Security of quantum cryptography using balanced homodyne detection

Namiki, Ryo; Hirano, Takuya

doi:10.1103/physreva.67.022308

Cited by 66 publications

(72 citation statements)

References 27 publications

(48 reference statements)

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“…These values demonstrate the excellent performance of balanced homodyne detection. For a given value of n sig there exists an optimal X + (X − ) that maximizes the product of the effective detection efficiency and the mutual information between Alice and Bob [20].…”

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confidence: 99%

“…Here we stress the advantage of the homodyne scheme that permits the measurement of quadrature amplitude distributions. A detailed analysis is presented in a separate paper [20]. In the intercept-resend eavesdropping strategy, Eve intercepts selected light pulses and mea-sures them, and then sends an appropriate state to Bob.…”

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confidence: 99%

“…For the latter case, because the simultaneous measurement of two noncommuting observables introduces extra noise [12,22], her error rate is larger than the one for the single basis measurement. For example, when n sig =1, the probability that she correctly differentiates between four phase shifts is only 0.708 (we assume that she must always choose one phase shift) [20].…”

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Quantum cryptography using pulsed homodyne detection

et al. 2003

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We report an experimental quantum key distribution that utilizes balanced homodyne detection, instead of photon counting, to detect weak pulses of coherent light. Although our scheme inherently has a finite error rate, it allows high-efficiency detection and quantum state measurement of the transmitted light using only conventional devices at room temperature. When the average photon number was 0.1, an error rate of 0.08 and "effective" quantum efficiency of 0.76 were obtained.PACS numbers: 03.67. Dd, 42.50.Lc According to quantum mechanics, one cannot obtain information about a single quantum system without disturbing its state [1] nor can one clone an unknown state [2]. Quantum cryptography is a technique for realizing secure communications exploiting these principles. The most popular protocol is quantum key distribution (QKD) in which two non-orthogonal states (B92 protocol) [3] or four states (BB84 protocol) [4] are sent via a quantum channel in order to generate random keys owned only by the legitimate sender (usually called Alice) and the receiver (Bob). These keys are then used to encode messages.In practice, a faint laser pulse is usually used as the quantum system, and keys are encoded by its polarization or its phase. Ideally, a single photon is desirable, but it is very difficult to generate it experimentally. Most of the previous experimental and theoretical studies on QKD used or postulated photon counting as a means to detect weak light. However, the usage of photon counting gives rise to two limitations. One is a technical limitation that at present there exists no efficient photon counter for infrared light, especially for 1.55-µm where optical loss in an optical fiber is minimum. State-of-the-art experiments used a specially designed photon-counting system made up of cooled avalanche photo diode operated in a gated Geiger mode [5][6][7]. For example, a quantum efficiency of 7% for 1.55µm with a dark-count probability of 10 −4 per 2.6-nsec time-window was reported [8]. Another limitation is a more fundamental: the quantum state of the transmitted light cannot be directly measured; the state alternation is inferred only from the change of the error rate. For example, when the eavesdropper (usually called Eve) changes the photon number distribution of the transmitted light while keeping the polarization (or the phase) and the mean photon number, Bob cannot notice the presence of Eve. This feature allows Eve to perform many kinds of attacks and leads to security holes (one example is the photon number splitting attack [9]). * Electronic address: hirano@qo.phys.gakushuin.ac.jpIn this paper, we propose using balanced homodyne detection for implementing the BB84 protocol with phase coding [10]. As we will explain, the above limitations associated with photon counting can be resolved by using balanced homodyne detection. In order to demonstrate the experimental feasibility of our scheme, we have performed QKD by sending light pulses at 1.55-µm wavelength through an optical fiber of 20-cm length. Whe...

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Quantum cryptography using pulsed homodyne detection

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“…Thus, the security analysis including experimental imperfections seems to become qualitatively different from that of the discrete schemes. * Electric address: namiki@qo.phys.gakushuin.ac.jpIn our previous work [9] we estimated G of a hybrid type scheme applying a postselection [8] for a given loss, provided Eve performs quadrature measurement for the lost part of the signal. In this case it is shown that G can be positive if the loss is less than unity by setting a large postselection threshold.…”

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“…The second one is an intercept-resend attack. It imposes a practical loss limit on every coherent state scheme.The protocol we study here is a four state protocol using phase modulation of weak coherent pulse and balanced homodyne detection applying a postselection process [8,9]. Alice randomly chooses one of the four coherent states | √ ne imπ/2 with the pulse intensity (the mean photon number per pulse) n > 0, m = 0, 1, 2, 3 and sends it to Bob.…”

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Practical Limitation for Continuous-Variable Quantum Cryptography using Coherent States

Namiki

Hirano

2004

Phys. Rev. Lett.

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In this letter, first, we investigate the security of a continuous-variable quantum cryptographic scheme with a postselection process against individual beam splitting attack. It is shown that the scheme can be secure in the presence of the transmission loss owing to the postselection. Second, we provide a loss limit for continuous-variable quantum cryptography using coherent states taking into account excess Gaussian noise on quadrature distribution. Since the excess noise is reduced by the loss mechanism, a realistic intercept-resend attack which makes a Gaussian mixture of coherent states gives a loss limit in the presence of any excess Gaussian noise.The security of quantum cryptography is degraded by the presence of realistic experimental imperfections. In particular the transmission loss limits the performance of schemes for a long distance transmission [1].Recently several continuous-variable quantum cryptographic schemes have been proposed [2,3,4,5,6,7,8,9]. Those are sorted into either all-continuous type or hybrid type [5], the all-continuous scheme distributes a continuous key and the hybrid scheme distributes a discrete key. A loss limit, in the sense that the mutual information between Alice and Bob I AB cannot be greater than the Shannon information of an Eavesdropper (Eve) I E , is given for an all-continuous scheme [6] and it is shown that this limitation can be removed by introducing a postselection process for a hybrid scheme [7,8,9]. The existence of loss limit is an open question.The reliable security measure for discrete quantum cryptographic schemes against individual attacks is the secure key gain G which ensures that I E can be arbitrarily small in the long key limit if G is positive [10,11]. The question is how high G can be for a given loss or transmission distance in realistic conditions. The estimations are given for BB84 protocol [11], entangled photon protocol [12], and B92 protocol [13]. The estimation of G for continuous schemes, if possible, is important as a comparison with discrete schemes. At least, the framework [14,15] can be adapted to hybrid schemes.For these discrete schemes, the experimental imperfections are mostly determined by observed bit error rate and dark count rate of single photon detectors [11,12,13]. In continuous-variable schemes, the experimental imperfections appear as the change of quadrature distributions. Experimentally, quadrature measurement is performed slightly above the standard quantum limit and observed quadrature distribution has additional Gaussian noise upon the minimum uncertainty Gaussian wavepacket [8]. Thus, the security analysis including experimental imperfections seems to become qualitatively different from that of the discrete schemes. * Electric address: namiki@qo.phys.gakushuin.ac.jpIn our previous work [9] we estimated G of a hybrid type scheme applying a postselection [8] for a given loss, provided Eve performs quadrature measurement for the lost part of the signal. In this case it is shown that G can be positive if the loss is less...

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Homodyne detection of weak coherent optical pulse: Applications to quantum cryptography

Sabban

Gallion

2009

Micro & Optical Tech Letters

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We propose a quantum key distribution system using balanced homodyne detection, in which strong reference pulses are time‐multiplexed with weak signal pulses by means of fiber interferometer. We use a dual‐threshold decision scheme for postdetection to improve the system performance in terms of BER, with a trade‐off in effective key generation rate. We conduct experimental measurements at 64 km SMF fiber at 1550 nm wavelength, and we prove that the system security can also be enhanced with properly selected threshold parameters. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 1934–1939, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24471

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Security of quantum cryptography using balanced homodyne detection

Cited by 66 publications

References 27 publications

Quantum cryptography using pulsed homodyne detection

Quantum cryptography using pulsed homodyne detection

Practical Limitation for Continuous-Variable Quantum Cryptography using Coherent States

Homodyne detection of weak coherent optical pulse: Applications to quantum cryptography

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