The key issue for the implementation of a metamaterial is to demonstrate the existence of collective modes corresponding to coherent oscillations of the meta-atoms. Atoms of natural materials interact with electromagnetic fields as quantum two-level systems. Artificial quantum two-level systems can be made, for example, using superconducting nonlinear resonators cooled down to their ground state. Here we perform an experiment in which 20 of these quantum meta-atoms, so-called flux qubits, are embedded into a microwave resonator. We observe the dispersive shift of the resonator frequency imposed by the qubit metamaterial and the collective resonant coupling of eight qubits. The realized prototype represents a mesoscopic limit of naturally occurring spin ensembles and as such we demonstrate the AC-Zeeman shift of a resonant qubit ensemble. The studied system constitutes the implementation of a basic quantum metamaterial in the sense that many artificial atoms are coupled collectively to the quantized mode of a photon field.

We study the loss rate for a set of $\lambda /2$ coplanar waveguide resonators at millikelvin temperatures (20–900 mK) and different applied powers $\left(3\cdot {10}^{-19}–{10}^{-12}\text{W}\right)$ . The loss rate becomes power independent below a critical power. For a fixed power, the loss rate increases significantly with decreasing temperature. We show that this behavior can be caused by two-level systems in the surrounding dielectric materials. Interestingly, the influence of the two-level systems is of the same order of magnitude for the different material combinations. That leads to the assumption that the nature of these two-level systems is material independent.

We present a portable optically pumped magnetometer instrument for ultrasensitive measurements within the Earth’s magnetic field. The central part of the system is a sensor head operating a MEMS-based Cs vapor cell in the light-shift dispersed $M_z$ mode. It is connected to a compact, battery-driven electronics module by a flexible cable. We briefly review the working principles of the device and detail the realization of both the sensor head and electronics. We show shielded and unshielded measurements within a static magnetic field amplitude of $50\mu \mathrm{T}$, demonstrating a noise level of the sensor system down to $140\mathrm{fT}/\sqrt{\mathrm{Hz}}$ and a sensor bandwidth of several 100 Hz. In a detailed analysis of sensor noise, we reveal the system to be limited by technical sources with straightforward strategies for further improvement toward its fundamental noise limit of $12\mathrm{fT}/\sqrt{\mathrm{Hz}}$. We assess the parameters defining the sensor bandwidth by theoretical modeling based on the Bloch equations. Finally, we compare our sensors’ performance to a commercial superconducting quantum interference device system in a measurement environment typical for geomagnetic observatory practice and geomagnetic prospection.

We construct a microwave detector based on the voltage switching of an underdamped Josephson junction that is positioned at a current antinode of a $\lambda /4$ coplanar waveguide resonator. By measuring the switching current and the transmission through a waveguide capacitively coupled to the resonator at different drive frequencies and temperatures, we are able to fully characterize the system and assess its detection efficiency and sensitivity. Testing the detector by applying a classical microwave field with the strength of a single photon yields a sensitivity parameter of 0.5, in qualitative agreement with theoretical calculations.

We demonstrate the narrow switching distribution of an underdamped Josephson junction from the zero to the finite voltage state at millikelvin temperatures. We argue that such junctions can be used as ultrasensitive detectors of the single photons in the GHz range, operating close to the quantum limit: a given initial (zero voltage) state can be driven by an incoming signal to the finite voltage state. The width of the switching distribution at a nominal temperature of about T = 10 mK was 4.5 nA, which corresponds to an effective noise temperature of the device below 60 mK.

We demonstrate amplification of a microwave signal by a strongly driven two-level system in a coplanar waveguide resonator. The effect, similar to the dressed-state lasing known from quantum optics, is observed with a single quantum system formed by a persistent current (flux) qubit. The transmission through the resonator is enhanced when the Rabi frequency of the driven qubit is tuned into resonance with one of the resonator modes. Amplification as well as linewidth narrowing of a weak probe signal has been observed. The stimulated emission in the resonator has been studied by measuring the emission spectrum. We analyzed our system and found an excellent agreement between the experimental results and the theoretical predictions obtained in the dressed-state model.