Technical challenges for realizing superconducting quantum computers

Yasunobu Nakamura

Jiawei Qiu, Yang Liu, Niu, Jingjing +10 more

Quantum teleportation~\cite{Bennett1993} is of both fundamental interest and great practical importance in quantum information science. To date, quantum teleportation has been implemented in various physical systems~\cite{Pirandola2015}, among which superconducting qubits are of particular practical significance as they emerge as a leading system to realize large-scale quantum computation~\cite{Arute2019,Wu2021}. Nevertheless, the number of superconducting qubits on the same chip is severely limited by the available chip size, the cooling power, and the wiring complexity. Realization of quantum teleportation and remote computation over qubits on distant superconducting chips is a key quantum communication technology to scaling up the system through a distributed quantum computational network~\cite{Gottesman1999,Eisert2000,Jiang2007,Kimble2008,Monroe2014}. However, this goal has not been realized yet in experiments due to the technical challenge of making a quantum interconnect between distant superconducting chips and the inefficient transfer of flying microwave photons over the lossy interconnects~\cite{Kurpiers2018,Axline2018,Campagne2018,Magnard2020}. Here we demonstrate deterministic teleportation of quantum states and entangling gates between distant superconducting chips connected by a 64-meter-long cable bus featuring an ultralow loss of 0.32~dB/km at cryogenic temperatures, where high fidelity remote entanglement is generated via flying microwave photons utilizing time-reversal-symmetry~\cite{Cirac1997,Korotkov2011}. Apart from the fundamental interest of teleporting macroscopic superconducting qubits over a long distance, our work lays a foundation to realization of large-scale superconducting quantum computation through a distributed computational network~\cite{Gottesman1999,Eisert2000,Jiang2007,Kimble2008,Monroe2014}.

Yutaka Tabuchi

Qing-Xian Lv, Sai Li, Hai-Tao Tu +4 more

The hybrid quantum system composed of superconductor and cold atoms is expected to achieve fast quantum gates, long-life quantum storage and long-distance transmission through optical fibers, making it one of the most promising hybrid quantum systems to realize optical interconnection between two superconducting quantum computers. In this paper, we comprehensively review the recent research advancements in the optical interconnection of two superconducting quantum computers, based on the superconductor and cold atoms hybrid quantum system, specifically the review covers the coherent coupling between superconducting chips and cold atoms, the coherent microwave-to-optics conversion, and the long-range microwave interconnection between superconducting qubits and quantum converters. The system is expected to provide a physical and technical foundation for practical optical-fiber interconnection of two superconducting quantum computers, and have broad applications in distributed superconducting quantum computation and hybrid quantum networks.

Frank K. Wilhelm, Kouichi Semba

Shuji Nakamura, Teruaki Yoshioka, S. A. Lemzyakov +8 more

Photon-absorption-based quantum circuit refrigeration is crucial to developing superconducting quantum computers, particularly for initializing qubits. However, precise and rapid measurement of residual photons in the circuit immediately after absorption has been a significant technical challenge. This study employs the ac Stark shift of superconducting qubits to detect sub-photon-level energies in the circuit, confirming the effectiveness of the fast refrigeration even in the quantum regime. These results will facilitate the development of more efficient quantum refrigeration devices and faster, high-fidelity qubit initialization.