SINGLEOUT · Single-Photon Microwave Devices: era of quantum optics outside cavities

The past couple of years have witnessed the rise of on-chip quantum optics. This has been enabled by the fabrication of high-finesse superconducting resonators made out of coplanar waveguides, and by the coupling of these resonators to superconducting quantum bits, qubits. This so-called circuit quantum electrodynamics (cQED) has proven superior compared with the standard cavity QED with photons coupled to atoms in three-dimensional space. Namely, the coupling of the cavity photons with the qubit has reached strengths completely out of reach with traditional techniques. The energy levels and their populations in the qubits can be controlled in-situ, which has also offered the possibility prepare the quantum mechanical state of the photons in the cavity to arbitrary superpositions of the low-lying photon number states.Although great focus is put worldwide into cQED in superconducting cavities, the field of manipulation and measurement of single microwave photons outside cavities is essentially missing. This ERC starting grant project, aims to expand the power of microwave photons witnessed in cavities to free photons in waveguides. The cornerstone is the design and implementation of a single-photon click detector for microwaves. The detector will allow for the single-shot measurement of the photon state in the waveguide in a similar fashion as the photon detectors are routinely used in optical quantum computing (OQC). Thus together with the already demonstrated single-photon source, the detector will be a critical step towards quantum information processing with microwave photons. In addition to opening this novel field in physics, the detector can be utilized in the characterization of microwave components and devices at ulra-high sensitivities. In this project, we will implement a platform for such characterization and build several circuit elements to manipulate single microwave photons in the same way as beam splitters are used in OQC.