The strange process of quantum entanglement, which Albert Einstein himself famously derided as "spooky action at a distance" has just become more extensive thanks to a breakthrough experiment that has demonstrated quantum entanglement at a greater distance via conventional telecommunications fiber optics than ever before.
In a paper published in Physical Review Letters, Chinese physicists have entangled two quantum memory units over a distance of 420 kilometers (261 miles) through optical fiber cables, setting a new record for the maximum range of matter-based quantum entanglement.
Led by physicists Xi-Yu Luo, Chao-Yang Wang, and Ming-Yang Zheng, who are all part of the University of Science and Technology of China (USTC), along with a research team long run by the renowned physicist Jian-Wei Pan, the scientists behind the research project are among those credited with ensuring the quantum supremacy of China in the field of quantum communication.
The difference between their work and other experiments that previously explored entanglement lies in their emphasis on matter-based entanglement rather than photon entanglement as their main point of interest.
Indeed, quantum memory plays an indispensable role when it comes to constructing any kind of a future quantum network that will be able to send more than one signal.
For the purpose of this study, the two memory units were called Alice and Bob after the classic characters from a number of physics-related thought experiments and consisted of rubidium atom clouds cooled with lasers to extremely low temperatures. Between them there existed a third unit named Charlie 420 kilometers (261 miles) away that worked as a receiver.
Alice and Bob generated photons with quantum-encoded information that got to Charlie via the optical fiber cables and when Charlie received certain signals about an interference pattern produced by incoming photons, it proved that the entanglement of Alice and Bob has occurred.
Creating entanglement over large distances has remained a key challenge in quantum communications for years. As light passes through fiber-optic cables, the quantum states they carry become susceptible to degradation in the form of absorption and scattering, and their entanglement gets affected by any disturbance.
Such degradation is the reason why all matter-based entanglement demonstrations were done only over a city scale, with the prior record, set by the same research lineage, being 50 kilometers just a couple of years ago.
In order to bypass these problems and extend the distance, the researchers used phase-stabilization techniques in order to protect the entanglement from small vibrations and changes in temperatures that may disrupt it during the transmission.
In addition, they used the standard telecommunication wavelength, as opposed to the wavelength originally emitted by the rubidium atoms, since these wavelengths suffer much lower losses in fiber optics.
This new feat has been described as the platform for the applications of the quantum network, which goes beyond the city scale in the published paper.
This recent breakthrough comes after years of investments by China in the research on quantum communications, and has its roots in one of the major headlines in physics back in 2017 when a Pan's team managed to generate entangled pairs of photons between the Micius satellite and the ground stations in China separated by 1,200 kilometers. Later on, the distance of satellite-based entanglement was increased to almost 1,400 kilometers.
While satellite-based quantum communications are effective when the distance exceeds what can be covered by the existing network of fiber-optics, fiber-based system, such as the recently introduced 420-kilometers link, is considered a much more realistic approach for the quantum internet infrastructure of the future, which is supposed to connect data centers, major cities and even banks using communications which are nearly impossible to hack.
According to outside experts, the achievement is nothing short of a breakthrough in the field. Outside physicists have emphasized that fiber and satellite based systems will probably play complimentary roles in the future, rather than being competitors.
The appeal of quantum entanglement for communication is based on the security aspect since measurement and interception of one particle from a particle pair destroy quantum state of both particles and thus are possible to notice. In other words, one can build communication networks which can be used only if there are no spies – at least theoretically speaking.
To implement this idea and provide reliable quantum communication network over long distance we need to create a chain of memory nodes or so-called quantum repeaters able to preserve quantum states.
Reliable entanglement of 420-kilometer separation memory nodes indicates that maybe building such chain is not such an impossible task after all.
