Researchers have demons­trated entanglement between a multimode quantum memory and a photon, after propagation in metro­politan fiber links. This type of remote light-matter entanglement lays the groundwork for fully independent quantum nodes that could be used to build quantum networks for highly secure information trans­mission. Samuele Grandi from the Institut de Ciencies Fotoniques (ICFO) at the Barcelona Institute of Science and Technology has presented this research at the Optica Quantum 2.0 Conference and Exhibition in Denver.

“Compatibility with the telecom network and with a solid-state quantum memory are some of the crucial properties required for the building blocks of the future quantum internet, which will connect remote quantum computers in a similar manner as the current classical internet,” explained Grandi. Delivering information over quantum networks requires sharing entangle­ment between the remote nodes of the network. One approach to accomplish this is by light to distribute entangle­ment across the current telecom network, connecting matter-based quantum systems located at the nodes.

These quantum nodes serve as storage, manipulation, and application units for quantum information. Nonetheless, any system designed for long-distance quantum communication must overcome various obstacles to establish quantum correlations between a quantum memory and a telecom photon. One significant challenge involves implementing quantum-based multi­plexed communi­cation, enabling optical fibers to transmit multiple data channels simultaneously. In the new work, the researchers performed a set of experiments over a quantum network testbed using a solid-state multi-mode quantum memory based on a praseodymium-doped crystal, which could be used for multiplexed quantum communication. They used spon­taneous parametric down-conversion to create photon pairs that contained an idler photon at telecom wavelength for propagation in optical fibers and a signal photon which was stored in the quantum memory.

The researchers confirmed entanglement between the multimode quantum memory and a telecom photon after its trans­mission through up to about 50 kilometers of deployed optical fiber in a metropolitan area. Non-classical correlations and light-matter entanglement were maintained after the transmission, with degra­dation coming only from a reduced signal-to-noise ratio.& As an additional step toward a field-deployable system, the researchers moved the idler detection station to a separate location 17 kilometers away and separated by about 47 kilo­meters of fiber. With this setup, which required additional synchronization of the remote setups, they were also able to demonstrate non-classical correlations between two locations.

Grandi added, “The next step will involve increasing the storage time in the quantum memory, to ensure that it is still storing its photon when the telecom one has reached its destination. This will remove the need for post-selection, and open the way to experi­ments featuring entangled and remote quantum memories.” (Source: Optica)

Links: Optica Quantum 2.0 Conference and Exhibition in Denver, USA • Quantum Photonics with Solids and Atoms Group, Institute of Photonic Sciences ICFO, Castelldefels, Spain