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arXiv:1910.07338 · Physical review. B./Physical review. B

Relaxation dynamics in double-spin systems

Philipp Stegmann, Jürgen König, Björn Sothmann

Published Feb 10, 2020• 9 citations
arXivHTMLPDF

Abstract

We consider the relaxation dynamics of two spins coupled to a common bosonic bath. The time evolution is simulated by a generalized master equation derived within a real-time diagrammatic approach. Interference effects due to the coherent coupling to the common bath give rise to characteristic features in the relaxation dynamics after a quench or during a periodic external driving. In particular, we find that the long-time behavior during periodic driving depends sensitively on the initial state as well as on system parameters such as coupling asymmetries. When coupled to more than a single reservoir, the interference effects can lead to a cooling mechanism for one of the bosonic reservoirs.

DOI
10.1103/physrevb.101.075411

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We consider the relaxation dynamics of two spins coupled to a common bosonic bath. The time evolution is simulated by a generalized master equation derived within a real-time diagrammatic approach. Interference effects due to the coherent coupling to the common bath give rise to characteristic features in the relaxation dynamics after a quench or during a periodic external driving. In particular, we find that the long-time behavior during periodic driving depends sensitively on the initial state as well as on system parameters such as coupling asymmetries. When coupled to more than a single reservoir, the interference effects can lead to a cooling mechanism for one of the bosonic reservoirs.
Spin collective phenomena including superradiance are even today being intensively investigated with experimental tests performed based on state-of-the-art quantum technologies. Such attempts are not only for the simple experimental verification of predictions from the last century, but also as a motivation to explore new applications of spin collective phenomena and the coherent control of the coupling between spin ensembles and reservoirs. In this paper, we investigate the open quantum dynamics of two spin ensembles (double-spin domains) coupled to a common bosonic reservoir. We analyze in detail the dynamics of our collective state and its structure by focusing on both the symmetry and asymmetry of this coupled spin system. We find that when the spin size of one of the double domains is larger than that of the other domain, at the steady state this system exhibits two unusual collective behaviors: the negative-temperature-state relaxation in the smaller spin domain and the reservoir-assisted quantum entanglement between the two domains. These results are the consequence of the asymmetry of this system and the decoherence driven by the common reservoir.
We study the classic problem concerning the dissipative dynamics of a system of two interacting spin-1/21/21/2 particles, coupled to a thermal bath. In particular, we consider the case of a resonant excitation of the initial single-spin coherences and examine the consequences of the interplay between this excitation and the spin-spin dipolar coupling. Using a fluctuation-regulated Quantum Master Equation (frQME) [Phys. Rev. A 97, 063837 (2018)] we show that the relevant dissipator consists of cross-terms between the drive and the dipolar Hamiltonian, apart from regular self-terms responsible for ordinary relaxation effects. The drive-dipole cross-correlations provide for a novel second-order coupling of single and two-spin observables in the Zeeman basis, which cannot be classified as a dissipative effect. We show that the presence of these unique second-order non-dissipative terms lead to the well-known spin-locked steady-states in dipolar spin-ensembles. Importantly, the mathematical form of the steady-state magnetization in the spin-locked phase, obtained from our theory, matches with the prediction of the traditional spin-temperature based methods used to describe such phenomena.
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In this thesis, a scheme for calculating the dynamics of two coupled dissipative spins is developed, where each of the spins is coupled to its own boson bath. We derive analytic path sum results both in the Markov-regime and in the one-boson exchange regime. The analysis is also performed with the Bloch-Redfield method. It is shown that the two different approaches lead to identical results for the dynamics. This is not obvious a priori because both methods are based on different procedures. The path sum method gives detailed insight into the internal dynamics of two coupled spins because we consider every path sequence that contributes to the time evolution. While most studies of coupled spins are restricted to one type of coupling, e.g., Ising type, we will generalize here to linear combinations of possible couplings. Especially interesting is the occurrence of a frustration of decoherence, if the spins are interacting via a linear combination of longitudinal and transverse coupling and, when in addition, some of the eigenfrequencies become degenerate. Our analysis shows that degenerate but mutual exclusive ground states lead to increased coherence times. Maximization of coherence is one of the crucial goals of quantum state engineering. Another topic of interest is the impact of non-linear quantum environments, formed by surrounding dissipative spins. A distribution of bistable background charges is known to be responsible for 1/f1/f1/f noise in solid state devices, like the superconducting quantum interference devices. Since 1/f1/f1/f noise is seemingly the dominating source of decoherence at very low temperature, there is a profound theoretical interest in modeling it. Therefore, we analyze two interacting spins, where one of them is coupled to a boson bath and thereby represents a resonant non-linear quantum environment. We study the crossover from a non-linear to a linear bath and study the corresponding time scale for the relevant bath correlations. Interestingly, for large and increasing temperature we find a decreasing decoherence for the central spin.
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