Quantum Electric Transport Meets Quantum Optics: Josephson Photonics with Strong Charge-Light Coupling

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Photonics and quantum electronics are currently among the fastest developing fields in physics due to their paramount relevance for future information processing, communication, and sensing. These fields deal with two fundamental quanta in nature, namely, photons and charge carriers, bound together according to the theory of quantum electrodynamics. However, a development which explores the quantum optics of quantum conductors has emerged only very recently as an interdisciplinary field with new prospects in creating and controlling quantum microwave radiation via quantum electronics. Particularly powerful are devices based on the key components of superconducting quantum electrical circuits, namely, dc-voltage biased Josephson junctions (JJ) and microwave resonators. They allow reaching the domain of strong charge-light interaction in combination with a basically perfect conversion of electrical into photonic energy. The goal of this project is to push forward the full potential of this new class of devices by designing new light sources for quantum microwaves and by exploring the passage from weak to strong charge-photon coupling. The platforms we will use are conceptually related to those developed in circuit Quantum Electrodynamics (cQED), the solid state analog of cavity QED that gained Haroche and Wineland the Nobel Prize in 2012. However, in Josephson photonics a dc-voltage bias is applied to the JJ which allows to address the Cooper pair transfer directly and to induce charge-photon coupling far from equilibrium. This regime is particularly useful for fast quantum microwave devices but forms the last gap in our understanding of Josephson physics: the crossover from the conventional Josephson regime, where the superconducting phase difference across the JJ is almost a classical variable, to the Coulomb blockade regime, where the transferred charge is almost a good quantum number that evolves via incoherent tunnel events. Quantum mechanically, phase difference and number of transferred Cooper pairs form a set of conjugate variables linked by a Heisenberg uncertainty relation. The intermediate regime, where neither phase nor charge are good quantum numbers, has not been investigated in depth.Specifically, we will show that the flow of Cooper pairs through dc-biased JJs can be exploited to produce useful devices in form of bright sources for non-classical light and amplifiers, whose noise temperature approaches the limits allowed by quantum mechanics. After demonstrating the principles at work in the microwave range, we will adapt some of these devices to the THz range. In a complementary effort, we will investigate the quantum to classical transition of JJs. We will use a particular feature of these devices which allows monitoring both: non-classical photon states and charge current noise. This project brings together the profound expertise of two experimental and one theory group.