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arXiv:1905.09029 · Journal of Physics B Atomic Molecular and Optical Physics

Passive-state preparation in continuous-variable measurement-device-independent quantum key distribution

Dongyun Bai, Peng Huang, Hongxin Ma, Tao Wang, Guihua Zeng

Published Feb 27, 2019• 22 citations
arXivHTMLPDF

Abstract

Abstract Continuous-variable measurement-device-independent quantum key distribution (CV-MDI QKD) with Gaussian modulation, or with active state preparation, is immune to all detection-related security loopholes. Here, we first propose a scheme where Alice and Bob both passively prepare quantum states using a true thermal source, which is of lower cost and will contribute to the commercialization of CV-MDI QKD. In particular, Alice and Bob first split the output of a thermal source into two correlated modes via a beam splitter at their respective side. Then they measure one of the two modes using two conjugate homodyne detectors and transmit the other attenuated mode to Charlie independently for the Bell state measurement. Moreover, the homodyne detectors, with bright local oscillators, can act as a filter to selectively measure one single mode, which removes the rigid requirement of a single-mode thermal source. This specific conceptual scheme will contribute to the simplification of the experimental implementation of CV-MDI QKD.

DOI
10.1088/1361-6455/ab0b2a

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Abstract Continuous-variable measurement-device-independent quantum key distribution (CV-MDI QKD) with Gaussian modulation, or with active state preparation, is immune to all detection-related security loopholes. Here, we first propose a scheme where Alice and Bob both passively prepare quantum states using a true thermal source, which is of lower cost and will contribute to the commercialization of CV-MDI QKD. In particular, Alice and Bob first split the output of a thermal source into two correlated modes via a beam splitter at their respective side. Then they measure one of the two modes using two conjugate homodyne detectors and transmit the other attenuated mode to Charlie independently for the Bell state measurement. Moreover, the homodyne detectors, with bright local oscillators, can act as a filter to selectively measure one single mode, which removes the rigid requirement of a single-mode thermal source. This specific conceptual scheme will contribute to the simplification of the experimental implementation of CV-MDI QKD.
In the Gaussian-modulated coherent-states (GMCS) quantum key distribution (QKD) protocol, Alice prepares quantum states actively: For each transmission, Alice generates a pair of Gaussian-distributed random numbers, encodes them on a weak coherent pulse using optical amplitude and phase modulators, and then transmits the Gaussian-modulated weak coherent pulse to Bob. Here we propose a passive state preparation scheme using a thermal source. In our scheme, Alice splits the output of a thermal source into two spatial modes using a beam splitter. She measures one mode locally using conjugate optical homodyne detectors, and transmits the other mode to Bob after applying appropriate optical attenuation. Under normal conditions, Alice's measurement results are correlated to Bob's, and they can work out a secure key, as in the active state preparation scheme. Given the initial thermal state generated by the source is strong enough, this scheme can tolerate high detector noise at Alice's side. Furthermore, the output of the source does not need to be single mode, since an optical homodyne detector can selectively measure a single mode determined by the local oscillator. Preliminary experimental results suggest that the proposed scheme could be implemented using an off-the-shelf amplified spontaneous emission source.
Measurement-device-independent quantum key distribution (MDI-QKD) can remove all detector loopholes. When it is combined with the decoy-state method, the final key is unconditionally secure, even if Alice and Bob do not have strict single-photon sources. However, active modulation of source intensity, which is used to generate the decoy state, may leave side channels and leak additional information to Eve. In this paper, we consider the MDI-QKD with a passive decoy state, in which both Alice and Bob send pulses to an untrusted third party, Charlie. Then, in order to estimate the key generation rate, we derive two tight formulas to estimate the lower bound of the yield and the upper bound of the error rate that both Alice and Bob send a single-photon pulse to Charlie. Furthermore, the statistical fluctuation due to the finite length of data is also taken into account based on the standard statistical analysis.
Continuous-variable measurement-device-independent quantum key distribution (CV-MDI-QKD) allows remote parties to share information-theoretical secure keys while defending all the side-channel attacks on measurement devices. However, the secure transmission distance and the secret key rate are quite limited due to the high untrusted equivalent excess noise in the Gaussian modulation. More particularly, extremely high-efficiency homodyne detections are required for even non-zero secure transmission distances, which directly restrict its practical realization. Here, we propose a CV-MDI-QKD protocol by encoding the key information into matched discrete phases of two groups of coherent states, which decreases the required detection efficiency for ideally asymmetric cases, and makes it possible to practically achieve secure key distribution with current low-efficiency homodyne detections. Besides, a proof-of-principle experiment with a locally generated oscillator is implemented, which, for the first time, demonstrates the realizability of CV-MDI-QKD using all fiber-based devices. The discrete-modulated phase-matching method provides an alternative direction of an applicable quantum key distribution with practical security.
Continuous-variable measurement-device-independent quantum key distribution (CV-MDI-QKD) is immune to all detector side channel attacks, however its security requires trusted sources. We investigate the effects of source intensity errors, the most crucial source error, on the security of CV-MDI-QKD protocol with Gaussian-modulated coherent states. We establish a general source intensity errors model and derive the secret key rate based on different assumptions on the abilities of legal parties Alice and Bob. To avoid the adverse effects of the sources' intensity errors on CV-MDI-QKD, we present different data-processing schemes. We also assess the security of the protocol against collective Gaussian attacks in the finite-size scenario with composable security. Taking source intensity errors into consideration, we achieve improved security compared to previous demonstrations. Our results will provide a useful reference for practical implementations of CV-MDI-QKD.
Continuous-variable measurement-independent-device quantum key distribution (CV-MDI-QKD) can offer high secure key rate at metropolitan distance and remove all side channel loopholes of detection as well. However, there is no in-field experimental demonstration of CV-MDI-QKD due to the remote distance phase-locking techniques challenge. Here, we present a new optical scheme to overcome this difficulty and also removes the requirement of two identical independent lasers. Furthermore, we give an alternate but detailed proof of the minimized key rate condition to extract the secure key rate. We anticipate that our new scheme can be used to demonstrate the in-field CV-MDI-QKD experiment and build the CV-MDI-QKD network with untrusted source.