Quantum computers allow to process a large number of data simultaneously and therefore pledge to solve complex problems much faster than computers can do today. Both, academia and companies such as Google or IBM research the underlying physics to realize quantum computers. An international team of German and Russian researchers from Leibniz-Institute of Photonics Technology Jena (Leibniz-IPHT), Karlsruhe Institute of Technology (KIT), and National University of Science and Technology (NUST MISIS) in Moscow realized the first quantum metamaterial showing unique interaction with microwave photons. The meta material consists of a linear array of 15 meta atoms, called quantum bits (qubits): loops of few microns diameter made of aluminum. They are superconducting at their working temperature of ca. -273°C, meaning they transport electrical current without resistance. At some positions the aluminum loops are interrupted by nanoscale tunnel structures, called Josephson junctions. These qubits present superconducting circuits in which current flows in two accurately defined states.
Magnetic field-controlled switching of properties
Now, the scientists for the first time created a quantum metamaterial from so-called “twin” qubits. A twin qubit consists of two connected qubit loops and therefore possesses five instead of three Josephson junctions. The unusual structures have been realized using electron lithography in the cleanroom of Leibniz-IPHT. “We investigated the double-loop qubits’ behavior by switching them into two different configurations using an external magnetic field. Here, we observed unexpected properties of the metamaterial. Through the applied magnetic field, we can exactly control its transmission for microwaves. We have been surprised about the possibility to switch the microwave transmission of these particular quantum metamaterials via the ground state configuration of the qubits. So far, that was unknown,” explains Leibniz-IPHT scientist Prof. Evgeni Il’ichev the discovery. The results of the research work, which was headed by Prof. Alexey Ustinov at NUST MISIS, are published in the highly-cited scientific journal Nature Communications.
Qubits: One system in two concurrent states
In contrast to the operating units (bits) of a conventional computer, qubits not only exist in the states 0 and 1. The obey the laws of quantum mechanics that allows them to be in a superposition of the two states, being 0 and 1 at once. In case of superconducting qubit circuits, the magnetic field-induced electrical current flows anti-clockwise (0) and clockwise (1) at the same time. Though, the superposition of states only exists until the system is read out – in the moment the system is measured, it takes either one of the values 0 or 1. It is this superposition that enables quantum computers to process a high number of information simultaneously, whereas conventional computers process data successively. The number of operations rises exponentially with the number of qubits. The company IBM offers online access to a superconductor-based quantum computer operating with 20 qubits.