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Frontiers in Optics

Decoy-state method for N-Channel SCM FC-QKD

Sudeshna Bhattacharya, Pradeep K Kumar

Published Jan 1, 2012• 0 citations
Publisher

Abstract

We propose decoy-state method for N-channel SCM FC-QKD implementing B92 protocol. Carrier ratio optimization improves key length and secure transmission distance over traditional methods by a factor of 5 and 40 km with N=3.
DOI
10.1364/fio.2012.fw3a.52

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We propose a decoy-state method to increase key generation rate of frequency-coded (FC) implementation of B92 quantum key distribution (QKD) protocol. Direct application of decoy-state method to FC-QKD results in a security loophole. We overcome this loophole by optimizing the ratio of decoy-to-signal carrier photon number to unity. Simulation results for a single-channel FC-QKD show 16 kbits of raw key length at a transmission distance of 70 km. We then extend the decoy-state method to subcarrier-multiplexed FC-QKD. We show that key generation rate and secure link length improves by a factor of 5 and 40 km, respectively, compared to single-carrier with decoy-state method.
In practical quantum key distribution (QKD), weak coherent states as the photon source have a limit in the secure key rate and transmission distance because of the existence of multi-photon pulses and heavy loss in transmission line. The decoy-state method and the nonorthogonal encoding protocol are two important methods to combat these effects. Here, we combine both the methods and propose an efficient method that can substantially improve the performance of QKD. We find a 78-km increase over the prior record using the decoy-state method and a 123-km increase over the result of the SARG04 protocol in transmission distance.
We propose a decoy-pulse protocol for frequency-coded implementation of B92 quantum key distribution protocol. A direct extension of decoy-pulse method to frequency-coding scheme results in security loss as an eavesdropper can distinguish between signal and decoy pulses by measuring the carrier photon number without affecting other statistics. We overcome this problem by optimizing the ratio of carrier photon number of decoy-to-signal pulse to be as close to unity as possible. In our method the switching between signal and decoy pulses is achieved by changing the amplitude of RF signal as opposed to modulating the intensity of optical signal thus reducing system cost. Our simulation results show a key generation rate of 1.5×10-4/pulse for link lengths up to 70 km. Finally, we discuss the optimum value of average photon number of signal pulse for a given key rate while also optimizing the carrier ratio.
Decoy state quantum key distribution (QKD), being capable of beating PNS attack and being unconditionally secure, has become attractive recently. However, in many QKD systems, disturbances of transmission channel make the quantum bit error rate (QBER) increase, which limits both security distance and key bit rate of real-world decoy state QKD systems. We demonstrate the two-intensity decoy QKD with a one-way Faraday-Michelson phase modulation system, which is free of channel disturbance and keeps an interference fringe visibility (99%) long period, over a 120km single mode optical fibre in telecom (1550 nm) wavelength. This is the longest distance fibre decoy state QKD system based on the two-intensity protocol.
This paper simulated the operation of vacuum state and single decoy state protocols in the BB84 and SARG04 QKD schemes by utilizing the features of the commercial ID-3000 QKD system. Numerical modeling identified an optimal signal-to-decoy state ratio of 0.95:0.05 and an intensity of μ=0.85 for the signal state and ν1=0.05 for the decoy state, ensuring the highest key generation rate and a secure distance of up to 50 km. These protocols were validated experimentally over various transmission distances with standard telecom fiber, using the ID-3000 QKD system in a conventional bi-directional plug-and-play setup. Simulations predicted secure key rates of 1.2 × 10 5 bits/s for SARG04 and 8.5 × 104 bits/s for BB84 at 10 km, with secure distances of 45 km and 35 km, respectively. The experimental results confirmed these predictions, showing a 30% higher key rate and 20% longer secure distance compared to non-decoy methods. The SARG04 protocol surpassed BB84 in key rate and secure distance, highlighting the two-photon component's role in key generation. This study concludes that the decoy-state method significantly enhances key generation rates and secure distances, optimizing QKD protocols for secure quantum communication.
Decoy states have recently been proposed as a useful method for substantially improving the performance of quantum key distribution (QKD). Here, we present a general theory of the decoy state protocol based on only two decoy states and one signal state. We perform optimization on the choice of intensities of the two decoy states and the signal state. Our result shows that a decoy state protocol with only two types of decoy states---the vacuum and a weak decoy state---asymptotically approaches the theoretical limit of the most general type of decoy state protocol (with an infinite number of decoy states). We also present a one-decoy-state protocol. Moreover, we provide estimations on the effects of statistical fluctuations and suggest that, even for long-distance (larger than 100 km) QKD, our two-decoy-state protocol can be implemented with only a few hours of experimental data. In conclusion, decoy state quantum key distribution is highly practical.