The innovative atomic device enables a simpler way to connect quantum computers

Princeton researchers present a new way to build quantum repeaters using light ready for communications from a single ion, with calcium tungstate emerging as the ideal material. Future work aims to improve the quantum state storage duration.

A new atomic device transmits high-resolution quantum information over fiber-optic networks.

Researchers have revealed a new way to connect quantum devices over long distances, a necessary step toward allowing the technology to play a role in future communication systems.

While today’s classical data signals can be amplified across a city or ocean, quantum signals cannot. They must be repeated at intervals, i.e. stopped, copied and passed around, by specialized machines called quantum repeaters. Many experts believe that these quantum repeaters will play a key role in future communications networks, allowing for enhanced security and enabling communications between distant quantum computers.

A new approach to quantum repeaters

The Princeton Study, published today (August 30) in nature, details the basis for a new approach to constructing quantum repeaters. It sends a communications-ready light emitting from a single ion implanted in a crystal. This effort has taken many years, according to Jeff Thompson, lead author of the study. The work combines advances in optical design and materials science.

Other pioneering quantum repeater designs emit light in the visible spectrum, which is rapidly decayed by optical fibers and must be converted before traveling long distances. The new device is based on a single rare earth ion implanted in a host crystal. Because this ion emits light at the ideal infrared wavelength, it does not require such signal conversion, which could lead to simpler and more robust networks.

Researchers from Princeton University have devised a new way to link quantum computers to high-resolution signals using wavelengths of light in the communications range. Credit: Photo by Samir A. Khan / Photobody

Device design and functionality

The device consists of two parts: a calcium tungstate crystal inlaid with a few erbium ions, and a silicon nanopiece etched into a J-shaped channel. The ion is pulsed with a special laser, emitting light through the crystal. But the silicon piece, a piece of semiconductor stuck to the top of the crystal, captures individual photons and directs them into the fiber-optic cable.

Ideally, this Photon It would be encoded with information from the ion, Thompson said. Or more specifically, from a quantum property of the ion called spin. In a quantum repeater, collecting signals from distant nodes and interfering with them would create entanglement between their spins, allowing quantum states to be transmitted end-to-end despite losses along the way.

Material selection and testing

Thompson’s team first began working with erbium ions several years ago, but early versions used different crystals that contained a lot of noise. In particular, this noise caused the frequency of the emitted photons to jump randomly in a process known as spectral diffusion. This prevented the subtle quantum interference necessary for the operation of quantum networks. To solve this problem, his lab began working with Natalie de Leon, assistant professor of electrical and computer engineering, and Robert Cava, a leading solid-state materials scientist and the Russell Wellman Moore Professor of Chemistry at Princeton University, to explore new materials that could host single organisms. Erbium ions are much less noisy.

They sorted the list of candidate materials from hundreds of thousands to a few hundred, then a few dozen, then three. Each of the finalists took half a year to audition. Turns out the first article wasn’t quite clear enough. The second caused erbium to have weak quantum properties. But the third, calcium tungstate, was just right.

Demonstrate the potential of new materials

To prove that the new material is suitable for quantum networks, the researchers built an interferometer in which photons randomly pass through one of two paths: a short path several feet long, or a long path 22 miles long (made of coiled optical fibers). the fiber). The photons emitted by the ion can either go the long path or the short path, and in about half the time, successive photons take opposite paths, arriving at the output at the same time.

When such a collision occurs, quantum interference causes the photons to leave the output in pairs that are essentially indistinguishable – and have the same shape and frequency. Otherwise, they leave the interferometer individually. By observing strong suppression – up to 80% – of single photons at the interferometer’s outputs, the team demonstrated conclusively that the erbium ions in the new material emit indistinguishable photons. According to Salim Orari, a graduate student who co-led the research, that puts the signal well above the hi-fi threshold.

future work

While this work crosses an important threshold, additional work is needed to improve the storage time of quantum states in the spin of an erbium ion. The team is currently working to manufacture calcium tungstate of high purity, with fewer impurities that disturb quantum spin states.

Reference: “Indistinguishable telecom band photons from a single erbium ion in the solid state” Slim Orari, Lukasz Dusanowski, Sebastian B. Caffa, Natalie B. De Leon and Jeff D. Thompson, Aug. 30, 2023, Available here. nature.
doi: 10.1038/s41586-023-06281-4

The paper has been published in the journal nature With support from the US Department of Energy, Office of Science, National Centers for Quantum Information Science Research, and the Joint Design Center for Quantum Advantage (C2QA). In addition to Thompson, Cava, and Orari, authors include Lukas Dusanowski, Sebastian B. Horvath, Muhammad T Uysal, Christopher M. Fennessy, Paul Stephenson, Muktik Raha, Songtao Chen, and Natalie De Leon. Orari, Dusanowski, Horvarth, and Uysal all contributed equally.