Direct measurement of subtle current phase relationship shows potential for more stable superconducting qubits

In recent years, quantum physicists and engineers have made significant progress toward developing high-performance quantum computing systems. However, to achieve a quantum advantage over classical computing systems and enable the stable operation of quantum devices, it will be necessary to develop new building blocks for these devices and other aspects underlying their proper functioning.

Researchers at Université Grenoble Alpes recently demonstrated the direct measurement of a subtle effect, namely a sin(2𝜑) current phase relationship, in a graphene-based superconducting quantum interference device based on Josephson junctions of gate-tunable graphene. Their method for collecting this measurement, described in an article published in Physical Examination Letterscould contribute to the development of more stable superconducting qubits that are less prone to decoherence.

Josephson junctions, the components on which the team’s device was based, connect two superconducting materials together via a weak bond. In quantum technology, these junctions enable the storage and processing of quantum information with minimal losses by allowing current to flow through devices without resistance, a specific property of superconductors below their transition temperature.

As part of their recent study, researchers at Grenoble Alpes University set out to directly measure how this current flow depended on the superconducting phase difference between the two sides of the gate-tunable graphene-based Josephson junctions in their device. . This measurement is of critical importance because it can be exploited to develop superconducting quantum circuits with carefully tailored properties.

“Looking at the existing literature, we realized that while the community has shown increasing interest in recent years in the phase relationships between current sin(2𝜑) in superconducting circuits, there was no direct measurement of this relationship in devices currently in use,” Julien Renard, lead author of the paper, told Phys.org. “We decided to design an experiment that would enable this measurement, giving a direct visualization of such a current phase relationship.”

In their experiment, Renard and his colleagues measured voltages in a graphene superconducting quantum interference device they developed based on externally controlled parameters, such as a magnetic field. Their setup relied on an advanced method to simultaneously control and read the current phase relationship of a pair of Josephson junctions in their device.

“The magnetic field allows us to vary the phase in the superconducting interference device,” Renard explained. “The measured signals, on the other hand, allow the current to be extracted. This is how we can directly measure the current phase relationship of the device.”

Direct measurements collected by this research team showed that their device can behave like a sin(2𝜑) element. This essentially means that the current flowing through their device follows a distinct pattern, represented by sin(2𝜑), which is not influenced by the simpler sin(𝜑) pattern characterizing current flow through more conventional Josephson junctions.

The experimental methods employed by Renard and his colleagues and the distinct current phase relationship they observed in their device could soon contribute to the advancement of quantum computing technologies. In their next studies, the researchers plan to build on their recent paper with the aim of developing new quantum bits protected from decoherence.

“We have shown that by combining two graphene Josephson junctions in a superconducting quantum interference device, we can obtain a current phase relationship sin(2𝜑) through the control of interference effects between Cooper pairs with a field magnetic,” Renard said. “Such a graphene superconducting quantum interference device could be the cornerstone of a future generation of quantum bits, protected from decoherence. We will now work to find the appropriate circuit geometry to construct such a quantum bit.”

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