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Practical Quantum Key Distribution Based on the BB84 Protocol

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Practical Quantum Key Distribution Based on the BB84 Protocol Practical Quantum Key Distribution based on the BB84 protocol A. Ruiz-Alba, D. Calvo, V. Garcia-Muñoz, A. Martinez, W. Amaya, J.G. Rozo, J. Mora, J. Capmany Instituto de Telecomunicaciones y Aplicaciones Multimedia, Universitat Politècnica de València, 8G Building-access D-Camino de Vera s/n-46022 Valencia (Spain) Corresponding author: jcapmany@iteam.upv.es Abstract relies on photon sources and the so-called pho- ton guns are far from ready. An alternative and This paper provides a review of the most impor- also a complementary approach to increase the tant and widely used Quantum Key Distribution bit rate is to make QKD systems work in paral- systems and then describes our recently pro- lel. This simple engineering approach becomes a posed scheme based on Subcarrier Multiplexing challenge when working with quantum systems: that opens the possibility of parallel Quantum The system should work in parallel but still rely Key Distribution. We report the first-ever ex- on just one source and its security should still be perimental implementation of parallel quantum guaranteed by the principles of quantum me- key distribution using this technique showing a chanics both when Alice prepares the photons maximum multiplexing gain. and when Bob “measures” the incoming states. At the same time the multiplexing technique Keywords: Optical fiber communications, quan- should be safe to Eve gaining any information. tum information, quantum key distribution, sub- carrier multiplexing. In this context, recent progress in the literature [4] remarks that photonics is a key enabling technology for implementing commercial QKD 1. Introduction systems to become a competitive technology in Information Security. For this to happen at a rea- Quantum Cryptography features a unique way sonable cost, it is fundamental to take advantage of sharing a random sequence of bits between of the currently available generation of photonic users with a certifiably security not attainable components developed for applications in the with either public or secret-key classical cryp- telecommunications field. Therefore, our over- tographic systems. This is achieved by means all objective is to implement and demonstrate of Quantum Key Distribution (QKD) techniques, practical QKD systems based on available opti- which rely on exploiting the laws of quantum cal communication devices and optical multi- mechanics [1]. Indeed, only systems based on plexing techniques feature securing operation QKD techniques can offer unconditional security outperforming currently available alternatives. without imposing any computational assump- tions [2]. The paper is organized as follows. In section 2 we provide some basic notions of QKD, includ- A main limitation of QKD systems, and still to ing the description of the generic settings and be solved, is the fact that bit rates are very low techniques so far proposed for its implementa- compared to those used in classical telecommu- tion and the most typical protocol, known as nications [3]. One of the main restrictions to date BB84. In section 3, we present different configu- 4 ISSN 1889-8297 / Waves · 2011 · year 3 rations reported in the literature that allow the urement in general modifies the state of the implementation of the BB84 protocol by using measure systems and secondly, the no-cloning The fact that optical polarization, optical phase or frequency theorem which states that one cannot duplicate security can modulation as encoding techniques. Section 4 an unknown quantum state while keeping the be based presents the theoretical principles of a quantum original intact [5]. Both these arguments are ex- on general key distribution system that opens the possibil- ploited in the BB84 protocol, probably the best principles ity of parallel: quantum key distribution based known and most widely employed scheme in of quantum on subcarrier multiplexing (SCM) technique or QKD systems. SCM-QKD. In section 5 we report some prelimi- physics nary results of an experimental prototype for a The BB84 protocol is a discrete variable coding suggests the SCM-QKD implementation operating with two named after its inventors Charles Bennet and possibility of radiofrequency subcarriers. The obtained re- Gilles Brassard and the year of its first publica- unconditional sults in quantum and classical regime demon- tion (1984) [1]. In this protocol, Alice prepares security strate the system feasibility. and sends to Bob a set of random qubits. These are selected from the following set of four states: 2. Basic notions of Quantum ⎧ ψ 0 = 0 key distribution Base 1 ←⎯→ ⎨ ⎩ ψ 1 = 1 Figure 1 illustrates a typical QKD system. Two ⎧ 1 ψ + = [0 + 1 ] partners who are traditionally called Alice and ⎪ Base 2 ←⎯→ 2 Bob want to establish a secret key at a distance. ⎨ 1 ⎪ They need to be connected by two channels: a ψ − = [0 − 1 ] ⎪ 2 quantum channel, allowing them to share quan- ⎩ tum signals; and a classical channel, -which needs to be authenticated-, (i.e Alice and Bob must cer- (1) tifiably identify themselves). On the other hand, a third person can listen to the conversation but The first two states in (1) form one base of a cannot participate in it. The quantum channel, quantum two dimensional system while the however, is open to any possible manipulation other two from a second one. Therefore, the from a third party. Specifically, the task of Alice conditions and correspon- and Bob is to guarantee security against an ad- ding to scalar products between states have to versarial eavesdropper, usually called Eve, tap- be satisfied. At the same time, the states in the ping on the quantum channel and listening to different basis of (1) are not orthogonal and have the exchanges on the classical channel [2, 3]. maximum overlapping. As a consequence, there is no measurement procedure that can determi- ne with 100% certainty the specific state which is prepared by Alice and it is sent to Bob. For every incoming state from Alice, Bob ran- domly chooses one of the two basis to measure, either Base 1 or Base 2, performs the measure- ment and records the results. This measurement procedure employed by Bob allows the deter- mination of the bit sent by Alice. Once quantum communication has finished, Bob and Alice use a public channel to communicate certain proper- ties relative to the states that she has sent and he has measured. Specifically, Bob announces the Figure 1. Diagram of a QKD system where Alice position of the detected bits and the basis used, and Bob are connected by a quantum channel into this stream is called raw key. Alice and Bob re- which Eve can tap without any restriction other tain the bits for which the basis employed in the than the laws of physics; and by an authenticated encoding and the decoding processes coincide classical channel, into which Eve can only listen to. and discard the rest. In this base reconciliation, Alice does not reveal the particular value of the state and Bob does not indicate the results of the measurement. This roughly results in a subset of The fact that security can be based on general around half of the bits originally detected by principles of quantum physics suggests the pos- Bob. This subset is called the sifted key. sibility of unconditional security, i.e. the possibil- ity of guaranteeing security without imposing In the process of qubit sending from Alice to Bob any restriction on the power of the eavesdrop- an eavesdropper, known as Eve, can intercept per. Indeed, the origin of security of QKD can some or all of them interfering the communi- be traced back to some fundamental principles cation process. Eve can implement this attack of quantum physics. Firstly, a well-know princi- through a kind of channel measurement, for ple of quantum physics establishes that meas- example a projective measurement carried over Waves · 2011 · year 3 / ISSN 1889-8297 5 the qubit, by means of an intercept and re-re- number of photons lower than 1, and sent along BB84 protocol send scheme. As it has been mentioned before, the quantum channel to Alice. It is essential that is a uncondi- in the second part of this protocol Alice and Bob the pulses remain polarized for Bob to be able to tionally make public the basis employed to encode and extract the information encoded by Alice. secure detect the bits respectively the outcome of this solution for process resulting in the generation of the raw When the pulses reach Bob’s location they are quantum key. In principle, it is thought possible for the extracted from the fiber and travel through a set key eavesdropper Eve to know part of the key but of waveplates used to recover the initial polari- distribution the QKD system permits Alice and Bob to detect zation states by compensating for the transfor- the presence of Eve. Note that Eve has to choose mation induced by the optical fiber. After that, one of the basis to perform the projective mea- the pulses reach a symmetric beamsplitter (BS), surement and determine incoming state. There- implementing the random base choice. The Base fore, she has a 50% probability of employing the 1 is performed with the rotation of the polariza- incorrect base in her measurements resulting in tion state with a waveplate by π/4. Transmitted a disagreement between what Bob detects and photons are analyzed in this vertical-horizontal Alice has sent. In this way, Alice and Bob have base with a polarizing BS and two photon- a very simple way to detect the presence of Eve counting detectors. Also, the photons are ana- by performing an error detection check by sha- lyzed with a second set of polarizing BS and pho- ring a subset of the bits. If the error rate of this ton-counting detectors that characterize Base 2.
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