A quantum cryptography network could detect earthquakes

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According to an estimate by the American Geological Survey (USGS), 500,000 earthquakes occur worldwide each year. Most are too weak to be felt. However, some of them, such as the earthquake off the coast of Japan in 2011, could cause large buildings to collapse, cutting off access to electricity, water and communications. They can also cause devastating landslides. Seismic detection is a crucial issue. In addition, a Chinese team recently demonstrated that a quantum communications network would be sensitive enough to detect small vibrations, warning signs of potential earthquakes. This discovery thus suggests a possible further use of a secure future quantum network.

In the age of ultra-connectivity, communication security is crucial. Quantum technologies provide a revolutionary solution for secure communication needs. A quantum communication network is thus dependent on quantum key distribution (QKD). It is a set of protocols for distributing an encryption key between two remote interlocutors, while ensuring transmission security thanks to the laws of quantum physics and information theory.

Specifically, the network consists of local quantum communication nodes between two users. These connection points generate photons that are used to transport information. The quantum properties of these light particles allow them to travel long distances through an optical fiber between local nodes. Quantum key distribution therefore uses lasers to transmit keys and data. The quantum properties of photons in laser beams are binary coded (with 1s and 0s). While a third party can intercept the beam, by being observed, the quantum properties change, making the keys useless to an attacker.

Recently, a Chinese research team led by Jian-Wei Pan of the University of Science and Technology of China attempted to reduce the number of nodes needed to perform information between two interlocutors without losing data. In addition, the researchers demonstrated that this ultra-sensitivity could be used to detect changes in the environment around the fiber, such as otherwise imperceptible vibrations. This property makes it potentially useful for detecting earthquakes and landslides. Their results were published in the journal Physical review letters.

A double-field quantum network

The research team focused on two-field quantum key distribution (TF-QKD), a common fiber protocol for secure long-distance encryption using photonic interference. It promises ultra-long secure key distribution by reducing the number of trusted nodes in the long-distance quantum network. Significant efforts have been made to implement TF-QKD.

Specifically, a TF-QKD experiment consists of two optical configurations, called A and B, at opposite ends of an optical fiber. Each configuration generates a random string of bits (0s and 1s) and sends it as an optical signal down the fiber to an intermediate node called C. At node C, the two signals (photonic interference) interfere, and the resulting optical signals are then forwarded to A and B, which uses the interference results to generate a shared key.

Based on this principle, Pan and his colleagues first demonstrated the operation of TF-QKD in a laboratory with an optical fiber wrapped around coils. In other words, it managed to send encrypted data over a 658 kilometer cable with minimal data loss. This is one of the longest distances traveled by any QKD system to date.

In addition, the detection of these photonic interferences requires constant monitoring of the light signals that users share on each fibrillar joint. The authors realized that they could use this continuous data stream to detect vibrations along the fiber. They point out that this recovery of vibration-sensitive data is possible without adding new fiber or hardware resources to a TF-QKD network.

Compensates for network fluctuations

As is common for the TF-QKD method, the researchers designed their system to correct for fluctuations in the photon phase passing through the fiber. For example, if a sudden movement briefly increases the length of the fiber at one end, a dip in the light wave would arrive at the intermediate node instead of a peak. Thus, a TF-QKD system must continuously make adjustments to compensate for phase fluctuations. This is what the authors realized when they designed their TF-QKD system. All environmental disturbances, such as vibrations in the ground or temperature variations, also affect this phase and can potentially be detected.

To demonstrate this possible detection of seismic vibration, the researchers installed a piezoelectric device that caused the fiber coming from one end to move at a specific location. The vibration frequency was set between 1 and 1000 Hz, which is the relevant range for seismic detection. The vibrations give phase shifts between 0.9 and 50 radians, which the oscillation compensation system has captured.

As a result, the team performed a similar test on the frequency calibration link, a separate fiber required by TF-QKD systems to lock the frequencies of lasers coming from both ends of the fiber. The researchers used this link to locate the source of the vibration with an accuracy of 1 km. They conclude: Our results not only set a new quantum key distribution distance record, but also demonstrate that redundant information from TF-QKD can be used for channel vibration remote measurement, which can find applications in three-dimensional detection and landslide monitoring, in addition to secure communication “.

This study is included in a set of previous works based on the same concept. In 2018, an experiment used underwater telecommunication fibers to detect earthquakes. The data transfer speed must be improved before the technology can be integrated into a large-scale quantum communication network and be operational for seismic detection. Nevertheless, in 2021, China claimed to have installed the world’s first integrated quantum communications network. Setting up a detection system on this network is a real challenge for China, one of the countries hardest hit by earthquakes in the world. Just for the month of May 2022, there are no fewer than 10 earthquakes, the strongest of which had a magnitude of 6.3 on the Richter scale.

Source: Physical Review Letters

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