| 1. |
- Aurell, Erik, et al.
(author)
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Frohlich-coupled qubits interacting with fermionic baths
- 2020
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In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - : AMER PHYSICAL SOC. - 1539-3755 .- 1550-2376. ; 102:1
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Journal article (peer-reviewed)abstract
- We consider a quantum system such as a qubit, interacting with a bath of fermions as in the Frohlich polaron model. The interaction Hamiltonian is thus linear in the system variable and quadratic in the fermions. Using the recently developed extension of Feynman-Vernon theory to nonharmonic baths we evaluate quadratic and the quartic terms in the influence action. We find that for this model the quartic term vanish by symmetry arguments. Although the influence of the bath on the system is of the same form as from bosonic harmonic oscillators up to effects to sixth order in the system-bath interaction, the temperature dependence is nevertheless rather different, unless rather contrived models are considered.
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| 2. |
- Aurell, Erik, et al.
(author)
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Real-time dynamics in diluted quantum networks
- 2022
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In: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - : AMER PHYSICAL SOC. - 2469-9926 .- 2469-9934. ; 105:2
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Journal article (peer-reviewed)abstract
- We introduce an approach to characterize the dynamics of disordered quantum networks. Each quantum element (i.e., each node) of the network experiences the other nodes as an effective environment that can be self-consistently represented by a Feynman-Vernon influence functional. For networks having the topology of locally treelike graphs, these Feynman-Vernon (FV) functionals can be determined by a new version of the cavity or belief propagation (BP) method. Here, we find the fixed point solution of this version of BP for a network of uniform quantum harmonic oscillators. Then, we estimate the effects of the disorder in these networks within the replica symmetry ansatz. We show that over a large time interval, at small disorder, the real part of the FV functional induces decoherence and classicality while at sufficiently large disorder the Feynman-Vernon functional tends to zero and the coherence survives, signaling in a time setting, the onset of an Anderson's transition.
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| 3. |
- Tuziemski, Jan
(author)
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Out-of-time-ordered correlation functions in open systems : A Feynman-Vernon influence functional approach
- 2019
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In: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - : AMER PHYSICAL SOC. - 2469-9926 .- 2469-9934. ; 100:6
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Journal article (peer-reviewed)abstract
- Recent theoretical and experimental studies have shown the significance of quantum information scrambling (i.e., the spread of quantum information over a system's degrees of freedom) for problems encountered in high-energy physics, quantum information, and condensed matter. Due to the complexity of quantum many-body systems it is plausible that new developments in this field will be achieved by experimental explorations. Since noise effects are inevitably present in experimental implementations, a better theoretical understanding is needed of quantum information scrambling in systems affected by noise. To address this problem we study indicators of quantum scrambling-out-of-time-ordered correlation functions (OTOCs) in open quantum systems. As most experimental protocols for measuring OTOCs are based on backward time evolution, we consider two possible scenarios of joint system-environment dynamics reversal: In the first one the evolution of the environment is reversed, whereas in the second it is not. We derive general formulas for OTOCs in those cases and study in detail the model of a spin chain coupled to the environment of harmonic oscillators. In the latter case we derive expressions for open-system OTOCs in terms of the Feynman-Vernon influence functional. Subsequently, assuming that dephasing dominates over dissipation, we provide bounds on open-system OTOCs and illustrate them for a spectral density known from the spin-boson problem. In addition to being significant for quantum information scrambling, the results also advance the understanding of decoherence in processes involving backward time evolution.
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| 4. |
- Tuziemski, Jan, et al.
(author)
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Reexamination of the decoherence of spin registers
- 2019
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In: Physical Review A: covering atomic, molecular, and optical physics and quantum information. - 2469-9926 .- 2469-9934. ; 99:2
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Journal article (peer-reviewed)abstract
- We revisit the decoherence process of a multiqubit register interacting with a thermal bosonic bath. We generalize the previous studies by considering not only the register's behavior but also a part of its environment. In particular, we are interested in information flow from the register to the environment, which we describe using recently introduced multipartite quantum state structures called spectrum broadcast structures. Working in two specific cases of (i) two-qubit register and (ii) collective decoherence, we identify the regimes where the environment acquires almost complete information about the register state. We also study in more detail the interesting causal aspects related to the finite propagation time of the field disturbances between the qubits. Finally, we describe quantum state structures which appear due to the presence of protected spaces.
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