Establishing the foundation for future quantum communication frameworks, engineers at Caltech have showcased the effective functioning of a quantum network comprising two nodes, each housing numerous quantum bits, or qubits—the essential information-holding elements of quantum computers.
To accomplish this, the researchers devised a novel protocol for distributing quantum information in a parallel approach, thereby generating multiple channels for data transmission, or multiplexing. This achievement was realized by embedding ytterbium atoms into crystals and linking them to optical cavities—nanoscale architectures that capture and guide light. This platform possesses unique characteristics that render it ideal for utilizing multiple qubits to relay quantum information-carrying photons in parallel.
“This marks the first demonstration of entanglement multiplexing in a quantum network of discrete spin qubits,” remarks Andrei Faraon (BS ’04), the William L. Valentine Professor of Applied Physics and Electrical Engineering at Caltech. “This technique greatly enhances quantum communication rates between nodes, signifying a substantial advancement in the field.”
The findings are detailed in a paper released on February 26 in the journal Nature. The primary authors of the publication are Andrei Ruskuc (PhD ’24), currently a postdoctoral fellow at Harvard University, and Chun-Ju Wu, a graduate student at Caltech, who completed the study in Faraon’s laboratory.
Much like how the internet links the classical computers we utilize today, the quantum networks of tomorrow will interconnect quantum computers situated in various physical locations.
In the quantum domain, researchers engage with the minuscule dimensions of individual atoms and photons, the fundamental particles of light. At this scale, matter does not conform to classical physics; rather, quantum mechanics govern behavior.
One of the most crucial and peculiar principles in quantum mechanics is entanglement, where two or more entities such as atoms or photons are irrevocably associated, irrespective of their physical distances. This connection is so intrinsic that one particle cannot be entirely described without considering the other. Consequently, measuring the quantum state of one also reveals information about the other, which is essential for quantum communication.
In quantum communication, the objective is to utilize entangled atoms as qubits to share or teleport quantum information. The principal obstacle that has so far restricted communication rates is the duration required to prepare qubits and transmit photons.
“Entanglement multiplexing addresses this issue by utilizing multiple qubits per processor, or node. By preparing qubits and sending photons simultaneously, the entanglement rate can be proportionally increased based on the number of qubits,” explains Ruskuc.
In the newly established system, the two nodes are nanofabricated constructs composed of yttrium orthovanadate (YVO4) crystals. Lasers are employed to excite ytterbium atoms (Yb3+), a rare-earth element, within these crystals, resulting in each atom emitting a photon that remains entangled with it. Photons from atoms in two distinct nodes subsequently travel to a central site where they are detected. This detection process initiates a quantum processing protocol that leads to the formation of entangled states between pairs of ytterbium atoms.
Each node consists of numerous ytterbium atoms within the YVO4 crystal, ensuring ample qubits are available. However, each of these atoms displays a slightly varied optical frequency due to defects within the crystal.
“This scenario resembles a double-edged sword,” Ruskuc comments. On one side, the varying frequencies enable the researchers to finely adjust their lasers to focus on specific atoms. Conversely, scientists previously thought that the corresponding disparities in photon frequencies would render it impossible to produce entangled qubit states.
“That’s where our protocol comes into play. It is an innovative method for generating entangled states of atoms even when their optical transitions differ,” Ruskuc states.
In the new protocol, the atoms undergo a sort of customized quantum processing in real-time once the photons are detected at the central location. The researchers refer to this processing as “quantum feed-forward control.”
“Essentially, our protocol harnesses the information received from the photon arrival time and applies a quantum circuit: a sequence of logic gates specifically designed for the two qubits. After implementing this circuit, we are left with an entangled state,” Ruskuc elucidates.
The team’s YVO4 platform can support many qubits—in this research, each node contained around 20. “However, it might be feasible to elevate that number by at least an order of magnitude,” asserts co-author Wu.
“The distinctive traits of rare-earth ions combined with our demonstrated protocol set the stage for networks housing hundreds of qubits per node,” Faraon remarks. “We believe this research establishes a solid foundation for high-performance quantum communication systems based on rare-earth ions.”
Additional Caltech contributors to the paper, “Multiplexed Entanglement of Multi-emitter Quantum Network Nodes,” include graduate student Emanuel Green; AWS Quantum Postdoctoral Scholar Research Associate Sophie L. N. Hermans; graduate student William Pajak; and Joonhee Choi of Stanford University, a former postdoctoral scholar from Faraon’s lab. Device nanofabrication was conducted at the Kavli Nanoscience Institute at Caltech. The research received primary support from the Air Force Office of Scientific Research and IQIM, a National Science Foundation Physics Frontiers Center at Caltech, which is backed by the Gordon and Betty Moore Foundation. Additional funding was provided by the NSF.