Scientists at the University of Basel have made a groundbreaking achievement by coupling two Andreev qubits coherently over a considerable distance for the very first time. They accomplished this by utilizing microwave photons produced in a narrow superconducting resonator. Their findings have been documented in a recent publication in Nature Physics, paving the way for the potential use of linked Andreev qubits in the realms of quantum communication and quantum computing.
Quantum communication and computing rely on quantum bits (qubits) as their fundamental units of information, similar to how bits work in classical computers. Among the various methods that are under exploration globally, Andreev pair qubits present a promising option.
Andreev qubits are created at the junctions between a metal and a superconductor through a process called Andreev reflection. In this process, an electron from the metal enters the superconductor, becoming part of an electron pair (specifically, a Cooper pair), while a hole, functioning like a positive particle, reflects back into the metal. This mechanism generates distinct pairs of bound states at the interface of these materials, known as Andreev bound states, which can act as the fundamental states of a qubit. These states tend to be relatively stable against external disturbances, and the coherence time—meaning the period during which superposition is maintained—is also notably long. Additionally, they can be easily manipulated and integrated into contemporary electronic circuits. All of these characteristics make them highly favorable for the advancement of dependable and scalable quantum computers.
Interconnection Between Two Quantum Systems
The researchers have succeeded in establishing a robust quantum mechanical coupling between two Andreev qubits, which are each situated in a semiconducting nanowire. The experimental outcomes align perfectly with theoretical predictions.
“We managed to couple the two Andreev pair qubits over a significant distance, positioned at opposite ends of a long, superconducting microwave resonator. This setup facilitates the exchange of microwave photons between the resonator and the qubits,” explains Professor Christian Schönenberger from the Department of Physics and the Swiss Nanoscience Institute at the University of Basel, whose team conducted the experiments.
The microwave resonator serves two key functions: One allows the researchers to read the qubits using the resonator, providing insights about their quantum states. The other mode enables the coupling of the two qubits to each other, enabling them to “communicate” without the loss of microwave photons. As a result, the qubits cease to act as independent entities and instead share a new combined quantum state—an essential development for quantum communication and computing.
“In our research, we integrate three quantum systems to enable photon exchange among them. Our qubits are roughly 100 nanometers in size, yet we connect them over a macroscopic distance of 6 millimeters,” states Dr. Andreas Baumgartner, a co-author of the article. “This demonstrates that Andreev pair qubits are viable as compact and scalable solid-state qubits.”
This research was a collaborative effort involving teams from the universities of Basel, Copenhagen, Karlsruhe, and Yale, under the European FET Open initiative AndQC.
