According to prevailing wisdom, long-distance quantum computing requires the ability to entangle qubits made from matter, in which information is stored and manipulated locally, with photonic qubits that can transmit the information. Entanglement between single trapped atoms and photons is limited in scalability, which is a significant challenge for long-distance quantum computing and communications. A promising approach to building scalable quantum networks, from the Max Planck Institute of Quantum Optics (MPQ), uses optical tweezers to entangle atoms that are trapped in an optical resonator with individual photons. The entanglement between atom and photon is maintained as the photon travels. With this technique, the MPQ team hopes to forge a possible route to distributed quantum information processing for future quantum communication and computing applications. In 2012, the MPQ team entangled an atom in one resonator with a second atom in another resonator, using a glass fiber 60 m long to transmit a photon. With the help of the transmitted photon, the researchers formed an extended entangled quantum object from the two atoms. However, the team needed a way to ensure that the photon would not become lost in the fiber during longer transmissions. A visualization of rubidium atoms trapped in the optical resonator and addressed individually using a highly focused laser beam is shown. This approach allows the researchers to entangle the trapped atoms with individual photons. Courtesy of the Max Planck Institute of Quantum Optics. For its current investigation, the team formed an optical resonator by trapping ultracold rubidium atoms between two almost perfect mirrors. This setup supports efficient interaction between the trapped atom and the photon. The team made one mirror slightly more transparent than the other. This compels the photon to travel in a predetermined direction, ensuring reliable coupling into an optical fiber. The researchers used multiplexing to overcome transmission losses. To achieve multiplexing in a quantum network, the atoms need to be entangled in parallel, with one photon per atom. Therefore, the position of each atom had to be identified. “Without multiplexing, even our current Internet would not work,” said researcher Emanuele Distante, who supervised the research. “But transferring this method to quantum information systems is a particular challenge.” To implement multiplexing, the team had to load several atoms into a resonator as resting qubits and address each atom individually. For this task, the team used optical tweezers with both the strength and sensitivity needed to capture an atom and move it to the desired location. The researchers developed a technique for inserting the tweezers into the narrow resonator. “The mirrors are only about half a millimeter apart,” researcher Lukas Hartung said. With this technique, up to six tweezers can be used to arrange six floating rubidium atoms in the optical cavity, forming a qubit lattice. To focus six different beams into the resonator, the researchers positioned a microscope lens objective with μm precision above the resonator. To control each beam individually, they generated the tweezer beams using acousto-optic deflectors. Adjusting the laser tweezers to focus within the narrow optical resonator was not easy. “Mastering this challenge was the cornerstone for the success of the experiment,” researcher Stephan Welte said. So far, the MPQ team has been able to manipulate up to six rubidium atoms as quantum bits in the optical resonator using the light tweezers. To make the atoms visible, the researchers excite them to emit light. In theory, the resonator can hold up to 200 atoms. Courtesy of the Max Planck Institute of Quantum Optics. The team stimulated the emission of a photon from each atom using the beam from the tweezers. The atoms can easily remain in the optical trap for a minute (a small eternity in quantum physics), which allows plenty of time for each atom to be entangled with a photon. The researchers demonstrated multiplexed atom-photon entanglement with a generation-to-detection efficiency approaching 90%. “This works almost one hundred percent of the time,” Distante said. The entanglement distribution works deterministically, making the outcome of the process more predictable. Multiplexing could provide more secure transmission over longer distances in a future quantum internet and local quantum networks. For example, in a quantum computer network comprising several processors connected via short optical fibers, the resting qubits could be entangled more reliably by multiplexing with flying qubits, resulting in a more powerful distributed network. Interfaces between resting qubits and flying qubits are critical when quantum information must be transmitted over long distances. “One aspect is the communication of quantum information over long distances in a future quantum internet,” Distante said. “The second aspect is the goal of connecting many qubits in a distributed network to form a more powerful quantum computer. Both applications require efficient interfaces between qubits at rest and qubits in motion.” The new technique has the potential to be scaled up to considerably more qubits without losses. The team estimates that up to 200 atoms could be controlled effectively in its optical resonator. As the interface can feed almost 100% of the entangled photons into the optical fiber, a network of many resonators, each with 200 atoms as resting qubits, is not out of the question, the researchers said. The research was published in Science (www.doi.org/10.1126/science.ado6471).