teleportation no-cloning limit.Comment: 4 pages, 4 figure
Non-linear and quantum optics of a type II OPO containing a birefringent element Part 2 : bright entangled beams generation
We describe theoretically the quantum properties of atype-II Optical
Parametric Oscillator containing a birefringent plate which induces a linear
coupling between the orthogonally polarized signal and idler beams and results
in phase locking between these two beams. As in a classical OPO, the signal and
idler waves show large quantum correlations which can be measured
experimentally due to the phase locking between the two beams. We study the
influence of the waveplate on the various criteria characterizing quantum
correlations. We show in particular that the quantum correlations can be
maximized by using optimized quadratures.Comment: to be published in Eur. Phys. J.
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