The low-noise amplification of weak microwave signals is crucial for countless protocols in quantum information processing. Quantum mechanics sets an ultimate lower limit of half a photon to the added input noise for phase-preserving amplification of narrowband signals, also known as the standard quantum limit (SQL). This limit, which is equivalent to a maximum quantum efficiency of 0.5, can be overcome by employing nondegenerate parametric amplification of broadband signals. We show that, in principle, a maximum quantum efficiency of unity can be reached. Experimentally, we find a quantum efficiency of 0.69 +/- 0.02, well beyond the SQL, by employing a flux-driven Josephson parametric amplifier and broadband thermal signals. We expect that our results allow for fundamental improvements in the detection of ultraweak microwave signals.

A quantum memory has to meet the conflicting requirements of strong coupling for fast readout and weak coupling for long storage. Multimode rectangular superconducting 3D cavities are known to satisfy both properties. Here, we systematically study the external coupling to the two lowest-frequency modes of an aluminum cavity. First, we introduce a general analytical scheme to describe the capacitive coupling of the antenna pin and validate this model experimentally. On this basis, we engineer an antenna which is overcoupled to the first mode, but undercoupled to the second mode.

Vom wissenschaftlichen Nischenthema zum international anerkannten Forschungsfeld: Quantenmikrowellen eröffnen viele Anwendungsperspektiven, für die sich auch die Industrie interessiert.

We have fabricated and studied a system of two tunable and coupled nonlinear superconducting resonators. The nonlinearity is introduced by galvanically coupled dc superconducting quantum interference devices. We simulate the system response by means of a circuit model, which includes an additional signal path introduced by the electromagnetic environment. Furthermore, we present two methods allowing us to experimentally determine the nonlinearity. First, we fit the measured frequency and flux dependence of the transmission data to simulations based on the equivalent circuit model. Second, we fit the power dependence of the transmission data to a model that is predicted by the nonlinear equation of motion describing the system. Our results show that we are able to tune the nonlinearity of the resonators by almost two orders of magnitude via an external coil and two on-chip antennas. The studied system represents a basic building block for larger systems, allowing for quantum simulations of bosonic many-body systems with a larger number of lattice sites.

Mit dem Sycamore-Quantenprozessor von Google gelang zum ersten Mal überzeugend ein Experiment, in dem ein Quantensystem ein Problem besser löst als derzeit verfügbare herkömmliche Supercomputer. Die Hardware basiert auf der Technologie der supraleitenden Quantenschaltkreise. Ihr wird schon länger ein besonders großes Skalierungspotenzial hin zu mehr Quantenbits bescheinigt. Der verwendete Chip besitzt 53 Qubits. Sie sind in einem zweidimensionalen quadratischen Gitter angeordnet und durch Nächste-Nachbar-Wechselwirkung gekoppelt. Somit stellt das Experiment einen großen technologischen Fortschritt für das gesamte Feld der Quantenwissenschaften und -technologien dar. Obwohl der praktische Nutzen derzeit noch gering erscheint, sind die Arbeiten des Google-Teams ein wichtiger Schritt hin zu skalierbarem Quantenrechnen. Damit erscheint erstmals eine fehlerkorrigierte, supraleitende Quantencomputer-Architektur in nicht allzu ferner Zukunft möglich.

Light-matter interaction in optomechanical systems is the foundation for ultra-sensitive detection schemes as well as the generation of phononic and photonic quantum states. Electromechanical systems realize this optomechanical interaction in the microwave regime. In this context, capacitive coupling arrangements demonstrated interaction rates of up to 280Hz. Complementary, early proposals and experiments suggest that inductive coupling schemes are tunable and have the potential to reach the single-photon strong-coupling regime. Here, we follow the latter approach by integrating a partly suspended superconducting quantum interference device (SQUID) into a microwave resonator. The mechanical displacement translates into a time varying flux in the SQUID loop, thereby providing an inductive electromechanical coupling. We demonstrate a sideband-resolved electromechanical system with a tunable vacuum coupling rate of up to 1.62kHz, realizing sub-aNHz(-1/2) force sensitivities. The presented inductive coupling scheme shows the high potential of SQUID-based electromechanics for targeting the full wealth of the intrinsically nonlinear optomechanics Hamiltonian. Recently, inductively-coupled optomechanical systems have been realized. They represent an important step forward towards achieving strong light-matter interaction, offer extreme sensitivity to mechanical displacement, and allow to study quantum phenomena on a single quantum level. In this work, a superconducting device is inductively coupled to a microwave resonator forming an electromechanical system operating at the single-photon level.

Quantum communication protocols based on nonclassical correlations can be more efficient than known classical methods and offer intrinsic security over direct state transfer. In particular, remote state preparation aims at the creation of a desired and known quantum state at a remote location using classical communication and quantum entanglement. We present an experimental realization of deterministic continuous-variable remote state preparation in the microwave regime over a distance of 35 cm. By employing propagating two-mode squeezed microwave states and feedforward, we achieve the remote preparation of squeezed states with up to 1.6 dB of squeezing below the vacuum level. Finally, security of remote state preparation is investigated by using the concept of the one-time pad and measuring the von Neumann entropies. We find nearly identical values for the entropy of the remotely prepared state and the respective conditional entropy given the classically communicated information and, thus, demonstrate close-to-perfect security.