Experimental demonstration of counterfactual quantum key distribution

Abstract

Counterfactual quantum key distribution provides natural advantage against the eavesdropping on the actual signal particles. It can prevent the photon-number-splitting attack when a weak coherent light source is used for the practical implementation. We realized the counterfactual quantum key distribution in an unbalanced Mach-Zehnder interferometer of 12.5-km-long quantum channel with a high-fringe visibility of 96.4%. As a result, we obtained secure keys against the noise-induced attack (eg. the vacuum attack) and passive photon-number-splitting attack.PACS numbers: 03.67.Dd Quantum key distribution (QKD) provides an unconditionally secure communication between two remote parties (Alice and Bob), where the security is guaranteed by the fundamentals of quantum mechanics [1,2]. However, current techniques cannot support to implement the ideal quantum cryptography experiment as originally proposed due to the lack of efficient single-photon or entangled photon-pair sources in the near-infrared region. So far, in practical long-distance fiber-based QKD systems, weak coherent light sources have been used instead of ideal single photons [3][4][5]. Intelligent methods have been invented to prevent the photon-numbersplitting (PNS) attack based on the multi-photon pulse of the weak coherent light source [6][7][8][9]. Among them, the decoy-state QKD protocol [10][11][12] could prevent the PNS attack by statistical security analysis of a large yield of the photon clicks [13,14]. On the other hand, quantum counterfactual effect has been discussed in interactionfree measurement [15][16][17] and developed for the quantum computation [18][19][20]. Recently, an interesting counterfactual QKD protocol was proposed [21] to distribute secret keys without any secret information-carrier qubits transmitting in the quantum channel, which can in principle prevent the eavesdropper to directly access the entire quantum system of each qubit. For example, secret key can be established by using the quantum counterfactual effect in a Michelson interferometer between Alice and Bob. One arm of the interferometer is in Alice's secure site, and the other arm is used to connect Bob as the quantum channel. Unlike previous QKD protocols which generate secret keys by transmitting the signal particles through the actual quantum channel, the proposed counterfactual QKD generates secret keys in the case that the photonic qubits only transmit in Alice's arm but never travel through the quantum channel to Bob. Interestingly, as a weak coherent light source is used, the quantum counterfactual effect provides a natural advantage to prevent the PNS attack by avoiding Eve's access to the information-carrier photons. Experimental demonstra- * Electronic address: gwu@phy.ecnu.edu.cn † Electronic address: hpzeng@phy.ecnu.edu.cn tion of such a counterfactual QKD requires a stabilized Michelson interferometer [22].In this letter, we experimentally demonstrate the counterfactual QKD based on a round-way unbalanced MachZehnder interferometer with 25 km fiber lengt...

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“…[5,6,15,17,18] It was later shown [19] that the distribution process of secret keys in QKD protocols can be realised with counterfactual phenomena, without the secret key particles ever traveling between the communicating parties. Such schemes have been experimentally realised [20][21][22], and their security advantages over other QKD protocols have been studied [23][24][25]. Whilst the secret key is generated counterfactually, the classical public channel communication of these schemes requires particle transfer.…”