Another world record for quantum communication 248 km of cross-country entanglement distributionm

Recently, researchers at the Institute for Quantum Optics and Quantum Information in Vienna successfully distributed polarization-entangled photon pairs directly in a 248-km transnational telecommunication fiber from Austria to Slovakia, the longest distance for real-world fiber-based entanglement distribution to date. on October 17, the related results, titled "Continuous entanglement distribution over a 248-km long transnational fiber link" [1], were published in Nature Communications.

01Fiber-based quantum entanglement distribution: suitable for short-range communication

Entanglement is the basis for many quantum applications. One of the most technically mature is quantum key distribution (QKD), which uses the quantum correlation of entangled photons to generate provable, unbreakable, secure keys.

A key challenge in this area is to establish continuously working, reliable long-range entangled distribution. However, satellite links do not support interruption-free operation, and deployed fibers have so far been limited to <100 km and a few hours of operation due to losses, or the use of trusted nodes. Fiber-based QKD systems operate stably, are not affected by meteorological conditions, significantly reduce maintenance efforts, directly mitigate limited key effects, and allow the use of already deployed telecommunication infrastructure.

These advantages over satellite links compensate for their higher photon losses. Thus, while intercontinental quantum links are most likely to be operated using satellites, short distances of a few hundred kilometers can also be covered by fiber optic links. Deploying an entanglement-based QKD metro fiber network has the added advantage of directly connecting many users with classical signals for wavelength multiplexing. However, losses in the fiber, imperfect preparation of the entanglement state, chromatic dispersion (CD), polarization mode dispersion and temporal accuracy of single photon detection hinder stable operation over long distances.

Until today, the longest distance of entangled distribution in deployed fibers is along a 96 km fiber between Malta and Sicily; in addition, the same submarine cable is used for a total of 192 km of round-trip links. The longest uninterrupted run using actively stabilized entanglement distribution has proven to be 6 hours of operation along a 10 km link of the deployment.

02Dual-channel configuration for QKD entanglement distribution: 248 km of continuous operation

In this work, Austrian scientists combined state-of-the-art equipment and optimal use of the quantum properties of polarization entangled photons to demonstrate continuous operation of entangled distribution along a deployed telecommunication fiber at a distance of 248 km: connecting Bratislava, Slovakia, to Sankt Pölten, Austria, and through Vienna.

248 km of entanglement distribution.

In the experiment, the source generating the entangled photon pairs is located in Vienna; the scientific team generates polarization-entangled photon pairs at two different telecommunication wavelengths by pumping a nonlinear crystal (NLC) with a 775 nm laser (PL) and collecting down-converted photons with a single-mode fiber (SMF) connected to a wavelength division (de)multiplexer (WDM). The idle photons pass through a dispersion compensation module (DCM), which nonlocally recovers the tight time dependence of the entangled state amplified by the dispersion on the link. The idle photons are then directed along 129 km of fiber to the polarization measurement module (PMM) in St. Pölten, Lower Austria. The signal photon passes through an automated intra-fiber piezoelectric polarization controller (PPC), which non-locally realigns the phase of the entangled state if its quality decreases. Afterwards, it travels to the PMM in Bratislava, which has the same design as the Austrian PMM. In these PMMs, photons are randomly guided to two mutually incompatible linear polarization bases in orthogonal measurements. The photons impact onto a superconducting nanowire single photon detector (SNSPD) and a time-tag module (TTM) controlled by a GPS clock records the detection time, the measurement basis and the results. Through a classical Internet link, the detection events of the two measurement stations are compared and the degree of overlap is calculated. If their quantum bit error rate increases, Vienna starts a polarization alignment.

This is the longest distance known to date for fiber-based entanglement distribution in the real world, the article says.

In addition, the team demonstrated ground-based QKD entanglement distribution in a real-life two-channel configuration, while one of the channels crossed the national border between Austria and Slovakia without any intermediate trusted nodes. Despite an unprecedented total loss of 79 dB, an average entanglement pair rate of 9s-1 and a progressively secure key rate of 1.4 bits/sec were achieved. The scientific team actively stabilized the polarization in an efficient, non-local manner, allowing the link to operate continuously for 110 hours, achieving a duty cycle of 75% and a total key of 258 kbit, taking into account finite key effects.

The quantum bit error rate (QBER) and the conforming counts (coincident counts) vary with time, and the conforming counts are tCC=114 ps.

Over time, it is in principle possible to create a quantum secure key as long as the quantum bit error rate (QBER) stays below 11%. The polarization alignment procedure in the experiment allowed a total of 82 hours of QBER to remain in the key creation state for the entire 110 hours of link operation. The red line gives the average key rate (1.4 bits/sec) calculated from all conforming events for these 82 hours.

QBER is described over time along with weather and construction site data. More severe weather conditions may still cause polarization drift.

03The quantum Internet of the future: long-range, low-maintenance, and ultra-stable

In this experiment, the research team deployed a high-brightness, high-fidelity source of entangled photon pairs at telecommunication wavelengths, and a high-end SNSPD system; and operated the link under current state-of-the-art limiting conditions and demonstrated an ultra-stable, polarization-based intra-fiber entanglement distribution scheme: capable of creating quantum-secure keys over a length of 248 km and a time span of 110 hours, overcoming the problem of the long distance along two nearly symmetric fiber links with a total of 79 dB of loss.

This work paves the way for low-maintenance, ultra-stable quantum communication over long distances, independent of weather conditions and time of day, thus constituting an important step towards the quantum Internet. Although the analysis in this paper focuses mainly on QKD, the performance impact of this new technology on other implementations (e.g., quantum computing or blind computing) remains to be evaluated and could have far-reaching implications.

This work paves the way for the distribution of quantum entanglement over long-haul fibers for various applications in continuous operation, most notably, perhaps, not limited to quantum key distribution.

Reference：

https://www.nature.com/articles/s41467-022-33919-0