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.

Keywords: polarization-entangled photons; quantum cryptography; quantum entanglement; quantum key distribution.

Fig. 1.

Setup and location of the experiment. The fiber optic cable used here links the Mediterranean islands of Malta and Sicily. A continuous wave laser at 775 nm bidirectionally pumped an MgO-doped periodically poled lithium niobate crystal (MgO:ppLN; PPLN) crystal and created, via the process of spontaneous parametric down-conversion, photon pairs that are entangled in polarization due to the Sagnac geometry. Signal and idler photons are separated from the pump beam with a dichroic mirror (DM) and then split by frequency into two different fibers by the band-pass filters 100-GHz band-pass filter (center wavelength: 1,548.52 nm; WDM1) and 100-GHz band-pass filter (center wavelength: 1,551.72 nm; WDM2); one photon is detected locally in Malta in a polarization analysis module consisting of a half-wave plate in front of a polarizing beam splitter (PBS) and two SNSPDs. The other photon is detected by SPADs in Sicily after transmission through the 96-km submarine telecommunications fiber. Mirrors and fiber couplers are not labeled, lenses are omitted. TTM1 and TTM2 are time-tagging modules, and $\lambda /4$ and $\lambda /2$ are wave plates. AMP, 50-dB voltage amplifier; LPF, 780-nm long-pass filter; PC, fiber polarization controller; PD, fast InGaAs photodiode; Sig. gen., 10-MHz signal generator; ${\mathrm{Y}\mathrm{V}\mathrm{O}}_4$, yttrium orthovanadate plate. Map images courtesy of NASA Worldview.

Fig. 2.

The cross-correlation function between the time tags from Malta and Sicily shows a peak at a relative delay of ∼532,281 ns, which corresponds to the length of the fiber when we take into account the different latencies of the detection systems. Coincident events are counted if they fall within 500 ps from the central peak position. The FWHM of ∼0.7 ns is attributed to timing uncertainty of the SPADs in Sicily (∼400 ps), the dispersion of the fiber link (∼500 ps), and other effects dominated by the timing uncertainty of the time-tagging units and their synchronization ($<$300 ps), including the uncertainty of the SNSPD system in Malta ($<$100 ps).

Fig. 3.

Coincidence count rates for one detector pair and two different measurement angles in Sicily [V (red) and A (green)] as a function of the measurement angle for the analyzer in Malta, ${\varphi }_{\text{M}}$, starting from H (red) or D (green). Poissonian statistics are assumed for the data as indicated by the error bars.

Fig. 4.

CHSH quantity $S\left({\varphi }_{\text{M}}\right)$ as a function of the measurement angle for the analyzer in Malta, ${\varphi }_{\text{M}}$, which resembles the relative angle between the two mutually unbiased bases that were used in Malta and Sicily each. Error bars are included but fit within the data markers; the SD is $\le 0.014$ for all of the points shown. Data outside the gray region (shown as squares) exclude local realistic theories. This function is computed using data similar to those shown in Fig. 3. The solid red curve is obtained from a fit to the coincidence rates (as in Fig. 3) and not from a fit to the data shown here, and it yields a CHSH value of $2.534\pm 0.08$. The green horizontal line shows the CHSH value of $2.421\pm 0.008$ obtained in a measurement run at the fixed value of ${\varphi }_{\text{M}}=157.5°$ (this includes a measurement at $22.5°$; i.e., the theoretically optimal angles).