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arXiv:2004.01058 · Physical review. B./Physical review. B

Triplet-triplet decoherence in singlet fission

Max Marcus, William Barford

Published Jul 20, 2020• 17 citations
arXivHTMLPDF

Abstract

Singlet fission is commonly defined to involve a process by which an overall singlet state with local triplet structure spin decoheres into two triplet states, thereby completing the fission process. This process, often defined in loose terms involving the multiplicity of the overall state, is investigated here using a uniform Heisenberg spin chain subject to a dephasing environmental interaction. We introduce results from quantum information theory which enables the quantification of coherence and entanglement in a bi- and multipartite system. The calculated measures of these quantum effects can be linked to observables, such as magnetization and total spin, with simulations of the model and using theoretical methods. We demonstrate that these observables can act as a proxy for the coherence and entanglement measures. The decay of both of these between the two local triplets can be monitored, enabling a clear definition of the spin-decoherence process in singlet fission.

DOI
10.1103/physrevb.102.035134

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Singlet fission is commonly defined to involve a process by which an overall singlet state with local triplet structure spin decoheres into two triplet states, thereby completing the fission process. This process, often defined in loose terms involving the multiplicity of the overall state, is investigated here using a uniform Heisenberg spin chain subject to a dephasing environmental interaction. We introduce results from quantum information theory which enables the quantification of coherence and entanglement in a bi- and multipartite system. The calculated measures of these quantum effects can be linked to observables, such as magnetization and total spin, with simulations of the model and using theoretical methods. We demonstrate that these observables can act as a proxy for the coherence and entanglement measures. The decay of both of these between the two local triplets can be monitored, enabling a clear definition of the spin-decoherence process in singlet fission.
Singlet fission is a photoconversion process that generates a doubly excited, maximally spin entangled pair state. This state has applications to quantum information and computing that are only beginning to be realized. In this article, we construct and analyze a spin-exciton hamiltonian to describe the dynamics of the two-triplet state. We find the selection rules that connect the doubly excited, spin-singlet state to the manifold of quintet states and comment on the mechanism and conditions for the transition into formally independent triplets. For adjacent dimers that are oriented and immobilized in an inert host, singlet fission can be strongly state-selective. We make predictions for electron paramagnetic resonance experiments and analyze experimental data from recent literature. Our results give conditions for which magnetic resonance pulses can drive transitions between optically polarized magnetic sublevels of the two-exciton states, making it possible to realize quantum gates at room temperature in these systems.
The spin-entangled exciton states produced by singlet fission provide an experimental route to generate entangled states with nondegenerate branches. Nondegenerate entangled pair states possess an internal "clock" that leads to quantum beating in various detected quantities. The implications of this internal clock for Bell's inequality measurements and correlated particle detection are analyzed using two- and three-state spin models. In a Bell's inequality experiment, we find that the choice of detection times can determine whether quantum or classical correlations are observed. The conditions under which the detection events could be time- or spacelike separated are analyzed in order to clarify how the temporal evolution of one particle can influence the time-dependent detection probability of the other. Possible routes to the detection of individual correlated triplet excitons are discussed, emphasizing both physical questions concerning the separation and propagation of triplet excitons over macroscopic distances and experimental challenges concerning decoherence, detection, and interpretation of the signals. We argue that spin-entangled triplet exciton states produced by singlet fission could provide a new way to probe entangled state detection and collapse, complementing schemes based on polarization-entangled photon states.
Recently, singlet-triplet measurements in double dots have emerged as a powerful tool in quantum information processing. In parallel, quantum dot arrays are being envisaged as analog quantum simulators of many-body models. Thus motivated, we explore the potential of the above singlet-triplet measurements for probing and exploiting the ground state of a Heisenberg spin chain in such a quantum simulator. We formulate an efficient protocol to discriminate the achieved many-body ground state with other likely states. Moreover, the transition between quantum phases, arising from the addition of frustrations in a ${J}_{1}\ensuremath{-}{J}_{2}$ model, can be systematically explored using the same set of measurements. We show that the proposed measurements have an application in producing long distance heralded entanglement between well separated quantum dots. Relevant noise sources, such as nonzero temperatures and nuclear spin interactions, are considered.
Entanglement is the superposition of two or more particles in multiple states. An example is given for the double-well potential; another involves particles with spin 1/2. Measurement of spin is carried out by a Stern–Gerlach apparatus, using the deflection of a spin by a magnet. Entangled spin-1/2 states, namely triplet states having total spin 1, and the singlet state, with spin 0, are also examined; the latter state appears in same form when expressed in spin up/down along any Cartesian coordinate. The determination of one of the spins in an entangled singlet state immediately determines the other, resulting in a “spooky action at a distance.” Einstein, Podolsky, and Rosen (EPR) claimed to show, on the basis of local realism, that quantum mechanics must be incomplete because of this feature. Their logic suggests that one might explain the instantaneous results by invoking “hidden variables” not contained in a quantum treatment.