Abstract The adiabatic quantum-flux-parametron (AQFP), which can operate with ultra-low energy consumption, has attracted attention as an alternative to semiconductor circuits. The development of reconfigurable AQFP devices is essential for advancing AQFP integrated circuits. However, research on reconfigurable devices at the logic gate level remains limited. We propose an AQFP circuit capable of reconfiguring its logic function by controlling the superconducting phase shift through externally applied magnetic flux. In AQFP logic, logic ‘1’ and ‘0’ are represented by the polarity of the current. The proposed circuit can switch between the Buffer function, which propagates the input current with its polarity unchanged, and the NOT function, which propagates it with the polarity inverted. We analytically derived the parameter conditions for correct operation and determined the parameters within this range to achieve adiabatic operation and sufficient element margins for experimental demonstration. We fabricated the proposed reconfigurable circuit using the 10 kA cm −2 Nb four-layer superconducting circuit fabrication process, and experimentally confirmed correct operation for both the Buffer and NOT functions. The control current margins, which are responsible for the functional reconfiguration, were −2.3% to +31.2% for chip 1 and +0.8% to +35.1% for chip 2. Both margins had widths comparable to the simulation result. This work presents the first experimental demonstration of an AQFP circuit whose function can be reconfigured using only a control current. We believe this work will contribute to the advancement of reconfigurable AQFP devices.
We present an approach to achieve efficient single-photon frequency conversion in the microwave domain based on coherent control in superconducting quantum circuits, which consist of a driven artificial atom coupled to a semi-infinite transmission line. Using the full quantum-mechanical method, we analyze the single-photon scattering process in this system and find that single-photon frequency up- or down-conversion with efficiency close to unity can be achieved by adjusting the parameters of the control field applied to the artificial atom. We further show that our approach is experimentally feasible in currently available superconducting flux qubit circuits.
Hybrid quantum systems play an essential role in efforts towards the advancements of quantum information technology and quantum sensing by bridging distinct technological platforms. In particular, hybrid systems comprising superconducting microwave circuits are being pursued intensely in light of existing achievements in superconducting qubits and precise quantum control in microwave circuits. Their realization relies on the design of superconducting resonators capable of coupling to an oscillatory mode of a second quantum system, facilitating the probing and manipulation of one via the other. In order to accommodate the coupled system, its particular constraints and requirements must be respected, which may be quite extreme from the perspective of established superconducting circuit design and operation. Overcoming this challenge has proved to be a substantial obstacle for the attainment of strongly coupled hybrid systems, despite spectacular advances in superconducting microwave circuits individually. In this work, we examine two approaches tackling this issue for different hybrid quantum systems, focusing on the optimization and characterization of the superconducting circuits. The first device aims at a superconductor-atom hybrid system through the coupling to Rydberg-Rydberg transitions in ultra-cold Rb-87 atoms trapped near the superconducting chip. Using simulations in conjunction with theoretical considerations, we demonstrate a comprehensive geometry parameter optimization procedure for maximum coupling rate. Presenting an experimental implementation of the resulting circuit, we validate its suitability for use under the intended operation conditions, including the option of tuning the electronic transition frequencies using a dc voltage. Secondly, we investigate a device designed for applications in large magnetic in-plane fields used to mediate coupling to integrated micromechanical oscillators; more specifically it is a flux-tunable SQUID resonator comprising Josephson nano-constrictions as nonlinear inductive components. Performing a thorough characterization of its frequency and Kerr anharmonicity tuning behavior at fields up to several hundred mT, we discover a pronounced asymmetry in its flux response at large fields. We show this asymmetry to be caused by the nano-constrictions turning into Josephson diodes, develop an intuitive macroscopic model for its field-induced emergence and reconstruct the diode current-phase relation from the flux tuning data. Several figures of merit informing the suitability of the SQUID resonators for optomechanical systems as well as other applications are improved by the diode effect, highlighting the potential of Josephson diodes in superconducting microwave circuits. Our results provide a promising stepping stone for the advancement of hybrid systems involving superconducting microwave circuits, putting new and exciting operation and coupling regimes within reach.