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arXiv:0907.0493 · Physical Review A

Robust unconditionally secure quantum key distribution with two nonorthogonal and uninformative states

Marco Lucamarini, Giovanni Di Giuseppe, Kiyoshi Tamaki

Published Sep 23, 2009• 27 citations
arXivHTMLPDF

Abstract

We present an unconventional form of decoy-state technique to make the single-photon Bennett 1992 protocol robust against losses and noise of a communication channel. Two uninformative states are prepared by the transmitter in order to prevent the unambiguous state discrimination attack and improve the phase-error rate estimation. The presented method does not require strong reference pulses, additional electronics, or extra detectors for its implementation.

DOI
10.1103/physreva.80.032327

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We present an unconventional form of decoy-state technique to make the single-photon Bennett 1992 protocol robust against losses and noise of a communication channel. Two uninformative states are prepared by the transmitter in order to prevent the unambiguous state discrimination attack and improve the phase-error rate estimation. The presented method does not require strong reference pulses, additional electronics, or extra detectors for its implementation.
The security of two-state quantum-key distribution against individual attack is estimated when the channel has losses and noises. We assume that Alice and Bob use two nonorthogonal single-photon polarization states. To make our analysis simple, we propose a modified B92B92B92 protocol in which Alice and Bob make use of inconclusive results, and Bob performs a kind of symmetrization of received states. Using this protocol, Alice and Bob can estimate Eve's information gain as a function of a few parameters that reflect the imperfections of devices, or Eve's disturbance. In some parameter regions, Eve's maximum information gain shows counterintuitive behavior, namely, it decreases as the amount of disturbances increases. For a small noise rate, Eve can extract perfect information in the case where the angle between Alice's two states is small or large, while she cannot extract perfect information for intermediate angles. We also estimate the secret key gain, which is the net growth of the secret key per pulse. We show the region where the modified B92B92B92 protocol over a realistic channel is secure against individual attack.
We discuss the unconditional security of a quantum key distribution protocol in which bit values are encoded in the phase of a weak coherent-state pulse relative to a strong reference pulse, which is essentially the one proposed by Bennett in 1992 (the B92 scheme). In the BB84 protocol with a perfect single photon source, the key rate decreases linearly with the transmission T of the channel. If we simply replace this source with a weak coherent-state pulse, the key rate drops more rapidly (as O(η2)) since the presence of multiple photons favors the eavesdropper, Eve. The B92 protocol, if modified to be encoded on the polarization of a single photon, also shows O(η2) dependence. This comes from Eve's option of sending the vacuum state, which is always registered as an inconclusive result and never causes bit errors. In the original B92 scheme with phase coding, Eve has no such option--replacing the weak pulse by the vacuum cannot avoid errors completely, thanks to the strong reference pulse. This should make the scheme stronger against the loss in the channel even coherent-state pulses from conventional lasers are used. We have obtained an unconditional security proof allowing the presence of small bit errors, and confirmed the above observation in a rigorous way. The key rate is found to drop linearly with the transmission η, and it is lower than the BB84 with a perfect single-photon source only by a constant.
Quantum key distribution (QKD) allows two parties to communicate in absolute security based on the fundamental laws of physics. Up till now, it is widely believed that unconditionally secure QKD based on standard Bennett-Brassard (BB84) protocol is limited in both key generation rate and distance because of imperfect devices. Here, we solve these two problems directly by presenting new protocols that are feasible with only current technology. Surprisingly, our new protocols can make fiber-based QKD unconditionally secure at distances over 100km (for some experiments, such as GYS) and increase the key generation rate from O(η2) in prior art to O(η) where η is the overall transmittance. Our method is to develop the decoy state idea (first proposed by W.-Y. Hwang in "Quantum Key Distribution with High Loss: Toward Global Secure Communication", Phys. Rev. Lett. 91, 057901 (2003)) and consider simple extensions of the BB84 protocol. This part of work is published in "Decoy State Quantum Key Distribution", . We present a general theory of the decoy state protocol and propose a decoy method based on only one signal state and two decoy states. We perform optimization on the choice of intensities of the signal state and the two decoy states. Our result shows that a decoy state protocol with only two types of decoy states—a vacuum and a weak decoy state—asymptotically approaches the theoretical limit of the most general type of decoy state protocols (with an infinite number of decoy states). We also present a one-decoy-state protocol as a special case of Vacuum+Weak decoy method. Moreover, we provide estimations on the effects of statistical fluctuations and suggest that, even for long distance (larger than 100km) 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. This part of work is published in "Practical Decoy State for Quantum Key Distribution", . We also have done the first experimental demonstration of decoy state quantum key distribution, over 15km of Telecom fibers. This part of work is published in "Experimental Decoy State Quantum Key Distribution Over 15km", .
We prove the unconditional security of the original Bennett 1992 protocol with strong reference pulse. We show that we may place a projection onto suitably defined qubit spaces before the receiver, which makes the analysis as simple as qubit-based protocols. Unlike the single-photon-based qubits, the qubits identified in this scheme are almost surely detected by the receiver even after a lossy channel. This leads to the key generation rate that is proportional to the channel transmission rate for proper choices of experimental parameters.
We propose an unconditionally secure quantum key distribution protocol, which uses a relatively strong signal pulse. While our protocol shares similar security bases as the Bennett 1992 protocol with a strong reference pulse, our scheme uses a smaller number of detectors and it is robust against Rayleigh scattering in an optical fiber. We derive a lower bound of secret key generation rate of our protocol and show that our protocol can cover relatively long distances, assuming precise phase modulations and stable interferometers.