This paper studies the effect of the momentum conservation on tunneling with dissipation. Two coupled macroscopic particles, interacting with a thermal bath of harmonic oscillators are considered. The interaction between the particles is chosen such that it has minima at two different separations. The exact effective action is obtained for the linear coupling with the bath by the direct calculation of the imaginary-time path integral at arbitrary temperature, and by applying Leggett's prescription at T=0. The case of the Ohmic dissipation is studied in detail. It is proved that the tunneling, at any strength of the dissipation, has a simple description in terms of the effective reduced mass that depends on temperature and classical friction coefficients. The connection with the M\"ossbauer effect is discussed. \textcopyright{} 1996 The American Physical Society.
This paper studies the effect of the momentum conservation on tunneling with dissipation. Two coupled macroscopic particles, interacting with a thermal bath of harmonic oscillators are considered. The interaction between the particles is chosen such that it has minima at two different separations. The exact effective action is obtained for the linear coupling with the bath by the direct calculation of the imaginary-time path integral at arbitrary temperature, and by applying Leggett's prescription at T=0. The case of the Ohmic dissipation is studied in detail. It is proved that the tunneling, at any strength of the dissipation, has a simple description in terms of the effective reduced mass that depends on temperature and classical friction coefficients. The connection with the M"ossbauer effect is discussed. \textcopyright{} 1996 The American Physical Society.
We present a comprehensive study of the effect of a transverse magnetic field on the macroscopic quantum tunneling of two identical coupled particles with dissipation. Two coupled particles of identical masses but opposite charges in the plane, in the presence of a constant transverse external magnetic field and an external potential interacting with a bath of harmonic oscillators are studied. In this case, the problem cannot be mapped to a one-dimensional problem, it strictly remains two-dimensional. We calculate the exact effective action both for the case of linear coupling to the bath and without a linear coupling to the bath using imaginary time path integral and Leggett's prescription at T=0. In the limit of zero magnetic field we recover a two dimensional version of the result found in \cite{em1} for the case of two identical particles. We find that in the limit of weak and strong dissipation, the effective action reduces to a two dimensional version of the Caldeira-Leggett form in terms of the reduced mass and magnetic field. The case of Ohmic dissipation with the motion of the two particles damped by the Ohmic frictional constant η is studied in detail.
Two coupled particles of identical mass but opposite charge are studied, with a constant transverse external magnetic field and an external potential, interacting with a bath of harmonic oscillators. We show that the problem cannot be mapped to a one-dimensional problem like the one in Ao (Phys. Rev. Lett. 72, 1898 (1994)), it strictly remains two-dimensional. We calculate the effective action both for the case of linear coupling to the bath and without a linear coupling using imaginary time path integral at finite temperature. At zero temperature we use Leggett’s prescription to derive the effective action. In the limit of zero magnetic field we recover a two-dimensional version of the result derived in Chudnovsky (Phys. Rev. B, 54, 5777 (1996)) for the case of two identical particles. We find that in the limit of strong dissipation, the effective action reduces to a two-dimensional version of the Caldeira–Leggett form in terms of the reduced mass and the magnetic field. The case of ohmic dissipation with the motion of the two particles damped by the ohmic frictional constant η is studied in detail.
Usually one finds that dissipation tends to make a quantum system more classical in nature. We study the effect of momentum dissipation on a quantum system. The momentum of the particle is coupled bi-linearly to the momenta of a harmonic oscillator heat bath. For a harmonic oscillator system we find that the position and momentum variances for momentum coupling are, respectively, identical to momentum and position variances for spatial friction. This implies that momentum coupling leads to an increase in the fluctuations in position as the temperature is lowered, exactly the opposite of the classical-like localization of the oscillator, found with spatial friction. For a parabolic barrier, momentum coupling causes an increase in the unstable normal mode barrier frequency, as compared to the lowering of the barrier frequency in the presence of purely spatial coupling. This increase in the frequency leads to an enhancement of the thermal tunneling flux, which below the crossover temperature becomes exponentially large. The crossover temperature between tunneling and thermal activation increases with momentum friction so that quantum effects in the escape are relevant at higher temperatures.
Usually one finds that dissipation tends to make a quantum system more classical in nature. In this paper we study the effect of momentum dissipation on a quantum system. The momentum of the particle is coupled bilinearly to the momenta of a harmonic oscillator heat bath. For a harmonic oscillator system we find that the position and momentum variances for momentum coupling are respectively identical to momentum and position variances for spatial friction. This implies that momentum coupling leads to an increase in the fluctuations in position as the temperature is lowered, exactly the opposite of the classical like localization of the oscillator, found with spatial friction. For a parabolic barrier, momentum coupling causes an {\it increase} in the unstable normal mode barrier frequency, as compared to the lowering of the barrier frequency in the presence of purely spatial coupling. This increase in the frequency leads to an enhancement of the thermal tunneling flux, which below the crossover temperature becomes exponentially large. The crossover temperature between tunneling and thermal activation {\it increases} with momentum friction so that quantum effects in the escape are relevant at higher temperatures.
For the realistic theoretical descriptions of quantum devices, we have to take the unavoidable coupling to their surrounding environment into account. A lot of effort in theoretical physics has been made to describe decoherence effects and the corresponding quantum to classical transition in the past. In these studies, baths are coupled to certain observables making their behavior more classical. In the most general case, a quantum device is affected by environments coupled to different observables. In this thesis, we study the differences emerging from coupling the bath to different observables. In particular, we investigate the situation, where two baths are coupled simultaneously to two conjugate variables (e.g. xˆ and pˆ, denoted dissipative frustration). We study how the interplay of these two baths can lead to interesting effects. After an introduction of the topic, we discuss the results of this thesis in Part II and Part III. In Part II, we study the effect of quantum tunneling in presence of a dissipative interaction with two Ohmic environments to the position xˆ and/or to the momentum pˆ of a particle. In chapter 2, we consider a symmetric double well potential which is important in the context of coherent state preparation, as its ground state is a superposition of being in the left and in the right well. Using the semiclassical approximation to calculate instanton paths within the path integral method, we find that momentum dissipation enhances the coherent tunnel splitting. In presence of dissipative couplings to xˆ and pˆ, the momentum dissipation acts in favor of the delocalization of the particle shifting the critical coupling strength of the dissipative phase transition induced by the position dissipation. Chapter 3 contains a study of the escape rate out of a metastable well in presence of dissipation. Here we find that the incoherent tunneling rate is exponentially enhanced in presence of momentum dissipation. We further show that the influence of momentum dissipation depends on the slope of the part of barrier the particle escaping through. In presence of both dissipative couplings, we find a non-monotonic behavior of the escape rate as a function of the dissipative couplings strengths. In Part III, we show that it is possible to realize the situation of "dissipative frustration" in Josephson junction chains. In particular, we introduce charge dissipation as a physical realization of the afore introduced momentum coupling. After presenting the circuit elements needed for charge dissipation, we study a chain of Josephson junctions affected by Ohmic baths coupled to the charge and/or the phase. Using a self-consistent harmonic approximation, we determine the phase diagram at zero temperature which exhibits a quantum phase transition between an ordered phase, corresponding to the superconducting state, and a disordered phase, corresponding to the insulating state with localized superconducting charge. Interestingly, we find that the critical line separating the two phases has a non-monotonic behavior as a function of the dissipative coupling strength. Moreover, within the self-consistent harmonic approximation, we analyze the dissipation induced crossover between a first and second order phase transition, showing that dissipative frustration increases the range in which the phase transition is second order. The non-monotonic behavior is also reflected in the purity of the system that quantifies the degree of correlation between the system and the environment, and in the logarithmic negativity as an entanglement measure that encodes the internal quantum correlations in the chain.
Abstract Macroscopic tunneling of Cooper pairs of electrons in a system of two superconductors separated by an insulator was modeled by Caldeira and Leggett by quantum-mechanical motion of a particle through the potential barrier. Friction (conductivity of the Josephson contact in experiments) was modeled by a one-dimensional phonon thermostat that interacts with the particle. Then the tunneling exponent was expressed in terms of the friction coefficient determined by the thermostat parameters. Alternatively, we propose a direct introduction of the friction coefficient into a classical action in the tunneling exponent using the method of imaginary time. As a result, the numerical coefficient of the classical friction contribution to the tunneling exponent is obtained.