Abstract
When connecting a voltage-biased Josephson junction in series to several microwave cavities, a Cooper-pair current across the junction gives rise to a continuous emission of strongly correlated photons into the cavity modes. Tuning the bias voltage to the resonance where a single Cooper pair provides the energy to create an additional photon in each of the cavities, we demonstrate the entangling nature of these creation processes by simple witnesses in terms of experimentally accessible observables. To characterize the entanglement properties of the such created quantum states of light to the fullest possible extent, we then proceed to more elaborate entanglement criteria based on the knowledge of the full density matrix and provide a detailed study of bi- and multipartite entanglement. In particular, we illustrate how due to the relatively simple design of these circuits changes of experimental parameters allow one to access a wide variety of entangled states differing, e.g., in the number of entangled parties or the dimension of state space. Such devices, besides their promising potential to act as a highly versatile source of entangled quantum microwaves, may thus represent an excellent natural testbed for classification and quantification schemes developed in quantum information theory.
Highlights
The concept of entanglement is at the heart of quantum physics: as one of the cornerstones at the foundation of quantum mechanics and, at the same time, as a key ingredient in emerging quantum technologies introducing a new quantum resource into communication, computing, and sensing.These two interconnected aspects of entanglement are reflected in a branching of research interests
A more abstract, mathematical direction of quantum information theory maps out the boundary between classical and quantum world and further charts quantum states into various classes of entanglement
The creation of multipartite or other more complex entangled states requires increasingly complex pulse schemes. These are reachable in such systems since one can build on the immense research effort which has been spent on these standard circuit-quantumelectrodynamics (QED) setups as part of the larger quest for universal quantum computing
Summary
The concept of entanglement is at the heart of quantum physics: as one of the cornerstones at the foundation of quantum mechanics and, at the same time, as a key ingredient in emerging quantum technologies introducing a new quantum resource into communication, computing, and sensing. The creation of multipartite or other more complex entangled states requires increasingly complex pulse schemes These are reachable in such systems since one can build on the immense research effort (and the resulting amazing progress in performance and control) which has been spent on these standard circuit-quantumelectrodynamics (QED) setups as part of the larger quest for universal quantum computing. The goals of the present work are twofold: to make detailed and quantitative predictions about entanglement properties of this new class of superconducting devices and to give instructions how to vary and design experimental parameters In this respect, the simple bipartite case is only the first step. Josephson-photonics architectures allow one by comparatively simple changes of experimental parameters to access multipartite situations, the characterization of which requires the most sophisticated classification and quantification schemes developed in abstract quantum information theory.
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