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Friday, 14 February 2020

New record: researchers have tangled quantum memory for more than 50 kilometers


Chinese scientists have succeeded in obtaining two quantum memories entangled on 50 kilometers of fiber optic cables, almost 40 times the previous record! This feat makes the idea of ​​an ultra-fast and ultra-secure quantum Internet much more plausible.

You should know that quantum communication is based on quantum entanglement : where two particles become inextricably linked and depend on each other, even if they are not in the same place. Indeed, in quantum mechanics, quantum entanglement is a phenomenon in which two particles (or groups of particles) form a linked system and have quantum states dependent on each other whatever the distance between them.

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In other words, quantum memory is the quantum equivalent of conventional computer memory:  to store information for later use, and if we ever want to develop and use practical and useful quantum computers, we must understand how to use this memory.

"The main purpose of this article is to extend the entanglement distance in [optical] fiber between quantum memories across the city," said Jian-Wei Pan, of the University of Science and Technology of China, head of the research team. The study was published in the journal Nature.



Quantum entanglement over a long distance

With regard to the entanglement of photon particles (light), scientists today know how to manage it in an empty space as well as at great distances. However, adding quantum memory to this equation makes the process much more complicated. Therefore, the researchers suggest that another type of approach might be preferable for this: the entanglement between atoms and photons on successive nodes (where the atoms are the nodes and the photons transmit the messages).

In other words, it is entanglement of photons with a twist, where atomic matter is added to the mixture to obtain a gain of efficiency, reliability and stability. According to scientists, with a good network of such nodes, we could create better basic technology for a future quantum Internet than with quantum entanglement using only photons.

During the experiment, the team used two storage units for quantum memory, which concretely were rubidium atoms cooled to a low energy state. Associated with entangled photons, they are then each part of an entangled system. Unfortunately, the more the photons have to travel by moving between the atoms, the more risks there are for the system to be disturbed. This is why this new record is so impressive.

According to the researchers, the key to this improvement and this record distance lies in a technique called “cavity improvement”, which reduces the loss of photon coupling during entanglement. The method involves placing the atoms of quantum memory in special rings, which reduces random noise that could interfere and destroy memory. The cavity has the additional advantage of improving the recovery of quantum information.

Thus, the coupled atoms and the photons produced by the improvement of the cavity constitute the node. Then, the photons are brought to a frequency suitable for transmission, through telecommunications networks (in this specific case, a network the size of a city).

You should know that Pan's scientific team has already established a quantum entanglement record, transmitting entangled photons between a satellite and the Earth over a distance of 1200 km, in 2017. This satellite system works well in space, but in the Earth's atmosphere, with all the interference present, it is less efficient and results in a loss of signal.


Diagram of the system for generating remote entanglement between atomic sets. Two distant quantum memory nodes are connected by a fiber optic channel and an intermediate station for photon measurement. In node A (B), an atomic set of 87Rb is placed inside a ring cavity. All atoms are first prepared in the basic state. A local entanglement is first created between the atomic set and a writing photon by applying a writing pulse (blue arrow). Then, the writing photon is collected clockwise (reverse) from the cavity and sent to the QFC module. Using a PPLN waveguide chip (PPLN-WG) and a 1950 nm pump laser (green arrow), the 795 nm (wavelength) writing photon is converted in O telecommunications band. After filtering the noise, two write photons are transmitted through long fibers, interfering in a base station and detected by two devices (SNSPD), with an efficiency of around 50%. The effective interference in the central station announces the entanglement of two sets. Fiber polarization controllers (PC) and polarization beam splitters (PBS) before interference from the base station are designed to actively compensate for polarization offset in long fiber. To recover the state of the atom, the researchers apply a read pulse (red arrow) in counter-propagation to the write pulse. Thanks to the phase correspondence between the spin wave and the improvement of the cavity, the atomic state is effectively recovered in a counter-clockwise direction from the ring cavity. Credits: Jian-Wei Pan / University of Science and Technology of China Fiber polarization controllers (PC) and polarization beam splitters (PBS) before interference from the base station are designed to actively compensate for polarization offset in long fiber. To recover the state of the atom, the researchers apply a read pulse (red arrow) in counter-propagation to the write pulse. Thanks to the phase correspondence between the spin wave and the improvement of the cavity, the atomic state is effectively recovered in a counter-clockwise direction from the ring cavity. Credits: Jian-Wei Pan / University of Science and Technology of China Fiber polarization controllers (PC) and polarization beam splitters (PBS) before interference from the base station are designed to actively compensate for polarization offset in long fiber. To recover the state of the atom, the researchers apply a read pulse (red arrow) in counter-propagation to the write pulse. Thanks to the phase correspondence between the spin wave and the improvement of the cavity, the atomic state is effectively recovered in a counter-clockwise direction from the ring cavity. Credits: Jian-Wei Pan / University of Science and Technology of China researchers apply a read pulse (red arrow) in counter-propagation to the write pulse. Thanks to the phase correspondence between the spin wave and the improvement of the cavity, the atomic state is effectively recovered in a counter-clockwise direction from the ring cavity. Credits: Jian-Wei Pan / University of Science and Technology of China researchers apply a read pulse (red arrow) in counter-propagation to the write pulse. Thanks to the phase correspondence between the spin wave and the improvement of the cavity, the atomic state is effectively recovered in a counter-clockwise direction from the ring cavity. Credits: Jian-Wei Pan / University of Science and Technology of China



During this experiment, the nodes of the atoms were in the same laboratory, but the photons still had to travel through cables extending over 50 kilometers. It is difficult to separate atoms more, but the proof of concept is there: "Despite enormous progress, at present, the maximum physical separation achieved between two nodes is 1.3 km, and challenges for longer distances remain,” explain the researchers.

"Our experiment could be extended to nodes physically separated by similar distances, which would thus form a functional segment of the atomic quantum network, opening the way to the establishment of an atomic entanglement on many nodes and on much longer distances.», They add.

Towards a quantum Internet?

This is where things get really interesting: while quantum memories could be the equivalent of computer memory in classical physics, the quantum version should be able to do a lot more, like for example processing more information in less time, or even solve problems that go far beyond those handled by our current computers.



When it comes to data communication, quantum technology promises to improve transmission speeds and secure data transfers thanks to the laws of quantum physics, provided we can make them work reliably over long distances.

According to the researchers, a quantum Internet that would connect remote quantum processors should allow a number of revolutionary applications, such as globalized quantum computing. "Its realization will be based on the entanglement of quantum memories over long distances", concluded the researchers.

Bibliography:


Entanglement of two quantum memories via fibres over dozens of kilometres.

Nature 578, 240–245 (2020).

https://doi.org/10.1038/s41586-020-1976-7

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