Quantum computers are considered the next big evolu­tionary step in information techno­logy. They are expected to solve computing problems that today's computers simply cannot solve – or would take ages to do so. Research groups around the world are working on making the quantum computer a reality. This is anything but easy, because the basic components of such a computer, the quantum bits or qubits, are extremely fragile. One type of qubits consists of the intrinsic angular momentum of a single electron. It is hard enough to keep such a fragile system intact. It is even more difficult to interconnect two or more of these qubits. So how can a stable exchange of infor­mation between qubits be achieved? The two Konstanz physicists Benedikt Tissot and Guido Burkard have now developed a theo­retical model of how the information exchange between qubits could succeed by using photons as a “means of transport” for quantum information.

The general idea: The information content – the electron spin state – of the material qubit is converted into a “flying qubit” – a photon. The special feature of the new model: stimu­lated Raman emissions are used for converting the qubit into a photon. This procedure allows more control over the photons. “We are proposing a paradigm shift from optimizing the control during the generation of the photon to directly optimi­zing the temporal shape of the light pulse in the flying qubit,” explains Guido Burkard.

Benedikt Tissot compares the basic procedure with the Internet: “In a classic computer, we have our bits, which are encoded on a chip in the form of electrons. If we want to send information over long distances, the infor­mation content of the bits is converted into a light signal that is transmitted through optical fibers”. The principle of infor­mation exchange between qubits in a quantum computer is very similar: “Here, too, we have to convert the information into states that can be easily transmitted – and photons are ideal for this”, explains Tissot.

“We need to consider several aspects”, says Tissot: “We want to control the direction in which the infor­mation flows – as well as when, how quickly and where it flows to. That's why we need a system that allows for a high level of control”. The researchers’ method makes this control possible by means of resonator-enhanced, stimulated Raman emissions. Behind this term is a three-level system, which leads to a multi-stage procedure. These stages offer the physicists control over the photon that is created. “We have more buttons here that we can operate to control the photon”, Tissot illus­trates.

timulated Raman emission are an established method in physics. However, using them to send qubit states directly is unusual. The new method might make it possible to balance the conse­quences of environ­mental pertur­bations and unwanted side effects of rapid changes in the temporal shape of the light pulse, so that information transport can be implemented more accurately. (Source: U. Konstanz)

Reference: B. Tissot & G. Burkard: Efficient high-fidelity flying qubit shaping, Phys. Rev. Research 6, 013150 (2024); DOI: 10.1103/PhysRevResearch.6.013150

Link: Condensed matter theory and quantum information, Dept. of Physics, University of Konstanz, Konstanz, Germany