April 13, 2024

Transmitting the entanglement between light and matter in the Barcelona metropolitan network

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Measuring non-classical correlations between remote locations. Map of the Barcelona metropolitan region, with three highlighted locations: ICFO, where the SPDC memory and source are located; CTTI, where the two fiber optic segments are connected; i2CAT, where intermediate photons are detected. Credit: ICFO

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Measuring non-classical correlations between remote locations. Map of the Barcelona metropolitan region, with three highlighted locations: ICFO, where the SPDC memory and source are located; CTTI, where the two fiber optic segments are connected; i2CAT, where intermediate photons are detected. Credit: ICFO

As efforts to realize powerful quantum computers and quantum simulators continue, there is a parallel program that aims to achieve the quantum analogue of the classical Internet.

This new quantum network will provide ultra-secure, quantum cybersecurity and will eventually be dedicated to exchanging qubits, the unitary elements of quantum information and the language of quantum computers. In effect, it will provide a network through which different quantum computers can connect, like classical processors are connected in cloud computing.

An initial choice for the future quantum Internet infrastructure is, in fact, the existing telecommunications network, which provides a nearly ubiquitous channel through which light can travel very large distances with limited absorption. Because of this low absorption and high speed, light is a great candidate as a carrier of information, whether classical or quantum.

Bright laser light can be easily used to transfer classical information on the Internet, while light attenuation in optical fibers is compensated by light amplifiers placed every ten km within these fibers. However, the transfer of quantum information – quantum communication – requires much more sophisticated means.

Quantum bits are still encoded in light, specifically in single photons, but this quantum encoding cannot be amplified because the rules of quantum mechanics prevent it; If you try to amplify quantum coding, you will seriously damage the information contained in the photons. Therefore, amplifiers used in classical networks cannot be used for quantum bits. This means that a radically new technology is needed to build a quantum version of the Internet: the quantum repeater.

As light amplifiers ensure connectivity between distant locations, quantum repeaters will enable long-distance communication by distributing entanglement between them.

Entanglement is a uniquely quantum property of two objects that presents correlations that cannot be reproduced by classical means, and is one of the main components of quantum communication. It can be used to transfer quantum information, for example through quantum teleportation between two nodes of a quantum repeater system.

One way to establish remote entanglement between two nodes is through direct transmission: an entangled pair of photons can be generated, with one remaining stationary while the other travels to the other location. This means that the latter must be compatible with optical fiber transmission, while the former must be stored in a quantum memory, leading to entanglement between light and matter.

Now, a set of quantum repeaters is needed to pair several of these nodes to achieve long-distance entanglement between quantum memories. One promising architecture for these quantum repeater nodes relies on pairing the spontaneous generation of photon pairs, a process known as spontaneous downconversion (SPDC), with an external quantum memory.

This is the approach adopted by ICFO researchers. In a new study published in arXiv preprint server, Jelena Rakonjac, Samuele Grandi, Soren Wengerowsky, Dario Lago-Rivera and Felicien Appas, led by ICREA Prof. at ICFO Hugues de Riedmatten demonstrate the transmission of light-matter entanglement over tens of kilometers of optical fiber.

In their experiment, they generated pairs of photons, where one is emitted at the telecommunications wavelength of 1436nm, while the other is emitted at 606nm, compatible with the solid-state quantum memories used, carried out in special crystals doped with earth atoms. rare.

They then accessed the Barcelona metropolitan network, connecting their system to two fibers running from the ICFO in Castelldefels to the Catalan Telecommunications Center (CTTI) in Hospitalet de Llobregat. By connecting the two centers, they created a 50 km ring, sending photons to the center of Barcelona and back to ICFO.

With this, they demonstrated that after a complete journey of 50 km, the light generated in the laboratory maintains its quantum characteristics without substantial decrease, showing that photonic qubits do not manifest decoherence when traveling tens of km in an optical fiber cable, even in a Metropolitan area. In short, quantum light has left the laboratory and has finally been detected at its source.

However, quantum communication requires the use and verification of entanglement between remote locations, where entangled photons are detected at locations well separated in space and time. Following in this direction, the researchers expanded their network to include a new node, this time located in the i2CAT foundation, a building in Barcelona, ​​approximately 44 km from ICFO via the local fiber optic network and 17 km online. straight.

There, they installed a telecommunications detector to measure the arrival of photons that passed through one of the fibers while the other fiber was connected to a transducer, which transformed the electrical signal from the detector into light and sent it along the fiber optic line.

In this way, information could be transmitted to ICFO with high precision, even if the photon was detected about 17 km away. Furthermore, they used the same transducers to send synchronization signals between the two nodes of this basic network, where the generation and detection of quantum correlations were completely separated between two independent but connected nodes.

The experiment validated the system used by researchers to generate light-matter entanglement and proved to be one of the pioneering candidates for realizing a quantum repeater node, the technology that enables long-distance quantum communication. Proof-of-principle demonstrations have already been performed in the laboratory and the group is now working to improve memory and font performance.

Furthermore, researchers have partnered with Cellnex (Xarxa Roberta de Catalunya), and a new laboratory is available in the Collserola tower in the context of the QNetworks and EuroQCI Spain projects for realizing an entangled state of remote quantum memories.

The realization of a long-distance backbone for entanglement distribution between quantum memories is also one of the main goals of the Quantum Internet Alliance (QIA), the main European effort in realizing the quantum Internet of which ICFO is the main partner.

The results of this study, “namely the transmission of light-matter entanglement over fibers deployed in a metropolitan area, are the initial springboard towards the realization of a complete quantum Internet, with our source and quantum memory node at its core,” comments Samuele Grandi, ICFO researcher and co-author of the study.

As Hugues de Riedmatten, ICREA professor at ICFO, concludes, “Light-matter entanglement is a key feature for quantum communication and has been demonstrated many times in the laboratory. Demonstrating it in the installed fiber network is a first step towards realizing a bank testing ground for quantum repeater technologies in the Barcelona area, setting the stage for fiber-based long-distance networks.”

More information:
Jelena V. Rakonjac et al, Transmission of light-matter entanglement in a metropolitan network, arXiv (2023). DOI: 10.48550/arxiv.2304.05416

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