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Generating pairs of entangled microwave photons by Josephson-photonics devices

Simon Dambach, A. Peugeot, Juha Leppäkangas, Björn Kubala, M. Westig, Yuri Mukharsky, Carles Altimiras, D. Vion, D. Estève, F. Portier, Joachim Ankerhold

Published Jan 1, 2019• 0 citations
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Multiphoton Generator on a ChipA device for producing up to six photons in a single event could open new doors to quantum technologies.By Philip Ball P roducing entangled photons, whose properties are interdependent, is key to several quantum communication technologies.Most techniques for generating these photons make them exclusively in pairs, and those that can produce more than two photons are difficult to implement.But researchers have now made a chip-sized device that can generate bunches of up to six microwave photons at once [1].The device may be useful for quantum technologies such as extremely secure quantum communication.Pairs of entangled photons are often made in a process known Making light of Cooper pairs.In the circuit used to make photon multiplets, a voltage (battery) enables superconducting Cooper pairs of electrons (S) to tunnel across an insulating gap in a Josephson junction (green) and to enter a resonating LC circuit made from a pair of capacitor plates and an inductor coil.The number of microwave photons emitted from the circuit depends on the energy imparted to the Cooper pair by the applied voltage.
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.
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.