Laying the groundwork for quantum communication systems of the future, engineers at Caltech have demonstrated the successful operation of a quantum network of two nodes, each containing multiple quantum bits, or qubits—the fundamental information-storing building blocks of quantum computers.
To achieve this, the researchers developed a new protocol for distributing quantum information in a parallel manner, effectively creating multiple channels for sending data, or multiplexing. The work was accomplished by embedding ytterbium atoms inside crystals and coupling them to optical cavities—nanoscale structures that capture and guide light. This platform has unique properties that make it ideal for using multiple qubits to transmit quantum information-carrying photons in parallel.
"This is the first-ever demonstration of entanglement multiplexing in a quantum network of individual spin qubits," says Andrei Faraon (BS '04), the William L. Valentine Professor of Applied Physics and Electrical Engineering at Caltech. "This method significantly boosts quantum communication rates between nodes, representing a major leap in the field."
The work is described in a paper published on February 26 in the journal Nature. The lead authors of the paper are Andrei Ruskuc (PhD '24), now a postdoctoral fellow at Harvard University, and Chun-Ju Wu, a graduate student at Caltech, who completed the work in Faraon's lab.
Just as the internet connects the classical computers we are accustomed to using today, the quantum networks of the future will connect quantum computers that exist in different physical locations.
When working with the quantum realm, researchers are dealing with the miniscule scale of individual atoms and of photons, the basic particles of light. At this scale, matter does not behave according to classical physics; instead, quantum mechanics are at play.
One of the most important and bizarre concepts in quantum mechanics is that of entanglement, where two or more objects such as atoms or photons are inextricably linked regardless of their physical separation. This connection is so fundamental, that one particle cannot be fully described without reference to the other. As a result, measuring the quantum state of one also provides information about the other, which is key to quantum communication.
In quantum communication, the goal is to use entangled atoms as qubits to share, or teleport, quantum information. The key challenge that has thus far limited communication rates is the time it takes to prepare qubits and to transmit photons.
"Entanglement multiplexing overcomes this bottleneck by using multiple qubits per processor, or node. By preparing qubits and transmitting photons simultaneously, the entanglement rate can be scaled proportionally to the number of qubits," says Ruskuc.
In the new system, the two nodes are nanofabricated structures made from crystals of yttrium orthovanadate (YVO4). Lasers are used to excite ytterbium atoms (Yb3+), a rare-earth metal, within these crystals, causing each atom to emit a photon that remains entangled with it. Photons from atoms in two separate nodes then travel to a central location where they are detected. That detection process triggers a quantum processing protocol that leads to the creation of entangled states between pairs of ytterbium atoms.
Each node has many ytterbium atoms within the YVO4 crystal, so there are plenty of available qubits. However, each of those atoms has a slightly different optical frequency caused by imperfections within the crystal.
"This is like a double-edged sword," Ruskuc says. On one hand, the differing frequencies allow the researchers to fine-tune their lasers to target specific atoms. On the other, scientists previously believed that the corresponding differences in photon frequencies would make it impossible to generate entangled qubit states.
"That's where our protocol comes in. It is an innovative way to generate entangled states of atoms even when their optical transitions are different," Ruskuc says.
In the new protocol, the atoms undergo a kind of tailored quantum processing in real time once the photons are detected at the central location. The researchers call this processing "quantum feed-forward control."
"Basically, our protocol takes this information that it received from the photon arrival time and applies a quantum circuit: a series of logic gates that are tailored to the two qubits. And after we've applied this circuit, we are left with an entangled state," Ruskuc explains.
The team's YVO4 platform can accommodate many qubits—in this work, each node contained approximately 20. "But it may be possible to increase that number by at least an order of magnitude," says co-author Wu.
"The unique properties of rare-earth ions combined with our demonstrated protocol pave the way for networks with hundreds of qubits per node," Faraon says. "We believe this work lays a robust foundation for high-performance quantum communication systems based on rare-earth ions."
Additional Caltech authors of the paper, "Multiplexed Entanglement of Multi-emitter Quantum Network Nodes," are 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 performed in the Kavli Nanoscience Institute at Caltech. The work was supported primarily by the Air Force Office of Scientific Research and IQIM , a National Science Foundation Physics Frontiers Center at Caltech that receives support from the Gordon and Betty Moore Foundation. Additional funding came from the NSF.
