Experimental study on the Gaussian-modulated coherent-state quantum key distribution over standard telecommunication fibers

Physical Review A - Atomic, Molecular, and Optical Physics

Volumn 76, Issue 5, 2007, Pages

  Qi, Bing ^a   Huang, Lei Lei ^a   Qian, Li ^a   Lo, Hoi Kwong ^a  

^a UNIVERSITY OF TORONTO   (Canada)

Author keywords

[No Author keywords available]

Indexed keywords

ACOUSTIC VARIABLES CONTROL; GAUSSIAN NOISE (ELECTRONIC); LIGHT MODULATION; POLARIZATION; SIGNAL ANALYSIS; TELECOMMUNICATION SYSTEMS;

GAUSSIAN-MODULATED COHERENT-STATE (GMCS); MACH-ZEHNDER INTERFEROMETER (MZI); NOISE ANALYSIS; QUANTUM SIGNAL;

QUANTUM CRYPTOGRAPHY;

EID: 36448962010     PISSN: 10502947     EISSN: 10941622     Source Type: Journal    
DOI: 10.1103/PhysRevA.76.052323     Document Type: Article

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28. and corresponds to 0 =0.1). In GMCS QKD, Bob's measurement result of X quadrature becomes X′ =Xcos 0 +Psin 0. If 0 1 and its change during the time of one frame transmission (40 ms in our experiment) is negligible, the excess noise contributed by the phase drift can be estimated by (X′ -X) 2 P2 02 = VA 02. With a modulation variance of VA =20, a 0.1 phase drift will result in an excess noise of 0.2. Note that the secure bound (of excess noise) for a reverse reconciliation protocol is around 0.5. An excess noise of 0.2 would lower the secure key rate dramatically. In our system, numerical simulation shows that the secure key rate would drop from 0.3 bit pulse to 0.1 bit pulse.
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32. In this semiclassical picture, a coherent laser pulse can be treated as a classical electromagnetic wave plus the vacuum noise. Since we are estimating the excess noise (the noise above vacuum noise) here, we can treat the leakage LE classically.
33. In BB84 QKD with a perfect single photon source, the secure key rate is given by R= 1 2 Q1 [1-f (e1) H2 (e1) - H2 (e1)]. Here Q1 is the overall gain. e1 is the QBER. f (x) is the bidirectional error correction efficiency and H2 (x) is the binary entropy function. Assuming e1 =3% and f (e1) =1.22, a 10% error on determining e1 will result in a 3% change of the secure key rate, which is tolerable. In decoy QKD, the equation for calculating the secure key rate is more complicated. Nevertheless, as we showed in Ref. moderate errors on determining system parameters are acceptable.
34. This is illustrated in Fig. 4. Note the similarity between Fig. 4 (experimental results) and Fig. 4 (simulation result under the assumption of no excess noise).