New breakthrough in quantum networks First long-distance entanglement of two ion quantum bits

Captured ions were previously only entangled in the same laboratory. Now, a team led by Tracy Northup and Ben Lanyon from the University of Innsbruck has entangled two ions at a distance of 230 meters [1]: long-range entanglement of two calcium ions, each located in a different building, has been achieved, showing that captured ions can be used to create quantum networks. The latest results have been published in Physical Review Letters [2].

Optical cavity traps atoms, fiber sends to achieve entanglement

A two-node quantum network. (a) Satellite image. Nodes A and B are located in different buildings, connected by 520(2) m optical fibers with a line-of-sight separation of 230 m. (b) The node consists of an ion, a linear Paul trap (four yellow electrodes) and a cavity composed of two mirrors. the PBSM device contains a beam splitter (BS), polarization beam splitter (PBS) and photon detectors. (c) Energy level mapping of Ca ions.

The nodes of this network are housed in two laboratories in Innsbruck, Austria. The experiment shows that captured ions are a promising platform for future quantum networks that span cities and eventually continents.

Captured ions are one of the main systems for building quantum computers and other quantum technologies. In order to connect multiple such quantum systems, interfaces are needed through which quantum information can be transmitted.

In recent years, a method has been developed by the Northup-Lanyon team at the Department of Experimental Physics at the University of Innsbruck that allows quantum information to be efficiently transferred to light particles by trapping atoms in an optical cavity. For each quantum bit, they use a dual-wavelength laser to excite the ion, prompting it to emit a photon. The polarization of the photon depends on which of the two laser wavelengths the ion absorbs, causing the photon to entangle with the final state of the ion. To entangle the two ions, the team then transmitted photons from one ion through a 510-meter-long fiber to a beam splitter near the other ion, where the two photons interacted. Entanglement was successfully achieved when the researchers then detected a pair of photons with a specific separate polarization.

The researchers say their use of an optical cavity was a key factor in achieving long-range entanglement because it allowed them to efficiently generate photons. In addition, unlike previous experiments with captured ions, they manipulated two quantum bits using independent control systems, suggesting that they could overcome challenges of time, frequency and phase stability that could affect real-world applications.

Entanglement between ionic quantum bits.

Captured ions: a platform for building distributed quantum networks

The two quantum systems are set up in two laboratories, one in the building of the Department of Experimental Physics and one in the building of the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences.

Quantum network nodes based on captured ions

"Until now, captured ions could only be entangled with each other within a few meters in the same laboratory. These results were also achieved using shared control systems and photons whose wavelengths were not suitable for propagation over longer distances." Lanyon explained that after years of research and development, physicists at Innsbruck have now successfully entangled the two ions.

"To do this, we send individual photons entangled with the ions through a 500-meter-long fiber optic cable and stack them on top of each other, exchanging the entanglement for two remote ions, Northup said in describing the experiment, "Our results suggest that capturing ions is a way to enable future quantum computers, quantum sensors and distributed networks of atomic clocks as a promising platform."

Lanyon and Northup's team is part of the Quantum Internet Consortium, an international project under the European Union's Quantum Flagship Program.

Reference links：

[1]https://phys.org/news/2023-02-entangle-ions-meter-quantum-network.html

[2]https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.050803