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arXiv:1309.1425 · Physical Review A

Thermal amplification of field-correlation harvesting

Eric G. Brown

Published Dec 27, 2013• 34 citations
arXivHTMLPDF

Abstract

We study the harvesting of quantum and classical correlations from a hot scalar field in a periodic cavity by a pair of spatially separated oscillator-detectors. Specifically, we utilize nonperturbative and exact (non-numerical) techniques to solve for the evolution of the detectors-field system and then we examine how the entanglement, Gaussian quantum discord, and mutual information obtained by the detectors change with the temperature of the field. While (as expected) the harvested entanglement rapidly decays to zero as temperature is increased, we find remarkably that both the mutual information $and$ the discord can actually be increased by multiple orders of magnitude via increasing the temperature. We go on to explain this phenomenon by a variety of means and are able to make accurate predictions of the behavior of thermal amplification. By doing this we also introduce a new perspective on harvesting in general and illustrate that the system can be represented as two dynamically decoupled systems, each with only a single detector. The thermal amplification of discord harvesting represents an exciting prospect for discord-based quantum computation, including its use in entanglement activation.

DOI
10.1103/physreva.88.062336

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We study the harvesting of quantum and classical correlations from a hot scalar field in a periodic cavity by a pair of spatially separated oscillator-detectors. Specifically, we utilize nonperturbative and exact (non-numerical) techniques to solve for the evolution of the detectors-field system and then we examine how the entanglement, Gaussian quantum discord, and mutual information obtained by the detectors change with the temperature of the field. While (as expected) the harvested entanglement rapidly decays to zero as temperature is increased, we find remarkably that both the mutual information andandand the discord can actually be increased by multiple orders of magnitude via increasing the temperature. We go on to explain this phenomenon by a variety of means and are able to make accurate predictions of the behavior of thermal amplification. By doing this we also introduce a new perspective on harvesting in general and illustrate that the system can be represented as two dynamically decoupled systems, each with only a single detector. The thermal amplification of discord harvesting represents an exciting prospect for discord-based quantum computation, including its use in entanglement activation.
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In the framework of the theory of open systems based on completely positive quantum dynamical semigroups, we give a description of the continuous-variable quantum entanglement and quantum discord for a system consisting of two noninteracting modes embedded in a thermal environment. Entanglement and discord are used to quantify the quantum correlations of the system. For all values of the temperature of the thermal reservoir, an initial separable Gaussian state remains separable for all times. We study the time evolution of logarithmic negativity, which characterizes the degree of entanglement, and show that in the case of an entangled initial Gaussian state, entanglement suppression (entanglement sudden death) takes place, for non-zero temperatures of the environment. Only for a zero temperature of the thermal bath the initial entangled state remains entangled for finite times. We analyze the time evolution of the Gaussian quantum discord, which is a measure of all quantum correlations in the bipartite state, including entanglement, and show that quantum discord decays asymptotically in time under the effect of the thermal bath. This is contrast with the sudden death of entanglement. Before the suppression of the entanglement, the qualitative evolution of quantum discord is very similar to that of the entanglement. We describe also the time evolution of the degree of classical correlations and of quantum mutual information, which measures the total correlations of the quantum system.
We study the harvesting of entanglement and mutual information by Unruh-DeWitt particle detectors from thermal and squeezed coherent field states. We prove (for arbitrary spatial dimensions, switching profiles and detector smearings) that while the entanglement harvesting ability of detectors decreases monotonically with the field temperature TTT, harvested mutual information grows linearly with TTT. We also show that entanglement harvesting from a general squeezed coherent state is independent of the coherent amplitude, but depends strongly on the squeezing amplitude. Moreover, we find that highly squeezed states (i) allow for detectors to harvest much more entanglement than from the vacuum, and (ii) ensure that the entanglement harvested does not decay with their spatial separation. Finally, we analyze the spatial inhomogeneity of squeezed states and its influence on harvesting, and investigate how much entanglement one can actually extract from squeezed states when the squeezing is bandlimited.
We study the ability of qubit detectors to i) extract correlations from, and ii) transmit quantum information through, a quantum field. \n \nWe start by perturbatively studying the harvesting of correlations from thermal and squeezed coherent field states. We find that an increase in field temperature is detrimental to entanglement harvesting, but beneficial to mutual information harvesting. We also show that entanglement harvesting is independent of the field's coherent amplitude - which we relate to fundamental results regarding the entanglement structure of coherent field states - but strongly dependent on the field's squeezing amplitude. We conclude by analyzing the practical feasibility of entangling qubits using squeezed field states. \n \nWe then go on to study, non-perturbatively, the entanglement extraction by targets A and B from a quantum source S. After proving a general no-go theorem which applies for any A, B and S, we apply this theorem to the entanglement harvesting setup to prove that a wide class of i) degenerate, or ii) point-in-time coupled, detectors cannot harvest entanglement from any field state. We also discuss the role of communication in the process of entanglement extraction, and we end the chapter by presenting the simplest successful example of a non-perturbative entanglement harvesting protocol. \n \nWe conclude by studying the ability of flat spacetime observers Alice and Bob to transmit quantum information through a quantum field. We construct a perfect, field-mediated quantum channel, each use of which allows Alice to transmit a full qubit of information to Bob. This construction provides us with an understanding of how quantum information propagates through a relativistic field, which we find to be consistent with our understanding of the strong Huygens principle. Lastly, we analyze the possibility of simultaneously broadcasting a quantum message through a quantum field to multiple receivers, and discover severe fundamental limitations to such a setup.
In this paper we examine the decay of quantum correlations for the radiation field in a two-mode squeezed thermal state in contact with local thermal reservoirs. Two measures of the evolving quantum correlations between the modes are compared: the entanglement of formation and the quantum discord. We derive analytic expressions of the entanglement-death time in two special cases: when the reservoirs for each mode are identical, as well as when a single reservoir acts on the first mode only. In the latter configuration, we show that all the pure Gaussian states lose their entanglement at the same time, determined solely by the field-reservoir coupling. Also investigated is the evolution of the Gaussian quantum discord for the same choices of thermal baths. We notice that the discord can increase in time above its initial value in a special situation, namely, when the input state is mixed, and local measurements are performed on the attenuated mode. This enhancement of discord is stronger for zero-temperature reservoirs and increases with the input degree of mixing.