Physicists at the University of Bonn have demonstrated quantum entanglement between a stationary qubit (a quantum system with two states) and a photon directly connected to an optical fiber.
Encrypting data in such a way as to ensure secure communications is an ever-growing problem as critical components of modern encryption systems cannot withstand the quantum computers of the future. Therefore, researchers around the world are working on technologies for new encryption methods that are also based on quantum effects. The phenomenon of quantum entanglement plays a particularly important role here.
Quantum entanglement is a quantum mechanical phenomenon in which the quantum states of two or more objects are interdependent. This means that in a quantum network, the stationary qubits of the network are entangled in a communication channel, which usually consists of photons (light particles). For the first time Physicists from the University of Bonn have demonstrated quantum entanglement between a stationary qubit (a quantum system with two states) and a photon directly connected to an optical fiber (a photon of a fiber-optic resonator). The research results are published by the npj Quantum Information magazine.
Quantum systems are part of the technology of the future. When carriers of quantum information (quantum nodes) are interconnected by quantum channels, a quantum network is formed. Since 2009, scientists at the University of Bonn have been working on the implementation of a quantum network node, in which a separate ion in the form of a memory qubit is connected to an optical resonator as an interface between light and matter.
However, for the distribution of quantum information in the network, the stationary qubits of the network must be associated with a communication channel. The problem is that a quantum state cannot be copied and transmitted in the classical way. As a communication channel, photons are usually used, which are difficult to store, but they allow information to be transmitted quickly. Implementing efficient interfaces between photons and stationary qubits is critical to the information transfer rate and scalability of a quantum network.
In their experimental setup, scientists have implemented a special interface between light and matter. For this, physicists used an optical resonator consisting of two opposite mirrors at the ends of two fibers. The scientists also removed part of the optical fiber using a laser pulse, and then covered the ends with a reflective coating.
The design and combination of such a resonator with one is an experimental problem. The fibers and the ion must be positioned with a relative accuracy of about one micrometer relative to each other. However, a small cavity volume increases the interaction of light with matter. This provides high bandwidth for distributing quantum information over the network. Another advantage is that the fiber cavity results in the internal coupling of photons to the optical fibers. This greatly simplifies their distribution on the web.
Using their experimental setup, scientists were the first to demonstrate quantum entanglement between a stationary qubit and a photon in a fiber-optic resonator. They noticed that even at a distance of one and a half meters, a single ion and a photon shared a common entangled quantum state.
The research results will be useful in distributed quantum computing. Physicists plan to further develop their system, for example, by improving the stability of the light-matter interface and using a device for distributing quantum keys.