Harvesting entanglement from quantum fields: from theory to proposed superconducting implementations

Abstract

Entanglement harvesting is a relativistic quantum information protocol through which initially uncorrelated particle detectors become entangled by locally interacting with a quantum field. This process allows one to extract and repurpose the entanglement naturally present in quantum fields, even from spacelike-separated regions. Entanglement harvesting has not been realized in the lab, but multiple experimental platforms to realize it have been proposed, among which superconducting circuits stand out due to their controllability and ability to implement strong interactions of detectors with 1+1 dimensional quantum fields.

In this thesis, we start by investigating entanglement harvesting using particle detectors coupled to a massless scalar field through its derivative, a coupling that captures important features of light-matter interaction and is naturally realized in superconducting circuits. We show that detectors in causal contact can still harvest genuine entanglement from the field, with harvested entanglement peaking when the detectors are fully light-connected. Additionally, we find that communication and harvesting contributions to the detectors’ entanglement can interfere both constructively and destructively. Surprisingly, this implies that the presence of entanglement in the field can sometimes inhibit, rather than enhance, the entangling of the detectors.

We then broaden the analysis to more general entanglement harvesting protocols involving detectors with arbitrary number of energy levels and a general class of couplings to the field. Furthermore, we study the longitudinal (diagonal) and transversal (off-diagonal) components of the detector-field interaction. We show that at leading order, entanglement harvesting is dominated by the component transversal to the detectors’ initial state. Through an explicit qubit model, we further illustrate how increasing the strength of longitudinal coupling can suppress harvested entanglement via higher-order effects.

Finally, motivated by the prospect of experimental realizations, we introduce a variable-gap detector model that bridges the gap between idealized Unruh-DeWitt particle detectors and existing implementations in superconducting circuits. Using parameters tailored to potential experimental setups, we investigate entanglement harvesting in both spacelike-separated and causally connected scenarios. We find that, while variations in the energy gap reduce the ability to harvest entanglement in spacelike scenarios, detectors in causal contact detectors can still become entangled through their interaction with the field. Notably, our analysis shows that (due to the derivative coupling nature of the model) even for causally connected detectors, there are setups where entanglement primarily originates from the field's correlations. This demonstrates the potential for genuine entanglement harvesting in the lab and opens the door to near-future entanglement harvesting experiments in superconducting circuits.