An Optically Controlled Quantum Network

Researchers at the Univer­sity of Rochester and Cornell University have taken an important step toward developing a communications network that exchanges information across long distances by using photons, mass-less measures of light that are key elements of quantum computing and quantum communi­cations systems. The research team has designed a nanoscale node made out of magnetic and semi­conducting materials that could interact with other nodes, using laser light to emit and accept photons. The development of such a quantum network – designed to take advantage of the physical properties of light and matter charac­terized by quantum mechanics – promises faster, more efficient ways to communicate, compute, and detect objects and materials as compared to networks currently used for computing and communi­cations.

This illustration of a nanoscale node shows a closeup of one of an array pillars, each a mere 120 nanometers high. Each pillar serves as a location marker for a quantum state that can interact with photons. (Source: M. Osadciw, U. Rochester)

The node consists of an array of pillars a mere 120 nanometers high. The pillars are part of a platform containing atomically thin layers of semi­conductor and magnetic materials. The array is engi­neered so that each pillar serves as a location marker for a quantum state that can interact with photons and the associated photons can poten­tially interact with other locations across the device and with similar arrays at other locations. This potential to connect quantum nodes across a remote network capitalizes on the concept of entanglement, a pheno­menon of quantum mechanics that, at its very basic level, describes how the properties of particles are connected at the subatomic level. “This is the beginnings of having a kind of register, if you like, where different spatial locations can store information and interact with photons,” says Nick Vamivakas, professor of quantum optics and quantum physics at Rochester.

The project builds on work the Vamivakas Lab has conducted in recent years using tungsten dise­lenide (WSe2) in Van der Waals hetero­structures. That work uses layers of atomically thin materials on top of each other to create or capture single photons. The new device uses a novel alignment of WSe2 draped over the pillars with an underlying, highly reactive layer of chromium triiodide (CrI3). Where the atomically thin, 12-micron area layers touch, the CrI3 imparts an electric charge to the WSe2, creating a hole alongside each of the pillars. Each posi­tively charged hole also has a binary north/south magnetic property associated with it, so that each is also a nanomagnet.

When the device is bathed in laser light, further reactions occur, turning the nanomagnets into indi­vidual optically active spin arrays that emit and interact with photons. Whereas classical infor­mation processing deals in bits that have values of either 0 or 1, spin states can encode both 0 and 1 at the same time, expanding the possi­bilities for information processing. “Being able to control hole spin orientation using ultrathin and 12-micron large CrI3, replaces the need for using external magnetic fields from gigantic magnetic coils akin to those used in MRI systems,” says graduate student Arunabh Mukherjee. “This will go a long way in minia­turizing a quantum computer based on single hole spins.”

Two major challenges confronted the researchers in creating the device. One was creating an inert environment in which to work with the highly reactive CrI3. This was where the colla­boration with Cornell University came into play. “They have a lot of expertise with the chromium triiodide and since we were working with that for the first time, we coor­dinated with them on that aspect of it,” Vamivakas says. For example, fabri­cation of the CrIwas done in nitrogen-filled glove boxes to avoid oxygen and moisture degra­dation.

The other challenge was deter­mining just the right configuration of pillars to ensure that the holes and spin valleys associated with each pillar could be properly registered to eventually link to other nodes. And therein lies the next major challenge: finding a way to send photons long distances through an optical fiber to other nodes, while preserving their properties of ent­anglement. “We haven’t yet engi­neered the device to promote that kind of behavior,” Vamivakas says. “That’s down the road.” (Source: U. Rochester)

Reference: A. Mukherjee et al.: Observation of site-controlled localized charged excitons in CrI3/WSe2 heterostructures, Nat. Commun. 11, 5502 (2020); DOI: 10.1038/s41467-020-19262-2

Link: Quantum Nanophotonics Group, Institute of Optics, University of Rochester, Rochester, USA

Further reading: Overview Quantum Technologies, PhotonicsViews 6/2020

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