NTT paving the way for removing defects in quantum processors

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Nippon Telegraph and Telephone Corporation (NTT) and the National Institute of Advanced Industrial Science and Technology (AIST) have realized a novel technique for the detection of parasitic defects in superconducting qubits.

Superconducting qubits are among the most promising hardware platforms for building a quantum computer, but the realization of a full-scale fault-tolerant quantum computer remains a challenging task due to limited lifetimes of the superconducting qubits.

One of the major noise mechanisms limiting the qubit lifetimes is due to unwanted two-level-system (TLS) defects*3 in superconducting qubits. Therefore, investigation and mitigation of noise contributions from TLS defects are crucial for the further improvement of superconducting quantum processors.

In this work, NTT and AIST introduce a novel method of defect spectroscopy in superconducting qubits that allows one to distinguish different types of TLS defects. The reported method will become a valuable diagnostic tool for quantifying TLS defects and finding optimal techniques and materials for the fabrication of defect-free superconducting qubits.

This research was reported in the American journal PRX Quantum on December 21, 2022. This work was partially supported by JST CREST (JPMJCR1774) and JST Moonshot R&D (JPMJMS2067).

The performance of conventional computers is expected to reach its physical limits in the near future, and scientists around the world actively explore new ways of computing.

Quantum computation is an emerging computing paradigm which is based on well-studied, but not yet fully exploited quantum phenomena such as quantum superposition and entanglement. The key building block of a quantum computer is a qubit – a computing element that can store quantum information – and the mainstream technology of quantum computing is based on superconducting qubits where quantum information is encoded in quantum states of superconducting electric circuits.

Despite the impressive progress in building modest-scale superconducting qubit systems in recent years, realization of a universal quantum computer is still a very challenging task. The major obstacle is the rapid loss of quantum information due to the interaction between a qubit and a noisy environment.

One of the main noise sources in superconducting qubits are atomic-scale parasitic defects which are located inside oxide layers formed on the top of superconducting materials. In our work we introduce a novel microwave technique that allows one to identify the type of a parasitic defect. The presented approach complements methods for the characterization of other types of noises in superconducting qubits, enabling further improvement in the performance of superconducting quantum processors.

The research group proposed and demonstrated a method for identifying different types of two-level-system defects in superconducting qubits. Although the existence of different types of TLS defects was predicted previously by several theoretical works, this is the first time the experimental demonstration of two different types of TLS defects has been reported.

2.1 Detection condition of two level system defects
There are two main types of the interaction between superconducting qubits and TLS defects. First, the charge fluctuation of the two-level system can cause the displacement of the charge in the Josephson junction, resulting in the Coulomb interaction between the TLS defect and the qubit charge degree of freedom

Second, the charge fluctuation of the two-level system defect can cause a change in the supercurrent through the Josephson junction, thereby coupling the TLS defect to the superconducting phase fluctuations of the qubit.

Spectrum of two-level system defects

The difference in the described detection conditions can be visualized by measuring the spectra of two-level system defects. Here, we measured the spectra of two level system defects while sweeping the magnetic flux applied to the flux qubit (Fig. 2b, c). The spectral lines of two-level system defects TLS1-TLS3 shown in Figure 2 c demonstrates the same curvature as the spectrum of the qubit in Figure 2 a, indicating that it is a charge-type-interaction TLS defect in which one qubit excitation is transferred into one excitation of the two-level system. On the other hand, the spectral lines TLS4, TLS5 of two-level system defects shown in Fig. 2c demonstrate twice the curvature of the qubit spectrum, indicating a critical-current-type interaction in which two qubit excitations are transferred into one excitation of the two-level system defect.

Future plans

The reported method can be applied to identify different types of two-level system defects, which is expected to help elucidate the physics of defects in superconducting qubits. In addition, the method can be used to provide feedback for the sample fabrication process to optimize the fabrication process and materials, enabling the realization of defect-free and long-lived superconducting qubits.

In a short term, the reported method can be used to extract parameters necessary to model noise due to two-level system defects, which can be applied to qubit gate operation optimization necessary to further improve the performance of small-scale quantum computer prototypes that are currently available.