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Published online: 25 March 2009 | doi:10.1038/nchina.2009.60

Quantum physics: Resonate with excitement

Felix Cheung

Introduction

© (2009) AIP

Superconducting qubits — loops of superconducting material interrupted by Josephson junctions — are the subject of intense research because of their long-term potential for quantum computing. They are often called 'artificial atoms' because they have many properties that are analogous to classical atoms. Guozhu Sun, Yang Yu and co-workers at Nanjing University¹ have demonstrated a controllable population inversion in an artificial atom, which might make a micromaser (a one-atom maser, the maser being the microwave equivalent of a laser) based on quantum transitions come true.

It is well known that population inversion, which maintains most atoms in excited states rather than in the ground state, is a necessary step in the workings of lasers and masers. Previous studies have suggested the possibility of generating population inversion in superconducting quantum circuits subjected to microwave radiation.

The researchers fabricated a quantum circuit by depositing superconducting materials on a silicon wafer through a standard photolithography process. The design of the quantum circuit (pictured) is based on a one-junction superconducting qubit, known as radio-frequency superconducting quantum interference device (commonly known as rf SQUID).

The researchers tweaked the design such that the quantum circuit becomes analogous to an atom moving in a double well potential. The lowest states in each of the double wells serve as the qubit's transitional states. The researchers also implemented features into the design to adjust the height and tilt of the well in situ.

When microwave irradiation is applied to the circuit, the circuit absorbs the photons and a population inversion is generated — and a microwave resonates between the qubit's transitional states. The population of atoms in the excited state can be modulated by the power of the microwave. The researchers envision the technology will eventually be used to make a micromaser.

The authors of this work are from:
Department of Electronic Science and Engineering, Research Institute of Superconductor Electronics, Nanjing University, Nanjing, China; Department of Physics, National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, China; Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas, USA.

Reference

1. Sun, G. et al. Population inversion induced by Landau–Zener transition in a strongly driven rf superconducting quantum interference device. Appl. Phys. Lett. 94, 102502 (2009).
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