Non-Markovian thermal reservoirs for autonomous entanglement distribution

Joan Agustí1,2,3,4, Christian M. F. Schneider1,2,3, Kirill G. Fedorov1,2,3, Stefan Filipp1,2,3, and Peter Rabl1,2,3

1Technical University of Munich, TUM School of Natural Sciences, Physics Department, 85748 Garching, Germany
2Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, 85748 Garching, Germany
3Munich Center for Quantum Science and Technology (MCQST), 80799 Munich, Germany
4Institute of Fundamental Physics IFF-CSIC, Calle Serrano 113b, 28006 Madrid, Spain

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Abstract

We describe a novel scheme for the generation of stationary entanglement between two separated qubits that are driven by a purely thermal photon source. While in this scenario the qubits remain in a separable state at all times when the source is broadband, i.e. Markovian, the qubits relax into an entangled steady state once the bandwidth of the thermal source is sufficiently reduced. We explain this phenomenon by the appearance of a quasiadiabatic dark state and identify the most relevant nonadiabatic corrections that eventually lead to a breakdown of the entangled state, once the temperature is too high. This effect demonstrates how the non-Markovianity of an otherwise incoherent reservoir can be harnessed for quantum communication applications in optical, microwave, and phononic networks. As two specific examples, we discuss the use of filtered room-temperature noise as a passive resource for entangling distant superconducting qubits in a cryogenic quantum link or solid-state spin qubits in a phononic quantum channel.

Entanglement — the defining quantum correlation between distant particles — is a key resource for quantum networks, but generating it typically requires carefully engineered coherent control and low-noise conditions. Thermal noise, the random fluctuations present in any warm environment, is usually considered the enemy: it destroys quantum coherences and prevents the formation of entangled states. This work turns that intuition on its head.

We show that two distant qubits connected by a quantum channel and driven by a purely thermal photon source — such as the filtered Johnson-Nyquist noise of a room-temperature resistor — can spontaneously relax into a highly entangled steady state, without any coherent driving or feedback. The key ingredient is reducing the bandwidth of the thermal source sufficiently. When the source is broadband, the qubits remain separable, as one would expect. But when the bandwidth is narrow enough, the system enters a non-Markovian regime in which the qubits adiabatically track an entangled dark state that is largely insensitive to the phase and amplitude fluctuations of the thermal field.

We develop a complete theoretical framework to describe this mechanism across a wide range of temperatures and bandwidths, and show that the effect is robust against realistic imperfections. As specific applications, we analyze superconducting qubits in a cryogenic microwave link and spin qubits in a phononic crystal waveguide, finding that near-maximal entanglement is achievable with current technology using only room-temperature noise as the resource.

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