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Optics Letters

Quantum key distribution over a fiber-optic channel by means of pulse position modulation

Moshe Nazarathy

Published Jun 15, 2005• 12 citations
Publisher

Abstract

New methods are proposed for extending the range of fiber-optic one-way quantum key distribution (QKD), inspired by classical optical communication formats. A new time-domain technique based on pulse position modulation (PPM) is combined with differential phase-shift keying to optically implement a six-state protocol. Finally, some recently proposed PPM schemes are critically reviewed and applied to synthesize a new QKD asymmetric optical realization, applicable to local or metropolitan area networks.

DOI
10.1364/ol.30.001533

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New methods are proposed for extending the range of fiber-optic one-way quantum key distribution (QKD), inspired by classical optical communication formats. A new time-domain technique based on pulse position modulation (PPM) is combined with differential phase-shift keying to optically implement a six-state protocol. Finally, some recently proposed PPM schemes are critically reviewed and applied to synthesize a new QKD asymmetric optical realization, applicable to local or metropolitan area networks.
The binary (one-bit-per-photon) encoding that most existing quantum key distribution (QKD) protocols employ puts a fundamental limit on their achievable key rates, especially under high channel loss conditions associated with long-distance fiber-optic or satellite-to-ground links. Inspired by the pulse-position-modulation (PPM) approach to photon-starved classical communications, we design and demonstrate the first PPM-QKD, whose security against collective attacks is established through continuous-variable entanglement measurements that also enable a novel decoy-state protocol performed conveniently in post processing. We achieve a throughput of 8.0 Mbit/s (2.5 Mbit/s for loss equivalent to 25 km of fiber) and secret-key capacity up to 4.0 bits per detected photon, thus demonstrating the significant enhancement afforded by high-dimensional encoding. These results point to a new avenue for realizing high-throughput satellite-based or long-haul fiber-optic quantum communications beyond their photon-reception-rate limits.
We demonstrate the realization of quantum key distribution (QKD) when combined with classical optical communication, and synchronous signals within a single optical fiber. In the experiment, the classical communication sources use Fabry-Pérot (FP) lasers, which are implemented extensively in optical access networks. To perform QKD, multistage band-stop filtering techniques are developed, and a wavelength-division multiplexing scheme is designed for the multi-longitudinal-mode FP lasers. We have managed to maintain sufficient isolation among the quantum channel, the synchronous channel and the classical channels to guarantee good QKD performance. Finally, the quantum bit error rate remains below a level of 2% across the entire practical application range. The proposed multiplexing scheme can ensure low classical light loss, and enables QKD over fiber lengths of up to 45 km simultaneously when the fibers are populated with bidirectional FP laser communications. Our demonstration paves the way for application of QKD to current optical access networks, where FP lasers are widely used by the end users.
This paper presents novel quantum key distribution (QKD) schemes that use differential phases of sequential pulses as an information carrier. Alice sends a photon in a linear superposition state of three temporal slots, in which each amplitude is phase-modulated by {0,π}. Bob measures the phase difference of an asymmetric interferometer, and tells Alice time-instance at which the photon was counted. From this time information and her modulation data, Alice knows which detector counted the photon in Bob s site. Then, the two parties create a secret key. The scheme is suitable for fiber transmission systems and offers key creation efficiency higher than conventional phase-encoding QKD. The above scheme utilizes fully non-orthogonal four states. Differential phase shift QKD utilizing two non-orthogonal states is also presented. Alice sends a coherent pulse train with less than one photon per pulse, which is phase modulated by {0,π} for each pulse, and Bob measures the pulse train by a one-bit delay circuit. The system has a simple configuration such that Alice has no interferometer and sends no intense reference pulse, unlike to conventional QKD scheme using two non-orthogonal states, and also has an advantage of high key creation efficiency.
We present an implementation of the time-frequency (TF) quantum key distribution (QKD) protocol realized mainly with standard telecommunication components at 1550 nm. TF-QKD is implemented with modulations in time and frequency, namely pulse position modulation (PPM) and frequency shift keying (FSK). The time-frequency uncertainty relation ensures the security of the protocol. We further demonstrate free-space optical QKD transmission over a 388-m distance. Since the QKD setup is single-mode fiber (SMF) -based, precise coupling in and out of the SMFs is crucial. Thus, we implemented an optical tracking system. The QKD signal is wavelength-multiplexed with a strong beacon for optical tracking. Strong spectral filtering is necessary to separate beacon and QKD signal. Together with spatial filtering caused by the SMF most background light is filtered, enabling daylight QKD transmission.
Pulse-position modulation (PPM) is a promising technique that can be used to improve the efficiency of quantum key distribution (QKD) based on a Poisson photon source. In this paper, we first investigate a simple entanglement-and- PPM-based QKD protocol and demonstrate the improvement in secret key rate. However, such a PPM-based QKD protocol that utilizes only frames with a single click is still inefficient because it ignores frames with two and more clicks. For this reason we propose to use such multi-click frames to further improve the efficiency of PPM-based QKD by employing a better sifting strategy. Specifically, we focus on using the frames with two clicks in addition to those with a single click. Finally, we analyze the secret key rate under various noise levels in the scenario of high channel loss, which has been faced by most QKD applications. With the analytical results, we show the advantage of the proposed PPM-based QKD.
In this paper, we consider quantum key distribution (QKD) using pulse-position modulation (PPM) and propose the PPM-based BB84 QKD protocol. With the adoption of PPM, the efficiency of utilizing weak laser pulses can be improved and hence the key rate. We first analyse the potential of using PPM from the perspective of raw key rate. We then provide detailed description of the proposed protocol, the encoding rule of which has to be specifically designed in order to be secure. Lastly, we consider the intercept-resend and photon-number splitting attacks and analyse the secret key rate of the proposed QKD protocol. Our analysis shows that in high-loss channel, the proposed PPM-based BB84 protocol can achieve higher secret key rates compared to the conventional BB84 protocol.