Entanglement distribution over a 96-km-long submarine optical fiber

Abstract

Quantum entanglement is one of the most extraordinary effects in quantum physics, with many applications in the emerging field of quantum information science. In particular, it provides the foundation for quantum key distribution (QKD), which promises a conceptual leap in information security. Entanglement-based QKD holds great promise for future applications owing to the possibility of device-independent security and the potential of establishing global-scale quantum repeater networks. While other approaches to QKD have already reached the level of maturity required for operation in absence of typical laboratory infrastructure, comparable field demonstrations of entanglement-based QKD have not been performed so far. Here, we report on the successful distribution of polarization-entangled photon pairs between Malta and Sicily over 96 km of submarine optical telecommunications fiber. We observe around 257 photon pairs per second, with a polarization visibility above 90%. Our results show that QKD based on polarization entanglement is now indeed viable in long-distance fiber links. This field demonstration marks the longest-distance distribution of entanglement in a deployed telecommunications network and demonstrates an international submarine quantum communication channel. This opens up myriad possibilities for future experiments and technological applications using existing infrastructure.

Passively stable distribution of polarisation entanglement over 192 km of deployed optical fibre

“…Spectral filtering using wavelengthdivision multiplexers separates the photons into two frequency channels -ITU Grid channels 32 (1551.72 nm) and 36 (1548.51 nm) -with the spacing between adjacent channels being 100 GHz and with each channel having a full-width at half maximum (FWHM) of 0.6 nm (similar to that described in Ref. [16]). One photon from each pair was sent to a polarisation analysis and detection module located in Malta close to the source.…”

“…The former has been demonstrated over two 150 km arms [13], while the latter has been demonstrated over a single arm consisting of a 100 km fibre spool [15]. The longest distribution of polarisation entanglement over deployed fibre so far is 96 km [16]. Polarisation entanglement/encoding, in particular, provides a conceptually higher key generations rate.…”

“…In the present work, one photon from each entangled pair was detected right at the source in Malta, whilst the second was sent through the submarine cable to Sicily and back to Malta via a second dark fibre, where it was eventually detected. Our demonstration is carried out at the same location as in [16]. The experiment was conducted without the use of either compensation of chromatic dispersion, or any active stabilisation techniques.…”

Decoherence-assisted detection of entanglement of two qubit states

“…Thus, aside from all other contexts, like the Bell nonlocality test, the CHSH inequality is a criterion for detecting the entanglement between two qubits. Indeed, as it was observed in e.g., [44][45][46][47], it is possible to repurpose the CHSH scheme as an entanglement witness. This is done by disassociating the correlator (1) from the initial state̺ by defining the corresponding hermitian operatorŝ…”

Quantum information processing with space-division multiplexing optical fibres

“…The successful distribution of photonic entanglement over long optical fibres is an important operational toolbox in quantum information. Many experiments have been performed using different degrees of freedom of a singlephoton over optical fibres, mainly using polarisation [75][76][77][78] and energy-time/time-bin [79][80][81][82]. For many years the distribution of spatial entanglement (i.e., entanglement between quantum systems encoded in terms of the transverse optical modes of light) over optical fibres has been out of reach.…”