COL 11(9), 090101(2013) CHINESE OPTICS LETTERS September 10, 2013
Scintillation discriminator improves free-space
quantum key distribution
Feng Tang (
///
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) and Bing Zhu (
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XXX
)
∗
Department of Electronic Engineering and Information Science,
University of Science and Technology of China, Hefei 230026, China
∗
Corresponding author: zbing@ustc.edu.cn
Received March 26, 2013; accepted June 28, 2013; posted online September 3, 2013
We present an analysis of the impact of afluctuating-loss channel on free-space quantum key distribution
(QKD). Considering the characteristics of the fluctuating-loss channel, a scintillation discriminator that
acts according to the information of instant channel loss is proposed to help improve the performance of
a free-space QKD system, which suffers from the influence of atmospheric turbulence. Theoretical and
numerical results show that this discriminator is a useful tool for increasing secure key rates, especially for
long-range free-space QKD.
OCIS codes: 010.1330, 060.5565, 270.5568.
doi: 10.3788/COL201311.090101.
In rec e nt years, quantum key distribution (QKD) has at-
tracted significant research attention because it offers
absolute security based on the fundamental laws of
physics
[1,2]
. QKD is currently the most promising imple-
mentation method based on quantum information the-
ory. Two types of QKD implementations are availa ble
according to the quantum transmission medium used:
QKD based on optical fibers and free-space QKD. Fo r
free-space links and long-range free-spac e QKD between
orbit satellites and ground stations, quantum signals are
subject to various environmental distortions. Thus, the
channel characteristics of the atmosphere are important
considerations fo r a practical free-space QKD sys tem.
When a laser beam propagates through the earth’s
atmosphere, it e ncounters a variety of deleterious effects
releva nt to phase and amplitude distortions due to ran-
dom refractive-index variations resulting from atmo-
spheric turbulence
[3]
. These disturbances cause insta-
bility of atmospheric losses and permanent fluctuation in
turbulence channels, which are referred to as fluctuating-
loss channels
[4]
. The impact of turbulence-induced scin-
tillation usually plays an important role in nois e in optical
communications
[5]
, wherein deep, long-lived scintillation
fades, ultimately limiting the system performance in
high-reliability operations. Shapiro
[6]
demonstrated that
the related noise arising from propagation through at-
mospheric turbulence does not significantly affect the
sift and error probabilities of a free-space QKD system
that employs weak coherent pulse s with decoy states. In
other words, almost no difference is observed be tween
a static channel and a fluctuating-loss channel so long
as the average channel loss remains the same
[7]
, since
fluctuations in quantum bit error rate (QB ER) induced
by variations in channel loss are averaged over time. As
only the average QBER is applied to the QKD system,
it has virtually no influence on the proc e ss of generat-
ing a final secure key. Erven et al.
[8]
showed that the
total secret key generated in a QKD sy stem can be sig-
nificantly increased by throwing away data blo cks where
the signal-to-noise ratio (SNR) is lower than a certain
threshold. In their work, atmospheric turbulence fluctu-
ations in free-space links were measured using entangled
photons. Capraro et al.
[9]
also suggested that the SNR
of a quantum signal can be improved by probing the
transmission of the channel by means of a classical signal
and acquiring only single-photon signals when the in-
stantaneous transmission of the channel is ab ove a given
threshold. Both studies indicate that the exploitation of
turbulence can be used as an improvement technique for
free-space QKD.
In this letter, unlike previous key generatio n calcula-
tions presented in Refs. [7,8], the fluctuating QBER is
directly addressed to the resulting secure key rate, which
is the pr incipal figure-of-merit ofa QK D system, before
averaging. Such fluctuations are useful for improving the
key rate of a free-space QKD system. A sc intillation-
based key data grouping scheme that takes advantage of
the fluctuating-loss channel is then proposed. The rest
of this letter is organized as follows: firstly, we calculate
the secure key rate of a free-space QKD system under
a fluctuating-loss channel. The scintillation-based key
data grouping scheme, which introduces a discriminator
acting according to the information of channel loss to
the key process , is then described to increase the secure
key rate of QKD. Finally, the efficiency of the pro posed
scheme is evaluated in a virtual optical scintillation sce-
nario of 20 km by employing the numerical simulation
method o f multiple phase screens.
As shown in Fig. 1, an additional reference link utiliz-
ing periodic bright light pulses is employed to per form
channel loss monitoring. The instantaneous atmospheric
information obtained from this classical link is used to
Fig. 1. (Color online) Schematic of QKD system with an ad-
ditional reference link.
1671-7694/2013/090101(4) 090101-1
c
2013 Chinese Optics Letters
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