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Routledge eBooks

Macroscopic Quantum Phenomena

James E. Mercereau

Published Mar 29, 2018• 2 citations
Publisher

Abstract

Superconductivity is a macroscopic quantum phenomena. This chapter examines Fritz London's ideas and describes some of the experiments establishing its validity. It was London's realization that electrodynamic behavior was a natural consequence of quantum condensation that led him to propose superconductivity as being fundamentally a macroscopic quantum phenomena. The chapter also discusses only those experiments immediately relevant to the concept of the Josephson junction as a quantum-phase detector for the purpose of experimentally examining the macroscopic quantum state. Because of the unusual voltage-dependent frequency the analysis of these effects is somewhat more complex than for the zero voltage current. However, at low frequencies it is usually adequate to describe the response simply as a complicated time modulation of the zero-voltage current. The existence of singularities within the macroscopic quantum state, however, leads to noise. Unless these singularities move with precision, a noise is developed associated with the random phase motion.

DOI
10.1201/9780203737965-8

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Superconductivity is a macroscopic quantum phenomena. This chapter examines Fritz London's ideas and describes some of the experiments establishing its validity. It was London's realization that electrodynamic behavior was a natural consequence of quantum condensation that led him to propose superconductivity as being fundamentally a macroscopic quantum phenomena. The chapter also discusses only those experiments immediately relevant to the concept of the Josephson junction as a quantum-phase detector for the purpose of experimentally examining the macroscopic quantum state. Because of the unusual voltage-dependent frequency the analysis of these effects is somewhat more complex than for the zero voltage current. However, at low frequencies it is usually adequate to describe the response simply as a complicated time modulation of the zero-voltage current. The existence of singularities within the macroscopic quantum state, however, leads to noise. Unless these singularities move with precision, a noise is developed associated with the random phase motion.
Experiments are described that demonstrate the quantum behavior of a macroscopic degree of freedom, namely the phase difference \ensuremath{\delta} across a current-biased Josephson tunnel junction. The behavior of \ensuremath{\delta} was deduced from measurements of the escape rate \ensuremath{\Gamma} of the junction from its zero-voltage state. The relevant parameters of the junction, that is, its critical current and shunting admittance, were determined in situ in the thermal regime from the dependence of \ensuremath{\Gamma} on bias current and from resonant activation in the presence of microwaves. It was found that the shunting capacitance was dominated by the self-capacitance of the junction while the shunting conductance was dominated by the bias circuitry. For an underdamped junction in the quantum regime, \ensuremath{\Gamma} became independent of temperature at low temperatures with a value that, with no adjustable parameters, was in excellent agreement with predictions for macroscopic quantum tunneling at T=0. When the critical current was reduced with a magnetic field so that the junction remained in the thermal regime at low temperatures, \ensuremath{\Gamma} followed the predictions of the thermal model, thereby showing the influence of extraneous noise to be negligible. In a further series of experiments, the existence of quantized energy levels in the potential well of the junction was demonstrated spectroscopically. The positions of the energy levels agreed quantitatively with quantum-mechanical predictions involving junction parameters measured in the thermal regime. The relative heights and widths of the resonances are in reasonable agreement with the predictions of a simple model.
A macroscopic variable, the phase difference across a Josephson junction, is shown to obey quantum mechanics by observing the phenomena of energy-level quantization and quantum tunneling.The relevant parameters of the junction were determined .!.!! .ill!:!, in the thermal regime from the dependence of the escape rate on bias current and from resonant activation in the presence of microwaves.The first observation of quantized energy levels for this system is reported.The positions of these energy levels are in quantitative agreement with the quantum mechanical predictions based on the measured junction parameters.A model is presented that predicts correctly the observed resonance shapes.The measured value of the escape rate out of the zero-voltage state for an underdamped (Q -30) junction was found to be independent of temperature at low temperatures.This low temperature escape rate was in excellent agreement with the prediction for macroscopic quantum tunneling, with no adjustable parameters.Jolw rJt..
The Josephson effect is a privileged access to the macroscopic quantum nature of superconductors. We review some ideas and experimental techniques on macroscopic quantum decay phenomena occurring in Josephson structures. The attention is mainly addressed to intermediate levels of dissipation which characterize a large majority of low critical current Josephson devices and are therefore an avoidable consequence of nanotechnology applied more and more to Josephson devices. Phase diffusion phenomena take over thermal activation in some temperature ranges also affecting the transition to macroscopic quantum tunneling, enriching the phase diagram mostly defined by the Josephson energy, the temperature and the level of dissipation.
Experiments to investigate the quantum behavior of a macroscopic degree of freedom, namely the phase difference across a Josephson tunnel junction, are described. The experiments involve measurements of the escape rate of the junction from its zero voltage state. Low temperature measurements of the escape rate for junctions that are either nearly undamped or moderately damped agree very closely with predictions for macroscopic quantum tunneling, with no adjustable parameters. Microwave spectroscopy reveals quantized energy levels in the potential well of the junction in excellent agreement with quantum-mechanical calculations. The system can be regarded as a "macroscopic nucleus with wires."
It is widely believed that the microscopic, that is, atomic-scale or smaller, world is governed by quantum mechanics while the macroscopic world, where our direct and daily experiences reside, is governed by classical mechanics. But why? Why do we not observe quantum phenomena such as quantum tunneling and quantum superposition in the macroscopic world? Where is the boundary between the microscopic and macroscopic worlds when it comes to the governing physical principles? Macroscopic quantum phenomena were first examined theoretically by A. Leggett in the 1970s. This boldly audacious idea was experimentally demonstrated in a series of groundbreaking works on Josephson junctions by John Clarke, Michel Devoret, and John Martinis in 1985, that have been celebrated by the 2025 Nobel Prize in Physics. The trio’s work spurred the development of various qubit types based on superconducting circuits, ultimately leading to the current advancement of superconducting quantum computers.