We present an experimental investigation of stochastic switching of a bistable Josephson junctions array resonator with a resonance frequency in the GHz range. As the device is in the regime where the anharmonicity is on the order of the linewidth, the bistability appears for a pump strength of only a few photons. We measure the dynamics of the bistability by continuously observing the jumps between the two metastable states, which occur with a rate ranging from a few Hz down to a few mHz. The switching rate strongly depends on the pump strength, readout strength and the temperature, following Kramer's law. The interplay between nonlinearity and coupling, in this little explored regime, could provide a new resource for nondemolition measurements, single photon switches or even elements for autonomous quantum error correction.The non-linearity provided by atoms and Josephson junctions is a necessary ingredient to observe quantum mechanical effects in cavity quantum-electro-dynamics (QED) and circuit QED (cQED) systems. Strong nonlinearites, much larger than the linewidth of the transition, are required to realize qubits 1 , implement quantum information protocols 2,3 and realize textbook quantum optics experiments 4,5 . Non-linearities much smaller than the linewidth of the transition are typically exploited for parametric processes 6-8 like amplification or frequency conversion at the quantum level.Besides quantum information applications, there has been a growing interest to exploit cavity QED for ultralow-power classical logic elements 9-11 . This interest has been sparked by the ever growing all optical communication networks. Remarkably, a single photon transistor 12 , reminiscent of an electronic transistor, has been implemented for the optical domain. In this device a single photon can switch a large optical field. Realizing such devices has been a challenging endeavour as the required non-linearity is hard to realize, due to the weak interaction of optical light with atoms.Much stronger light matter interactions can be achieved in the microwave regime using the cQED platform. In this context Josephson junction arrays (JJAs) have proven to be an ideal circuit element to build superconducting qubits with excellent coherence properties and unique tuning capabilities [13][14][15] . Similarly, JJAs have also been used to build quantum limited parametric amplifiers 8,[16][17][18] . Recently, the coherence properties of the self resonances of JJAs 19,20 , as well as their self-Kerr and cross-Kerr coefficients have been measured 21 . The measured Kerr coefficient showed good agreement with a model based on a second order expansion of the Josephson potential 22 . A regime of particular interest arises when the selfKerr K i and cross-Kerr K ij nonlinear coefficients are on the order of the linewidth κ of the system. In this regime the system will show a pronounced bistability 23,24 at the single to few photon level. Bistability is a phenomenon which is relevant in many fields, ranging from chemistry 25 and biology ...

Fast magnetic flux control is a crucial ingredient for circuit quantum electrodynamics (cQED) systems. So far, it has been a challenge to implement this technology with the high coherence 3D cQED architecture. In this paper, we control the magnetic field inside a superconducting waveguide cavity using a magnetic hose, which allows flux control of 3D transmon qubits on time scales less than 100 ns while maintaining a cavity quality factor larger than 106. The magnetic hose is designed as an effective microwave filter to not compromise the energy relaxation time of the qubit. The magnetic hose is a promising tool for fast magnetic flux control in various platforms intended for quantum information processing and quantum optics.

One of the cornerstones of quantum communication is the unconditionally secure distribution of classical keys between remote parties. This key feature of quantum technology is based on the quantum properties of propagating electromagnetic waves, such as entanglement, or the no-cloning theorem. However, these quantum resources are known to be susceptible to noise and losses, which are omnipresent in open-air communication scenarios. In this work, we theoretically investigate the perspectives of continuous-variable open-air quantum key distribution at microwave frequencies. In particular, we present a model describing the coupling of propagating microwaves with a noisy environment. Using a protocol based on displaced squeezed states, we demonstrate that continuous-variable quantum key distribution with propagating microwaves can be unconditionally secure at room temperature up to distances of around 200 meters. Moreover, we show that microwaves can potentially outperform conventional quantum key distribution at telecom wavelength at imperfect weather conditions.