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Quantum networks, systems consisting of connected quantum computers, quantum sensors or other quantum devices, hold the potential of enabling faster and safer communications. The establishment of these networks relies on a quantum phenomenon known as entanglement, which entails a link between particles or systems, with the quantum state of one influencing the other even when they are far apart.

The atom-based qubits used to establish quantum networks so far operate at visible or ultraviolet wavelength, which is not ideal for the transmission of signals over long distances via optical fibers. Converting these signals to telecom-band wavelengths, however, can reduce the efficiency of communication and introduce undesirable signals that can disrupt the link between qubits.

A research team at University of Illinois at Urbana-Champaign, led by Prof. Jacob P. Covey recently realized telecom-band wavelength quantum networking using an array of ytterbium-171 atoms. Their paper, in Nature Â鶹ÒùÔºics, introduces a promising approach to realize high-fidelity entanglement between atoms and optical photons generated directly in the telecommunication band.

"Networks of quantum devices with shared entanglement present new opportunities in quantum information science," Xiye Hu, co-author of the paper, told Â鶹ÒùÔº.

"Ytterbium-171, conventionally employed in optical atomic clocks due to its long-lived metastable state, has emerged as a resourceful candidate in the atom array community with novel applications in quantum computation and metrology."

To realize their quantum network, Hu and his colleagues leveraged the unique properties of ¹⁷¹Yb atom arrays, which are known to be promising for long-range communications. Their network marks a significant step towards the realization of a network of quantum processors that can support distributed computing or a quantum network of atomic clocks for precise timekeeping and sensing applications.

"From the metastable state in ¹⁷¹Yb exists a moderately broad transition at 1389-nm, which we utilized to realize time-bin encoded entanglement between a single atom and a telecom-band single photon with high-fidelity," explained Hu.

"By imaging our one-dimensional atom array onto a commercial fiber array, we showed that the collection of single photons, and the subsequent generation of entanglement, can be parallelized across the array."

Hu and his colleagues demonstrated the feasibility of their parallelized quantum networking approach in a series of tests and found that it yielded a uniformly high entanglement fidelity and negligible crosstalk across different sites on the network. They then also engineered a 'mid-circuit networking protocol," a tool that allows the coherence of data qubits to be preserved during networking attempts.

"We studied in detail both the physical and technical factors that limit the achieved time-bin encoded atom-photon entanglement fidelity, and provided concrete solutions for improvements," said Hu.

"Crucially, we showed that 99% fidelity is readily achievable with technical upgrades. Second, we confirmed that the fiber array does not introduce additional error sources that may hinder entanglement fidelity."

A key feature of the ¹⁷¹Yb atom array employed by the researchers is its geometrical resemblance to a fiber array. Hu and his colleagues believe that their network could thus be useful for tackling generalized parallelization tasks (i.e., tasks that can be divided into smaller subtasks and completed simultaneously by different qubits or devices in a network).

The design strategies and mid-circuit networking protocol developed by these researchers could soon be used by other research teams to realize parallelized quantum networks. The protocol proved to be highly promising for scheduling networking tasks, while persevering computation or storage coherence on a single quantum processor within a larger network.

"One of the most substantial improvements we can make as part of our future work is to switch from using an objective lens to using a cavity for single photon collection," said Hu. "Among others, cavity provides orders of magnitude improvement to collection efficiency which greatly enhances networking rate."

Researchers at the Covey Lab are currently designing a new second-generation ytterbium experiment aimed at realizing high-rate and long-distance communication within a quantum network. In this experiment, the team plan to place their atom array inside a macroscopic confocal cavity that is coated for the 1,389-nm transition.

"The time-bin encoded atom-photon entanglement demonstrated as part of our recent work will also ultimately be extended to realize remote atom-atom entanglement, either between two atoms within a single apparatus, or between two atoms in two different apparatuses," added Hu.

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