601 research outputs found
A balanced homodyne detector for high-rate Gaussian-modulated coherent-state quantum key distribution
We discuss excess noise contributions of a practical balanced homodyne
detector in Gaussian-modulated coherent-state (GMCS) quantum key distribution
(QKD). We point out the key generated from the original realistic model of GMCS
QKD may not be secure. In our refined realistic model, we take into account
excess noise due to the finite bandwidth of the homodyne detector and the
fluctuation of the local oscillator. A high speed balanced homodyne detector
suitable for GMCS QKD in the telecommunication wavelength region is built and
experimentally tested. The 3dB bandwidth of the balanced homodyne detector is
found to be 104MHz and its electronic noise level is 13dB below the shot noise
at a local oscillator level of 8.5*10^8 photon per pulse. The secure key rate
of a GMCS QKD experiment with this homodyne detector is expected to reach
Mbits/s over a few kilometers.Comment: 22 pages, 11 figure
Security proof of a three-state quantum key distribution protocol without rotational symmetry
Standard security proofs of quantum key distribution (QKD) protocols often
rely on symmetry arguments. In this paper, we prove the security of a
three-state protocol that does not possess rotational symmetry. The three-state
QKD protocol we consider involves three qubit states, where the first two
states, |0_z> and |1_z>, can contribute to key generation and the third state,
|+>=(|0_z>+|1_z>)/\sqrt{2}, is for channel estimation. This protocol has been
proposed and implemented experimentally in some frequency-based QKD systems
where the three states can be prepared easily. Thus, by founding on the
security of this three-state protocol, we prove that these QKD schemes are, in
fact, unconditionally secure against any attacks allowed by quantum mechanics.
The main task in our proof is to upper bound the phase error rate of the qubits
given the bit error rates observed. Unconditional security can then be proved
not only for the ideal case of a single-photon source and perfect detectors,
but also for the realistic case of a phase-randomized weak coherent light
source and imperfect threshold detectors. Our result on the phase error rate
upper bound is independent of the loss in the channel. Also, we compare the
three-state protocol with the BB84 protocol. For the single-photon source case,
our result proves that the BB84 protocol strictly tolerates a higher quantum
bit error rate than the three-state protocol; while for the coherent-source
case, the BB84 protocol achieves a higher key generation rate and secure
distance than the three-state protocol when a decoy-state method is used.Comment: 10 pages, 3 figures, 2 column
Quantum Hacking: Experimental demonstration of time-shift attack against practical quantum key distribution systems
Quantum key distribution (QKD) systems can send signals over more than 100 km
standard optical fiber and are widely believed to be secure. Here, we show
experimentally for the first time a technologically feasible attack, namely the
time-shift attack, against a commercial QKD system. Our result shows that,
contrary to popular belief, an eavesdropper, Eve, has a non-negligible
probability (~4%) to break the security of the system. Eve's success is due to
the well-known detection efficiency loophole in the experimental testing of
Bell inequalities. Therefore, the detection efficiency loophole plays a key
role not only in fundamental physics, but also in technological applications
such as QKD.Comment: 5 pages, 3 figures. Substantially revised versio
The Relationship between Ischemic Stroke Patients with and without Retroflex Tongue: A Retrospective Study
Background. Patients suffering from stroke exhibit different levels of capability in retroflex tongues, in our clinical observation. This study aims to derive the association of tongue retroflexibility with the degree of severity for stroke patients. Methods. All ischemic stroke patients were collected from August 2010 to July 2013 in the Stroke Center, Changhua Christian Hospital, Taiwan. All participants underwent medical history collection and clinical examination, including tongue images captured by ATDS. Statistical analysis was performed to compare the differences of ischemic stroke patients with and without retroflex tongue. Result. Among the total of 308 cases collected, 123 patients cannot retroflex their tongues, that is, the non-RT group. The length of stay in the non-RT group, 32.0 ± 21.5, was longer than those of the RT counterparts, 25.9 ± 14.4 (p value: 0.007). The NIHSS on admission, 14.1 ± 7.8 versus 8.9 ± 5.2, was higher and the Barthel Index upon admission, 18.6 ± 20.7 and 35.0 ± 24.2, was lower for the non-RT patients than that of the RT counterparts. Also, the non-RT patients account for 60.2% and 75.6% for Barthel Index ≤ 17 and NIHSS ≥ 9, respectively. Conclusion. The stroke patients in non-RT group showed significantly poor prognosis and were more serious in the degree of severity and level of autonomy than RT group, indicating that the ability to maneuver tongue retroflex can serve as a simple, reliable, and noninvasive means for the prognosis of ischemic stroke patients
Vacuum ultraviolet photoabsorption spectra of nitrile ices for their identification on Pluto
Icy bodies, such as Pluto, are known to harbor simple and complex molecules. The recent New Horizons flyby of Pluto has revealed a complex surface composed of bright and dark ice surfaces, indicating a rich chemistry based on nitrogen (N2), methane (CH4), and carbon monoxide (CO). Nitrile (CN) containing molecules such as acetonitrile (CH3CN), propionitrile (CH3CH2CN), butyronitrile (CH3CH2CH2CN), and isobutyronitrile ((CH3)2CHCN) are some of the nitrile molecules that are known to be synthesized by radiative processing of such simple ices. Through the provision of a spectral atlas for such compounds we propose that such nitriles may be identified from the ALICE payload on board New Horizons</i
Preliminary Surgical Results of Single-Incision Transumbilical Laparoscopic Bariatric Surgery
Feasibility of quantum key distribution through dense wavelength division multiplexing network
In this paper, we study the feasibility of conducting quantum key
distribution (QKD) together with classical communication through the same
optical fiber by employing dense-wavelength-division-multiplexing (DWDM)
technology at telecom wavelength. The impact of the classical channels to the
quantum channel has been investigated for both QKD based on single photon
detection and QKD based on homodyne detection. Our studies show that the latter
can tolerate a much higher level of contamination from the classical channels
than the former. This is because the local oscillator used in the homodyne
detector acts as a "mode selector" which can suppress noise photons
effectively. We have performed simulations based on both the decoy BB84 QKD
protocol and the Gaussian modulated coherent state (GMCS) QKD protocol. While
the former cannot tolerate even one classical channel (with a power of 0dBm),
the latter can be multiplexed with 38 classical channels (0dBm power each
channel) and still has a secure distance around 10km. Preliminary experiment
has been conducted based on a 100MHz bandwidth homodyne detector.Comment: 18 pages, 5 figure
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