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Quantum Key Distribution System Using Dual-Threshold Homodyne Detection Quantum Key Distribution System using Dual-threshold Homodyne Detection Qing Xu, Manuel Sabban, Philippe Gallion Francisco Mendieta Ecole Nationale Supérieure des Télécommunications on sabbatical leave from CICESE (TELECOM ParisTech, CNRS LTCI) km.107 Carr. Tijuana, Ensenada, Baja California 22800, 75013 Paris, France México [email protected] Abstract— In this work we present the principles of a flexible chosen encryption algorithm to encrypt (and decrypt) a quantum key distribution (QKD) system using quadrature- message, which can then be transmitted over a standard phase-shift-keying (QPSK) base and symbol encoding and dual- communication channel. The algorithm most commonly threshold balanced homodyne detection (BHD) scheme. We give associated with QKD is OTP, as it is up to now considered as its security proofs and we compare its performance unbreakable when used with a perfectly secret, random key. experimentally with a photon counting detection scheme. QC is now moving from the promise of physics to the hard Keywords- quantum key distribution, quantum cryptography, reality of electrical engineering world and is obviously balanced homodyne detection, QPSK modulation, BB84 protocol, handling with the full quantum nature of light. Optical QKD photon counting, phase shift keying, unconditional security system is based on the use of single-photon Fock states in which any state of the Fock space is with a well-defined I. INTRODUCTION number of particles. Unfortunately, these states are up to now difficult to realize experimentally. A more practical choice of The ongoing boom of information technology (IT) and our days is faint laser pulses [3-5], i.e. weak coherent states telecommunications infrastructure is a main driving force of (WCP) or entangled photon pairs [6,7], in which both the the technological and social changes in the late 20th and early photon and the photon-pair number distribution, obey Poisson 21st centuries. The confidentiality of the IT based applications statistics [8]. and communications systems, i.e. the information security is becoming one of the biggest concerns in governments, military, Then the key issue in a QKD system turns to be the homeland security, financial institutions, hospitals, and private detection of quantum level Qbits, such as the reliable and businesses. inexpensive WCP. Today, the Geiger gated-mode avalanche diode, also called photon counter (PC), is widely used. PC The security of the conventional public-key cryptosystems works under low and precise temperature control, i.e. around - such as Data Encryption Standard (DES), Advanced 30ºC, and exhibits inherent low quantum efficiency around 0.1 Encryption Standard (AES) relies on the computational and the inevitable residual after-pulse noise due to the difficulty of certain mathematical functions, and can provide macroscopic avalanche [9,10] at the C band, i.e. 1550nm neither any indication of eavesdropping nor guarantee of key widely used in optical communications. Moreover its security. It is threatened by the calculation capacity potential operational frequency is limited to 4-8MHz due to the improvement of super computer, or eventually quantum necessary quenching process. computer. On the other hand, information theory shows that traditional private-key (secret-key) cryptosystems cannot be On the other hand, coherent optical communication is one totally secure unless the key is used once only, being at least as of the most promising ways to achieve highest receiver long as the enciphered text. This algorithm is also called one- sensitivity, excellent spectral efficiency and longest time pad (OTP) [1]. transmission distance for the next generation of optical communication systems. Already in the late 1980s and early Based on the laws of quantum mechanics, in contrast to 1990s coherent systems attracted a lot of attention [11-20] as it traditional public key cryptography, quantum cryptography was a promising way to improve the receiver sensitivity. (QC), as a system that guarantees unconditional security of communications [2], has been extensively studied recently in Balanced homodyne detection (BHD), using high search for high-speed, long distance operational scheme that is efficiency, high bandwidth and low cost positive-intrinsic- compatible with the today’s optical networks. negative diode (PIN) operating at room temperature, is also sensitive to phase and polarization matching. It allows a noise An important point is that QC is only used to produce and free single quadrature measurement of the optical field. This distribute a key, not to transmit any message data itself. In such leads to a frequency selection scheme that is useful for a quantum key distribution (QKD) system the security is background radiation rejection as for the compatibility with the checked a posteriori and the key can then be used with any current WDM networks. It allows signal phase encoding more suitable for optical fiber communications than polarization be generated separately (the symbols 0 and 1 in two different encoding in one-way systems because it benefits from the bases), constituting a QPSK constellation. At Bob’s end, the mixing with a strong local optical field. Operating near the base coincidences turn to a BPSK constellation; base anti- quantum limit, it is potentially capable of overcoming the non- coincidences are not considered since their results are desirable effects such as afterpulses and “dark counts” discarded and do not contribute to the bit error rate (BER). characteristics of the single photon detection measurement (SPDM). Optical phase encoding is well known to overcome Alice’s choices of bases and symbols and Bob’s choices of bases, as well as the key coincidence/anti-coincidence are the fiber impairments of the historical polarization encoding. Optical phase and information recoveries are to be solved both shown in Table I. by the receiver Bob and the eavesdropper Eve. TABLE I. QPSK BB84 PROTOCOL Post-detection, threshold and symbol synchronization stages must be properly designed as in BHD the decision Alice Bob process is carried out a posteriori [21,22], in opposite to Base Bit Ф Ф Φ Base Φ Φ -Φ Key photon counting that inherently performs built-in decision 1 2 A B A B B1 π/4 0 0 [9,10], making a compromise between detection efficiency and 0 0 π/2 π/4 B2 -π/4 π/2 ? bit error rate (BER). A1 B1 π/4 π 1 1 π -π/2 -3π/4 In this paper we first recall the principles of the BB84 QKD B2 -π/4 -π/2 ? B1 π/4 -π/2 ? protocol [2], and then we discuss the homodyne detection using 0 0 -π/2 -π/4 B2 -π/4 0 0 both photon counting and BHD. Next we analyze the security A2 B1 π/4 π/2 ? issues of the BHD QKD system under the “intercept-resend” 1 π π/2 3π/4 attack and the “intermediate base” attack. Furthermore we B2 -π/4 π 1 present the experimental setup of a one-way BB84 QKD system using weak coherent pulses (WCP) QPSK format encoding at Alice’s end and BPSK base switching at Bob’s end. Photon counting and dual-threshold BHD are performed with optical synchronization for phase drift compensation. Finally we compare the system performance of the two receivers in terms of detection efficiency and BER. II. HOMODYNE QKD SYSTEM A. BB84 protocol and unconditional security Charles H. Bennett and Gilles Brassard proposed the first protocol for QC in 1984, as usually referred as BB84 [2]. In this protocol, the two protagonists Alice and Bob use two Figure 1. BB84 Protocol in quantum channel channels of communications: one quantum channel and another classical channel. The quantum channel allows the The steps of BB84 protocol in quantum channel are: 1 - transmission of quantum objects that have to be very weak so Alice chooses a random series of bits; 2 - Alice sends each bit that quantum effects are measurable. The eavesdropper, with a random base choice (base 1 or base 2); 3 - Bob detects namely Eve, is supposed to have full access to this quantum each bit using another random choice of the base (base 1 or channel although the quantum channel nature limits her base 2). actions. These quantum objects are prepared in such a way that Eve’s tentative of acquiring the information will induce, in accordance with the quantum mechanics, by a perturbation of the signals that Alice and Bob could measure by comparing the communications through the classical channel. The classical channel that permits Alice and Bob to communicate can be a standard telephone line, cable, radio frequency, or even the same fiber link as quantum channel. Eve can listen to the conversation between Alice and Bob, but she will not modify the information. In other terms, this classical channel should be authenticated, which is possible by the classical cryptography algorithm, since Alice and Bob share a Figure 2. BB84 Protocol classic channel priori some secret key. The steps of BB84 protocol in classical channel are: 4 - The protocol BB84 is a group of strict rules that is Bob publicly announces his series of base choices (but not the indispensable for a QKD system to be implemented as an measurement result!); 5 - Alice publicly announces the base unconditionally secure communication. From two orthogonal coincidences, i.e. the bits correctly detected by Bob; 6 - Bob bases chosen randomly by Alice, four quantum eigen states can and Alice use this bit sequence as the key, a raw key is thus D. Dual-threshold balanced homodyne detection generated through this “reconciliation” process. BHD consists of mixing the weak signal filed with the >> When there is base coincidence between Bob and Alice, the strong LO field before intensity detection, i.e. ELO E S . bit is correctly detected and when there is base anti- BHD technique is potentially capable of overcoming the non- coincidence, the result of the measurement is random.
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