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Progress in physics

QUANTUM JUMP APPROACH IN SUPERCONDUCTING QUBITS

Yang Yu

Published Jan 1, 2009• 0 citations
OpenAlex

Abstract

Quantum mechanics had been considered as a theory of ensembles for a long time since its foundation.However,quantum jumps demonstrated in atomic system after 1980s leads to new approaches,such as quantum jumps approach,that can be used to describe the trajectory of single quantum system.Here we summarized our recent efforts to develop quantum jump approach to the Josephson-tunnel-junction based superconducting qubits.We experimentally demonstrated quantum jumps in a macroscopic quantum system.In addition,quantum jumps can serve as a useful tool to investigate the properties of the two-level systems that are ubiquitous in solid-state systems.

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Quantum mechanics had been considered as a theory of ensembles for a long time since its foundation.However,quantum jumps demonstrated in atomic system after 1980s leads to new approaches,such as quantum jumps approach,that can be used to describe the trajectory of single quantum system.Here we summarized our recent efforts to develop quantum jump approach to the Josephson-tunnel-junction based superconducting qubits.We experimentally demonstrated quantum jumps in a macroscopic quantum system.In addition,quantum jumps can serve as a useful tool to investigate the properties of the two-level systems that are ubiquitous in solid-state systems.
With microwave irradiation, the switching current of a Josephson junction coupled to a microscopic two-level system jumps randomly between two discrete states. We modeled the switching process of the coupled system with quantum jump approach that was generally used in quantum optics. The parameters that affect the character of the quantum jumps between macroscopic quantum states are discussed. The results obtained from our theoretical analysis agree well with those of the experiments and provide a clear physical picture for the macroscopic quantum jumps in Josephson junctions coupled with two-level systems. In addition, quantum jumps may serve as a useful tool to investigate the microscopic two-level structures in solid-state systems.
The achievements of quantum computation theory, e.g. Shor's factoring algorithm, motivate efforts to realize quantum computers. Among systems proposed for quantum computing, macroscopic superconducting circuits of Josephson junctions appear promising for integration in electronic circuits and large-scale applications. Recently, a superconducting tunnel junction circuit was designed and a sufficiently high quality factor of quantum coherence has been obtained. This indicates that decoherence need not be among the obstacles in building quantum computers with macroscopic Josephson circuits. In this paper we present the setup of some elementary quantum logic with macroscopic Josephson qubits, strengthened by some simulation work, and then study the feasibility of implementing Shor's quantum factoring algorithm on them. It is shown that it would be eventually possible to build a 2-dimensional Josephson qubit array, possibly accompanied by classical computing components, capable of performing useful quantum computations.
Superconducting quantum devices have drawn the attention of physicists greatly in recent years, not only because they are one of the most favorable candidates for developing a solid state quantum computer, but also because they can be employed as test systems for understanding quantum mechanics at nearly macro scale. Superconducting quantum devices can be divided into two groups: the superconducting resonators (linear devices), which have equally spaced discrete energy levels, and the artificial atoms (non-linear devices) consisting of Josephson junctions which have unequally spaced discrete energy levels. By combining these two kinds of devices together, a solid state counterpart of optical cavity quantum electrodynamics (QED) known as circuit QED emerges. Experiments of cavity QED and quantum optics can be reproduced in either circuit QED systems or in bare superconducting artificial atoms. As an example, we have observed the Autler-Townes effect in a three-level artificial atom called phase qutrit. \nMore than one decade has passed since the first experimental demonstration of superconducting two-level artificial atom (qubit). However, to build a practical quantum computer with thousands of superconducting qubits there is still a long way to go (we are at the stage of three qubits now). One of the main obstacles preventing us from scaling up a superconducting quantum computer, as well as some other kinds of quantum computers, has been decoherence, which is believed to be dominated by the low frequency noise due to the two-level system (TLS) fluctuators located inside the Josephson junction barrier. We have experimentally simulated the dynamics of a qubit longitudinally coupled to a randomly fluctuating TLS. A phenomenon known as motional averaging has been observed. \nIn this thesis, theoretical models for decoherence in both resonator and artificial atoms have been established and used for explaining phenomena observed in the experiments performed in our laboratory. The phenomenon of decoherence in coupled bipartite systems has been also studied, and besides the already known entanglement sudden death phenomenon, stable entanglement (robust under decoherence) generated from an arbitrary initial state has been found in these systems.
In 1913, when Bohr’s atom allowed electron jumps between discrete orbits, it was impossible to experiment on single atoms and study individually these jumps. Many people considered them not real, but something artificial for the explanation of the linear spectra of atoms. Especially Schrödinger rejected the image of negatively charged electrons orbiting around the nucleus and in particular their jumps from orbit to orbit. Finally, in 1986 single atoms (ions) laser cooled and confined in an electromagnetic trap gave the possibility to investigate experimentally electron jumps to and fro a metastable state. Recently, physicists experimenting on a macroscopic superconducting circuit with discrete quantum states analogous to a natural atom were able to halt and reverse quantum jumps mid-flight revealing very interesting details of them.
Recent demonstrations of macroscopic quantum coherence in Josephson junction based electronic circuits have opened an entirely new dimension for research and applications in the established field of Josephson electronics. In this article we discuss basic Josephson circuits for qubit applications, methods of quantum description of these circuits, and circuit solutions for qubit couplings. Principles of manipulation and readout of superconducting qubits are reviewed and illustrated with recent experiments using various qubit types.