High-dimensional quantum key distribution based on multicore fiber using silicon photonic integrated circuits

Abstract

Quantum key distribution provides an efficient means to exchange information in an unconditionally secure way. Historically, quantum key distribution protocols have been based on binary signal formats, such as two polarization states, and the transmitted information efficiency of the quantum key is intrinsically limited to 1 bit/photon. Here we propose and experimentally demonstrate, for the first time, a high-dimensional quantum key distribution protocol based on space division multiplexing in multicore fiber using silicon photonic integrated lightwave circuits. We successfully realized three mutually unbiased bases in a four-dimensional Hilbert space, and achieved low and stable quantum bit error rate well below both the coherent attack and individual attack limits. Compared to previous demonstrations, the use of a multicore fiber in our protocol provides a much more efficient way to create high-dimensional quantum states, and enables breaking the information efficiency limit of traditional quantum key distribution protocols. In addition, the silicon photonic circuits used in our work integrate variable optical attenuators, highly efficient multicore fiber couplers, and Mach-Zehnder interferometers, enabling manipulating high-dimensional quantum states in a compact and stable manner. Our demonstration paves the way to utilize state-of-the-art multicore fibers for noise tolerance high-dimensional quantum key distribution, and boost silicon photonics for high information efficiency quantum communications.npj Quantum Information (2017) 3:25 ; doi:10.1038/s41534-017-0026-2 INTRODUCTION Quantum key distribution (QKD) is an attractive quantum technology that provides a means to securely share secret keys between two clients (Alice and Bob).1-4 Traditional QKD is based on binary signal formats, such as the BB84 protocol where the quantum information is encoded in the polarization domain.5 Four polarization states create a set of two mutually unbiased basis (MUBs) in a two-dimensional Hilbert space which are used for establishing quantum keys between two parties. In these binary QKD systems the information efficiency is limited to 1 bit/photon. Recently, tremendous efforts have been put into developing novel protocols to increase the information efficiency.6-10 Highdimensional QKD (HD-QKD) based on qudit encoding (unit of information in a N dimension space) is an efficient technique to achieve high information efficiency for QKD systems.

Boosting the secret key rate in a shared quantum and classical fibre communication system

“…As an example, high-speed classical communication at 10.16 Pbit s −1 has been demonstrated thanks to SDM combined with dense wavelength division multiplexing, polarization modulation and advanced coding [29]. Likewise, both multicore and few-mode fibres have been exploited for encoding and transmitting high-dimensional (Hi-D) quantum states [30][31][32].In particular, the cores or the modes of these special fibres have been used to increase the dimensionality of the Hilbert space, thus allowing higher photon information efficiency. Although Hi-D quantum states are suitable for high rate quantum communications, in specific channel conditions (low noise channel), it was experimentally demonstrated that SDM is suitable for very high rate secure communications [33].…”

“…It also requires the modulation and detection of only three states, instead of the usual four that constitute the two full mutually unbiased bases, which means that in principle only two single photon detectors are required. Moreover, it is known from literature that the modulation, stabilisation and detection of superposition states (those in the Fourier basis) are more challenging [30,31,35]: hence the use of this threestates protocol results in a further simplification of the system. Finally, our analysis is carried out in the finite key regime, with a block size corresponding to the actual sifted rate over 30 seconds (meaning that the time acquisition of our block is exactly 30 seconds).…”

High‐Dimensional Quantum Communication: Benefits, Progress, and Future Challenges

“…Multicore fibers -Multicore fibers present wellperformant characteristics: they offer low losses (comparable with the standard single-mode fibers) and low cross-talk between cores (fundamental for reliable transmission of the qudits) [161]. Previous experiments already demonstrated the capability of transferring spatial modes of light with high-fidelity up to a dimension equal to four [162][163][164]. In particular, Y. Ding and coauthors used two silicon photonic platforms, connected by a 3 m MCF, for preparing and measuring the quantum states [162], as reported in Figure 8.…”

Stable Transmission of High-Dimensional Quantum States Over a 2-km Multicore Fiber

“…However, based on the results of the other two previous experiments and this work, any entanglement distribution setup would also require phase stabilisation when implemented on longer distance. In our previous work [17], both transmitter and receiver were integrated on silicon photonics circuits that coupled light to and from the fiber cores directly, through apodized grating couplers positioned in such a way they correspond to the fiber cores position within the cladding. The fiber used in the demonstration is a 3 m long 7-core multicore fiber.…”

“…Multicore fibers propagation losses are similar to those of standard single mode fibers, and their inter-core cross-talk is low enough to ensure the reliable transmission of quantum states [20]- [22]. Indeed, these fibers have already been investigated as a mean for highdimensional quantum communication, however limitations in terms of stability affected the achievable distance [17], [19], [23]. Here, we investigate the phase stability of path encoded qudits propagated in a 2 km long multicore fiber using a phaselocked loop system.…”